Aircraft Maintenance Engineering-Avionics

Very High Frequency Communication VHF

2011-12-11T07:22:00.000-08:00

Very High Frequency Communication is the standard civil short range communication facility using the band of frequencies between 118.000 MHz and 137 MHz. VHF is used by ground control facilities and aircraft or by aircraft and other aircraft on one of 760 possible frequency channels with 25 kHz spacing between channels. Another VHF service available is weather information transmitted from ground stations

Current International Civil Aviation Organisation (ICAO) regulations require a new channel spacing one third of the current spacing ie. 8.33 kHz apart. This will treble the number of available channels to 2280 (3760). Frequency selection is by means of 2 from 5 system converted to Binary Coded Decimal (BCD), from which control of phase-locked loops (PLL) determine transmitter Tx and receiver Rx frequencies.

VHF uses line of sight space wave transmissions with a theoretical range of 123 miles between an aircraft at a height of 10-,000 ft and a ground station at sea level. In practice, however, useable range will also vary depending upon factors such as transmitter power, receiver sensitivity, atmospheric and temperature conditions, ground station geographical situation etc. and any obstruction in between aircraft and ground station eg. mountains, hills and trees. As a general rule, satisfactory two-way communication can typically be maintained up to 200 miles, this range dependent on the aircraft height.

Speech amplitude modulation is used for radio telephony transmissions which are vertically polarised transmissions. A common aerial for transmit and receive purposes is used, this system being known as a single channel simplex system.

Basic Cable Coding System

2011-08-30T10:02:00.000-07:00

(1) (2) (3) (4) (5) (6)

1 E F 6 B 22 NMS V

Position 1	Unit number, used where components have identical circuits.
Position 2	Circuit function letter and circuit designation letter which indicate circuit function and the associated system.
Position 3	Cable number, allocated to differentiate.
Position 4	Cable segment letter, which identifies the segment of cable between two terminals or connections, and differentiates between segments of the circuit when the same cable number is used throughout. Segments are lettered in alphabetical sequence, excluding the letter I and O. A different letter is used for each of the cable segments having a common terminal or connection.
Position 5	Cable size.
Position 6	Suffix data, used to indicate the type of cable and to identify its connection function. For example, in the example code NMS V indicates nyvinmetsheath ungrounded cable in a single-phase system.

Air Data Computer-Anologue

2011-06-21T23:39:00.000-07:00

The air data computer (ADC) is an additional and improved means of supplying the pilot and co-pilot with airspeed, altitude, vertical speed and mach number. Previously, air pressure was supplied to the primary pitot/static flight instruments by long lengths of P/S pipelines. To minimize these, a central unit was developed where the pitot and static pressures could be fed. This unit is known as ‘The Centralized Air Data Computer’. The outputs from the CADC are in the form of synchro signals therefore, the CADC may be referred to as being an analogue device. These are still widely used, although later generation aircraft have switched to digital units.

The simple pitot static system is still fitted in almost all aircraft and will be used as a safety/comparison factor against the CADC and vice versa. Two independent CADC systems would be installed, the input to each would be from independent sources for safety reasons.

A modular arrangement is shown along with a typical transmission system.

Aircraft Antennas

2011-05-06T07:46:00.000-07:00

Air carrier operations ideally require uninterrupted communications between both air traffic control agencies, and company operations, and/or out-station handling agents. Much of these communications are achieved by voice, using either short range VHF, or HF for long distance communications. This situation is changing in that information is being digitized and routed over data links including satellites with print-outs available as required. The antenna map shows the typical equipment installed in a modern jet transport for communication purpose.

Obviously there are many equipment manufacturers producing communications and navigation equipment. In these lessons no attempt is made to portray any specific type, and the diagrams should be regarded as typical of the many and various models available.

Velocity Control Servomechanisms

2011-04-26T08:09:00.000-07:00

In some applications it is the rotational speed of a shaft and not its position that must be controlled. A Rate Servo is shown in the diagram . The input demand signal is used to control the angular velocity of the output shaft and not its position. To make the speed of the driving motor exactly proportional to the input demand a servomechanism is essential. If a servomechanism were not used the speed of the output motor would vary with changes in the supply voltage or any changes of the friction in the motor or its load.

