RESISTORS INTRODUCTION All materials oppose the flow of current through them, i e , they have resistance. ~ INFOTECHNO

4.1. INTRODUCTION

All materials oppose the flow of current through them, i e , they have resistance. When a wire or a piece of a material is used as a component for producing resistance in a circuit. It is called a resistor. Resistors used in electronic equipments are classified as under on the basis of their construction : carbon, wire wound, ballast, PTC, NTC, varistor, LDR, etc.

4.2. CARBON RESISTORS

Carbon is a non-metallic and ideal material for making a resistor. Resistors made of carbon or graphie are called carbon resistors. They are of following two main types :

I. Carbon Composition Resistor. Resistors made by compressing carbon powder while using suitable resin hinder are called composition resistors. They are of following two types :

(i) Pallet or Slug Type. For making this type of resistors, carbon black or graphite powder is moulded into small cylindrical pieces while using a suitable resin binder. During moulding, two leads of tinned copper wire are also joined, one on each side. A protective coating is made over moulded pieces. After heat treatment the resistor becomes ready for use.

Fig. 4.1. Slug type carbon composition resistor

(ii) Filament Type Carbon Composition Resistor. For making this type of resistors, carbon powder is deposited over a porcelain or glass tube with the help of a resin hinder. Tinned copper leads are joined on both the ends, one on each side. Finally, the resistor is covered with a phenolic (weather proof) layer.

Carbon composition resistors arc made in the range of 2 ohms to 10 mega ohms with PS, 1/4. 1/2, I and 2 watts power ratings.

Fig. 4.2. Filament type carbon composition resistor

2. Film Carbon Resistors. These types of resistors are made by depositing hydrocarbon vapours on a ceramic capillary pipe at 900- 1100⁰C temperature. Resistors of small values are made by depositing metallic oxide (of palladium, nickel, chromium metals) on a ceramic capillary, pipe. Resistors of first type are also known as pyrolytic resistors and of second type as metal oxide film resistors. Tinned copper or silver wires are used as leads for these resistors.

Fig. 4.3. Film carbon resistor

These types of resistors are made in the range of 1 ohm to 20 mega ohms with 1/2 watts power rating.

4 .3. CHARACTERISTICS OF CARBON RESISTORS

(a) Merits

(i) Their size may be quite small upto 6 mm X 1 mm.

(ii) Their mass may be quite small upto 1 gram.

(iii) High value resistors (1 mega ohm and above) can be made easily.

(iv)They are most suitable for high frequency circuits.

(v) Their temperature coefficient lies within 0.02 to 0.1.

(b) Demerits

(i) Temperature and humidity affect their values.

(ii) Their current carrying capacity is limited, since their wattage is limited upto 2 watts.

(iii) It is difficult to make a carbon resistor of a value below 2 ohms.

(iv)Their stability and reliability are poor.

4.4. COLOUR CODE

(a) Introduction. Carbon resistors are so tiny that it is difficult to print their resistance value on them. Therefore, it is more convenient to express their value by drawing a few circular lines (called bands) of different colours on their body. Moreover, this later method of expressing a resistor's value is most convenient while searching a resistor of desired value out of a container containing assorted resistors.

(b) Colour Code Chart

Colour	Ist Figure	IInd Figure	Multiplier	Tolerance
Black Brown Red Orange Yellow Green Blue Violet Grey White Golden Silver No Colour	0 1 2 3 4 5 6 7 8 9 x x x	0 1 2 3 4 5 6 7 8 9 x x x	1 10 100 10³ = k 10⁴ 10⁵ i 0⁶ = M - - - 0.1 0.01 x	- 1% 2% 3% 4% - - - - - 5% 10% 20%

(i) Band Type. In this method 3 or 4 circular rings of different colours are marked on the carbon resistor at suitable distance intervals between the two colour rings starting from one end. Here, the first colour band indicates first digit. second band indicates second digit and third band indicates the multiplier. The fourth hand indicates the tolerance of the resistor. If there is no fourth band then the tolerance is considered to be 20%.

Fig. 4.4. Colour marking methods

Some examples of band type resistors are tabulated as under :

S. No.	Colour of First Band	Colour of Second Band	Colour of Third Band	Colour of Fourth Band	Resistor’s Value
1. 2. 3. 4. 5. 6. 7.	Brown Yellow Blue Orange Red Yellow Red	Black Violet Grey Orange Red Violet Black	Blue Yellow Orange Red Brown Black Golden	Silver Golden Golden Red Silver Golden Red	10 MΩ 10% 470 kΩ 5% 68 kΩ 5% 3.3 kΩ 2% 220 Ω 10% 47 Ω 5% 2.0 Ω 2%

(ii) Body Type. In this method the colour of body indicates first digit, the colour of end indicates second digit and the colour of dot (marked on the body) indicates the multiplier. The tolerance is indicated by another dot or rectangular mark marked on other side of the body. If there is no tolerance dot then the tolerance is considered to be 20%.

