The Working of a Potentiometer

A potentiometer (also known as a pot or potmeter) is defined as a 3 terminal variable resistor (or variable resistor) consisting of two pairs of terminals, such that the voltage divider formed between the two potentiometer’s terminals represents the resistance of a particular adjustable resistance in an electrical circuit.

What is a potentiometer?

A potentiometer is a variable resistor. In other words, it’s a device which allows users to adjust the resistance of the device by rotating a shaft. It is actually a basic electronic component that performs three functions acting as a voltage divider, rheostat and variable resistor. Voltage dividers are widely used in electrical engineering devices such as amplifiers, computers and other high-end equipment.

The construction of a potentiometer is simple but might be a bit confusing at first sight. The basic construction consists of three terminals that are internally connected to separate wire leads (which are attached to a screw shaft) and an insulator material (resistor) mounted on an external casing. Following is the derivation used to explain the working principle of a potentiometer:

 V=IR

(Ohm’s law)

Where,

I: current

R: total resistance

V: voltage

Potentiometer working principle

The operation of a potentiometer is based on the voltage divider principle, which is used to convert a constant current into a variable voltage in the circuit. The type of resistance connected to each end of each terminal decides how much current flows through it from positive to negative terminals. By changing the connection from one side to another, one can change the amount of current and hence adjust its voltage output.

Circuit with potentiometer

Potentiometers are used to adjust the voltage in a circuit. The figure below depicts a circuit with a potentiometer:

The voltage source is connected across series R1 and R2. When the variable resistor is open, current from the negative terminal of the battery flows through R2, as shown by the arrow. It produces a voltage drop across as current flowing through it senses resistance between its terminals; this results in the application of power by connecting the +V source across terminals 1 and 2 of the potentiometer.

Potentiometer Types

1. Rotary potentiometer: 

A rotary potentiometer is used as a variable control element in automated machinery. This type of potentiometer is commonly seen in home stereos and other electrical appliances such as radios, tape players, graphic equalisers and volume control. It has three terminals: the wiper terminal, the slider terminal and the fixed terminal. The wiper terminal is electrically connected to a device that varies the resistance of the potentiometer (a control knob, encoder turns a shaft attached to an end of a resistive element or variable resistor). It is the slider terminal which moves in and out along the resistive element, thus varying the resistance between it and the wiper terminal. The slider is made of a conductive material that may be moved using mechanical force or by a motor. The fixed terminal acts as ground. The potentiometer has two resistances, R2 and R1, connected in series with each other, which are placed in parallel with the potentiometer’s three terminals, as shown in the circuit diagram given above.

2. Linear Potentiometer: 

A linear potentiometer is a type of resistance component which is used as a variable resistor. It consists of three terminals: fixed terminal, wiper terminal, and slider terminal. The wiper and slider terminals are usually arranged such that the slider moves along the element’s axis of rotation. The resistance R2 is connected in series with R1, which acts as a potentiometer’s wiper. A voltage V1 is applied across to the potentiometer’s fixed and slider terminals simultaneously. 

The voltage V2 across the potentiometer’s terminals itself creates a voltage drop from this, thus producing a variable voltage which depends on the amount of current drawn by it through its terminals.

Applications of a Potentiometer

1) Potentiometer is used to adjust the voltage across a circuit. For example, it is used to make the voltage of an electrical system “passive” rather than “active”; it’s applied in many electronic equipment such as TV sets, stereos, radios, etc.

2) Potentiometer is used to adjust the amplitude of an audio signal (volume) by means of a disk that spins or that slides along pins on a potentiometer. It can produce an audio signal from zero to full control to any level between those extremes. The rotary potentiometer and linear potentiometers are often utilised in electronic music equipment such as turntables and CD players.

3) It’s used as a tuning device for analogue radio frequency (RF) receivers. In this application, it is often referred to as a variable capacitor or varactor diode. The volume control of a car radio is another example of the use of the potentiometer.

4) It’s used to adjust the brightness of the display in an electronic indicator system. For example, it is used to adjust brightness and contrast in LCD screens, etc.

5) Potentiometers are also used as controls on electric guitars and basses. It is often seen in electric guitars, as well as in some headphone amplifiers.

Conclusion

This article explained the working principle of a potentiometer. A potentiometer is an electronic component which consists of three terminals, two resistances and an insulator material (protective casing) which acts as a voltage divider. The type of resistance connected to each end of each terminal decides how much current flows through it from positive to negative terminals. The slider terminal moves in and out along the resistive element and thus varies the resistance between it and the wiper terminal. Changing the connection from one side to another, one can change the amount of current and hence adjust its voltage output.