Toms Service Group

Inductor & Transformer

2011-04-29T08:42:00.004+07:00

ITS ABOUT INDUCTOR & TRANSFORMER

1. INDUCTOR
An inductor is a coil of wire which may have a core of air, iron or ferrite (a brittle material made from iron). Its electrical property is called inductance and the unit for this is the henry, symbol H. 1H is very large so mH and µH are used, 1000µH = 1mH and 1000mH = 1H. Iron and ferrite cores increase the inductance. Inductors are mainly used in tuned circuits and to block high frequency AC signals (they are sometimes called chokes). They pass DC easily, but block AC signals, this is the opposite of capacitors.
An inductor may be connected either way round and no special precautions are required when soldering.

A. Picture
Inductor with air core :

Inductor with iron core:

Inductor with Ferrite core:

Variable inductor:

B. Symbol
Inductor with air core:

Inductor with iron or ferrite core:

Variable inductor:

C. Inductor Marking
- printed 102 = mean 1000 micro henry (µH) = 1 milihenry (mH)
- printed 105 = mean 1000000 microHenry = 1000miliHenry = mean 1 Henry
-color code :

D. Inductor Network
Inductors in a parallel configuration each have the same potential difference (voltage). To find their total equivalent inductance (Leq):

The current through inductors in series stays the same, but the voltage across each inductor can be different. The sum of the potential differences (voltage) is equal to the total voltage. To find their total inductance:

These simple relationships hold true only when there is no mutual coupling of magnetic fields between individual inductors.

2. TRANSFORMER
A. Picture

Transformer Low voltage Singlephase:

Transformer High Voltage Multiphase:

B. Symbol

C. Basic Principles

If the secondary coil is attached to a load that allows current to flow, electrical power is transmitted from the primary circuit to the secondary circuit. Ideally, the transformer is perfectly efficient; all the incoming energy is transformed from the primary circuit to the magnetic field and into the secondary circuit. If this condition is met, the incoming electric power must equal the outgoing power:

giving the ideal transformer equation

P = Electric Power
Vp= Primary AC Voltage
Ip= Primary Current
Vs= Secondary AC Voltage
Is= Secundary current
Np= Turns of Primary
Ns= Turns of Secondary

D. Types of Transformer

In Generally Transformer can be classified :

1.Power Transformer
   - Step Up Transformer
   - Step Down Transformer

2. Instrument transformers
   - Voltage Transformer
   - Current Transformer
Used for metering and protection in high-voltage circuits.

3. RF (Radio Frequency) Transformer
Used in high frequency work.

4. Audio Transformer
Audio transformers are usually the factor which limit sound quality when used; electronic circuits with wide frequency response and low distortion are relatively simple to design.

References:
1. “Succes in Electronic” by Tom Duncan
2. Wikipedia.org
3. kpsec.freeuk.com

Capacitor

2011-04-29T08:35:00.009+07:00

ITS ABOUT CAPACITOR

A capacitor is an electronic device for storing charge. Capacitors can be found in almost all but the most simple electronic circuits. There are many different types of capacitor but they all work in essentially the same way. A simplified view of a capacitor is a pair of metal plates separated by a gap in which there is an insulating material known as the dielectric.

1. FIXED CAPACITOR
1.1. Picture

Electrolytic Capacitor

Ceramic , mica , polyester Capacitor

SMD Capacitor

1.2. Symbol

Polarised Capacitor

Unpolarised Capacitor

1.3. Capacitor Marking

A. Polarised Capacitor
A.1. Electrolytic Capacitor

Electrolytic capacitors are polarised and they must be connected the correct way round, at least one of their leads will be marked + or -. They are not damaged by heat when soldering.
There are two designs of electrolytic capacitors; axial where the leads are attached to each end (220µF in picture) and radial where both leads are at the same end (10µF in picture). Radial capacitors tend to be a little smaller and they stand upright on the circuit board.
It is easy to find the value of electrolytic capacitors because they are clearly printed with their capacitance and voltage rating. The voltage rating can be quite low (6V for example) and it should always be checked when selecting an electrolytic capacitor. If the project parts list does not specify a voltage, choose a capacitor with a rating which is greater than the project's power supply voltage. 25V is a sensible minimum for most battery circuits.

