There are different types of resistor depending upon their construction, power dissipation capacities and tolerance of the value. Such as
- Carbon Composition Resistor
- Wire Wound Resistor
- Metal Film Resistor
- Carbon Film Resistor
- Variable Resistor
- Light Dependent Resistor
Carbon Composition Resistor
These resistors are very commonly used. These resistors cost low. The construction of a carbon composition resistor is simple. It is also commonly referred to as carbon resistor. It is mainly made of carbon clay composition covered with a plastic case. The lead of the resistor is made of tinned copper.The main advantages of these resistors are that they are easily available in the local market and they are very durable too. These resistors are available in a wide range of values. It is available in as low as 1 Ω value and it is also available in as high as 22 Mega Ω value. But the main disadvantage is that they are very much temperature sensitive. The tolerance range in resistance of carbon composition resistor is of ± 5 to ± 20 %. Such resistor has a tendency of electric noise due to passage of electrical current from one carbon particle to other. Where low cost is the main criteria for designing a circuit rather than its perfection of performance, these resistors are normally used. Carbon resistors are provided with a different coloured band on their cylindrical body. These colour bands are code for the resistance values of resistors along with their tolerance range.
Wire Wound Resistor
In wire wound resistor a wire of manganin or constantan is wound around a cylinder of insulating material. The temperature coefficient of resistance of manganin and constantan is almost zero. So, resistance variation with temperatureof these resistors is negligible. The wounded wire is covered with an insulating cover such as baked enamel. This cover of insulating heat resistible material resists the effect of ambient temperature variation. Different sizes and ratings of wire wound resistor can easily be achieved by using different lengths and diameters of the wire.These resistors are easily available for a wide range of ratings. The range of resistance values varies from 1 Ω to 1 MΩ. Typical tolerance limit of these resistors varies from 0.01 % to 1 %. They can be used for high power applications of 5 to 200 W dissipation ratings. The cost of these resistors is much higher than carbon resistor. Normally wire wound resistor is used where carbon composition resistor cannot meet the purpose because of its limitations.
The main disadvantage of this resistor is the inductance that arises because of its coil-like structure. At high frequency, the behaviour of the circuit may be changed due to its reaction. This problem can be solved if one half of the wire is wound in one direction and another half in the opposite direction so that the inductance due to these two halves cancels each other hence the net inductive effect of the resistor becomes nil. The non – inductive wire wound resistor is ideal for the high-frequency circuit but it is costlier than an ordinary one.
Metal Film Resistor and Carbon Film Resistor
The resistor is constructed by means of deposition a thin film of conductive material such as pure carbon or metal on to an insulating core. The desired value of resistance of metal film resistor or carbon film resistor can easily be obtained by either trimming the layer of thickness or by cutting helical grooves of suitable pitch along its length.
Metallic contact cap is fitted at both ends of the resistor. The caps are in contact with the conductive film or helical grooves. The lead wire is welded to the end caps. Metal Film Resistor or Carbon Film Resistor can be made up to a value of 10,000 MΩ and size of this type of resistor is much smaller than wire wound resistor. Because of their constructional features, these resistors are fully non – inductive. The accuracy level of metal film resistors can be of order ± 1 % and they are suitable for high-grade applications. Carbon film resistors give lower tolerances and smaller values of electrical resistance than those available with metal film. However the carbon film posses a mildly negative temperature coefficient of resistance which is very useful for certain electronic circuits.
The variable resistor means its resistance value can be adjusted. There are a rotating shaft and a wiping contact. Basically, there is a resistive semicircular bar or coil and by wiping the contact we change the effective length of the resistive element and hence the resistance gets changed. One example of such resistors is rheostat.
The variable resistor or rheostat can also be a linear sliding type where the sliding contact moves on the resistive element linearly for adjustment of the effective resistance of the resistor.
The word thermistor means a thermal resistor. Its resistance value changes with the change in the temperature. Most of the thermistors have the negative temperature coefficient which means its resistance will fall down when the temperature increases. These are normally made of semiconductor materials. A resistance up to a few mega ohms can be obtained from thermistors. They are used to detect the small temperature changes, when there is a temperature change, however small, there will be a large change in the value of the resistance.
Non Linear Resistor or Varistor
They are also known as varistors. They are popular for having the non linear V-I characteristics curve. That is its resistance is not uniform and it does not obey OHM’s law. They are made of materials such as silicon carbides, zinc oxide.
They are of three types-
- Silicon carbide disc type varistor.
- Silicon carbide rod type varistor.
- Zinc oxide type varistor.
Light Dependent Resistor
As the name suggests its resistance depends on the intensity of light falling on it. This is made of cadmium sulfide which contains a small number of electrons when it is not illuminated. When a light ray falls on it, electrons get ejected and hence the conductivity of it increases. Hence, it offers low resistance when light falls on it and offers high resistance in the dark.