A component whose function in a circuit is to provide a specified value of resistance is called a resistor. The resistors are used to limit the circuit current, divide voltage and in certain cases, generate heat. Although there are a variety of different types of resistors, the following are the commonly used resistors in electrical and electronic circuits:-
- Carbon composition types
- Film resistors
- Wire-wound resistors
- Cermet resistors
Carbon Composition Type
This type of resistor is made with a mixture of finely ground carbon insulating filler and a resin binder. The ratio of carbon and insulating filler decides the resistance value. The mixture is formed into a rod and lead connections are made. The entire resistor is then enclosed in a plastic case to prevent the entry of moisture and other harmful elements from atmosphere.
Carbon resistors are relatively inexpensive to build. However, they are highly sensitive to temperature variations. The carbon resistors are available in power ratings ranging from 1/8 to 2 W.
In a film resistor, a resistive material is deposited uniformly onto a high grade ceramic rod. The resistive film may be carbon (carbon film resistor) or nickel-chromium (metal film resistor). In these types of resistors, the desired resistance value is obtained by removing a part of the resistive material in a helical pattern along the rod.
A wire wound resistor is constructed by winding a resistive wire of some alloy around an insulating rod. It is then enclosed in an insulating cover. Generally, nickel chromium alloy is used because of its very small temperature coefficient of resistance.
Wire-wound resistors can safely operate at higher temperatures than carbon types. These resistors have high power ratings ranging from 12 to 225 W.
A cermet resistor is made by depositing a thin film of metal such as nichrome or chromium cobalt on a ceramic substrate. They are cermet which is a contraction for ceramic and metal. These resistors have very accurate values.