A.2. Tantalum Bead Capacitors

Tantalum bead capacitors are polarised and have low voltage ratings like electrolytic capacitors. They are expensive but very small, so they are used where a large capacitance is needed in a small size.
Modern tantalum bead capacitors are printed with their capacitance, voltage and polarity in full. However older ones use a colour-code system which has two stripes (for the two digits) and a spot of colour for the number of zeros to give the value in µF. The standard colour code is used, but for the spot, grey is used to mean × 0.01 and white means × 0.1 so that values of less than 10µF can be shown. A third colour stripe near the leads shows the voltage (yellow 6.3V, black 10V, green 16V, blue 20V, grey 25V, white 30V, pink 35V). The positive (+) lead is to the right when the spot is facing you: 'when the spot is in sight, the positive is to the right'.
For example:   blue, grey, black spot   means 68µF
For example:   blue, grey, white spot   means 6.8µF
For example:   blue, grey, grey spot   means 0.68µF

B. Unpolarised Capacitor

Small value capacitors are unpolarised and may be connected either way round. They are not damaged by heat when soldering, except for one unusual type (polystyrene). They have high voltage ratings of at least 50V, usually 250V or so. It can be difficult to find the values of these small capacitors because there are many types of them and several different labelling systems!

Many small value capacitors have their value printed but without a multiplier, so you need to use experience to work out what the multiplier should be!
For example 0.1 means 0.1µF = 100nF.
Sometimes the multiplier is used in place of the decimal point:
For example:   4n7 means 4.7nF.

Capacitor Number Code
A number code is often used on small capacitors where printing is difficult:
•   the 1st number is the 1st digit,
•   the 2nd number is the 2nd digit,
•   the 3rd number is the number of zeros to give the capacitance in pF.
•   Ignore any letters - they just indicate tolerance and voltage rating.
For example:   102   means 1000pF = 1nF   (not 102pF!)
For example:   472J means 4700pF = 4.7nF (J means 5% tolerance).

Capacitor Colour Code
A colour code was used on polyester capacitors for many years. It is now obsolete, but of course there are many still around. The colours should be read like the resistor code, the top three colour bands giving the value in pF. Ignore the 4th band (tolerance) and 5th band (voltage rating).

For example:

brown, black, orange means 10000pF = 10nF = 0.01µF.
Note that there are no gaps between the colour bands, so 2 identical bands actually appear as a wide band.
For example:
wide red, yellow means 220nF = 0.22µF

Polystyrene Capacitors
This type is rarely used now. Their value (in pF) is normally printed without units. Polystyrene capacitors can be damaged by heat when soldering (it melts the polystyrene!) so you should use a heat sink (such as a crocodile clip). Clip the heat sink to the lead between the capacitor and the joint.

1.4. Capacitor Networks

a. Series
b. Paralel

Series
Consider the series network of capacitors shown in Figure a.) where the positve plate is conected to the negative plate of the next.What is the equivalent capacitance of the network? Look at the plates in the middle, these plates are physically disconected from the circuit so the total charge on them must remain constant. It follows that when a voltage is applied across both of the capacitors, the charge +Q on the positive plate of capacitor C1 must be balanced by the charge -Q on the negative plate of capacitor C2. The net result is that both capacitors possess the same charge Q. The potential drops V1 and V2 across the two capacitors are in general, different. However, the sum of these drops equals the total potential drop V applied across the input and output wires. V=V1 + V2. The equivalent capacitance of the pair is again CT=Q/V. Thus, 1/CT = V/Q = (V1 + V2)/Q = V1/Q + V2/Q giving

In general, for N capacitors connected in series, is

By connecting capacitors in seires you store less charge so does ever make sense to connect capacitors in series? It is sometimes done because capacitors have maximum working voltages, and with two 900 volt maximum capacitors in series, you can increase the working voltage to 1800 volts.

Parallel
For a parallel circuit such as in Figure b.) the voltages are the same across each component. However the total charge is divided between the two capacitors since it must distribute itself such that the voltage across the two is the same. Also, since the capacitors may have different capacitances C1 and C2 the charges Q1 and Q2 must also be different. The equivalent capacitance CT of the pair of capacitors is simply the ratio Q/V where Q=Q1+Q2 is the total stored charge. It follows that CT = Q/V = (Q1+Q2)/V = Q1/V + Q2/V giving

It is fairly obvious from the previous discussion that for N capacitors in parallel, the total capacitance is

The overall capacitance increases by added together capacitors in parallel so we create larger capacitances than is possible using a single capacitor. High-energy physics labs often have large banks of capacitors which can store large quantities of energy to be released in a very short time.

2. VARIABLE CAPACITOR
2.1. Tuning Capacitor
A. Picture

A variable capacitor is a capacitor whose capacitance may be intentionally and repeatedly changed mechanically or electronically.
Variable capacitors are mostly used in radio tuning circuits and they are sometimes called 'tuning capacitors'. They have very small capacitance values, typically between 100pF and 500pF (100pF = 0.0001µF)..
Many variable capacitors have very short spindles which are not suitable for the standard knobs used for variable resistors and rotary switches. It would be wise to check that a suitable knob is available before ordering a variable capacitor.
Variable capacitors are not normally used in timing circuits because their capacitance is too small to be practical and the range of values available is very limited. Instead timing circuits use a fixed capacitor and a variable resistor if it is necessary to vary the time period.

