Electronics

Connectors and Cables

2010-04-10T03:51:00.000-07:00

Connectors and Cables

Battery clips and holders:

The standard battery clip fits a 9V PP3 battery and many battery holders such as the 6 × AA cell holder shown. Battery holders are also available with wires attached, with pins for PCB mounting, or as a complete box with lid, switch and wires.

Many small electronic projects use a 9V PP3 battery but if you wish to use the project for long periods a better choice is a battery holder with 6 AA cells. This has the same voltage but a much longer battery life and it will work out cheaper in the long run.

Larger battery clips fit 9V PP9 batteries but these are rarely used now.

Terminal blocks and PCB terminals:

Terminal blocks are usually supplied in 12-way lengths but they can be cut into smaller blocks with a sharp knife, large wire cutters or a junior hacksaw. They are sometimes called 'chocolate blocks' because of the way they can be easily cut to size.

PCB mounting terminal blocks provide an easy way of making semi-permanent connections to PCBs. Many are designed to interlock to provide more connections.

Crocodile clips:

The 'standard' crocodile clip has no cover and a screw contact. However, miniature insulated crocodile clips are more suitable for many purposes including test leads. They have a solder contact and lugs which fold down to grip the cable's insulation, increasing the strength of the joint. Remember to feed the cable through the plastic cover before soldering! Add and remove the cover by fully opening the clip, a piece of wood can be used to hold the jaws open.

4mm plugs, sockets and terminals:

These are the standard single pole connectors used on meters and other electronic equipment. They are capable of passing high currents (typically 10A) and most designs are very robust. Shrouded plugs and sockets are available for use with high voltages where there is a risk of electric shock. A wide variety of colours is available from most suppliers.

Plugs:

Plugs may have a screw or solder terminal to hold the cable. Check if you need to thread the cable through the cover before connecting it. Some plugs, such as those illustrated, are 'stackable' which means that they include a socket to accept another plug, allowing several plugs to be connected to the same point - a very useful feature for test leads.

Sockets:

These are usually described as 'panel mounting' because they are designed to be fitted to a case. Most sockets have a solder contact but the picture shows other options. Fit the socket in the case before attaching the wire otherwise you will be unable to add the mounting nut.

Terminals:

In addition to a socket these have provision for attaching a wire by threading it through a hole (or wrapping it around the post) and tightening the top nut by hand. They usually have a threaded stud to fit a solder tag inside the case.

2mm plugs and sockets:

These are smaller versions of the 4mm plugs and sockets described above, but terminals are not readily available. The plugs illustrated are stackable. Despite their small size these connectors can pass large currents and some are rated at 10A.

DC power plugs and sockets:

These 2-pole plugs and sockets ensure that the polarity of a DC supply cannot be accidentally reversed. The standard sizes are 2.1 and 2.5mm plug diameter. Standard plugs have a 10mm shaft, 'long' plugs have a 14mm shaft. Sockets are available for PCB or chassis mounting and most include a switch on the outer contact which is normally used to disconnect an internal battery when a plug is inserted.

Miniature versions with a 1.3mm diameter plug are used where small size is essential, such as for personal cassette players.

Jack plugs and sockets:

These are intended for audio signals so mono and stereo versions are available. The sizes are determined by the plug diameter: ¼" (6.3mm), 3.5mm and 2.5mm. The 2.5mm size is only available for mono.

Screened plugs have metal bodies connected to the COM contact. Most connections are soldered, remember to thread cables through plug covers before soldering! Sockets are designed for PCB or chassis mounting.

¼" plug connections are similar to those for 3.5mm plugs shown below. ¼" socket connections are COM, R and L in that order from the mounting nut, ignore R for mono use. Most ¼" sockets have switches on all contacts which open as the plug is inserted so they can be used to isolate internal speakers for example.

The connections for 3.5mm plugs and sockets are shown below. Plugs have a lug which should be folded down to grip the cable's insulation and increase the strength of the joint. 3.5mm mono sockets have a switch contact which can be used to switch off an internal speaker as the plug is inserted. Ignore this contact if you do not require the switching action.

L = left channel signal

R = right channel signal

COM = common (0V, screen)

Do not use jack plugs for power supply connections because the contacts may be briefly shorted as the plug is inserted. Use DC power connectors for this.

Phono plugs and sockets:

These are used for screened cables carrying audio and video signals. Stereo connections are made using a pair of phono plugs and sockets. The centre contact is for the signal and the outer contact for the screen (0V, common). Screened plugs have metal bodies connected to the outer contact to give the signal additional protection from electrical noise. Sockets are available for PCB or chassis mounting, singly for mono, or in pairs for stereo. Line sockets are available for making extension leads.

Coax plugs and sockets:

These are similar to the phono plugs and sockets described above but they are designed for use with screened cables carrying much higher frequency signals, such as TV aerial leads. They provide better screening because at high frequencies this is essential to reduce electrical noise.

BNC plugs and sockets:

These are designed for screened cables carrying high frequency signals where an undistorted and noise free signal is essential, for example oscilloscope leads. BNC plugs are connected with a push and twist action, to disconnect you need to twist and pull.

Plugs and sockets are rated by their impedance (50Ω or 75Ω) which must be the same as the cable's impedance. If the connector and cable impedances are not matched the signal will be distorted because it will be partly reflected at the connection, this is the electrical equivalent of the weak reflection which occurs when light passes through a glass window.

DIN plugs and sockets:

These are intended for audio signals but they can be used for other low-current purposes where a multi-way connector is required. They are available from 3 way to 8 way. 5 way is used for stereo audio connections. The contacts are numbered on the connector, but they are not in numerical order! For audio use the 'common' (0V) wire is connected to contact 2. 5 way plugs and sockets are available in two versions: 180° and 270° (the angle refers to the arc formed by the contacts).

Plastic covers of DIN plugs (and line sockets) are removed by depressing the retaining lug with a small screwdriver. You may also need small pliers to extract the body from the cover but do not pull on the pins themselves to avoid damage. Remember to thread the cable through the cover before starting to solder the connections!

Soldering DIN plugs is easier if you clamp the insert with the pins. Wires should be pushed into the hollow pins - first 'tin' the wires (coat them with a thin layer of solder) then melt a little solder into the hollow pin and insert the wire while keeping the solder molten. Take care to avoid melting the plastic base, stop and allow the pin to cool if necessary.

Mini-DIN connectors are used for computer equipment such as keyboards and mice but they are not a good choice for general use unless small size is essential.

D connectors:

These are multi-pole connectors with provision for screw fittings to make semi-permanent connections, for example on computer equipment. The D shape prevents incorrect connection. Standard D-connectors have 2 rows of contacts (top picture); 9, 15 and 25-way versions are the most popular. High Density D-connectors have 3 rows of contacts (bottom picture); a 15-way version is used to connect computer monitors for example.

Note that covers (middle picture) are usually sold separately because both plugs and sockets can be fitted to cables by fitting a cover to a chassis mounted connector. PCB mounting versions of plugs and sockets are also available. The contacts are usually numbered on the body of the connector, although you may need a magnifying glass to see the very small markings. Soldering D-connectors requires a steady hand due to the closeness of the contacts, it is easy to accidently unsolder a contact you have just completed while attempting to solder the next one!

IDC communication connectors:

These multi-pole insulation displacement connectors are used for computer and telecommunications equipment. They automatically cut through the insulation on wires when installed and special tools are required to fit them. They are available as 4, 6 and 8-way versions.

The 8-way RJ45 is the standard connector for modern computer networks. If you regularly use these you may be interested in our network lead tester project.

Standard UK telephone connectors are similar in style but a slightly different shape. They are called BT (British Telecom) connectors.

Cables:

Cable... flex... lead... wire... what do all these terms mean?

A cable is an assembly of one or more conductors (wires) with some flexibility.

A flex is the proper name for the flexible cable fitted to mains electrical appliances.

A lead is a complete assembly of cable and connectors.

A wire is a single conductor which may have an outer layer of insulation (usually plastic).

Single core equipment wire:

This is one solid wire with a plastic coating available in a wide variety of colours. It can be bent to shape but will break if repeatedly flexed. Use it for connections which will not be disturbed, for example links between points of a circuit board.

Typical specification: 1/0.6mm (1 strand of 0.6mm diameter), maximum current 1.8A.

Stranded wire:

This consists of many fine strands of wire covered by an outer plastic coating. It is flexible and can withstand repeated bending without breaking. Use it for connections which may be disturbed, for example wires outside cases to sensors and switches. A very flexible version ('extra-flex') is used for test leads.

Typical specifications:

10/0.1mm (10 strands of 0.1mm diameter), maximum current 0.5A.

7/0.2mm (7 strands of 0.2mm diameter), maximum current 1.4A.

16/0.2mm (16 strands of 0.2mm diameter), maximum current 3A.

24/0.2mm (24 strands of 0.2mm diameter), maximum current 4.5A.

55/0.1mm (55 strands of 0.1mm diameter), maximum current 6A, used for test leads.

(speaker) cable:

cable consists of two stranded wires arranged in a figure of 8 shape. One wire is usually marked with a line. It is suitable for low voltage, low current (maximum 1A) signals where screening from electrical interference is not required. It is a popular choice for connecting loudspeakers and is often called 'speaker cable'.

Signal cable:

Signal cable consists of several colour-coded cores of stranded wire housed within an outer plastic sheath. With a typical maximum current of 1A per core it is suitable for low voltage, low current signals where screening from electrical interference is not required.

The picture shows 6-core cable, but 4-core and 8-core are also readily available.

Screened cable:

The diagram shows the construction of screened cable. The central wire carries the signal and the screen is connected to 0V (common) to shield the signal from electrical interference. Screened cable is used for audio signals and dual versions are available for stereo.

Co-axial cable:

This type of screened cable (see above) is designed to carry high frequency signals such as those found in TV aerials and oscilloscope leads.

Mains flex:

Flex is the proper name for the flexible cable used to connect appliances to the mains supply. It contains 2 cores (for live and neutral) or 3 cores (for live, neutral and earth). Mains flex has thick insulation for the high voltage (230V in UK) and it is available with various current ratings: 3A, 6A and 13A are popular sizes in the UK.

Mains flex is sometimes used for low voltage circuits which pass a high current, but please think carefully before using it in this way. The distinctive colours of mains flex should act as a warning of the mains high voltage which can be lethal; using mains flex for low voltage circuits can undermine this warning.

Reference:

www.kpsec.freeuk.com

Electronics

2009-10-12T09:03:00.001-07:00

Electronics

This article is about the engineering discipline. For consumer electronic devices, see Consumer electronics.

The field of electronics comprises the study and use of systems that operate by controlling the flow of electrons (or other charge carriers) in devices such as thermionic valves (vacuum tubes) and semiconductors. The design and construction of electronic circuits to solve practical problems is an integral technique in the field of electronics engineering and is equally important in hardware design for computer engineering. All applications of electronics involve the transmission of either information or power. Most deal only with information.

The study of new semiconductor devices and surrounding technology is sometimes considered a branch of physics. This article focuses on engineering aspects of electronics. Other important topics include electronic waste and occupational health impacts of semiconductor manufacturing.

Overview of electronic systems and circuits:

Electronic systems are used to perform a wide variety of tasks. The main uses of electronic circuits are:

The controlling and processing of data.

The conversion to/from and distribution of electric power.

Both these applications involve the creation and/or detection of electromagnetic fields and electric currents. While electrical energy had been used for some time prior to the late 19th century to transmit data over telegraph and telephone lines, development in electronics grew exponentially after the advent of radio.

One way of looking at an electronic system is to divide it into 3 parts:

Inputs – Electronic or mechanical sensors (or transducers). These devices take signals/information from external sources in the physical world (such as antennas or technology networks) and convert those signals/information into current/voltage or digital (high/low) signals within the system.

Signal processors – These circuits serve to manipulate, interpret and transform inputted signals in order to make them useful for a desired application. Recently, complex signal processing has been accomplished with the use of Digital Signal Processors.

Outputs – Actuators or other devices (such as transducers) that transform current/voltage signals back into useful physical form (e.g., by accomplishing a physical task such as rotating an electric motor).

For example, a television set contains these 3 parts. The television's input transforms a broadcast signal (received by an antenna or fed in through a cable) into a current/voltage signal that can be used by the device. Signal processing circuits inside the television extract information from this signal that dictates brightness, colour and sound level. Output devices then convert this information back into physical form. A cathode ray tube transforms electronic signals into a visible image on the screen. Magnet-driven speakers convert signals into audible sound.

Electronic devices and components:

An electronic component is any indivisible electronic building block packaged in a discrete form with two or more connecting leads or metallic pads. Components are intended to be connected together, usually by soldering to a printed circuit board, to create an electronic circuit with a particular function (for example an amplifier, radio receiver, or oscillator). Components may be packaged singly (resistor, capacitor, transistor, diode etc.) or in more or less complex groups as integrated circuits (operational amplifier, resistor array, logic gate etc). Active components are sometimes called devices rather than components.

Types of circuits:

Analog circuits:

Most analog electronic appliances, such as radio receivers, are constructed from combinations of a few types of basic circuits. Analog circuits use a continuous range of voltage as opposed to discrete levels as in digital circuits. The number of different analog circuits so far devised is huge, especially because a 'circuit' can be defined as anything from a single component, to systems containing thousands of components.

Analog circuits are sometimes called linear circuits although many non-linear effects are used in analog circuits such as mixers, modulators, etc. Good examples of analog circuits include vacuum tube and transistor amplifiers, operational amplifiers and oscillators.

Some analog circuitry these days may use digital or even microprocessor techniques to improve upon the basic performance of the circuit. This type of circuit is usually called 'mixed signal'.

Sometimes it may be difficult to differentiate between analog and digital circuits as they have elements of both linear and non-linear operation. An example is the comparator which takes in a continuous range of voltage but puts out only one of two levels as in a digital circuit. Similarly, an overdriven transistor amplifier can take on the characteristics of a controlled switch having essentially two levels of output.

Digital circuits:

Digital circuits are electric circuits based on a number of discrete voltage levels. Digital circuits are the most common physical representation of Boolean algebra and are the basis of all digital computers. To most engineers, the terms "digital circuit", "digital system" and "logic" are interchangeable in the context of digital circuits. In most cases the number of different states of a node is two, represented by two voltage levels labeled "Low" and "High". Often "Low" will be near zero volts and "High" will be at a higher level depending on the supply voltage in use.

Computers, electronic clocks, and programmable logic controllers (used to control industrial processes) are constructed of digital circuits. Digital Signal Processors are another example.

