Basically resistors are divided into two types namely fixed resistors and variable resistors. These two types are further classified into different types as shown below.
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| Types of Resistors |
I. FIXED RESISTORS:
Resistors, whose ohmic values remain fixed at a constant value, are known as fixed resistors.
1. Carbon Resistors:
Carbon resistors are made of carbon. These are of two types:
(i) Carbon-composition resistors
(ii) Carbon film or cracked carbon or pyrolytic resistors.
(i) Carbon-Composition Resistors:
The carbon-composition resistors are made of finely divided carbon or graphite mixed with a resin binder in suitable proportion needed for the desired resistance value.
These resistors are available in resistance values of 1Ω to 20 MΩ and power ratings of 1/10, 1/8 , ¼, ½, 1, 2 watts.
Advantages:
• Carbon-composition resistors are small in size compared with wire-wound resistors.
• A very wide resistance range is available.
• These are the cheapest resistors.
• These have good RF performance.
Disadvantages:
• Carbon-composition resistors have no precision, and have very high tolerance.
• These resistors get easily heated and crack down on soldering.
• The resistance values of these resistors vary with aging.
• They are not useful for applications involving power levels above 5 watts.
Applications:
Carbon composition resistors are used for all general purpose circuits including entertainment (Radio, T.V).
(ii) Carbon Film Resistors:
Carbon film resistors are manufactured by depositing very thin film of carbon on to a substrate of ceramic or glass tube. Depending on the thickness of the film we have thin film (< 5 μm) resistors and thick-film (> 5 μm) resistors.
Advantages:
• Carbon film resistors are available in all resistor values, from very low values (< 1Ω) to many MΩ.
• They are available in very miniature size
• They may also be used as resistors in ICs.
• They can replace wire wound resistors in high-voltage applications.
• Carbon-film resistors have good high-frequency properties.
• They cost low.
Disadvantages:
• Carbon film resistors cannot withstand high temperatures.
• They cannot with stand to mechanical shocks.
• They cannot with stand to atmospheric moisture and humidity.
• They are chemically reactive and hence unstable.
Applications:
These are used in good high frequency performance and stability circuits such as computers, telephone circuits and high fidelity amplifiers.
2. Wire Wound Resistors :
Wire wound resistors are constructed from a long fine wire (usually nickel-chromium wire) wound on a ceramic core. Wire wound resistors are manufactured in two types
(i) power wire wound resistors
(ii) precision wire wound resistors.
Power wire wound resistors are available in low, medium and high power types as : 3, 5, 10 Watts (low); 10 to 60 W (medium), 60-1000 watts (high power).
Precision wire wound resistors are wound on ceramic tubes and sometimes epoxy moulded tubes are used. The winding is so done so as to minimize inductive and capacitive effects at high frequencies (beyond 200 kHz). These precision wire wound resistors are available up to 5 W with ½ % and 1% tolerances, and are useful upto 5 to 10 MHz.
Advantages:
• Wire wound resistors can be designed to produce very accurate resistance values, with very low tolerance (± 0.01%).
• They can with stand large power dissipation.
• They can be used in high-temperature situations.
• They are capable of carrying extremely large currents.
• Wire-wound resistors can with stand mechanical shock and vibration.
• They can be used in high-voltage circuits.
• Wire-wound resistors have very stable resistance values which do not change much with aging.
Disadvantages:
• Wire wound resistors are very large in size and weight.
• They are very costly.
• Power type wire wound resistors are not suitable beyond 200 kHz because of interwinding inductance and the capacitance.
• In many situations, wires can break, leading to the breakdown of the circuit in which these resistors are used.
Applications:
Ordinary type wire wound resistors are used in power supplies control circuits, as loads in Television receivers. Precision type wire wound resistors are used in bridges, voltmeters and other instruments.
Comparison of Carbon and Wire Wound Resistors:
S.No | Feature | Carbon Resistor | Wire wound Resistor |
1. | Size | Small | Big |
2. | Power rating | Low | High |
3. | Tolerance | High | Low |
4. | Applications | Carbon composition resistors are used for all general purpose circuits including entertainment (Radio, T.V). Carbon film resistors are used in computers, telephone circuits and high fidelity amplifiers. | Ordinary types are used in power supplies Control circuits, as loads in T.V. Receivers. Precision types are used in bridges, voltmeters and other instruments. |
II. VARIABLE RESISTORS:
Variable resistors are the resistors whose resistance value can be varied. In some electrical/electronic circuits sometimes it is necessary to change the values of currents and voltages. For example it is often necessary to change the volume of sound and brightness in T.V, volume of sound and tone in radios and to regulate the speed of a fan. Such adjustments can be done by using variable resistors. Different types of variable resistors were mentioned in the classification of resistors.
Potentiometer:
The smaller variable resistors commonly used in electronic circuits are called potentiometers simply called as pots.
As shown in Figure, it is a three terminal variable resistor. The arrow indicates the movable contact on the resistive element. The position of the movable contact determines the resistance value in the circuit. Based on the material used for construction, potentiometers are classified into two types,
1. Carbon potentiometers and
2. Wire wound potentiometers.
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Basic Electronics