B. Symbol

C. Tuning Capacitor marking
A Tuning Capacitor value typically between 100pF and 500pF. Its value is printed:
- 100, mean this capacitor can be trimmed from 0-100pf
- 220, mean this capacitor can be trimmed from 0-220 pf, etc

2.2. Trimmer Capacitor
A. Picture

Trimmer capacitors (trimmers) are miniature variable capacitors. They are designed to be mounted directly onto the circuit board and adjusted only when the circuit is built.
A small screwdriver or similar tool is required to adjust trimmers. The process of adjusting them requires patience because the presence of your hand and the tool will slightly change the capacitance of the circuit in the region of the trimmer!
Trimmer capacitors are only available with very small capacitances, normally less than 100pF. It is impossible to reduce their capacitance to zero, so they are usually specified by their minimum and maximum values, for example 2-10pF.

B. Symbol

C. Trimmer Capacitor marking
A Trimmer Capacitor value normally less than 100pF. Its value is printed:
- 10, mean this capacitor can be trimmed from 0-10pf
- 20, mean this capacitor can be trimmed from 0-20pf ,etc

References:
1. “Succes in Electronic” by Tom Duncan
2. Wikipedia.org
3. kpsec.freeuk.com

Resistor, Thermistor, LDR

2011-04-28T15:16:00.007+07:00

ITS ABOUT RESISTOR, THERMISTOR & LDR

The job done by resistors include directing and controlling current, making changing currents produce canging voltages (as in a voltage amplifier) and obtaining variable voltages from fixed ones (as in a potensial divider). There are two main types of resistor. Those with fixed values and that are variable.

1. FIXED RESISTOR
1.1. Picture

Low Power Resistor (1/4 - 2 watt)

Resistor 5 watt

Resistor 25 watt

A single in line (SIL) resistor package with 8 individual, 47 ohm resistors. One end of each resistor is connected to a separate pin and the other ends are all connected together to the remaining (common) pin - pin 1, at the end identified by the white dot.

1.2. Symbol

The 'box' symbol for a fixed resistor is popular in the UK and Europe. A 'zig-zag' symbol is used in America and Japan

1.3.   Resistor Marking

Resistance is measured in ohms, the symbol for ohm is an omega .
1 is quite small so resistor values are often given in k and M .
1 k = 1000      1 M = 1000000 .
Resistor values are normally shown using coloured bands.
Each colour represents a number .

A.   4 bands Resistor:
•   The first band gives the first digit.
•   The second band gives the second digit.
•   The third band indicates the number of zeros.
•   The fourth band is used to shows the tolerance (precision) of the resistor.

This resistor has red (2), green (5), orange (3 zeros) and gold bands.
So its value is 25000 = 25 k .± 5%
On circuit diagrams the is usually omitted and the value is written 25K.

B.   5 Band Resistor
-   The first band gives the first digit
-   The second band gives the second digit
-   The third band gives the third digit
-   The fourth band is indicates the number of zeros
-   The fifth band is used to shows the tolerance (precision) of the resistor



This resistor has yellow (4), blue (6), black (0) orange (3 zeros) and brown bands.
So its value is 460000 = 460 k .± 1%
On circuit diagrams the is usually omitted and the value is written 460K

C.   6 Band Resistor
•   The first band gives the first digit
•   The second band gives the second digit
•   The third band gives the third digit
•   The fourth band is indicates the number of zeros
•   The fifth band is used to shows the tolerance (precision) of the resistor
•   The sixth band is used to shows temperature coefficient

This resistor has red (2), violet (7), blue (6) black (0 zeros/ no zeros) , gold, and brown bands.
So its value is 276 .± 5% , with 100ppm of coefficient temperature
On circuit diagrams the is usually omitted and the value is written 276.