Note that there is no position feedback.

Movement of the speed control potentiometer produces a voltage proportional to the demanded speed. The tacho-generator provides a voltage proportional to the angular velocity of the output shaft. If there is a difference between these two signals an error voltage will be fed to the amplifier. The output of the amplifier will accelerate or decelerate the motor until the output of the tacho-generator produces a voltage exactly equal to the input demand voltage and the motor will run at the demanded speed.

Coulomb Friction Damping

2011-04-08T07:56:00.000-07:00

In a servomechanism with no frictional forces, all the torque developed, which is proportional to the error is used to accelerate the load. However, when friction damping is introduced, the torque developed must first overcome the frictional force before the system can be set in motion.

If d represents the error that must exist before the system can develop sufficient torque to overcome the frictional torque, then if the error is less than d when the system is not in motion, the system will remain at rest with that amount of error. The response to a step function input of a servomechanism with coulomb friction damping is shown below; the response curve of an undamped system is also given so that a direct comparison can be made.

From Figure it can be seen that each successive oscillation of the output shaft is reduced in amplitude by the retarding effect of the friction damping until a point is reached when, with the output shaft velocity at zero, the error is less than d; at this point the output shaft will come to rest. However, as shown in Fig 8, the output shaft may not be at its correct position and a permanent error may exist; this error is known as positional error.

The response curve given in Figure indicates two overshoots before the system finally comes to rest, but the number of overshoots for a given degree of damping will depend upon the size of the initial error in relation to the amount of error required to overcome the friction torque; an initial error of 2d would produce no overshoot, whilst a large initial error compared with 2d would produce several overshoots. In practice, coulomb friction damping is not used and, although always present, it is kept to a minimum.

Synchro Types

2011-04-04T00:32:00.000-07:00

Synchro types may be classified as follows:

Torque transmitter
Torque receiver
Torque differential receiver
Torque differential transmitter
Control transmitter
Control transformer
Control differential transmitter
Resolver

Torque Transmitter - TX

Used to generate an electrical signal corresponding to the angular position of a mechanical component. The rotor is connected to the component and the stator kept stationary. The electrical signal is derived from the position of the rotor relative to the stator. The TX is generally used as the transmitting element in a remote position indicating system.

Torque Receiver - TR

The rotor of a torque receiver, which is free to turn, moves to a position dependent on the electrical angular information received from its connected torque transmitter or torque differential transmitter. The TR is generally used as the receiving element (indicator) in a remote position indicating system.

Torque Differential Receiver - TDR

The torque differential receiver is electrically connected to two torque transmitters. The rotor of the TDR, which is free to move, aligns with the stator field. The position of the stator field depends on the inputs from the two transmitters, and the way in which they are interconnected. By suitable connection, the TDR can be made to indicate the sum of the transmitter inputs, or the difference between them.

Torque Differential Transmitter - TDX

The torque differential transmitter has a stator that receives electrical positional information from a torque transmitter, and a rotor which is mechanically positioned. This enables it to transmit electrical information corresponding to the sum, or difference, between the electrical input and its own rotor angle.

Control Transmitter - CX

Used to generate an electrical signal corresponding to the angular position of a mechanical component. The rotor is connected to the component and the stator kept stationary. The electrical signal is derived from the position of the rotor relative to the stator. The TX is generally used as the position transmitting element in a remote position control system.

Control Transformer - CT

A CT is electrically connected to a CX and is used to produce an electrical signal for driving a servo system. The electrical signal produced, is an a.c. voltage with an amplitude and phase dependent on the position of the rotor relative to the stator.

Control Differential Transmitter - CTX

A CTX receives electrical information from a CX and has a rotor which can be mechanically moved. This enables it to transmit an electrical signal proportional to the sum or difference in angle between the electrical input and its own rotor position.

