A potentiometer is a passive electronic component. It is an electric instrument that used to measure the EMF (electromotive force) of a given cell, the internal resistance of a cell. Potentiometers work by varying the position of a sliding contact across a uniform resistance. These potentiometers are used in huge quantities in the manufacture of electronics equipment that provides a way of adjusting electronic circuits so that the correct outputs are obtained.
A potentiometer has the two terminals of the input source fixed to the end of the resistor. To adjust the output voltage the sliding contact gets moved along the resistor on the output side. This is a very basic instrument used for comparing the emf of two cells and for calibrating ammeter, voltmeter, and watt-meter. The basic working principle of a potentiometer is quite simple. Suppose we have connected two batteries in parallel through a galvanometer.
A battery Bt is connected between the ends A and B of a potentiometer wire through a key K. A steady current I flows through the potentiometer wire (Figure). This forms the primary circuit. A primary cell is connected in series with the positive terminal A of the potentiometer, a galvanometer, high resistance, and jockey. This forms the secondary circuit.
Fig: Principle of potentiometer
If the potential difference between A and J is equal to the emf of the cell, no current flows through the galvanometer. It shows zero deflection. AJ is called the balancing length. If the balancing length is l, the potential difference across AJ = Irl where r is the resistance per unit length of the potentiometer wire and I the current in the primary circuit.
∴ E = Irl,
since I and r are constants, E α l
Hence emf of the cell is directly proportional to its balancing length. This is the principle of a potentiometer.
Construction of a Potentiometer
A potentiometer consists of a long wire with a uniform area of cross-section. In some cases, the wire may be cut into some pieces and each piece is connected at the endpoints by means of a thick metallic strip. Usually, it will be copper strips. Each piece of wire has a length of one meter. Usually, there will be six pieces of wire and the total length of the wire is six meters.
The potentiometer consists of a driving circuit that consists of a battery, key, and rheostat. It also consists of a galvanometer and a jockey.
There are some applications of potentiometer that are given below: Potentiometers are widely used as user controls and may control a very wide variety of equipment functions.
- The potentiometer is used as a voltage divider in the electronic circuit.
- The potentiometer is used in radio and television (TV) receiver for volume control, tone control, and linearity control.
- The potentiometer is used in medical equipment.
- It is used in the wood processing machine.
- It is used in injection mold machines.