S. No.	Colour of Body	Colour of End	Colour of Dot	Colour of Tolerance Dot	Resistor’s Value
1. 2. 3. 4. 5. 6.	Brown Green Orange Green Grey Green	Black Black White Blue Red Black	Yellow Orange Brown Red Black Golden	Silver Silver Golden Silver Golden Golden	100 kΩ 10% 50 kΩ 10% 390 Ω 5% 5.6 kΩ 10% 82 Ω 5% 5.0 Ω 5%

4.5. TOLERANCE

Since carbon resistors are made by the deposition of carbon particle hence it is but natural that the density of carbon particles in identical pieces may differ. As a result such pieces ma' has e different resistance value. Besides it, the humidity and temperature variations may also cause a change' in the value of a resistor.

In this way, the percentage of change in the value of a resistor due to manufacturing defect or variation in humidity and temperature is called its tolerance. The tolerance of a resistor may be of the order of 1% to 20%.

4.6. POWER RATING

The maximum current carrying capability of a resistor is ca1led its power rating. It is expressed in watts. Since P = I². R, therefore, the wattage of a resistor depends on the current flowing through it.

The wattage of carbon resistors lie between 1/ 8 to 2 watts and that of wire wound resistors lie 5 to 50 watts. The wattage of a wire wound resistor is printed on its body while that of a carbon resistor is printed on its packing. (Wire wound resistors having a wattage of more than 50 was are also made for industrial use)

4.7. STABILITY

The ability of a resistor of maintaining a stable value under variable temperature and humidity conditions is called its stability. The stability of a wire wound resistor is much better than that of a carbon resistor.

4.8. RELIABILITY

The ability of a resistor of maintaining a stable value for a long period while in use is called its reliability. The reliability of a wire wound resistor is much better than that of a carbon resistor.

4.9. WIRE WOUND RESISTORS

These types of resistors are made by winding a wire on an insulated former. Generally, porcelain pipe pieces are used as former and Eureka wire (an alloy of 60% nickel and 40% copper) is used as resistance wire. These types of resistors are made in the range of 0.01 to I mega ohms with 5 to 50 watts power rating.

Wire wound resistors are classified as follows on the basis of their resistance control :

1. Fixed Value Resistor. A resistor having a fixed value of resistance is called a fixed value resistor. IT has no arrangement for changing its resistance value.

2. Tapped Resistor. A resistor having two, three or more values made in a single unit is called.

3. Adjustable Resistor. A resistor having arrangement for changing its value within a definite range is called an adjustable resistor. It may also have an arrangement for two or three values as in a tapped resistor. This type of resistor is used in mains operated valve receiver and it is known as 'mains resistor' e.g., 1500.

4. Rheostat. The resistance value of a rheostat can be varied quite easily. It is used to laboratories.

4.10. CHARACTERISTICS OF WIRE WOUND RESISTORS

Merits

(i) They have a lowest tolerance value upto 5%.

(ii) They have a high power rating.

(iii) It is easy to construct a low valued resistor — as low as 0.01 ohms.

(iv) They have a good stability and reliability.

Demerits

(i) Their size is quite big which is unsuitable for small sized equipments.

(ii) It is difficult to construct a high valued resistor.

(iii) It is difficult to construct a variable wire wound resistor and working of such resistors is not satisfactory.

(iv) They arc unsuitable for high frequency circuits because their effective resistance is increased due to presence of inductive reactance in them.

4.11. POTENTIOMETER

Fig 4.9 Potentiometer

A resistor whose resistance value can be varied easily is called a potentiometer. The resistor used as volume control and tone control in a radio receiver is a potentiometer.

It consists of circular strip which is equal to 3/4th of a ring. A carbon film is deposited on it. A moving arm which is connected to a shaft is mounted in such a manner that by rotating the shaft, any resistance value between zero and maximum can be obtained. Generally, the potentiometers are made of carbon granules as stated above but Moving wire wound potentiometers are also made for special purposes in the arm range of 1 to 100 ohms (e.g., for controlling the e.m.f. acting in a multimeter). Carbon potentiometers are of following two types :

(i) Linear Potentiometer. In this type of potentiometer, the variation in resistance value is proportional to the angle rotated by the moving arm.