Building-blocks:

logic gates

Adders

Binary Multipliers

flip-flops

counters

registers

multiplexers

Schmitt triggers

Highly integrated devices:

microprocessors

microcontrollers

Application-specific integrated circuit(ASIC)

Digital signal processor (DSP)

Field Programmable Gate Array (FPGA)

Mixed-signal circuits:

Mixed-signal circuits refers to integrated circuits (ICs) which have both analog circuits and digital circuits combined on a single semiconductor die or on the same circuit board. Mixed-signal circuits are becoming increasingly common. Mixed circuits contain both analog and digital components. Analog to digital converters and digital to analog converters are the primary examples. Other examples are transmission gates and buffers.

Heat dissipation and thermal management:

Heat generated by electronic circuitry must be dissipated to prevent immediate failure and improve long term reliability. Techniques for heat dissipation can include heatsinks and fans for air cooling, and other forms of computer cooling such as water cooling. These techniques use convection, conduction, & radiation of heat energy.

Noise:

Noise is associated with all electronic circuits. Noise is generally defined as any unwanted signal that is not present at the input of a circuit. Noise is not the same as signal distortion caused by a circuit.

Electronics theory:

Mathematical methods are integral to the study of electronics. To become proficient in electronics it is also necessary to become proficient in the mathematics of circuit analysis.

Circuit analysis is the study of methods of solving generally linear systems for unknown variables such as the voltage at a certain node or the current though a certain branch of a network. A common analytical tool for this is the SPICE circuit simulator. Also important to electronics is the study and understanding of electromagnetic field theory.

Electronic test equipment:

Electronic test equipment is used to create stimulus signals and capture responses from electronic Devices Under Test (DUTs). In this way, the proper operation of the DUT can be proven or faults in the device can be traced and repaired.

Practical electronics engineering and assembly requires the use of many different kinds of electronic test equipment ranging from the very simple and inexpensive (such as a test light consisting of just a light bulb and a test lead) to extremely complex and sophisticated such as Automatic Test Equipment.

Computer aided design (CAD):

Today's electronics engineers have the ability to design circuits using premanufactured building blocks such as power supplies, semiconductors (such as transistors), and integrated circuits. Electronic design automation software programs include schematic capture programs and pcb design programs. Popular names in the EDA software world are MultiSIM, Cadence (ORCAD), Eagle PCB and Schematic, Mentor (PADS PCB and LOGIC Schematic), Altium (Protel), and many others. The programs can cost a few hundred dollars up to tens of thousands, depending upon complexity. For example, some programs can automatically "route" a PCB with surprising accuracy, while some programs can perform thermal and electrical analysis.

Construction methods:

Many different methods of connecting components have been used over the years. For instance, in the beginning point to point wiring using tag boards attached to chassis were used to connect various electrical innards. Cordwood construction and wire wraps were other methods used. Most modern day electronics now use printed circuit boards (made of FR4), and highly integrated circuits. Health and environmental concerns associated with electronics assembly have gained increased attention in recent years, especially for products destined to the European Union, with its Restriction of Hazardous Substances Directive (RoHS) and Waste Electrical and Electronic Equipment Directive (WEEE), which went into force in July 2006.

Reference:

en.wikipedia.org

Capacitor

2009-10-12T09:02:00.002-07:00

Capacitor

A capacitor or condenser is a passive electronic component consisting of a pair of conductors separated by a dielectric. When a voltage potential difference exists between the conductors, an electric field is present in the dielectric. This field stores energy and produces a mechanical force between the plates. The effect is greatest between wide, flat, parallel, narrowly separated conductors.
An ideal capacitor is characterized by a single constant value, capacitance, which is measured in farads. This is the ratio of the electric charge on each conductor to the potential difference between them. In practice, the dielectric between the plates passes a small amount of leakage current. The conductors and leads introduce an equivalent series resistance and the dielectric has an electric field strength limit resulting in a breakdown voltage.
The properties of capacitors in a circuit may determine the resonant frequency and quality factor of a resonant circuit, power dissipation and operating frequency in a digital logic circuit, energy capacity in a high-power system, and many other important system characteristics.

History:
In October 1745, Ewald Georg von Kleist of Pomerania in Germany found that charge could be stored by connecting a generator by a wire to a volume of water in a hand-held glass jar. Von Kleist's hand and the water acted as conductors and the jar as a dielectric. Von Kleist found that after removing the generator, touching the wire resulted in a painful spark. In a letter describing the experiment, he said "I would not take a second shock for the kingdom of France." The following year, the Dutch physicist Pieter van Musschenbroek invented a similar capacitor, which was named the Leyden jar, after the University of Leyden where he worked. Daniel Gralath was the first to combine several jars in parallel into a "battery" to increase the charge storage capacity.
Benjamin Franklin investigated the Leyden jar, and proved that the charge was stored on the glass, not in the water as others had assumed. Leyden jars began to be made by coating the inside and outside of jars with metal foil, leaving a space at the mouth to prevent arcing between the foils. The earliest unit of capacitance was the 'jar', equivalent to about 1 nanofarad.
Leyden jar or flat glass plate construction was used exclusively up until about 1900, when the invention of wireless (radio) created a demand for standard capacitors, and the steady move to higher frequencies required capacitors with lower inductance. A more compact construction began to be used of a flexible dielectric sheet such as oiled paper sandwiched between sheets of metal foil, rolled or folded into a small package.
Early capacitors were also known as condensers, a term that is still occasionally used today. It was coined by Alessandro Volta in 1782 (derived from the Italian condensatore), with reference to the device's ability to store a higher density of electric charge than a normal isolated conductor. Most non-English European languages still use a word derived from "condensatore".

Theory of operation:
Charge separation in a parallel-plate capacitor causes an internal electric field. A dielectric (orange) reduces the field and increases the capacitance.A capacitor consists of two conductors separated by a non-conductive region. The non-conductive substance is called the dielectric medium, although this may also mean a vacuum or a semiconductor depletion region chemically identical to the conductors. A capacitor is assumed to be self-contained and isolated, with no net electric charge and no influence from an external electric field. The conductors thus contain equal and opposite charges on their facing surfaces, and the dielectric contains an electric field. The capacitor is a reasonably general model for electric fields within electric circuits.
An ideal capacitor is wholly characterized by a constant capacitance C, defined as the ratio of charge ±Q on each conductor to the voltage V between them:

Sometimes charge buildup affects the mechanics of the capacitor, causing the capacitance to vary. In this case, capacitance is defined in terms of incremental changes:

In SI units, a capacitance of one farad means that one coulomb of charge on each conductor causes a voltage of one volt across the device.

Energy storage:
Work must be done by an external influence to move charge between the conductors in a capacitor. When the external influence is removed, the charge separation persists and energy is stored in the electric field. If charge is later allowed to return to its equilibrium position, the energy is released. The work done in establishing the electric field, and hence the amount of energy stored, is given by:

Current-voltage relation:
The current i (t ) through a component in an electric circuit is defined as the rate of change of the charge q (t ) that has passed through it. Physical charges cannot pass through the dielectric layer of a capacitor, but rather build up in equal and opposite quantities on the electrodes: as each electron accumulates on the negative plate, one leaves the positive plate. Thus the accumulated charge on the electrodes is equal to the integral of the current, as well as being proportional to the voltage (as discussed above). As with any antiderivative, a constant of integration is added to represent the initial voltage v (t0). This is the integral form of the capacitor equation.

Taking the derivative of this, and multiplying by C, yields the derivative form.

The dual of the capacitor is the inductor, which stores energy in the magnetic field rather than the electric field. Its current-voltage relation is obtained by exchanging current and voltage in the capacitor equations and replacing C with the inductance L.

DC circuits:
A simple resistor-capacitor circuit demonstrates charging of a capacitor.A series circuit containing only a resistor, a capacitor, a switch and a constant DC source of voltage V0 is known as a charging circuit. If the capacitor is initially uncharged while the switch is open, and the switch is closed at t = 0, it follows from Kirchhoff's voltage law that

Taking the derivative and multiplying by C, gives a first-order differential equation,

At t = 0, the voltage across the capacitor is zero and the voltage across the resistor is V0. The initial current is then i (0) =V0 /R. With this assumption, the differential equation yields

where τ0 = RC is the time constant of the system.
As the capacitor reaches equilibrium with the source voltage, the voltage across the resistor and the current through the entire circuit decay exponentially. The case of discharging a charged capacitor likewise demonstrates exponential decay, but with the initial capacitor voltage replacing V0 and the final voltage being zero.

AC circuits:
Impedance, the complex sum of reactance and resistance, describes the phase difference and the ratio of amplitudes between sinusoidally varying voltage and sinusoidally varying current at a given frequency. Fourier analysis allows any signal to be constructed from a spectrum of frequencies, whence the circuit's reaction to the various frequencies may be found. The reactance and impedance of a capacitor are respectively

where j is the imaginary unit and ω is the angular velocity of the sinusoidal signal. The - j phase indicates that the AC voltage V = Z I lags the AC current by 90°: the positive current phase corresponds to increasing voltage as the capacitor charges, zero current corresponds to instantaneous constant voltage, etc.
Note that impedance decreases with increasing capacitance and increasing frequency. This implies that a higher-frequency signal or a larger capacitor results in a lower voltage amplitude per current amplitude—an AC "short circuit" or AC coupling. Conversely, for very low frequencies, the reactance will be high, so that a capacitor is nearly an open circuit in AC analysis—those frequencies have been "filtered out."
Capacitors are different from resistors and inductors in that the impedance is inversely proportional to the defining characteristic, i.e. capacitance.

Parallel plate model:
The simplest capacitor consists of two parallel conductive plates separated by a dielectric with permittivity ε. The model may also be used to make qualitative predictions for other device geometries. The plates are considered to extend uniformly over an area A and a charge density ±ρ = ±Q/A exists on their surface. Assuming that the width of the plates is much greater than their separation d, the electric field near the centre of the device will be uniform with the magnitude E = ρ/ε. The voltage is defined as the line integral of the electric field between the plates

Solving this for C = Q/V reveals that capacitance increases with area and decreases with separation.

The capacitance is therefore greatest in devices made from materials with a high permittivity.

Networks:
Capacitors in a parallel configuration each have the same applied voltage. Their capacitances add up. Charge is apportioned among them by size. Using the schematic diagram to visualize parallel plates, it is apparent that each capacitor contributes to the total surface area.

Several capacitors in series.Connected in series, the schematic diagram reveals that the separation distance, not the plate area, adds up. The capacitors each store instantaneous charge build-up equal to that of every other capacitor in the series. The total voltage difference from end to end is apportioned to each capacitor according to the inverse of its capacitance. The entire series acts as a capacitor smaller than any of its components.

Capacitors are combined in series to achieve a higher working voltage, for example for smoothing a high voltage power supply. The voltage ratings, which are based on plate separation, add up. In such an application, several series connections may in turn be connected in parallel, forming a matrix. The goal is to maximize the energy storage utility of each capacitor without overloading it.
Series connection is also used to adapt electrolytic capacitors for AC use.

Non-ideal behavior:
Capacitors deviate from the ideal capacitor equation in a number of ways. Some of these, such as leakage current and parasitic effects are linear, or can be assumed to be linear, and can be dealt with by adding virtual components to the equivalent circuit of the capacitor. The usual methods of network analysis can then be applied. In other cases, such as with breakdown voltage, the effect is non-linear and normal (i.e., linear) network analysis cannot be used, the effect must be dealt with separately. There is yet another group, which may be linear but invalidate the assumption in the analysis that capacitance is a constant. Such an example is temperature dependence.

Breakdown voltage:
Above a particular electric field, known as the dielectric strength Eds, the dielectric in a capacitor becomes conductive. The voltage at which this occurs is called the breakdown voltage of the device, and is given by the product of the dielectric strength and the separation between the conductors,

Vbd = Edsd
The maximum energy that can be stored safely in a capacitor is limited by the breakdown voltage. Due to the scaling of capacitance and breakdown voltage with dielectric thickness, all capacitors made with a particular dielectric have approximately equal maximum energy density, to the extent that the dielectric dominates their volume.
For air dielectric capacitors the breakdown field strength is of the order 107 V/m and will be rather a lot less when other materials are used for the dielectric. The absolute breakdown voltage of most capacitors is nowhere near such a high number because of the very small distance between the plates. Typical ratings for capacitors used for general electronics applications range from a few volts to 100V or so. For high voltage applications physically much larger capacitors have to be used. In this field, there are a number of factors that can dramatically reduce the breakdown voltage below that to be expected by considering the breakdown field strength of the dielectric alone. For one thing, the geometry of the capacitor conductive parts (plates and connecting wires) is important. In particular, sharp edges or points hugely increase the electric field strength at that point and can lead to a local breakdown. Once this starts to happen, the breakdown will quickly "track" through the dielectric till it reaches the opposite plate and cause a short circuit.
The usual breakdown route is that the field strength becomes large enough to pull electrons in the dielectric from their atoms thus causing conduction. Other scenarios are possible, such as impurities in the dielectric, and, if the dielectric is of a crystalline nature, imperfections in the crystal structure can result in an avalanche breakdown as seen in semi-conductor devices. Breakdown voltage is also affected by pressure, humidity and temperature.

Equivalent circuit:
Two equivalent circuits of a real capacitorAn ideal capacitor only stores and releases electrical energy, without dissipating any. In reality, all capacitors have imperfections within the capacitor's material that create resistance. This is specified as the equivalent series resistance or ESR of a component. This adds a real component to the impedance:

As frequency approaches infinity, the capacitive impedance (or reactance) approaches zero and the ESR becomes significant. As the reactance becomes negligible, power dissipation approaches PRMS. = VRMS.² /RESR.
Similarly to ESR, the capacitor's leads add equivalent series inductance or ESL to the component. This is usually significant only at relatively high frequencies. As inductive reactance is positive and increases with frequency, above a certain frequency capacitance will be canceled by inductance. High frequency engineering involves accounting for the inductance of all connections and components.
If the conductors are separated by a material with a small conductivity rather than a perfect dielectric, then a small leakage current flows directly between them. The capacitor therefore has a finite parallel resistance, and slowly discharges over time (time may vary greatly depending on the capacitor material and quality).

Ripple current:
Ripple current is the AC component of an applied source (often a switched-mode power supply) whose frequency may be constant or varying. Certain types of capacitors, such as electrolytic tantalum capacitors, usually have a rating for maximum ripple current (both in frequency and magnitude). This ripple current can cause damaging heat to be generated within the capacitor due to the current flow across resistive imperfections in the materials used within the capacitor, more commonly referred to as equivalent series resistance (ESR). For example electrolytic tantalum capacitors are limited by ripple current and generally have the highest ESR ratings in the capacitor family, while ceramic capacitors generally have no ripple current limitation and have some of the lowest ESR ratings.