Resolver

A resolver has two mutually perpendicular windings on the rotor and another two on the stator (4 windings in total). It can resolve an input signal into its sine and cosine components, perform the operations of vector addition and subtraction or convert polar to cartesian co-ordinates and vice versa.

Aircraft Servomechanisms

2011-04-01T09:27:00.000-07:00

Servomechanisms can be classified according to two main categories:

• Open loop systems.

• Closed loop systems.

OPEN LOOP

In an open loop system, the input demand generates an electrical equivalent of the demand position. This signal is amplified to the required power level and applied to a motor to position the load. The speed of response and the final position of the load depend on the following factors:

• Any variations in load conditions.

• Frictional forces within the motor and its load, and any mechanical interconnections.

• Variations in power supplies.

• The value of the demand voltage.

• Variations in amplifier gain.

As the open loop system suffers from the variable factors shown above, the output is unlikely to follow the input precisely and cannot provide the close tolerance required.

CLOSED LOOP

If the errors in the output of a system are detected and fed back to the input so that the necessary corrections can be made to eliminate the error, the system is said to be a closed loop system. A Closed loop system is shown below.

The essential features of a closed loop system are:

• Information concerning the behaviour of the load is fed back to the input. This is called feedback.

• The position of the output (feedback) is compared to that demanded by the input. Typically in a summing amplifier.

• The production of an error signal proportional to the difference between the demand and feedback signals.

• Power amplification of the error signal to control the load.

• Movement of the load in such a direction as to reduce the error signal to zero, at which point the output is the same as that demanded by the input.

Electrostatic Discharge Sensitive (ESDS) Devices

2011-03-26T07:40:00.000-07:00

Many electronic Line Replaceable Units (LRU's) on aircraft contain printed circuit boards containing components that are susceptible to damage from electrostatic discharges. Such components are referred to as electrostatic discharge sensitive (ESDS) devices. Decals installed on ESDS LRU's, indicate that special handling is required

Static Electricity & Electrostatic Discharge

The most common conception of static electricity and its accompanying discharge, is the miniature lighting shock you receive when you touch a metal door handle having walked across a nylon carpet. If the door handle is touched with a key first, the discharge will be seen but not felt.

The discharge occurs because different materials receive different levels of charge as materials are rubbed together or pulled apart. The different charge levels create potential differences between the different materials, and when materials of different electrical potential are brought into close proximity with each other, a discharge occurs as the potentials equalise.

The different levels of charge with respect to cotton (the reference material) are shown on the following page, in what is known as the Triboelectric Series.

The further up or down, the greater the charge and hence the greater the discharge when the two materials are brought together.

Triboelectric Series

Material

Air	Increasingly Positive
Human Hands
Asbestos
Rabbit Fur
Glass
Mica
Human Hands
Nylon
Wool
Fur
Lead
Silk
Aluminium
Paper
Cotton-----------------------------------------------	Increasingly Negative
Steel
Wood
Amber
Sealing Wax
Hard Rubber
Nickel Copper
Brass Silver
Gold Platinum
Sulphur
Acetate Rayon
Polyester
Celluloid
Orion
Saran
Polyurethane
Polyethylene
Polypropylene
PVC (vinyl)
Kelf (ctfe)
Silicon
Teflon

Electronic Instrument Systems EIS

2011-03-20T00:18:00.000-07:00

Modern technology has enabled some significant changes in the layout of flight instrumentation on most aircraft currently in service. The biggest change has been the introduction of Electronic Instrument systems. These systems have meant that many complex Electro-mechanical instruments have now been replaced by TV type colour displays. These systems also allow the exchange of images between display units in the case of display failures.

There are many different Electronic Instrument Systems, including:

i. Electronic Flight Instrument System (EFIS).

ii. Engine Indicating & Crew Alerting System (EICAS).

iii. Electronic Centralised Aircraft Monitoring (ECAM).

The Electronic Instrument System (EIS) also allows the flight crew to configure the instrument layout by allowing manual transfer of the Primary Flight Display (PFD) with the Navigation Display (ND) and the secondary Electronic Centralised Aircraft Monitoring (ECAM) display with the ND.