(ii) Logarithmic Potentiometers. In a radio receiver or an audio amplifier the A.F. signal strength has to be increased ten, hundred or thousand times for increasing the output one, two or three times respectively. Because of this reason only, the unit for the measurement of sound intensity has been selected a logarithmic unit called Decibel.

Hence, potentiometer used in radio receivers or an audio amplifiers are of logarithmic type. In this type of potentiometer, the variation in resistance value is not proportional to the angle rotated by the moving arm. For equal angle rotation, the order of variation in the value of resistance is 1, 10, 100, 1000 ohms etc.

Pre-set : It is a potentiometer of a very small size. It is generally mounted on the PCB of an equipment. Its ohmic value can be adjusted by means of a small screw driver during alignment of the equipment. Sometimes, a pre-set is referred as a 'POT' also. These are made in 'linear' as well as in 'logarithmic' types ranging from 100 ohms to 1 mega ohm. These are extensively used in TV receiver's etc.

4.12. SPECIAL TYPES OF RESISTORS

Besides carbon and wire wound resistors, some special types of resistors are also made for specific uses. They are as follows :

I . Ballast Resistor. It is made by a substance which has a positive temperature coefficient such as nickel iron alloy. It has a property of variation in its resistance value for a change in its temperature. Hence, an increase in the magnitude of current flowing through it causes a corresponding increase in its temperature

And which in turn result is an increase in it resistance value. The increase in the resistance value decrease the magnitude of current flowing through it. In the way, a ballast resistor stabilities the circuit current within the definite range. These resistor are used for current stabilization in various type of electronic equipments.

Fig 4.10 Ballast resistor

2. PTC (Positive Temperature Coefficient) Resistor. A resistor made of a substance which has a positive temperature coefficient is called a PTC. The resistance value PTC is directly proportional to its temperature. Hence, an increase in the magnitude of voltage applied or the external temperature causes as increase in its resistance which in turn reduces the magnitude of current flowing through it. A PTC is used for bias stabilisation in transistorised circuits. There, it is connected in the emitter circuit. If due to any reason the magnitude of emitter current rises above a definite value, it causes and increase in its resistance value. In turn, it results in a decrease in the emitter current. Therefore, the effect of increase in the magnitude of current is controlled by it. it is made by a mixture of nickel oxide and cobalt oxide. It is known as thermistor also.

3. NTC (Negative Temperature Coefficient) Resistor. A resistor made of a substance which has a negative temperature coefficient is called a NTC. In the beginning (after switching on the circuit), it has a very high resistance and thus the circuit current is quite small. Slowly and slowly its temperature starts to rise which results in an increase in the magnitude of current flowing through it. An increase in the magnitude of current results in an increase in its temperature which again results in an increase in the circuit current. In this way, after a few seconds its temperature rises to a sufficiently high value and the magnitude of current reaches at its maximum value. Under such conditions the resistance value of NTC is minimum. The device is used in timer circuits, valve filament circuits etc. If a valve filament is allowed to be heated up at a slow rate or in other words the magnitude of its heating current is controlled in such a manner that it rises slowly then the working life of a valve filament can be increased upto a great extent. It is made by graphite.

4. Varistor. It is also a special type of resistor and it has following two types :

(i) Symmetrical Varistor. It is made of carbide granules and a ceramic binder material. It has a property that its resistance value is controlled by the magnitude of current flowing through it. At high magnitude of current, the resistance is low and vice-versa. Since the magnitude of current in a circuit is proportional to supply voltage, therefore, a varistor can be used for controlling voltage fluctuations within a limit of few volts. It is also known as VDR (Voltage Dependent Resistor) and it is used for controlling voltage fluctuations.

(ii) Unsymmetrical Varistor. It is made of a semi-conductor material. It has a property of controlling the magnitude of current of a circuit by the direction of flow of current. It is also called a varistor diode. It is used as a rectifier also in certain types of circuits.

5. LDR (Light Dependent Resistor). It is made of selenium. It has a property that its resistance value decreases on the incidence of light rays on it and thus the magnitude of current flowing through it is increased. In other words, it is a photo sensitive resistor. It is used in light operated control circuits. You can make a counter for counting the persons passing through a narrow path, a garage door opener which can open the door automatically on the incidence of light rays from a torch or car headlight, an automatic street light control which can switch 'on' street lights in the night and .switch off them in the day light automatically.