Instability of capacitance:
The capacitance of certain capacitors decreases as the component ages. In ceramic capacitors, this is caused by degradation of the dielectric. The type of dielectric and the ambient operating and storage temperatures are the most significant aging factors, while the operating voltage has a smaller effect. The aging process may be reversed by heating the component above the Curie point. Aging is fastest near the beginning of life of the component, and the device stabilizes over time. Electrolytic capacitors age as the electrolyte evaporates. In contrast with ceramic capacitors, this occurs towards the end of life of the component.
Temperature dependence of capacitance is usually expressed in parts per million (ppm) per °C. It can usually be taken as a broadly linear function but can be noticeably non-linear at the temperature extremes. The temperature coefficient can be either positive or negative, sometimes even amongst different samples of the same type. In other words, the spread in the range of temperature coefficients can encompass zero. See the data sheet in the leakage current section above for an example.
Capacitors, especially older components, can absorb sound waves resulting in a microphonic effect. Vibration moves the plates, causing the capacitance to vary, in turn inducing AC current. Some dielectrics also generate piezoelectricity. The resulting interference is especially problematic in audio applications, potentially causing feedback or unintended recording. In the reverse microphonic effect, the varying electric field between the capacitor plates exerts a physical force, moving them as a speaker. This can generate audible sound, but drains energy and stresses the dielectric and the electrolyte, if any.

Capacitor types:
Practical capacitors are available commercially in many different forms. The type of internal dielectric, the structure of the plates and the device packaging all strongly affect the characteristics of the capacitor, and its applications.

Dielectric materials:
Most types of capacitor include a dielectric spacer, which increases their capacitance. These dielectrics are most often insulators. However, low capacitance devices are available with a vacuum between their plates, which allows extremely high voltage operation and low losses. Variable capacitors with their plates open to the atmosphere were commonly used in radio tuning circuits. Later designs use polymer foil dielectric between the moving and stationary plates, with no significant air space between them.
Several solid dielectrics are available, including paper, plastic, glass, mica and ceramic materials. Paper was used extensively in older devices and offers relatively high voltage performance. However, it is susceptible to water absorption, and has been largely replaced by plastic film capacitors. Plastics offer better stability and aging performance, which makes them useful in timer circuits, although they may be limited to low operating temperatures and frequencies. Ceramic capacitors are generally small, cheap and useful for high frequency applications, although their capacitance varies strongly with voltage, and they age poorly. They are broadly categorized as class 1 dielectrics, which have predictable variation of capacitance with temperature or class 2 dielectrics, which can operate at higher voltage. Glass and mica capacitors are extremely reliable, stable and tolerant to high temperatures and voltages, but are too expensive for most mainstream applications. Electrolytic capacitors and supercapacitors are used to store small and larger amounts of energy, respectively, ceramic capacitors are often used in resonators, and parasitic capacitance occurs in circuits wherever the simple conductor-insulator-conductor sequence is formed unintentionally.
Electrolytic capacitors use an aluminum or tantalum plate with an oxide dielectric layer. The second electrode is a liquid electrolyte, connected to the circuit by another foil plate. Electrolytic capacitors offer very high capacitance but suffer from poor tolerances, high instability, gradual loss of capacitance especially when subjected to heat, and high leakage current. The conductivity of the electrolyte drops at low temperatures, which increases equivalent series resistance. While widely used for power-supply conditioning, poor high-frequency characteristics make them unsuitable for many applications. Tantalum capacitors offer better frequency and temperature characteristics than aluminum, but higher dielectric absorption and leakage. OS-CON (or OC-CON) capacitors are a polymerized organic semiconductor solid-electrolyte type that offer longer life at higher cost than standard electrolytic capacitors.
Several other types of capacitor are available for specialist applications. supercapacitors store large amounts of energy. Supercapacitors made from carbon aerogel, carbon nanotubes, or highly porous electrode materials offer extremely high capacitance (as much as 2,500 farads) and can be used in some applications instead of rechargeable batteries. Alternating current capacitors are specifically designed to work on line (mains) voltage AC power circuits. They are commonly used in electric motor circuits and are often designed to handle large currents, so they tend to be physically large. They are usually ruggedly packaged, often in metal cases that can be easily grounded/earthed. They also tend to have rather high direct current breakdown voltages.

Structure:
The arrangement of plates and dielectric has many variations depending on the desired ratings of the capacitor. For small values of capacitance (microfarads and less), ceramic disks use metallic coatings, with wire leads bonded to the coating. Larger values can be made by multiple stacks of plates and disks. Larger value capacitors usually use a metal foil or metal film layer deposited on the surface of a dielectric film to make the plates, and a dielectric film of impregnated paper or plastic - these are rolled up to save space. To reduce the series resistance and inductance for long plates, the plates and dielectric are staggered so that connection is made at the common edge of the rolled-up plates, not at the ends of the foil or metalized film strips that comprise the plates.
The assembly is encased to prevent moisture entering the dielectric - early radio equipment used a cardboard tube sealed with wax. Modern paper or film dielectric capacitors are dipped in a hard thermoplastic. Large capacitors for high-voltage use may have the roll form compressed to fit into a rectangular metal case, with bolted terminals and bushings for connections. The dielectric in larger capacitors is often impregnated with a liquid to improve its properties.
Capacitors may have their connecting leads arranged in many configurations, for example axially or radially. "Axial" means that the leads are on a common axis, typically the axis of the capacitor's cylindrical body -- the leads extend from opposite ends. Radial leads might more accurately be referred to as tandem; they are rarely actually aligned along radii of the body's circle, so the term is inexact, although universal. The leads (until bent!) are usually in planes parallel to that of the flat body of the capacitor, and extend in the same direction; they are often parallel as manufactured.
Small, cheap discoidal ceramic capacitors have existed since the 1930s, and remain in widespread use. Since the 1980s, surface mount packages for capacitors have been widely used. These packages are extremely small and lack connecting leads, allowing them to be soldered directly onto the surface of printed circuit boards. Surface mount components avoid undesirable high-frequency effects due to the leads and simplify automated assembly, although manual handling is made difficult due to their small size.
Mechanically controlled variable capacitors allow the plate spacing to be adjusted, for example by rotating or sliding a set of movable plates into alignment with a set of stationary plates. Low cost variable capacitors squeeze together alternating layers of aluminum and plastic with a screw. Electrical control of capacitance is achievable with varactors (or varicaps), which are reverse-biased semiconductor diodes whose depletion region width varies with applied voltage. They are used in phase-locked loops, amongst other applications.

Applications:
Capacitors have many uses in electronic and electrical systems. They are so ubiquitous that it is a rare electrical product that does not include at least one for some purpose.

System Energy Storage:
A capacitor can store electric energy when disconnected from its charging circuit, so it can be used like a temporary battery. Capacitors are commonly used in electronic devices to maintain power supply while batteries are being changed. (This prevents loss of information in volatile memory.)
In car audio systems, large capacitors store energy for the amplifier to use on demand.
UPSes can be equipped with maintenance-free capacitors to extend service life.

Pulsed power and weapons:
Groups of large, specially constructed, low-inductance high-voltage capacitors (capacitor banks) are used to supply huge pulses of current for many pulsed power applications. These include electromagnetic forming, Marx generators, pulsed lasers (especially TEA lasers), pulse forming networks, radar, fusion research, and particle accelerators.
Large capacitor banks (reservoir) are used as energy sources for the exploding-bridgewire detonators or slapper detonators in nuclear weapons and other specialty weapons. Experimental work is under way using banks of capacitors as power sources for electromagnetic armour and electromagnetic railguns and coilguns.

Power conditioning:
Reservoir capacitors are used in power supplies where they smooth the output of a full or half wave rectifier. They can also be used in charge pump circuits as the energy storage element in the generation of higher voltages than the input voltage.
Capacitors are connected in parallel with the power circuits of most electronic devices and larger systems (such as factories) to shunt away and conceal current fluctuations from the primary power source to provide a "clean" power supply for signal or control circuits. Audio equipment, for example, uses several capacitors in this way, to shunt away power line hum before it gets into the signal circuitry. The capacitors act as a local reserve for the DC power source, and bypass AC currents from the power supply. This is used in car audio applications, when a stiffening capacitor compensates for the inductance and resistance of the leads to the lead-acid car battery.

Power factor correction:
In electric power distribution, capacitors are used for power factor correction. Such capacitors often come as three capacitors connected as a three phase load. Usually, the values of these capacitors are given not in farads but rather as a reactive power in volt-amperes reactive (VAr). The purpose is to counteract inductive loading from devices like electric motors and transmission lines to make the load appear to be mostly resistive. Individual motor or lamp loads may have capacitors for power factor correction, or larger sets of capacitors (usually with automatic switching devices) may be installed at a load center within a building or in a large utility substation.

Supression and coupling: Signal coupling:
Because capacitors pass AC but block DC signals (when charged up to the applied dc voltage), they are often used to separate the AC and DC components of a signal. This method is known as AC coupling or "capacitive coupling". Here, a large value of capacitance, whose value need not be accurately controlled, but whose reactance is small at the signal frequency, is employed.

Decoupling:
A decoupling capacitor is a capacitor used to decouple one part of a circuit from another. Noise caused by other circuit elements is shunted through the capacitor, reducing the effect they have on the rest of the circuit. It is most commonly used between the power supply and ground. An alternative name is bypass capacitor as it is used to bypass the power supply or other high impedance component of a circuit.

Noise filters and snubbers:
When an inductive circuit is opened, the current through the inductance collapses quickly, creating a large voltage across the open circuit of the switch or relay. If the inductance is large enough, the energy will generate a spark, causing the contact points to oxidize, deteriorate, or sometimes weld together, or destroying a solid-state switch. A snubber capacitor across the newly opened circuit creates a path for this impulse to bypass the contact points, thereby preserving their life; these were commonly found in contact breaker ignition systems, for instance. Similarly, in smaller scale circuits, the spark may not be enough to damage the switch but will still radiate undesirable radio frequency interference (RFI), which a filter capacitor absorbs. Snubber capacitors are usually employed with a low-value resistor in series, to dissipate energy and minimize RFI. Such resistor-capacitor combinations are available in a single package.
Capacitors are also used in parallel to interrupt units of a high-voltage circuit breaker in order to equally distribute the voltage between these units. In this case they are called grading capacitors.
In schematic diagrams, a capacitor used primarily for DC charge storage is often drawn vertically in circuit diagrams with the lower, more negative, plate drawn as an arc. The straight plate indicates the positive terminal of the device, if it is polarized (see electrolytic capacitor).

Motor starters:
In single phase squirrel cage motors, the primary winding within the motor housing isn't capable of starting a rotational motion on the rotor, but is capable of sustaining one. To start the motor, a secondary winding is used in series with a non-polarized starting capacitor to introduce a lag in the sinusoidal current through the starting winding. When the secondary winding is placed at an angle with respect to the primary winding, a rotating electric field is created. The force of the rotational field is not constant, but is sufficient to start the rotor spinning. When the rotor comes close to operating speed, a centrifugal switch (or current-sensitive relay in series with the main winding) disconnects the capacitor. The start capacitor is typically mounted to the side of the motor housing. These are called capacitor-start motors, and have relatively high starting torque. There are also capacitor-run induction motors which have a permanently-connected phase-shifting capacitor in series with a second winding. The motor is much like a two-phase induction motor.
Motor-starting capacitors are typically non-polarized electrolytic types, while running capacitors are conventional paper or plastic film dielectric types.

Signal processing:
The energy stored in a capacitor can be used to represent information, either in binary form, as in DRAMs, or in analogue form, as in analog sampled filters and CCDs. Capacitors can be used in analog circuits as components of integrators or more complex filters and in negative feedback loop stabilization. Signal processing circuits also use capacitors to integrate a current signal.

Tuned circuits:
Capacitors and inductors are applied together in tuned circuits to select information in particular frequency bands. For example, radio receivers rely on variable capacitors to tune the station frequency. Speakers use passive analog crossovers, and analog equalizers use capacitors to select different audio bands.
The resonant frequency f of a tuned circuit is a function of the inductance (L) and capacitance (C) in series, and is given by:

where L is in henries and C is in farads.

Sensing:
Most capacitors are designed to maintain a fixed physical structure. However, various factors can change the structure of the capacitor; the resulting change in capacitance can be used to sense those factors.
Changing the dielectric:
The effects of varying the physical and/or electrical characteristics of the dielectric can also be of use. Capacitors with an exposed and porous dielectric can be used to measure humidity in air. Capacitors are used to accurately measure the fuel level in airplanes; as the fuel covers more of a pair of plates, the circuit capacitance increases.
Changing the distance between the plates:
Capacitors with a flexible plate can be used to measure strain or pressure. Industrial pressure transmitters used for process control use pressure-sensing diagphragms, which form a capacitor plate of an oscillator circuit. Capacitors are used as the sensor in condenser microphones, where one plate is moved by air pressure, relative to the fixed position of the other plate. Some accelerometers use MEMS capacitors etched on a chip to measure the magnitude and direction of the acceleration vector. They are used to detect changes in acceleration, eg. as tilt sensors or to detect free fall, as sensors triggering airbag deployment, and in many other applications. Some fingerprint sensors use capacitors. Additionally, a user can adjust the pitch of a theremin musical instrument by moving his hand since this changes the effective capacitance between the user's hand and the antenna.
Changing the effective area of the plates:
Capacitive touch switches are now used on many consumer electronic products.

Hazards and safety:
Capacitors may retain a charge long after power is removed from a circuit; this charge can cause shocks or damage to connected equipment. For example, even a seemingly innocuous device such as a disposable camera flash unit powered by a 1.5 volt AA battery contains a capacitor which may be charged to over 300 volts. This is easily capable of delivering a shock. Service procedures for electronic devices usually include instructions to discharge large or high-voltage capacitors. Capacitors may also have built-in discharge resistors to dissipate stored energy to a safe level within a few seconds after power is removed. High-voltage capacitors are stored with the terminals shorted, as protection from potentially dangerous voltages due to dielectric absorption.
Some old, large oil-filled capacitors contain polychlorinated biphenyls (PCBs). It is known that waste PCBs can leak into groundwater under landfills. Capacitors containing PCB were labelled as containing "Askarel" and several other trade names. PCB-filled capacitors are found in very old (pre 1975) fluorescent lamp ballasts, and other applications.
High-voltage capacitors may catastrophically fail when subjected to voltages or currents beyond their rating, or as they reach their normal end of life. Dielectric or metal interconnection failures may create arcing that vaporizes dielectric fluid, resulting in case bulging, rupture, or even an explosion. Capacitors used in RF or sustained high-current applications can overheat, especially in the center of the capacitor rolls. Capacitors used within high-energy capacitor banks can violently explode when a short in one capacitor causes sudden dumping of energy stored in the rest of the bank into the failing unit. High voltage vacuum capacitors can generate soft X-rays even during normal operation. Proper containment, fusing, and preventive maintenance can help to minimize these hazards.
High-voltage capacitors can benefit from a pre-charge to limit in-rush currents at power-up of HVDC circuits. This will extend the life of the component and may mitigate high-voltage hazards.