As well as a manual transfer, the system will automatically transfer displays when either the PFD or the primary ECAM display fails. The PFD is automatically transferred onto the corresponding ND, with the ECAM secondary display used for the primary ECAM display.

The system will also automatically transfer the primary ECAM information onto the ND if a double failure of the ECAM display system occurs.

Slab Desynn

2011-03-18T22:43:00.000-07:00

If the voltage at the 3 tappings of the transmitter of a basic Desynn are measured as the wiper arms are rotated 360°, it will be seen that they produce a sawtooth waveform as opposed to a sinewave.
This results in the pointer of the indicator not following the transmitter exactly. In most instances the difference is insignificant, however their may be certain circumstances where it cannot be overlooked.

The solution is to use a modified Desynn transmitter called a 'slab Desynn'. In a slab Desynn, the resistor is wound on a slab former and has the power supply connected to it, whilst the wiper arms now provide the output to the receiver, there being 3 wiper arms each displaced from the next by 120°. The output from this device is a sinewave. It can be connected to the same type of indicator and operates in the same way as the basic Desynn.

Printed Circuit Board PCB

2011-03-17T01:27:00.000-07:00

The assembly of the various circuits which form part of the units employed in aircraft electronic systems, necessitates the interconnection of many components by means of electrical conductors. Before the introduction of printed wiring, these conductors were formed by wires which connected to the components either by soldering, or by screw and crimped terminal methods.

In the development of circuit technology, micro-miniaturisation, rationalisation of component layout and mounting, weight saving, and the simplification of installation and maintenance become essential factors; and as a result, the technique of printing the required circuits was adopted.

In this technique, a metallic foil is first bonded to a base board made from an insulating material, and a pattern is then printed and etched on the foil to form a series of current conducting paths, the pattern replacing the old method or wiring. Connecting points and mounting pads, for the soldering of components appropriate to the circuit, are also formed on the board, so that, as a single assembly, the board satisfies the structural and electrical requirements of the unit which it forms a part.

If the circuit is a simple one, the wiring may be formed on one side of a board, but, where a more complex circuit is required, wiring is continued on to the reverse side, which also serves as the mounting for components. In addition, complex circuits may be incorporated in multi-layer assemblies.

Synchronous Data Transmission

2011-03-15T07:31:00.000-07:00

Synchronous data transmission systems are designed to indicate the position of a component or control surface that cannot be directly observed. The systems fall into one of two categories; d.c. systems called 'Desynn Systems'

There are a variety of different types of Desynn systems available:

The Basic Desynn is generally operated by a rotary motion, however linear versions are also found. The conversion from linear to rotary motion being achieved by a push rod and gear wheel.

The Micro Desynn was designed to magnify the small movement obtained by such items as pressure measuring devices. They are operated by linear motion.

The Slab Desynn was designed to overcome signally errors inherent in the basic Desynn system. In the vast majority of instances the errors in the basic Desynn

Basic Desynn

In the basic Desynn system the transmitter comprises an endless resistance wound on a circular former, this arrangement being referred to as a 'Toroidal Resistance'. Equally spaced at 120° intervals around the resistor are 3 tappings, it is to these that the signal wires are connected. Running on the resistor are two wiper arm type contacts that are spaced apart by 180° and insulated from one another, it is to these that system power is applied.

The indicator comprises a two pole permanent magnet rotor, pivoted to rotate inside a soft iron stator, the pointer being attached to the spindle. The stator carries three star connected windings that are connected to the three wires coming from the tappings of the transmitter.

Methods Of Biasing

2011-03-09T04:41:00.000-08:00

Base Bias

The simplest form of biasing is to use a single resistor. A single source provides the biasing requirements of both PN junctions. The base of the transistor is returned to via and the value of determines the level of

This form of biasing is not as accurate or stable as the aforementioned potential divider bias.

Feedback base bias

A better more stable form of biasing arrangement is shown below, is to connect the bias resistor between the collector and base terminals of the transistor.