D. SMD Resistor

Surface mounted resistors are printed with numerical values in a code related to that used on axial resistors. Standard-tolerance surface-mount technology (SMT) resistors are marked with a three-digit code, in which the first two digits are the first two significant digits of the value and the third digit is the power of ten (the number of zeroes). For example:
334   = 33 × 104 ohms = 330 kilohms
222   = 22 × 102 ohms = 2.2 kilohms
473   = 47 × 103 ohms = 47 kilohms
105   = 10 × 105 ohms = 1.0 megohm
Resistances less than 100 ohms are written: 100, 220, 470. The final zero represents ten to the power zero, which is 1. For example:
100   = 10 × 100 ohm = 10 ohms
220   = 22 × 100 ohm = 22 ohms
Sometimes these values are marked as 10 or 22 to prevent a mistake.
Resistances less than 10 ohms have 'R' to indicate the position of the decimal point. For example:
4R7   = 4.7 ohms
R300   = 0.30 ohms
0R22   = 0.22 ohms
0R01   = 0.01 ohms

1.4.   Resistor in Series

In a series circuit, the current flowing is the same at all points. The circuit diagram shows two resistors connected in series with a 6 V battery:

                  Resistors in series

It doesn't matter where in the circuit the current is measured, the result will be the same. The total resistance is given by:

In this circuit, Rtotal=1+1=2 . What will be the current flowing? The formula is:

Substituting:

Notice that the current value is in mA when the resistor value is substituted in .
The same current, 3 mA, flows through each of the two resistors. What is the voltage across R1? The formula is:

Substituting:

What will be the voltage across R2? This will also be 3 V. It is important to point out that the sum of the voltages across the two resistors is equal to the power supply voltage.

1.5. Resistor in Paralel

The next circuit shows two resistors connected in parallel to a 6 V battery:

              Resistors in parallel

Parallel circuits always provide alternative pathways for current flow. The total resistance is calculated from:

This is called the product over sum formula and works for any two resistors in parallel. An alternative formula is:

This formula can be extended to work for more than two resistors in parallel, but lends itself less easily to mental arithmetic. Both formula are correct.
What is the total resistance in this circuit?

The current can be calculated from:

How does this current compare with the current for the series circuit? It's more. This is sensible. Connecting resistors in parallel provides alternative pathways and makes it easier for current to flow. How much current flows through each resistor? Because they have equal values, the current divides, with 6 mA flowing through R1, and 6 mA through R2.
To complete the picture, the voltage across R1 can be calculated as:

This is the same as the power supply voltage. The top end of R1 is connected to the positive terminal of the battery, while the bottom end of R1 is connected to the negative terminal of the battery. With no other components in the way, it follows that the voltage across R1 must be 6 V. What is the voltage across R2? By the same reasoning, this is also 6 V.
KEY POINT : When components are connected in parallel, the voltage across them is the same

2. VARIABLE RESISTOR

2.1. Potentiometer
A potentiometer is a three-terminal resistor with a sliding contact that forms an adjustable voltage divider. If only two terminals are used (one side and the wiper), it acts as a variable resistor or rheostat. Potentiometers are commonly used to control electrical devices such as volume controls on audio equipment. Potentiometers operated by a mechanism can be used as position transducers, for example, in a joystick.

A. Picture

B. Symbol

C. Potentiometer marking
As ussually a potentiometer has value :
- 5KB , it means 5 Kilo ohm with linear function
- 5KA , it means 5 Kilo ohm with logaritmic function

2.2. Trimpot/Trimmer
A. Picture

B. Symbol

C. Trimpot Marking
As ussually a trimpot has value
-103, it means 10 x 103 = 10000 = 10 Kilo ohm
-102, it means 10 x 102 = 1000 = 1 Kilo ohm

3. THERMISTOR

A thermistor is a type of resistor whose resistance varies significantly with temperature, more so than in standard resistors. The word is a portmanteau of thermal and resistor. Thermistors are widely used as inrush current limiters, temperature sensors, self-resetting overcurrent protectors, and self-regulating heating elements.

Picture :

Symbol :

Thermistors can be classified into two types, depending on the sign of k. If k is positive, the resistance increases with increasing temperature, and the device is called a positive temperature coefficient (PTC) thermistor, or posistor. If k is negative, the resistance decreases with increasing temperature, and the device is called a negative temperature coefficient (NTC) thermistor.

4. LIGHT DEPENDENT RESISTOR (LDR)

An LDR is an input transducer (sensor) which converts brightness (light) to resistance. It is made from cadmium sulphide (CdS) and the resistance decreases as the brightness of light falling on the LDR increases.

Picture:

Symbol:

A multimeter can be used to find the resistance in darkness and bright light, these are the typical results for a standard LDR:
•   Darkness: maximum resistance, about 1M .
•   Very bright light: minimum resistance, about 100 .
For many years the standard LDR has been the ORP12, now the NORPS12, which is about 13mm diameter. Miniature LDRs are also available and their diameter is about 5mm.
An LDR may be connected either way round and no special precautions are required when soldering.

References:
1. “Succes in Electronic” by Tom Duncan
2. Wikipedia.org
3. kpsec.freeuk.com