Reference:
en.wikipedia.org

Coil

2009-10-12T09:02:00.001-07:00

Coil

A coil is a series of loops. A coiled coil is a structure where the coil itself is in turn also looping.

General applications:
A helical spring. A coil is made up of materials, usually rigid, which can be fashioned into a spiral or helical shape. Flexible materials like wire, roe, hose, cable or paper can also be coiled into empty loops, or wound around a central drum or spindle. Some common applications of coils include:
- A coil spring is the most common type of spring.
- A spiral staircase, a stairway fashioned in a coil shape.
- A Slinky is a coil-shaped toy.
- Evaporator coils are used in air conditioning and other refrigeration cycles.
- A boiler coil is an element in a water heater.
- An Alpine coil, one of several coil knots, is a method for carrying a rope.
- Quilling coils use shaped paper to create artistic designs.
- Coils are also used electrically.

Electromagnetic coils:
Diagram of typical transformer configurations An electromagnetic coil (or simply a "coil") is formed when a conductor (usually a solid copper wire) is wound around a core or form to create an inductor or electromagnet. One loop of wire is usually referred to as a turn, and a coil consists of one or more turns. For use in an electronic circuit, electrical connection terminals called taps are often connected to a coil. Coils are often coated with varnish and/or wrapped with insulating tape to provide additional insulation and secure them in place. A completed coil assembly with taps etc. is often called a winding. A transformer is an electromagnetic device that has a primary winding and a secondary winding that transfers energy from one electrical circuit to another by magnetic coupling without moving parts. The term tickler coil usually refers to a third coil placed in relation to a primary coil and secondary coil A coil tap is a wiring feature found on some electrical transformers, inductors and coil pickups, all of which are sets of wire coils. The coil tap(s) are points in a wire coil where a conductive patch has been exposed (usually on a loop of wire that extends out of the main coil body). As self induction is larger for larger coil diameter the current in a thick wire tries to flow on the inside. The ideal use of copper is achieved by foils. Sometimes this means that a spiral is a better alternative. Multilayer coils have the problem of interlayer capacitance, so when multiple layers are needed the shape needs to be radically changed to a short coil with many layers so that the voltage between consecutive layers is smaller (making them more spiral like).

Analysis:
The inductance of single-layer air-cored coils can be calculated to a reasonable degree of accuracy with the simplified formula

where µH (microhenries) are units of inductance, R is the coil radius (measured in inches to the center of the conductor), N is the number of turns, and L is the length of the coil in inches. The online Coil Inductance Calculator calculates the inductance of any coil using this formula. Higher accuracy estimates of coil inductance require calculations of considerably greater complexity. A layperson's translation is:

Note that if the coil has a ferrite core, or one made of another metallic material, it's inductance cannot be calculated with this formula.
In calculating the distances, one centimeter is equal to 0.393700787 inches and one inch is equal to 2.54 centimeters. The inductance formula uses inches. The relationship between the radius and the circumference of a coil is , with r as the radius, c as the circumference, and π (the Greek letter pi) as the constant 3.141. The circumference of a coil can be calculated by , with d as the diameter of the coil and π as 3.141.

Coil examples:
Nikola Tesla's flat spiral coil. Some common electromagnetic coils include:
A bifilar coil is a coil that employs two parallel windings.
A Barker coil is used in low field NMR imaging.
A Balun is set of transformer coils for transmission lines.
A Braunbeck coil is used in geomagnetic research.
A degaussing coil is used in the process of removing permanent magnetism (magnetic hysteresis) from an object.
A choke coil (or choking coil) is low-resistance inductor used to block alternating current while passing direct current.
A Flat coil is used in thin electric motor.
A Garrett coil is used in metal detectors.
A Helmholtz coil is a device for producing a region of nearly uniform magnetic field.
A hybrid coil (or bridge transformer) is a single transformer that effectively has three windings.
An induction coil (or ignition coil) is an electrical device in common use as the ignition system (ignition coil or spark coil) of internal-combustion engines.
A loading coil is, in electronics, a coil (inductor) inserted in a circuit to increase its inductance. Archaically called Pupin coils.
A multiple coil magnet is an electromagnet that has several coils of wire connected in parallel.
A Maxwell coil is a device for producing almost a constant magnetic field.
A Micro coil use in security devices.
A Oudin coil is a disruptive discharge coil.
The polyphase coils are connected together in a polyphase system such as a generator or motor.
A relay coil is the copper winding part of a relay that produces a magnetic field that actuates the mechanism.
A Repeating coil is a voice-frequency transformer.
A Rogowski coil is an electrical device for measuring alternating current.
A single coil is a type of pickup for the electric guitar.
A solenoid is a mechanical device, based around a coil of wire, that usually converts energy into linear motion, however solenoids also come in a rotary motion (normally up to a turn of 90 degrees).
A telephone cord is usually manufactured in a coiled fashion, as to allow maximum length while taking up minimum space when not in use.
A Tesla coil is category of disruptive discharge coils, usually denoting a resonant transformer that generates very high voltages at radio frequencies.
A voice coil which is mounted to the moving cone of a loudspeaker.
Other applications of coils exist in the field of electromagnetic devices. A coilgun is a type of cannon that uses a series of electromagnetic coils to accelerate a magnetic shell to very high velocities. The filament of an incandescent light bulb has usually the shape of a coiled coil, in order to fit the long filament in a small space.

Chemistry, biology and medicine:
A chemistry coil is a tube of spiral form, used commonly to cool originating steam of the distillation and thus to condense them in liquid form. In the study of how molecules interact with each other, there are a few specific references to organic coils. During self-assembly, organic elements organize to form this structural pattern. Molecular self-assembly assembles the molecules, without guidance or management from an outside source, into these shapes.
Examples of these structural patterns include:
- A coiled coil is a structural motif found in many proteins.
- A random coil is a polymer conformation where the monomers are arranged at random.
As an acronym, COIL denotes the Chemical Oxygen Iodine Laser.
In medicine, the Guglielmi Detachable Coil is a platinum coil commonly used in intracranial non-invasive surgery, for the occlusion of brain aneurysms.

In Ceramics (Fine Arts):
Coiling has been used to shape clay into useful beautiful vessels for many of thousands of years. It ranges from Africa to Greece and from China to New Mexico. They have used this method in a variety of ways. Using the coiling technique, it is possible to build thicker or taller walled vessels, which may not have been possible using earlier methods. The technique lets you control the walls as you build them up and allows you to build on top of the walls to make the vessel look bigger and bulge outward or narrow inward with less danger of collapsing. There are many different ways you can build ceramic objects using the coiling technique.

Coil Construction:
Squeezing the clay into a coil or rolling between your hands are two different was to make coils. Using these techniques, it may prove very difficult to make a smooth preform due to the uneven pressure applied by your hands and fingers.
When rolling with your hands, use a smooth surface. By spreading your hands (to apply even pressure), gently roll the clay back and forth until you think the preform is of the right thickness.
The roll should be a little thicker than a pencil or pen. Now stack the coils on top of each other.
Now, for strength, force the clay together as hard as you can on the inside of the piece without messing the clay up. Use your fingers and scrape the top coil onto the coil underneath.
While smoothing the inside of the piece hold your other hand on the outside so you don’t damage what you have already done.
If you want a top level, gently turn your piece over and lightly tap it on a smooth surface.
Let it dry.

Slab pottery Construction:
How to build it
1- Large flat pieces of clay are rolled out with a rolling pin. The slabs are cut for the base and walls and are attached together.
2- Slabs work goes fast but lots of care must be given to make sure that the seams won’t crack, break or pull apart during the drying process.

Reference:
en.wikipedia.org

Electricity

2009-10-12T09:01:00.002-07:00

Electricity

Electricity (from Greek ήλεκτρον (electron) "amber") is a general term for the variety of phenomena resulting from the presence and flow of electric charge. Together with magnetism, it constitutes the fundamental interaction known as electromagnetism. It includes many well-known physical phenomena such as lightning, electric fields and electric currents, and is put to use in industrial applications such as electronics and electric power.

Concepts in electricity:
In casual usage, the term electricity is applied to several related concepts that are better identified by more precise terms:
Electric potential - the capacity of an electric field to do work, typically measured in volts.
Electric current - a movement or flow of electrically charged particles, typically measured in amperes.
Electric field - an effect produced by an electric charge that exerts a force on charged objects in its vicinity.
Electrical energy - the energy made available by the flow of electric charge through an electrical conductor.
Electric power - the rate at which electric energy is converted to or from another energy form, such as light, heat, or mechanical energy.
Electric charge - a fundamental conserved property of some subatomic particles, which determines their electromagnetic interactions. Electrically charged matter is influenced by, and produces, electromagnetic fields.

History of discovery:
The ancient Greeks and Parthians knew of static electricity from rubbing objects against fur. The ancient Babylonians may have had some knowledge of electroplating, based on the discovery of the Baghdad Battery, which resembles a Galvanic cell.
Benjamin Franklin conducted extensive research in electricity. His theories on the relationship between lightning and static electricity, including his famous kite-flying experiment, sparked the interest of later scientists whose work provided the basis for modern electrical technology. Most notably these include Luigi Galvani (1737–1798), Alessandro Volta (1745-1827), Michael Faraday (1791–1867), André-Marie Ampère (1775–1836), and Georg Simon Ohm (1789-1854). The late 19th and early 20th century produced such giants of electrical engineering as Nikola Tesla, Samuel Morse, Antonio Meucci, Thomas Edison, George Westinghouse, Werner von Siemens, Charles Steinmetz, and Alexander Graham Bell.

Concepts in detail:
Electric charge:
Electric charge is a property of certain subatomic particles (e.g., electrons and protons) which interacts with electromagnetic fields and causes attractive and repulsive forces between them. Electric charge gives rise to one of the four fundamental forces of nature, and is a conserved property of matter that can be quantified. In this sense, the phrase "quantity of electricity" is used interchangeably with the phrases "charge of electricity" and "quantity of charge". There are two types of charge: we call one kind of charge positive and the other negative. Through experimentation, we find that like-charged objects repel and opposite-charged objects attract one another. The magnitude of the force of attraction or repulsion is given by Coulomb's law.

Electric field:
The concept of electric fields was introduced by Michael Faraday. The electrical field force acts between two charges, in the same way that the gravitational field force acts between two masses. However, the electric field is a little bit different. Gravitational force depends on the masses of two bodies, whereas electric force depends on the electric charges of two bodies. While gravity can only pull two masses together, the electric force can be an attractive or repulsive force. If both charges are of same sign (e.g. both positive), there will be a repulsive force between the two. If the charges are opposite, there will be an attractive force between the two bodies. The magnitude of the force varies inversely with the square of the distance between the two bodies, and is also proportional to the product of the unsigned magnitudes of the two charges.

Electric potential:
The electric potential difference between two points is defined as the work done per unit charge (against electrical forces) in moving a positive point charge slowly between two points. If one of the points is taken to be a reference point with zero potential, then the electric potential at any point can be defined in terms of the work done per unit charge in moving a positive point charge from that reference point to the point at which the potential is to be determined. For isolated charges, the reference point is usually taken to be infinity. The potential is measured in volts. (1 volt = 1 joule/coulomb) The electric potential is analogous to temperature: there is a different temperature at every point in space, and the temperature gradient indicates the direction and magnitude of the driving force behind heat flow. Similarly, there is an electric potential at every point in space, and its gradient indicates the direction and magnitude of the driving force behind charge movement

Electric current:
An electric current is a flow of electric charge, and its intensity is measured in amperes. Examples of electric currents include metallic conduction, where electrons flow through a conductor or conductors such as a metal wire, and electrolysis, where ions (charged atoms) flow through liquids. The particles themselves often move quite slowly, while the electric field that drives them propagates at close to the speed of light. See electrical conduction for more information.
Devices that use charge flow principles in materials are called electronic devices.
A direct current (DC) is a unidirectional flow, while an alternating current (AC) reverses direction repeatedly. The time average of an alternating current is zero, but its energy capability (RMS value) is not zero.
Ohm's Law is an important relationship describing the behaviour of electric currents, relating them to voltage.
For historical reasons, electric current is said to flow from the most positive part of a circuit to the most negative part. The electric current thus defined is called conventional current. It is now known that, depending on the conditions, an electric current can consist of a flow of charged particles in either direction, or even in both directions at once. The positive-to-negative convention is widely used to simplify this situation. If another definition is used - for example, "electron current" - it should be explicitly stated.

Electrical energy:
Electrical energy is energy stored in an electric field or transported by an electric current. Energy is defined as the ability to do work, and electrical energy is simply one of the many types of energy. Examples of electrical energy include:
the energy that is constantly stored in the Earth's atmosphere, and is partly released during a thunderstorm in the form of lightning
the energy that is stored in the coils of an electrical generator in a power station, and is then transmitted by wires to the consumer; the consumer then pays for each unit of energy received
the energy that is stored in a capacitor, and can be released to drive a current through an electrical circuit

Electric power:
Electric power is the rate at which electrical energy is produced or consumed, and is measured in watts (symbol is: W).
A fossil-fuel, solar-thermal, nuclear or biomass power station converts heat to electrical energy, and the faster the station burns fuel, assuming positively-sloped efficiency of conversion, the higher its power output. The output of a power station is usually specified in megawatts (millions of watts). The electrical energy is then sent over transmission lines to reach the consumers.
Every consumer uses appliances that convert the electrical energy to other forms of energy, such as heat (in electric arc furnaces and electric heaters), light (in light bulbs and fluorescent lamps), or motion, i.e. kinetic energy (in electric motors). Like the power station, each appliance is also rated in watts, depending on the rate at which it converts electrical energy into another form. The power station must produce electrical energy at the same rate as all the connected appliances consume it.
In electrical engineering, the concepts of apparent power and reactive power are also used. Apparent power is the product of RMS voltage and RMS current, and is measured in volt-amperes (VA). Reactive power is measured in volt-amperes-reactive (VAr).
Non-nuclear electric power is categorized as either green or brown electricity.
Green power is a cleaner alternative energy source in comparison to traditional sources, and is derived from renewable energy resources that do not produce any nuclear waste; examples include energy produced from wind, water, solar, thermal, hydro, combustible renewables and waste.
Electricity from coal, oil, and natural gas is known as traditional power or "brown" electricity.