Now any increase in increases the potential difference across , lowers the quiescent collector potential set by the biasing arrangement, decreasing as before. However, this time the fall is fed back to the base via , decreasing the forward bias on the base , decreasing and ultimately decreasing , thus correcting the original rise owing to thermal runway.

This NEGATIVE FEEDBACK also occurs when an ac signal is applied to the base and causes a severe reduction in amplification. As the example above shows, the value of the biasing resistor is dependent upon the gain of the transistor and this circuit does NOT stabilise the quiescent conditions against changes in transistor parameters (production spread).

Photo Diodes

2011-02-24T04:20:00.000-08:00

The photodiode is another light-sensitive device which utilises a PN junction. It is constructed in a manner similar to the photovoltaic cell, but it is used in basically the same way as the photoconductive cell. In other words it is used essentially as a light-variable resistor.

The photodiode is a semiconductor device (usually made from silicon) and may be constructed in basically two ways. One type of photodiode utilises a simple PN junction as shown below. A P-type region is diffused into an N-type substrate as shown. This diffusion takes place through a round window that is etched into a silicon dioxide layer that is formed on top of the N-type substrate. Then a metal ring or window is formed over the silicon dioxide layer (through an evaporation process) as shown. This window makes electrical contact with the P-type region and serves as an electrode to which an external lead can be attached. however, the window also accurately controls the area that will receive or respond to light. A metal base is then formed on the bottom N-type layer. This metal layer serves as a second electrode to which another lead is attached

When used as a photovoltaic cell, the device is said to be operating in the photovoltaic mode and it will generate an output voltage (across its electrodes) that varies with the intensity of the light striking it’s P-type layer. However, the photodiode is most commonly subjected to a reverse bias voltage as shown in Fig 1A. In other words its P-type region is made negative with respect to it’s N-type region. Under these conditions a wide depletion region forms around the PN junction. When photons enter this region to create electron-hole pairs, the separated electrons and holes are pulled in opposite directions because of the influence of the charges that exist on each side of the junction and the applied reverse bias. The electrons are drawn toward the positive side of the bias source (the N-type region) and the holes are attracted toward the negative side of the bias voltage (the P-type region). The separated electrons and holes therefore support a small current flow in the reverse direction through the photodiode. As the light intensity increases, more photons produce more electron-hole pairs which further increase the conductivity of the photodiode resulting in a proportionally higher current. When a photodiode is used in this manner it is said to be operating in the photoconductive or photocurrent mode.

PN junction and PIN photodiodes are often mounted on an insulative platform or substrate and sealed within a metal case as shown in Fig 2. A glass window is provided at the top of the case, as shown, to allow light to enter and strike the photodiode. The two leads extend through the insulative base at the bottom of the case and are internally bonded (with fine wires) to the photodiode’s electrodes.

Multi Segment LED

2011-02-20T06:37:00.000-08:00

A further development of the standard LED package is the seven segment numerical indicator and the sixteen segment alpha-numeric indicator. In these devices, the PN junctions are elongated into a rectangular format and the light is emitted in a bar shape. The letter or number which a multi-segment display is required to produce is formed from a combination of illuminated segments.
Below shows the layout of the constituent light emitting diodes which are used in seven segment displays and shows the layout for sixteen segment indicators. A sample of the letters or figures which may also be produced by some of the possible combinations of illuminated diodes is also illustrated.

Because these displays are composed of linear segments (that is, there are no curls or twists which can be produced), some anomalies could exist between similarly formed letters or numbers. Any combination which may introduce a misinterpretation is usually not specified in the equipment manual which covers the interpretation of the display.
As an example, the number 1 and the letter I could easily be read one for the other, and the distinction will be shown in the display dictionary.

The diode junctions which form the segments of the display require both an anode and a cathode connection. For a sixteen segment display this would result in thirty two connectors. Fortunately, most situations allow for a common connection to all the cathodes (or, alternatively, all the anodes). The displays are then referred to as common anode connected or common cathode connected and the number of leads reduces to eight or seventeen for the two types of display.