Reference:
en.wikipedia.org

Engineering

2009-10-12T09:01:00.001-07:00

Engineering

Engineering is the design, analysis, and/or construction of works for practical purposes. One who practices engineering is called an engineer, and those licensed to do so have formal designations such as Professional Engineer or Chartered Engineer. Engineers use imagination, judgment, and reasoning to apply science, technology, mathematics, and practical experience. The result is the design, production, and operation of useful objects or processes. The broad discipline of engineering encompasses a range of specialized subdisciplines that focus on the issues associated with developing a specific kind of product, or using a specific type of technology.

Methodology:
The crucial and unique task of the engineer is to identify, understand, and interpret the constraints on a design in order to produce a successful result. It is usually not enough to build a technically successful product; it must also meet further requirements. Constraints may include available resources, physical or technical limitations, flexibility for future modifications and additions, and other factors, such as requirements for cost, marketability, producibility, and serviceability. By understanding the constraints, engineers derive specifications for the limits within which a viable object or system may be produced and operated.

Problem solving:
Engineers use their knowledge of science, mathematics, and appropriate experience to find suitable solutions to a problem. Creating an appropriate mathematical model of a problem allows them to analyze it (sometimes definitively), and to test potential solutions. Usually multiple reasonable solutions exist, so engineers must evaluate the different design choices on their merits and choose the solution that best meets their requirements. Genrich Altshuller, after gathering statistics on a large number of patents, suggested that compromises are at the heart of "low-level" engineering designs, while at a higher level the best design is one which eliminates the core contradiction causing the problem.
Engineers typically attempt to predict how well their designs will perform to their specifications prior to full-scale production. They use, among other things: prototypes, scale models, simulations, destructive tests, nondestructive tests, and stress tests. Testing ensures that products will perform as expected. Engineers as professionals take seriously their responsibility to produce designs that will perform as expected and will not cause unintended harm to the public at large. Engineers typically include a factor of safety in their designs to reduce the risk of unexpected failure. However, the greater the safety factor, the less efficient the design may be.

Computer use:
As with all modern scientific and technological endeavors, computers and software play an increasingly important role. As well as the typical business application software there are a number of computer aided applications (CAx) specifically for engineering.
One of the most widely used tools in the profession is computer-aided design (CAD) software which enables engineers to create 3D models, 2D drawings, and schematics of their designs. CAD together with Digital mockup (DMU) and CAE software such as finite element method analysis allows engineers to create models of designs that can be analyzed without having to make expensive and time-consuming physical prototypes. These allow products and components to be checked for flaws; assess fit and assembly; study ergonomics; and to analyze static and dynamic characteristics of systems such as stresses, temperatures, electromagnetic emissions, electrical currents and voltages, digital logic levels, fluid flows, and kinematics. Access and distribution of all this information is generally organized with the use of Product Data Management software.
There are also many tools to support specific engineering tasks such as Computer-aided manufacture (CAM) software to generate CNC machining instructions; Manufacturing Process Management software for production engineering; EDA for printed circuit board (PCB) and circuit schematics for electronic engineers; MRO applications for maintenance management ; and AEC software for civil engineering.
In recent years the use of computer software to aid the development of goods has collectively come to be known as Product Lifecycle Management (PLM).

Etymology:
The Oxford English Dictionary gives one, now obsolete, meaning of engineer (dating from 1325) as "A constructor of military engines". Engineering was originally divided into military engineering (which included construction of fortifications as well as military engines) and civil engineering (non-military construction of such as bridges).
The words engine and engineer (as well as ingenious) developed in parallel from the Latin root ingeniosus, meaning "skilled". An engineer is thus implied to be a clever, practical, designer.
With the rise of engineering as a profession in the nineteenth century the term became more narrowly applied to fields in which mathematics and science were applied to these ends. In some other languages, such as Arabic, the word for "engineering" also means "geometry".
In the nineteenth century in addition to military and civil engineering the fields then known as the mechanic arts became incorporated into engineering.

Engineering in a social context:
Engineering is a subject that ranges from large collaborations to small individual projects. Almost all engineering projects are beholden to some sort of financing agency: a company, a set of investors, or a government. The few types of engineering that are minimally constrained by such issues are pro bono engineering and open design engineering.
By its very nature engineering is bound up with society and human behaviour. Every product or construction used by modern society will have been influenced by engineering design. Engineering design is a very powerful tool to make changes to environment, society and economies, and its application brings with it a great responsibility, as represented by many of the Engineering Institutions codes of practice and ethics. Whereas medical ethics is a well-established field with considerable consensus, engineering ethics is far less developed, and engineering projects can be subject to considerable controversy. Just a few examples of this from different engineering disciplines are the develoment of nuclear weapons, the Three Gorges Dam, the design and use of Sports Utility Vehicles and the extraction of oil. There is a growing trend amongst western engineering companies to enact serious Corporate and Social Responsibility policies, but many companies do not have these.
Engineering is a key driver of human development. Sub-Saharan Africa in particular has a very small engineering capacity and as a result many African nations are unable to implement solutions to problems they face without outside intervention, even if the political and financial obstacles are overcome. The attainment of many of the Millennium Development Goals is primarily an engineering challenge and the achievement of sufficient engineering capacity is a prerequisite to achieving the MDGs.
All overseas development and relief NGOs make considerable use of engineers to apply solutions in disaster and development scenarios. A number of charitable organizations aim to use engineering directly for the good of mankind:
- Engineers Without Borders
- Engineers Against Poverty
- Registered Engineers for Disaster Relief
- Engineers for a Sustainable World

Cultural presence:
Engineering is a well respected profession. For example, in Canada it ranks as one of the public's most trusted professions .
Sometimes engineering has been seen as a somewhat dry, uninteresting field in popular culture, and has also been thought to be the domain of nerds. For example, the cartoon character Dilbert is an engineer.
This has not always been so - most British school children in the 1950s were brought up with stirring tales of 'the Victorian Engineers', chief amongst whom were the Brunels, the Stephensons, Telford and their contemporaries.
In science fiction engineers are often portrayed as highly knowledgeable and respectable individuals who understand the overwhelming future technologies often portrayed in the genre. The Star Trek characters Montgomery Scott and Geordi La Forge are famous examples.
Engineers are often respected and ridiculed for their intense beliefs and interests. Perhaps because of their deep understanding of the interconnectedness of many things, engineers such as Governor John H. Sununu, New York City Mayor Michael Bloomberg and Nuclear Physicist Edward Teller, are often driven into politics to "fix things" for the public good.
Occasionally, engineers may be recognized by the "Iron Ring"--a stainless steel or iron ring worn on the little (fourth) finger of the dominant hand. This tradition was originally developed in Canada in the Ritual of the Calling of an Engineer as a symbol of pride and obligation for the engineering profession. Some years later this practice was adopted in the United States. Members of the US Order of the Engineer accept this ring as a pledge to uphold the proud history of engineering. A Professional Engineer's name often has the post-nominal letters PE or P.Eng in North America. In much of Europe a professional engineer is denoted by the letters IR, while in the UK and much of the Commonwealth the term Chartered Engineer applies and is denoted by the letters CEng.

Legislation:
In most modern countries, certain engineering tasks, such as the design of bridges, electric power plants, and chemical plants, must be approved by a Professional Engineer or a Chartered Engineer.
Laws protecting public health and safety mandate that a professional must provide guidance gained through education and experience. In the United States, each state tests and licenses Professional Engineers. In much of Europe and the Commonwealth professional accreditation is provided by Engineering Institutions, such as the Institution of Civil Engineers from the UK. The engineering institutions of the UK are some of the oldest in the world, and provide accredition to many engineers around the world. In Canada the profession in each province is governed by its own engineering association. For instance, in the Province of British Columbia an engineering graduate with 5 or more years of experience in an engineering-related field will need to be certified by the Association for Professional Engineers and Geoscientists (APEGBC) in order to become a Professional Engineer.
The federal US government, however, supervises aviation through the Federal Aviation Regulations administrated by the Dept. of Transportation, Federal Aviation Administration. Designated Engineering Representatives approve data for aircraft design and repairs on behalf of the Federal Aviation Administration.
Even with strict testing and licensure, engineering disasters still occur. Therefore, the Professional Engineer or Chartered Engineer adheres to a strict code of ethics. Each engineering discipline and professional society maintains a code of ethics, which the members pledge to uphold.
Refer also to the Washington accord for international accreditation details of professional engineering degrees.

Comparison to other disciplines:
Science:
There exists a definitive overlap between the sciences and engineering practice; in engineering, one applies science. Students in mathematics and engineering both begin their education by studying the fundamental principles of mathematics, such as geometry, algebra, and especially, calculus. However, as the students progress, their foci diverge. The mathematics student continues to learn mathematics, consisting of advanced principles and concepts which typically build on those of the past. The engineering student begins to focus primarily on mathematical application. At the end of his education, the engineer will ideally understand most of the science necessary for the solution to physical problems. The scientist will know far more science than the engineer, but with much of this knowledge being too abstract for utility in real world applications.
Engineering is concerned with the design of a solution to a problem. A scientist may ask why a phenomenon arises, and proceed to research the answer to the question. Engineers do not deal with scientific phenomena, but may use science in solving a problem, and/or find out how to implement that solution. In other words, scientists attempt to explain phenomena, whereas engineers use that knowledge to construct solutions to problems.
Both areas of endeavor rely on accurate observation of materials and phenomenon. Both use mathematics and classification criteria to analyze and communicate observations. Scientists are expected to interpret their observations and to make expert recommendations for practical action based on those interpretations. Scientists may also have to complete engineering tasks, such as designing experimental apparatus or building prototypes. Conversely, in the process of developing technology engineers sometimes find themselves exploring new phenomena, thus becoming, for the moment, scientists.
However, engineering research has a character different from that of scientific research. First, it often deals with areas in which the basic physics and/or chemistry are well understood, but the problems themselves are too complex to solve in an exact manner. Examples are the use of numerical approximations to the Navier-Stokes equations to describe aerodynamic flow over an aircraft, or the use of Miner's rule to calculate fatigue damage. Second, engineering research employs many semi-empirical methods that are foreign to pure scientific research, one example being the method of parameter variation.

Other fields:
There are significant parallels between engineering and medicine. Both fields are well known for their pragmatism — the solution to real world problems often requires moving forward before phenomena are completely understood in a more rigorous scientific sense and therefore experimentation and empirical knowledge is an integral part of both. Part of medicine examines the function of the human body. The human body although biological has many functions similar to a machine. The heart for example functions much like a pump, the skeleton is like a linked structure with levers etc. This similarity has led to the development of the field of biomedical engineering that utilizes concepts developed in both disciplines.
There are also close connections between the workings of engineers and artists; they are direct in some fields, for example, architecture, landscape architecture and industrial design (even to the extent that these disciplines may sometimes be included in a University's Faculty of Engineering); and indirect in others. Artistic and engineering creativity may be fundamentally connected as the case of Leonardo Da Vinci indicates.
In Political science the term engineering has been borrowed for the study of the subjects of Social engineering and Political engineering that deal with forming political and social structures using engineering methodology coupled with political science principles.

Reference:
en.wikipedia.org

Electrical engineering

2009-10-12T09:00:00.002-07:00

Electrical engineering

Electrical engineering (sometimes referred to as electrical and electronic engineering) is a professional engineering discipline that deals with the study and application of electricity, electronics and electromagnetism. The field first became an identifiable occupation in the late nineteenth century with the commercialization of the electric telegraph and electrical power supply. The field now covers a range of sub-disciplines including those that deal with power, optoelectronics, digital electronics, analog electronics, computer science, artificial intelligence, control systems, electronics, signal processing and telecommunications.
The term electrical engineering may or may not encompass electronic engineering. Where a distinction is made, electrical engineering is considered to deal with the problems associated with large-scale electrical systems such as power transmission and motor control, whereas electronic engineering deals with the study of small-scale electronic systems including computers and integrated circuits. Another way of looking at the distinction is that electrical engineers are usually concerned with using electricity to transmit energy, while electronics engineers are concerned with using electricity to transmit information. William Gilbert, with his 1600 publication of De Magnete, was the originator of the term "electricity" and many regard him as the father of electrical engineering or of electricity.

History:
Early developments:
Electricity has been a subject of scientific interest since at least the 17th century, but it was not until the 19th century that research into the subject started to intensify. Notable developments in this century include the work of Georg Ohm, who in 1827 quantified the relationship between the electric current and potential difference in a conductor, Michael Faraday, the discoverer of electromagnetic induction in 1831, and James Clerk Maxwell, who in 1873 published a unified theory of electricity and magnetism in his treatise on Electricity and Magnetism.
During these years, the study of electricity was largely considered to be a subfield of physics. It was not until the late 19th century that universities started to offer degrees in electrical engineering. The Darmstadt University of Technology founded the first chair and the first faculty of electrical engineering worldwide in 1882. In 1883 Darmstadt University of Technology and Cornell University introduced the world's first courses of study in electrical engineering and in 1885 the University College London founded the first chair of electrical engineering in the United Kingdom. The University of Missouri subsequently established the first department of electrical engineering in the United States in 1886.
During this period, the work concerning electrical engineering increased dramatically. In 1882, Edison switched on the world's first large-scale electrical supply network that provided 110 volts direct current to fifty-nine customers in lower Manhattan. In 1887, Nikola Tesla filed a number of patents related to a competing form of power distribution known as alternating current. In the following years a bitter rivalry between Tesla and Edison, known as the "War of Currents", took place over the preferred method of distribution. AC eventually replaced DC for generation and power distribution, enormously extending the range and improving the safety and efficiency of power distribution.
The efforts of the two did much to further electrical engineering—Tesla's work on induction motors and polyphase systems influenced the field for years to come, while Edison's work on telegraphy and his development of the stock ticker proved lucrative for his company, which ultimately became General Electric. However, by the end of the 19th century, other key figures in the progress of electrical engineering were beginning to emerge.