Light Emitting Diodes

2011-02-18T06:10:00.000-08:00

Light emitting diodes are available in a wide variety of shapes and sizes, and they can be manufactured to fulfil a specific purpose or for general use. In size, they range from a miniature decimal point for use in multi-number configuration to a three-inch high single character display.

The A diagram shows the PN junction encapsulated in clear plastic with the light from the device focused by a lens. The lens is usually made of self-colored plastic to match, or modify slightly, the colour being emitted by the junction. The lens can be made to focus the light to a small pinpoint or to spread it over a wide area.

More detailed drawing of the PN wafer of the light emitting diode is shown in B, from this diagram, it can be seen that one face of the silicon is left clear of any obstructions or connecting wire which may impede the path of the light rays. For a single indicator diode, the junction uses a square section of silicon. During manufacture it is considerably easier to slice the silicon die into squares than it is to cut and trim complicated shapes; the desired outward appearance of the LED can be more easily obtained by shaping the lens of the device. Thus, from a square section of silicon, plastic lens caps of square, rectangular, circular or star shape can achieve the shapes required for a particular indicator.

Hot Carrier Diode (HCD)

2011-02-15T07:00:00.000-08:00

The hot carrier diode (HCD) is formed by placing an N-type semiconductor material (usually silicon) in contact with a metal such as gold, silver or aluminium to form a metal-to-semiconductor junction. This diode operates in a manner similar to ordinary PN junction diodes but there are several important differences. The barrier voltage developed within the device is approximately one half as great as the barrier voltage within an ordinary silicon diode. This means that the forward voltage drop across the diode is approximately 0.3 volts instead of 0.6 or 0.7 volts. Also, the HCD operates with majority carriers (electrons); virtually no minority carriers are involved. This means that the reverse or leakage current through the device is extremely small.

The term hot carrier diode is used because the electrons move from the N-type semiconductor material cathode across the junction to the metallic anode (the forward-biased direction of current flow) in a manner similar to the movement of electrons through a vacuum tube diode. In other words the electrons possess a high level of kinetic energy just like the electrons leaving the heated cathode of a vacuum tube.

The barrier voltage produced within the HCD is often referred to as the Schottky-barrier because the German scientist Schottky discovered the operating principle of the device in 1938. For this reason the HCD is also commonly referred to as a Schottky-barrier diode or simply a Schottky-diode.

The HCD is able to change operating states (turn on and off) much faster than ordinary PN junction diodes, and it is used extensively to process high frequency AC signals. This device finds extensive use in microwave electronic mixers (circuits which combine AC signals), detectors (circuits which use rectification as a means of extracting information from AC signals) , and high speed digital logic circuits

Bridge Rectifier Diodes

2011-02-13T02:37:00.000-08:00

Full Wave Bridge rectifier each half cycle of input produce current pulses.As disadvantage of the center-tap full wave rectifier is that because only half of the secondary winding is used at anytime the transformer has to produce twice the voltage required. The problem does not arise if four diodes are connected as a bridge network.

If A is positive with respect to B during the first half cycle, D₂ and D₄are forward biased and the current takes the path A,D2,R,D₄,B. On the next half cycle when B is positive (with respect to A), D₁and D₃ are conducting and the current follows the path B, D₃, R, D₁, A. The current through R is in the same direction during both half cycles of input and a varying DC output is obtained.

All four diodes are available in one package and the junction temperature of a diode must be kept low, therefore, in order to dissipate the heat associated with high current carrying rectifiers, they are mounted onto black painted aluminum sheet heat sinks.

Varactor Diodes

2011-02-12T01:32:00.000-08:00

When any PN junction diode is reverse-biased, majority carriers are swept away from the diode’s junction and a relatively wide depletion area is formed. When the diode is subjected to varying reverse bias voltage, the width or thickness of this depletion region will also vary. When the reverse bias voltage increases in value, the depletion region becomes wider. When the reverse voltage decreases, the depletion region becomes narrower. The depletion region acts like an insulator since it provides an area through which no conduction can take place. It also effectively separates the N and P sections of the diode in the same way that a dielectric separates the two plates of a capacitor. In fact, the entire PN junction diode is basically a small electronic capacitor that changes its capacitance as its depletion region changes in size.