Modern developments:
Emergence of radio and electronics:
During the development of radio, many scientists and inventors contributed to radio technology and electronics. In his classic UHF experiments of 1888, Heinrich Hertz transmitted (via a spark-gap transmitter) and detected radio waves using electrical equipment. In 1895, Nikola Tesla was able to detect signals from the transmissions of his New York lab at West Point (a distance of 80.4 km). In 1897, Karl Ferdinand Braun introduced the cathode ray tube as part of an oscilloscope, a crucial enabling technology for electronic television. John Fleming invented the first radio tube, the diode, in 1904. Two years later, Robert von Lieben and Lee De Forest independently developed the amplifier tube, called the triode. In 1920 Albert Hull developed the magnetron which would eventually lead to the development of the microwave oven in 1946 by Percy Spencer. In 1934 the British military began to make strides towards radar (which also uses the magnetron), under the direction of Dr Wimperis culminating in the operation of the first radar station at Bawdsey in August 1936.
In 1941 Konrad Zuse presented the Z3, the world's first fully functional and programmable computer. In 1946 the ENIAC (Electronic Numerical Integrator and Computer) of John Presper Eckert and John Mauchly followed, beginning the computing era. The arithmetic performance of these machines allowed engineers to develop completely new technologies and achieve new objectives, including the Apollo missions and the NASA moon landing.
The invention of the transistor in 1947 by William B. Shockley, John Bardeen and Walter Brattain opened the door for more compact devices and led to the development of the integrated circuit in 1958 by Jack Kilby and independently in 1959 by Robert Noyce. In 1968 Marcian Hoff invented the first microprocessor at Intel and thus ignited the development of the personal computer. The first realization of the microprocessor was the Intel 4004, a 4-bit processor developed in 1971, but only in 1973 did the Intel 8080, an 8-bit processor, make the building of the first personal computer, the Altair 8800, possible.

Education:
Electrical engineers typically possess an academic degree with a major in electrical engineering. The length of study for such a degree is usually four or five years and the completed degree may be designated as a Bachelor of Engineering, Bachelor of Science, Bachelor of Technology or Bachelor of Applied Science depending upon the university. The degree generally includes units covering physics, mathematics, computer science, project management and specific topics in electrical engineering. Initially such topics cover most, if not all, of the sub-disciplines of electrical engineering. Students then choose to specialize in one or more sub-disciplines towards the end of the degree.
Some electrical engineers also choose to pursue a postgraduate degree such as a Master of Engineering/Master of Science, a Master of Engineering Management, a Doctor of Philosophy in Engineering or an Engineer's degree. The Master and Engineer's degree may consist of either research, coursework or a mixture of the two. The Doctor of Philosophy consists of a significant research component and is often viewed as the entry point to academia. In the United Kingdom and various other European countries, the Master of Engineering is often considered an undergraduate degree of slightly longer duration than the Bachelor of Engineering.

Practicing engineers:
In most countries, a Bachelor's degree in engineering represents the first step towards professional certification and the degree program itself is certified by a professional body. After completing a certified degree program the engineer must satisfy a range of requirements (including work experience requirements) before being certified. Once certified the engineer is designated the title of Professional Engineer (in the United States, Canada and South Africa ), Chartered Engineer (in the United Kingdom, Ireland, India and Zimbabwe), Chartered Professional Engineer (in Australia and New Zealand) or European Engineer (in much of the European Union).
The advantages of certification vary depending upon location. For example, in the United States and Canada "only a licensed engineer may seal engineering work for public and private clients". This requirement is enforced by state and provincial legislation such as Quebec's Engineers Act. In other countries, such as Australia, no such legislation exists. Practically all certifying bodies maintain a code of ethics that they expect all members to abide by or risk expulsion. In this way these organizations play an important role in maintaining ethical standards for the profession. Even in jurisdictions where certification has little or no legal bearing on work, engineers are subject to contract law. In cases where an engineer's work fails he or she may be subject to the tort of negligence and, in extreme cases, the charge of criminal negligence. An engineer's work must also comply with numerous other rules and regulations such as building codes and legislation pertaining to environmental law.
Professional bodies of note for electrical engineers include the Institute of Electrical and Electronics Engineers (IEEE) and the Institution of Electrical Engineers (IEE). The IEEE claims to produce 30 percent of the world's literature in electrical engineering, has over 360,000 members worldwide and holds over 300 conferences annually. The IEE publishes 14 journals, has a worldwide membership of 120,000, and claims to be the largest professional engineering society in Europe. Obsolescence of technical skills is a serious concern for electrical engineers. Membership and participation in technical societies, regular reviews of periodicals in the field and a habit of continued learning are therefore essential to maintaining proficiency.
In countries such as Australia, Canada and the United States electrical engineers make up around 0.25% of the labour force (see note). Outside of these countries, it is difficult to gauge the demographics of the profession due to less meticulous reporting on labour statistics. However, in terms of electrical engineering graduates per-capita, electrical engineering graduates would probably be most numerous in countries such as Taiwan, Japan and South Korea.

Tools and work:
From the Global Positioning System to electric power generation, electrical engineers are responsible for a wide range of technologies. They design, develop, test and supervise the deployment of electrical systems and electronic devices. For example, they may work on the design of telecommunication systems, the operation of electric power stations, the lighting and wiring of buildings, the design of household appliances or the electrical control of industrial machinery.
Fundamental to the discipline are the sciences of physics and mathematics as these help to obtain both a qualitative and quantitative description of how such systems will work. Today most engineering work involves the use of computers and it is commonplace to use computer-aided design programs when designing electrical systems. Nevertheless, the ability to sketch ideas is still invaluable for quickly communicating with others.
Although most electrical engineers will understand basic circuit theory (that is the interactions of elements such as resistors, capacitors, diodes, transistors and inductors in a circuit), the theories employed by engineers generally depend upon the work they do. For example, quantum mechanics and solid state physics might be relevant to an engineer working on VLSI (the design of integrated circuits), but are largely irrelevant to engineers working with macroscopic electrical systems. Even circuit theory may not be relevant to a person designing telecommunication systems that use off-the-shelf components. Perhaps the most important technical skills for electrical engineers are reflected in university programs, which emphasize strong numerical skills, computer literacy and the ability to understand the technical language and concepts that relate to electrical engineering.
For most engineers technical work accounts for only a fraction of the work they do. A lot of time is also spent on tasks such as discussing proposals with clients, preparing budgets and determining project schedules. Many senior engineers manage a team of technicians or other engineers and for this reason project management skills are important. Most engineering projects involve some form of documentation and strong written communication skills are therefore very important.
The workplaces of electrical engineers are just as varied as the types of work they do. Electrical engineers may be found in the pristine lab environment of a fabrication plant, the offices of a consulting firm or on site at a mine. During their working life, electrical engineers may find themselves supervising a wide range of individuals including scientists, electricians, computer programmers and other engineers.

Sub-disciplines:
Electrical engineering has many sub-disciplines, the most popular of which are listed below. Although there are electrical engineers who focus exclusively on one of these sub-disciplines, many deal with a combination of them. Sometimes certain fields, such as electronic engineering and computer engineering, are considered separate disciplines in their own right.

Power:
Power engineering deals with the generation, transmission and distribution of electricity as well as the design of a range of related devices. These include transformers, electric generators, electric motors and power electronics. In many regions of the world, governments maintain an electrical network called a power grid that connects a variety of generators together with users of their energy. Users purchase electrical energy from the grid, avoiding the costly exercise of having to generate their own. Power engineers may work on the design and maintenance of the power grid as well as the power systems that connect to it. Such systems are called on-grid power systems and may supply the grid with additional power, draw power from the grid or do both. Power engineers may also work on systems that do not connect to the grid, called off-grid power systems, which in some cases are preferable to on-grid systems.

Control:
Control engineering focuses on the modelling of a diverse range of dynamic systems and the design of controllers that will cause these systems to behave in the desired manner. To implement such controllers electrical engineers may use electrical circuits, digital signal processors, microcontrollers and PLCs (Programmable Logic Controllers). Control engineering has a wide range of applications from the flight and propulsion systems of commercial airliners to the cruise control present in many modern automobiles. It also plays an important role in industrial automation.
Control engineers often utilize feedback when designing control systems. For example, in an automobile with cruise control the vehicle's speed is continuously monitored and fed back to the system which adjusts the motor's speed accordingly. Where there is regular feedback, control theory can be used to determine how the system responds to such feedback.

Electronics:
Electronic engineering involves the design and testing of electronic circuits that use the properties of components such as resistors, capacitors, inductors, diodes and transistors to achieve a particular functionality. The tuned circuit, which allows the user of a radio to filter out all but a single station, is just one example of such a circuit. Another example (of a pneumatic signal conditioner) is shown in the adjacent photograph.
Prior to the second world war, the subject was commonly known as radio engineering and basically was restricted to aspects of communications and radar, commercial radio and early television. Later, in post war years, as consumer devices began to be developed, the field grew to include modern television, audio systems, computers and microprocessors. In the mid to late 1950s, the term radio engineering gradually gave way to the name electronic engineering.
Before the invention of the integrated circuit in 1959, electronic circuits were constructed from discrete components that could be manipulated by humans. These discrete circuits consumed much space and power and were limited in speed, although they are still common in some applications. By contrast, integrated circuits packed a large number—often millions—of tiny electrical components, mainly transistors, into a small chip around the size of a coin. This allowed for the powerful computers and other electronic devices we see today.

Microelectronics:
Microelectronics engineering deals with the design of very small electronic circuit components for use in an integrated circuit or sometimes for use on their own as a general electronic component. The most common microelectronic components are semiconductor transistors, although all main electronic components (resistors, capacitors, inductors) can be created at a microscopic level.
Microelectronic components are created by chemically fabricating wafers of semiconductors such as silicon (at higher frequencies, gallium arsenide and indium phosphide) to obtain the desired transport of electronic charge and control of current. The field of microelectronics involves a significant amount of chemistry and material science and requires the electronic engineer working in the field to have a very good working knowledge of the effects of quantum mechanics.

Signal processing:
Signal processing deals with the analysis and manipulations of signals. Signals can be either analog, in which case the signal varies continuously according to the information, or digital, in which case the signal varies according to a series of discrete values representing the information. For analog signals, signal processing may involve the amplification and filtering of audio signals for audio equipment or the modulation and demodulation of signals for telecommunications. For digital signals, signal processing may involve the compression, error detection and error correction of digitally sampled signals.

Telecommunications:
Telecommunications engineering focuses on the transmission of information across a channel such as a coax cable, optical fibre or free space. Transmissions across free space require information to be encoded in a carrier wave in order to shift the information to a carrier frequency suitable for transmission, this is known as modulation. Popular analog modulation techniques include amplitude modulation and frequency modulation. The choice of modulation affects the cost and performance of a system and these two factors must be balanced carefully by the engineer.
Once the transmission characteristics of a system are determined, telecommunication engineers design the transmitters and receivers needed for such systems. These two are sometimes combined to form a two-way communication device known as a transceiver. A key consideration in the design of transmitters is their power consumption as this is closely related to their signal strength. If the signal strength of a transmitter is insufficient the signal's information will be corrupted by noise.

Instrumentation engineering:
Instrumentation engineering deals with the design of devices to measure physical quantities such as pressure, flow and temperature. The design of such instrumentation requires a good understanding of physics that often extends beyond electromagnetic theory. For example, radar guns use the Doppler effect to measure the speed of oncoming vehicles. Similarly, thermocouples use the Peltier-Seebeck effect to measure the temperature difference between two points.
Often instrumentation is not used by itself, but instead as the sensors of larger electrical systems. For example, a thermocouple might be used to help ensure a furnace's temperature remains constant. For this reason, instrumentation engineering is often viewed as the counterpart of control engineering.

Computers:
Computer engineering deals with the design of computers and computer systems. This may involve the design of new hardware, the design of PDAs or the use of computers to control an industrial plant. Computer engineers may also work on a system's software. However, the design of complex software systems is often the domain of software engineering, which is usually considered a separate discipline. Desktop computers represent a tiny fraction of the devices a computer engineer might work on, as computer-like architectures are now found in a range of devices including video game consoles and DVD players.

Related disciplines:
Mechatronics is an engineering discipline which deals with the convergence of electrical and mechanical systems. Such combined systems are known as electromechanical systems and have widespread adoption. Examples include automated manufacturing systems, heating, ventilation and air-conditioning systems and various subsystems of aircraft and automobiles.
The term mechatronics is typically used to refer to macroscopic systems but futurists have predicted the emergence of very small electromechanical devices. Already such small devices, known as micro electromechanical systems (MEMS), are used in automobiles to tell airbags when to deploy, in digital projectors to create sharper images and in inkjet printers to create nozzles for high-definition printing. In the future it is hoped the devices will help build tiny implantable medical devices and improve optical communication.
Biomedical engineering is another related discipline, concerned with the design of medical equipment. This includes fixed equipment such as ventilators, MRI scanners and electrocardiograph monitors as well as mobile equipment such as cochlear implants, artificial pacemakers and artificial hearts.

Reference:
en.wikipedia.org

Photoresistor

2009-10-12T09:00:00.001-07:00

Photoresistor

A photoresistor or light dependent resistor or cadmium sulfide (CdS) cell is a resistor whose resistance decreases with increasing incident light intensity. It can also be referred to as a photoconductor.
A photoresistor is made of a high resistance semiconductor. If light falling on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band . The resulting free electron (and its hole partner) conduct electricity, thereby lowering resistance.
A photoelectric device can be either intrinsic or extrinsic. An intrinsic semiconductor has its own charge carriers and is not an efficient semiconductor, e.g. silicon. In intrinsic devices the only available electrons are in the valence band , and hence the photon must have enough energy to excite the electron across the entire bandgap. Extrinsic devices have impurities, also called dopants, added whose ground state energy is closer to the conduction band; since the electrons don't have as far to jump, lower energy photons (i.e., longer wavelengths and lower frequencies) are sufficient to trigger the device. If a sample of silicon has some of its atoms replaced by phosphorus atoms (impurities), there will be extra electrons available for conduction. This is an example of an extrinsic semiconductor.

Applications:
Photoresistors come in many different types. Inexpensive cadmium sulfide cells can be found in many consumer items such as camera light meters, street lights, clock radios, alarms , and outdoor clocks.
They are also used in some dynamic compressors together with a small incandescent lamp or light emitting diode to control gain reduction.
Lead sulfide and indium antimonide LDRs are used for the mid infrared spectral region. Ge:Cu photoconductors are among the best far-infrared detectors available, and are used for infrared astronomy and infrared spectroscopy .

Circuit symbol:
Below is a symbol for a photoresistor as used in some circuit diagrams.
Phototransistors react much quicker to light change. As such, they're preferred for receiving data.

Reference:
www.absoluteastronomy.com

golssary

2009-10-12T08:59:00.001-07:00

golssary

Resistor:
|- align = "center"||width = "25"|| |- align = "center"||| Potentiometer|- align = "center"| || |- align = "top"| Resistor|| Variable resistor...
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Electrical resistance:
The electrical resistance of an object is a measure of its opposition to the passage of a steady electrical current. An object of uniform cross section will have a resistance proportional to its length and inversely proportional to its cross-sectional area, and proportional to the resistivity of the material....
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Semiconductor:
A semiconductor is a material that has electrical conductivity between those of a Electrical conductor and an electrical insulation; it can vary over that wide range either permanently or dynamically....
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Frequency:
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency....
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Photon:
In physics, the photon is an elementary particle, the quantum of the electromagnetic field and the basic unit of light and all other forms of electromagnetic radiation....
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Electron:
The electron is a subatomic particle that carries a negative electric charge. It has elementary particle and is believed to be a point particle....
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Conduction band:
In the physics field of semiconductors and Electrical insulations, the conduction band is the range of electron energy, higher than that of the valence band, sufficient to make the electrons free to accelerate under the influence of an applied electric field and thus constitute an electric current....
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Electron hole:
An electron hole is the conceptual and mathematical opposite of an electron, useful in the study of physics and chemistry. The concept describes the lack of an electron....
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Valence band:
In solids, the valence band is the highest range of electron energy where electrons are normally present at absolute zero.In semiconductors and Electrical insulations, there is a band gap above the valence band, followed by a conduction band above that....
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Cadmium sulfide:
Cadmium Sulfur is a chemical compound with the formula CdS. Cadmium sulfide is yellow in colour and is a semiconductor. It exists in nature as two different minerals, greenockite and hawleyite....
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Alarm:
An alarm gives an audible or visual warning about a problem or condition.Alarms include:* burglar alarms, designed to warn of burglaries; this is often a silent alarm: the police or guards are warned without indication to the burglar, which increases the chances of catching him or her....
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Lead sulfide:
Lead sulfide is an ionic compound of lead and sulfur, having two possible proportions:*Lead sulfide, the ionic compound containing one lead atom and one sulfur atom....
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Germanium:
Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, greyish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon....
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Copper:
Copper is a chemical element with the symbol Cu and atomic number 29.It is a ductile metal with very high thermal and electrical conductivity....
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Infrared:
Infrared radiation is electromagnetic radiation whose wavelength is longer than that of visible light , but shorter than that of terahertz radiation and microwaves ....
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Infrared astronomy:
Infrared astronomy is the branch of astronomy and astrophysics which deals with objects visible in infrared radiation. Visible radiation ranges from 400 nanometre to 700 nm ....
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Infrared spectroscopy:
Infrared spectroscopy is the subset of spectroscopy that deals with the infrared region of the electromagnetic spectrum. It covers a range of techniques, the most common being a form of absorption spectroscopy....
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Optoelectronics:
Optoelectronics is the study and application of electronics devices that source, detect and control light, usually considered a sub-field of photonics....

Diode

2009-10-12T08:58:00.000-07:00

Diode

In electronics, a diode is a two-terminal device (thermionic diodes may also have one or two ancillary terminals for a heater).
Diodes have two active electrodes between which the signal of interest may flow, and most are used for their unidirectional electric current property. The varicap diode is used as an electrically adjustable capacitor.
The unidirectionality most diodes exhibit is sometimes generically called the rectifying property. The most common function of a diode is to allow an electric current in one direction (called the forward biased condition) and to block the current in the opposite direction (the reverse biased condition). Thus, the diode can be thought of as an electronic version of a check valve.
Real diodes do not display such a perfect on-off directionality but have a more complex non-linear electrical characteristic, which depends on the particular type of diode technology. Diodes also have many other functions in which they are not designed to operate in this on-off manner.
Early diodes included “cat’s whisker” crystals and vacuum tube devices (also called thermionic valves). Today the most common diodes are made from semiconductor materials such as silicon or germanium.

History:
Although the crystal (solid state) diode was popularized before the thermionic diode, thermionic and solid state diodes were developed in parallel. The principle of operation of thermionic diodes was discovered by Frederick Guthrie in 1873. The principle of operation of crystal diodes was discovered in 1874 by the German scientist, Karl Ferdinand Braun.
At the time of their invention, such devices were known as rectifiers. In 1919, William Henry Eccles coined the term diode from Greek roots; di means "two", and ode (from ὅδος) means "path".

Principles:
Thermionic diode principles were rediscovered by Thomas Edison on February 13, 1880 and he was awarded a patent in 1883 (U.S. Patent 307,031), but developed the idea no further. Braun patented the crystal rectifier in 1899. Braun's discovery was further developed by Jagdish Chandra Bose into a useful device for radio detection.

Radio receivers:
The first radio receiver using a crystal diode was built by Greenleaf Whittier Pickard. The first thermionic diode was patented in Britain by John Ambrose Fleming (scientific adviser to the Marconi Company and former Edison employee) on November 16, 1904 (followed by U.S. Patent 803,684 in November 1905). Pickard received a patent for a silicon crystal detector on November 20, 1906 (U.S. Patent 836,531).

Thermionic and gaseous state diodes:
Thermionic diodes are thermionic-valve devices (also known as vacuum tubes, tubes, or valves), which are arrangements of electrodes surrounded by a vacuum within a glass envelope. Early examples were fairly similar in appearance to incandescent light bulbs.
In thermionic valve diodes, a current through the heater filament indirectly heats the cathode, another internal electrode treated with a mixture of barium and strontium oxides, which are oxides of alkaline earth metals; these substances are chosen because they have a small work function. (Some valves use direct heating, in which a tungsten filament acts as both heater and cathode.) The heat causes thermionic emission of electrons into the vacuum. In forward operation, a surrounding metal electrode called the anode is positively charged so that it electrostatically attracts the emitted electrons. However, electrons are not easily released from the unheated anode surface when the voltage polarity is reversed. Hence, any reverse flow is negligible.
For much of the 20th century, thermionic valve diodes were used in analog signal applications, and as rectifiers in many power supplies. Today, valve diodes are only used in niche applications such as rectifiers in electric guitar and high-end audio amplifiers as well as specialized high-voltage equipment.

Semiconductor diodes:
Most diodes today are based on semiconductor p-n junctions. In a p-n diode, conventional current is from the p-type side (the anode) to the n-type side (the cathode), but not in the opposite direction. Another type of semiconductor diode, the Schottky diode, is formed from the contact between a metal and a semiconductor rather than by a p-n junction.

Current–voltage characteristic:
A semiconductor diode's current–voltage characteristic, or I–V curve, is related to the transport of carriers through the so-called depletion layer or depletion region that exists at the p-n junction between differing semiconductors. When a p-n junction is first created, conduction band (mobile) electrons from the N-doped region diffuse into the P-doped region where there is a large population of holes (places for electrons in which no electron is present) with which the electrons "recombine". When a mobile electron recombines with a hole, both hole and electron vanish, leaving behind an immobile positively charged donor on the N-side and negatively charged acceptor on the P-side. The region around the p-n junction becomes depleted of charge carriers and thus behaves as an insulator.
However, the width of the depletion region (called the depletion width) cannot grow without limit. For each electron-hole pair that recombines, a positively-charged dopant ion is left behind in the N-doped region, and a negatively charged dopant ion is left behind in the P-doped region. As recombination proceeds and more ions are created, an increasing electric field develops through the depletion zone which acts to slow and then finally stop recombination. At this point, there is a "built-in" potential across the depletion zone.
If an external voltage is placed across the diode with the same polarity as the built-in potential, the depletion zone continues to act as an insulator, preventing any significant electric current flow. This is the reverse bias phenomenon. However, if the polarity of the external voltage opposes the built-in potential, recombination can once again proceed, resulting in substantial electric current through the p-n junction. For silicon diodes, the built-in potential is approximately 0.6 V. Thus, if an external current is passed through the diode, about 0.6 V will be developed across the diode such that the P-doped region is positive with respect to the N-doped region and the diode is said to be "turned on" as it has a forward bias.
A diode’s I–V characteristic can be approximated by four regions of operation (see the figure at right).
At very large reverse bias, beyond the peak inverse voltage or PIV, a process called reverse breakdown occurs which causes a large increase in current that usually damages the device permanently. The avalanche diode is deliberately designed for use in the avalanche region. In the zener diode, the concept of PIV is not applicable. A zener diode contains a heavily doped p-n junction allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material, such that the reverse voltage is "clamped" to a known value (called the zener voltage), and avalanche does not occur. Both devices, however, do have a limit to the maximum current and power in the clamped reverse voltage region. Also, following the end of forward conduction in any diode, there is reverse current for a short time. The device does not attain its full blocking capability until the reverse current ceases.
The second region, at reverse biases more positive than the PIV, has only a very small reverse saturation current. In the reverse bias region for a normal P-N rectifier diode, the current through the device is very low (in the µA range).
The third region is forward but small bias, where only a small forward current is conducted.
As the potential difference is increased above an arbitrarily defined "cut-in voltage" or "on-voltage" or "diode forward voltage drop (Vd)", the diode current becomes appreciable (the level of current considered "appreciable" and the value of cut-in voltage depends on the application), and the diode presents a very low resistance.
The current–voltage curve is exponential. In a normal silicon diode at rated currents, the arbitrary "cut-in" voltage is defined as 0.6 to 0.7 volts. The value is different for other diode types — Schottky diodes can be as low as 0.2 V and red light-emitting diodes (LEDs) can be 1.4 V or more and blue LEDs can be up to 4.0 V.
At higher currents the forward voltage drop of the diode increases. A drop of 1 V to 1.5 V is typical at full rated current for power diodes.

Shockley diode equation:
The Shockley ideal diode equation or the diode law (named after transistor co-inventor William Bradford Shockley, not to be confused with tetrode inventor Walter H. Schottky) is the I–V characteristic of an ideal diode in either forward or reverse bias (or no bias). The equation is:

where
I is the diode current,
IS is the reverse bias saturation current,
VD is the voltage across the diode,
VT is the thermal voltage,
and n is the emission coefficient, also known as the ideality factor. The emission coefficient n varies from about 1 to 2 depending on the fabrication process and semiconductor material and in many cases is assumed to be approximately equal to 1 (thus the notation n is omitted).
The thermal voltage VT is approximately 25.85 mV at 300 K, a temperature close to “room temperature” commonly used in device simulation software. At any temperature it is a known constant defined by:

where
q is the magnitude of charge on an electron (the elementary charge),
k is Boltzmann’s constant,
T is the absolute temperature of the p-n junction in kelvins
The Shockley ideal diode equation or the diode law is derived with the assumption that the only processes giving rise to current in the diode are drift (due to electrical field), diffusion, and thermal recombination-generation. It also assumes that the recombination-generation (R-G) current in the depletion region is insignificant. This means that the Shockley equation doesn’t account for the processes involved in reverse breakdown and photon-assisted R-G. Additionally, it doesn’t describe the “leveling off” of the I–V curve at high forward bias due to internal resistance.
Under reverse bias voltages (see Figure 5) the exponential in the diode equation is negligible, and the current is a constant (negative) reverse current value of -IS. The reverse breakdown region is not modeled by the Shockley diode equation.
For even rather small forward bias voltages (see Figure 5) the exponential is very large because the thermal voltage is very small, so the subtracted ‘1’ in the diode equation is negligible and the forward diode current is often approximated as

The use of the diode equation in circuit problems is illustrated in the article on diode modeling.

Small-signal behavior:
For circuit design, a small-signal model of the diode behavior often proves useful. A specific example of diode modeling is discussed in the article on small-signal circuits.

Types of semiconductor diode:
There are several types of junction diodes, which either emphasize a different physical aspect of a diode often by geometric scaling, doping level, choosing the right electrodes, are just an application of a diode in a special circuit, or are really different devices like the Gunn and laser diode and the MOSFET:
Normal (p-n) diodes, which operate as described above, are usually made of doped silicon or, more rarely, germanium. Before the development of modern silicon power rectifier diodes, cuprous oxide and later selenium was used; its low efficiency gave it a much higher forward voltage drop (typically 1.4–1.7 V per “cell”, with multiple cells stacked to increase the peak inverse voltage rating in high voltage rectifiers), and required a large heat sink (often an extension of the diode’s metal substrate), much larger than a silicon diode of the same current ratings would require. The vast majority of all diodes are the p-n diodes found in CMOS integrated circuits, which include two diodes per pin and many other internal diodes.

Avalanche diodes:
Diodes that conduct in the reverse direction when the reverse bias voltage exceeds the breakdown voltage. These are electrically very similar to Zener diodes, and are often mistakenly called Zener diodes, but break down by a different mechanism, the avalanche effect. This occurs when the reverse electric field across the p-n junction causes a wave of ionization, reminiscent of an avalanche, leading to a large current. Avalanche diodes are designed to break down at a well-defined reverse voltage without being destroyed. The difference between the avalanche diode (which has a reverse breakdown above about 6.2 V) and the Zener is that the channel length of the former exceeds the “mean free path” of the electrons, so there are collisions between them on the way out. The only practical difference is that the two types have temperature coefficients of opposite polarities.

Cat’s whisker or crystal diodes:
These are a type of point contact diode. The cat’s whisker diode consists of a thin or sharpened metal wire pressed against a semiconducting crystal, typically galena or a piece of coal. The wire forms the anode and the crystal forms the cathode. Cat’s whisker diodes were also called crystal diodes and found application in crystal radio receivers. Cat’s whisker diodes are obsolete.

Constant current diodes:
These are actually a JFET with the gate shorted to the source, and function like a two-terminal current-limiter analog to the Zener diode, which is limiting voltage. They allow a current through them to rise to a certain value, and then level off at a specific value. Also called CLDs, constant-current diodes, diode-connected transistors, or current-regulating diodes.

Esaki or tunnel diodes:
These have a region of operation showing negative resistance caused by quantum tunneling, thus allowing amplification of signals and very simple bistable circuits. These diodes are also the type most resistant to nuclear radiation.

Gunn diodes:
These are similar to tunnel diodes in that they are made of materials such as GaAs or InP that exhibit a region of negative differential resistance. With appropriate biasing, dipole domains form and travel across the diode, allowing high frequency microwave oscillators to be built.