Ordinary PN junction diodes possess only a small amount of internal junction capacitance and in most cases this capacitance is too small to be effectively used. However, special diodes are constructed so that they have an appreciable amount of internal capacitance and are used in much the same way that ordinary capacitors are used in electronic circuits. These special diodes are commonly referred to as VARACTOR DIODES or simply VARACTORS, or VARICAP on CAPACITANCE DIODES.

Flight Management System (FMS)

2011-02-09T05:21:00.000-08:00

A Flight Management System (FMS) is a computer-based flight control system and is capable of four main functions:

1. Automatic Flight Control.

2. Performance Management.

3. Navigation and Guidance.

4. Status and Warning Displays.

The FMS utilizes two Flight Management Computers (FMC) for redundancy purposes. During normal operation both computers crosstalk; that is, they share and compare information through the data bus. Each computer is capable of operating completely independently in the event of one failed unit.

The FMC receives input data from four sub-system computers:

1. Flight Control Computer (FCC).

2. Thrust Management Computer (TMC).

3. Digital Air Data Computer (DADC).

4. Engine Indicating & Crew Alerting System (EICAS).

Data inputs are received from flight deck controls, navigation aids and various airframe and engine sensors.

Integrated Circuits-Bistables

2011-02-08T06:37:00.000-08:00

Bistables, as stated, have two(Bi) stable states. The majority have at least two inputs and two outputs, one output being the complement of the other. A bistable is sometimes called a bistable multivibrator, binary trigger, and frequently by the American term of flip-flop.

The bistable is a device which is capable of having its output set by an input signal; the input signal may then be removed, but the bistable output will be unaffected. Provided that the normal power supplies are maintained, the device will remember’ the original input condition. Each bistable is, therefore, capable of storing one bit (logic 1 or 0) of information. Any number of bistables can then be linked together to form a register capable of remembering n bits of information.
There are various bistables as following

R-S
RST
JK
Edge triggered bistable
T bistable
D bistable

A bistable is often called a sequential device because its output depends not only on the present value of inputs, but also upon the past history of the inputs. Bistables are the basic building blocks for counters, registers, dividers and other sequential systems.

Rectifier Circuit

2011-02-07T05:23:00.000-08:00

Half-wave rectifier

In the elementary half wave rectifier circuit, the load is shown as a resistor but in practice it will be a piece of electronic equipment. During half cycles of alternating input voltage diode D conducts, creating a pulse of current. This produces a voltage across R of almost the same value as the peak input voltage, if the forward resistance of the diode is small compared with the value of R.

During the negative half cycles of input voltage, the diode is cut off so there is little or no current in the circuit and therefore the voltage across R is zero.

Centre-tap full wave rectifier

With a Full Wave rectifier each half cycle of input produce current pulses.

The circuit have two diodes D₁ and D₂ and a transformer with centre tapped secondary winding. Suppose that at the peak of the first half cycle of input, the voltage induced in winding AOB is 10v. If we take the centre tap 0 as the reference point at OV, then when A is at +5v, B will be at –5v. D1 then conducts causing a current pulse to flow in the direction;During the other half of the first cycle, B becomes positive to O (thus A is negative with respect to O). Diode D₂ therefore conducts giving current in the circuit .D₁is now cut off.

Rectifier Diode Characteristics And Parameters

2011-02-06T06:43:00.000-08:00

The forward characteristics for silicon and germanium diodes are shown below. For forward voltages below about 0.6 v for silicon and 0.2 v for germanium the current is very small. In the reverse bias condition the reverse saturation current (leakage current) is of the order of several nA to several mA. Leakage current is very temperature-dependent and it is found that the reverse saturation current approximately doubles for every 10°C rise in temperature.

At the reverse breakdown voltage (or peak inverse voltage, PIV) the diode will break down, and unless the current is limited by a resistor the device will be damaged. Diodes are not used on this part of the characteristic except in the case of zener diodes.