Light-emitting diodes (LEDs):
In a diode formed from a direct band-gap semiconductor, such as gallium arsenide, carriers that cross the junction emit photons when they recombine with the majority carrier on the other side. Depending on the material, wavelengths (or colors) from the infrared to the near ultraviolet may be produced. The forward potential of these diodes depends on the wavelength of the emitted photons: 1.2 V corresponds to red, 2.4 to violet. The first LEDs were red and yellow, and higher-frequency diodes have been developed over time. All LEDs produce incoherent, narrow-spectrum light; “white” LEDs are actually combinations of three LEDs of a different color, or a blue LED with a yellow scintillator coating. LEDs can also be used as low-efficiency photodiodes in signal applications. An LED may be paired with a photodiode or phototransistor in the same package, to form an opto-isolator.

Laser diodes:
When an LED-like structure is contained in a resonant cavity formed by polishing the parallel end faces, a laser can be formed. Laser diodes are commonly used in optical storage devices and for high speed optical communication.

Peltier diodes:
These diodes are used as sensors, heat engines for thermoelectric cooling. Charge carriers absorb and emit their band gap energies as heat.

Photodiodes:
All semiconductors are subject to optical charge carrier generation. This is typically an undesired effect, so most semiconductors are packaged in light blocking material. Photodiodes are intended to sense light(photodetector), so they are packaged in materials that allow light to pass, and are usually PIN (the kind of diode most sensitive to light). A photodiode can be used in solar cells, in photometry, or in optical communications. Multiple photodiodes may be packaged in a single device, either as a linear array or as a two-dimensional array. These arrays should not be confused with charge-coupled devices.

Point-contact diodes:
These work the same as the junction semiconductor diodes described above, but their construction is simpler. A block of n-type semiconductor is built, and a conducting sharp-point contact made with some group-3 metal is placed in contact with the semiconductor. Some metal migrates into the semiconductor to make a small region of p-type semiconductor near the contact. The long-popular 1N34 germanium version is still used in radio receivers as a detector and occasionally in specialized analog electronics.

PIN diodes:
A PIN diode has a central un-doped, or intrinsic, layer, forming a p-type/intrinsic/n-type structure. They are used as radio frequency switches and attenuators. They are also used as large volume ionizing radiation detectors and as photodetectors. PIN diodes are also used in power electronics, as their central layer can withstand high voltages. Furthermore, the PIN structure can be found in many power semiconductor devices, such as IGBTs, power MOSFETs, and thyristors.

Schottky diodes:
Schottky diodes are constructed from a metal to semiconductor contact. They have a lower forward voltage drop than p-n junction diodes. Their forward voltage drop at forward currents of about 1 mA is in the range 0.15 V to 0.45 V, which makes them useful in voltage clamping applications and prevention of transistor saturation. They can also be used as low loss rectifiers although their reverse leakage current is generally higher than that of other diodes. Schottky diodes are majority carrier devices and so do not suffer from minority carrier storage problems that slow down many other diodes — so they have a faster “reverse recovery” than p-n junction diodes. They also tend to have much lower junction capacitance than p-n diodes which provides for high switching speeds and their use in high-speed circuitry and RF devices such as switched-mode power supply, mixers and detectors.

Super Barrier Diodes:
Super barrier diodes are rectifier diodes that incorporate the low forward voltage drop of the Schottky diode with the surge-handling capability and low reverse leakage current of a normal p-n junction diode.

Gold-doped” diodes:
As a dopant, gold (or platinum) acts as recombination centers, which help a fast recombination of minority carriers. This allows the diode to operate at signal frequencies, at the expense of a higher forward voltage drop. Gold doped diodes are faster than other p-n diodes (but not as fast as Schottky diodes). They also have less reverse-current leakage than Schottky diodes (but not as good as other p-n diodes). A typical example is the 1N914.
Snap-off or Step recovery diodes:
The term ‘step recovery’ relates to the form of the reverse recovery characteristic of these devices. After a forward current has been passing in an SRD and the current is interrupted or reversed, the reverse conduction will cease very abruptly (as in a step waveform). SRDs can therefore provide very fast voltage transitions by the very sudden disappearance of the charge carriers.

Transient voltage suppression diode (TVS):
These are avalanche diodes designed specifically to protect other semiconductor devices from high-voltage transients. Their p-n junctions have a much larger cross-sectional area than those of a normal diode, allowing them to conduct large currents to ground without sustaining damage.

Varicap or varactor diodes:
These are used as voltage-controlled capacitors. These are important in PLL (phase-locked loop) and FLL (frequency-locked loop) circuits, allowing tuning circuits, such as those in television receivers, to lock quickly, replacing older designs that took a long time to warm up and lock. A PLL is faster than an FLL, but prone to integer harmonic locking (if one attempts to lock to a broadband signal). They also enabled tunable oscillators in early discrete tuning of radios, where a cheap and stable, but fixed-frequency, crystal oscillator provided the reference frequency for a voltage-controlled oscillator.

Zener diodes:
Diodes that can be made to conduct backwards. This effect, called Zener breakdown, occurs at a precisely defined voltage, allowing the diode to be used as a precision voltage reference. In practical voltage reference circuits Zener and switching diodes are connected in series and opposite directions to balance the temperature coefficient to near zero. Some devices labeled as high-voltage Zener diodes are actually avalanche diodes (see below). Two (equivalent) Zeners in series and in reverse order, in the same package, constitute a transient absorber (or Transorb, a registered trademark). The Zener diode is named for Dr. Clarence Melvin Zener of Southern Illinois University, inventor of the device.
Other uses for semiconductor diodes include sensing temperature, and computing analog logarithms.

Numbering:
A standardized 1N-series numbering system was introduced in the US by EIA/JEDEC (Joint Electron Device Engineering Council) about 1960. Among the most popular in this series were: 1N34A/1N270 (Germanium signal), IN914/1N4148 (Silicon signal) and 1N4001-1N4007 (Silicon 1A power rectifier).

Related devices:
- Rectifier
- Transistor
- Thyristor or silicon controlled rectifier (SCR)
- TRIAC
- Diac
- Varistor
In optics, an equivalent device for the diode but with laser light would be the Optical isolator, also known as an Optical Diode, that allows light to only pass in 1 direction. It uses a Faraday rotator as the main component.

Applications: Radio demodulation:
The first use for the diode was the demodulation of amplitude modulated (AM) radio broadcasts. The history of this discovery is treated in depth in the radio article. In summary, an AM signal consists of alternating positive and negative peaks of voltage, whose amplitude or “envelope” is proportional to the original audio signal. The diode (originally a crystal diode) rectifies the AM radio frequency signal, leaving an audio signal which is the original audio signal. The audio is extracted using a simple filter and fed into an audio amplifier or transducer, which generates sound waves.

Power conversion:
Rectifiers are constructed from diodes, where they are used to convert alternating current (AC) electricity into direct current (DC). Automotive alternators are a common example, where the diode, which rectifies the AC into DC, provides better performance than the commutator of earlier dynamo. Similarly, diodes are also used in Cockcroft–Walton voltage multipliers to convert AC into higher DC voltages.

Over-voltage protection:
Diodes are frequently used to conduct damaging high voltages away from sensitive electronic devices. They are usually reverse-biased (non-conducting) under normal circumstances. When the voltage rises above the normal range, the diodes become forward-biased (conducting). For example, diodes are used in ( stepper motor and H-bridge ) motor controller and relay circuits to de-energize coils rapidly without the damaging voltage spikes that would otherwise occur. (Any diode used in such an application is called a flyback diode). Many integrated circuits also incorporate diodes on the connection pins to prevent external voltages from damaging their sensitive transistors. Specialized diodes are used to protect from over-voltages at higher power (see Diode types above).

Logic gates:
Diodes can be combined with other components to construct AND and OR logic gates. This is referred to as diode logic.

Ionizing radiation detectors:
In addition to light, mentioned above, semiconductor diodes are sensitive to more energetic radiation. In electronics, cosmic rays and other sources of ionizing radiation cause noise pulses and single and multiple bit errors. This effect is sometimes exploited by particle detectors to detect radiation. A single particle of radiation, with thousands or millions of electron volts of energy, generates many charge carrier pairs, as its energy is deposited in the semiconductor material. If the depletion layer is large enough to catch the whole shower or to stop a heavy particle, a fairly accurate measurement of the particle’s energy can be made, simply by measuring the charge conducted and without the complexity of a magnetic spectrometer or etc. These semiconductor radiation detectors need efficient and uniform charge collection and low leakage current. They are often cooled by liquid nitrogen. For longer range (about a centimetre) particles they need a very large depletion depth and large area. For short range particles, they need any contact or un-depleted semiconductor on at least one surface to be very thin. The back-bias voltages are near breakdown (around a thousand volts per centimetre). Germanium and silicon are common materials. Some of these detectors sense position as well as energy. They have a finite life, especially when detecting heavy particles, because of radiation damage. Silicon and germanium are quite different in their ability to convert gamma rays to electron showers.
Semiconductor detectors for high energy particles are used in large numbers. Because of energy loss fluctuations, accurate measurement of the energy deposited is of less use.

Temperature measuring:
A diode can be used as a temperature measuring device, since the forward voltage drop across the diode depends on temperature, as in a Silicon bandgap temperature sensor. From the Shockley ideal diode equation given above, it appears the voltage has a positive temperature coefficient (at a constant current) but depends on doping concentration and operating temperature (Sze 2007). The temperature coefficient can be negative as in typical thermistors or positive for temperature sense diodes down to about 20 kelvins.

Current steering:
Diodes will prevent currents in unintended directions. To supply power to an electrical circuit during a power failure, the circuit can draw current from a battery. An Uninterruptible power supply may use diodes in this way to ensure that current is only drawn from the battery when necessary. Similarly, small boats typically have two circuits each with their own battery/batteries: one used for engine starting; one used for domestics. Normally both are charged from a single alternator, and a heavy duty split charge diode is used to prevent the higher charge battery (typically the engine battery) from discharging through the lower charged battery when the alternator is not running.
Diodes are also used in electronic musical keyboards. To reduce the amount of wiring needed in electronic musical keyboards, these instruments often use keyboard matrix circuits. The keyboard controller scans the rows and columns to determine which note the player has pressed. The problem with matrix circuits is that when several notes are pressed at once, the current can flow backwards through the circuit and trigger "phantom keys" that cause "ghost" notes to play. To avoid triggering unwanted notes, most keyboard matrix circuits have diodes soldered with the switch under each key of the musical keyboard. The same principle is also used for the switch matrix in solid state pinball machines.

Abbreviations:
Diodes are usually referred to as D for diode on PCBs. Sometimes the abbreviation CR for crystal rectifier is used.

Reference:
en.wikipedia.org

How does a transistor work?

2009-10-12T08:49:00.000-07:00

How does a transistor work?

Answer:
The design of a transistor allows it to function as an amplifier or a switch. This is accomplished by using a small amount of electricity to control a gate on a much larger supply of electricity, much like turning a valve to control a supply of water.
Transistors are composed of three parts – a base, a collector, and an emitter. The base is the gate controller device for the larger electrical supply. The collector is the larger electrical supply, and the emitter is the outlet for that supply. By sending varying levels of current from the base, the amount of current flowing through the gate from the collector may be regulated. In this way, a very small amount of current may be used to control a large amount of current, as in an amplifier. The same process is used to create the binary code for the digital processors but in this case a voltage threshold of five volts is needed to open the collector gate. In this way, the transistor is being used as a switch with a binary function: five volts – ON, less than five volts – OFF.
Semi-conductive materials are what make the transistor possible. Most people are familiar with electrically conductive and non-conductive materials. Metals are typically thought of as being conductive. Materials such as wood, plastics, glass and ceramics are non-conductive, or insulators. In the late 1940’s a team of scientists working at Bell Labs in New Jersey, discovered how to take certain types of crystals and use them as electronic control devices by exploiting their semi-conductive properties. Most non-metallic crystalline structures would typically be considered insulators. But by forcing crystals of germanium or silicon to grow with impurities such as boron or phosphorus, the crystals gain entirely different electrical conductive properties. By sandwiching this material between two conductive plates (the emitter and the collector), a transistor is made. By applying current to the semi-conductive material (base), electrons gather until an effectual conduit is formed allowing electricity to pass The scientists that were responsible for the invention of the transistor were John Bardeen, Walter Brattain, and William Shockley. Their Patent was called: “Three Electrode Circuit Element Utilizing Semiconductive Materials.”

There are two main types of transistors-junction transistors and field effect transistors. Each works in a different way. But the usefulness of any transistor comes from its ability to control a strong current with a weak voltage. For example, transistors in a public address system amplify (strengthen) the weak voltage produced when a person speaks into a microphone. The electricity coming from the transistors is strong enough to operate a loudspeaker, which produces sounds much louder than the person's voice.

JUNCTION TRANSISTORS:
A junction transistor consists of a thin piece of one type of semiconductor material between two thicker layers of the opposite type. For example, if the middle layer is p-type, the outside layers must be n-type. Such a transistor is an NPN transistor. One of the outside layers is called the emitter, and the other is known as the collector. The middle layer is the base. The places where the emitter joins the base and the base joins the collector are called junctions.
The layers of an NPN transistor must have the proper voltage connected across them. The voltage of the base must be more positive than that of the emitter. The voltage of the collector, in turn, must be more positive than that of the base. The voltages are supplied by a battery or some other source of direct current. The emitter supplies electrons. The base pulls these electrons from the emitter because it has a more positive voltage than does the emitter. This movement of electrons creates a flow of electricity through the transistor.
The current passes from the emitter to the collector through the base. Changes in the voltage connected to the base modify the flow of the current by changing the number of electrons in the base. In this way, small changes in the base voltage can cause large changes in the current flowing out of the collector.
Manufacturers also make PNP junction transistors. In these devices, the emitter and collector are both a p-type semiconductor material and the base is n-type. A PNP junction transistor works on the same principle as an NPN transistor. But it differs in one respect. The main flow of current in a PNP transistor is controlled by altering the number of holes rather than the number of electrons in the base. Also, this type of transistor works properly only if the negative and positive connections to it are the reverse of those of the NPN transistor.

FIELD EFFECT TRANSISTORS:
A field effect transistor has only two layers of semiconductor material, one on top of the other. Electricity flows through one of the layers, called the channel. A voltage connected to the other layer, called the gate, interferes with the current flowing in the channel. Thus, the voltage connected to the gate controls the strength of the current in the channel. There are two basic varieties of field effect transistors-the junction field effect transistor(JFET) and the metal oxide semiconductor field effect transistor (MOSFET). Most of the transistors contained in today's integrated circuits are MOSFETS's.