Potentiometer Working - Principle & Diagram


Potentiometer or pot is a three-terminal variable resistor. Resistor, a bundle of resistance, is one of the commonly used components in an electric circuit. Often used to regulate the current flow either by adding or subtracting resistance from the circuit, resistors are available in several different shapes and sizes. Apart from the classification based on these two features, i.e., size and shape, the resistors are categorized as fixed and variable. As the names suggest, a fixed resistor has a fixed value of resistance, whereas a variable resistor possesses resistance value over a defined range. Out of the various variable resistors available, the potentiometer is the most common one. In this article, we will discuss working, applications, and many other things related to the potentiometer. Nevertheless, to explain or understand potentiometer working, we first need to comprehend potentiometer. So, let's start.

 

What is Potentiometer

As discussed above, a Potentiometer is a resistor with three terminals. Out of the three terminals - one is varying, whereas the rest two are fixed. It is an electric component used to measure the unknown voltage by comparing it with the known one, which can be drawn by cell or any of the other supply sources. In other words, it is a device used for measuring the EMF (electromotive force), and internal resistance of a given cell. A Potentiometer can also be used to vary the resistance for measuring the potential differences. Although the most common use of Potentiometer is to control volume in radios and other audio-related electronic equipment, they are also helpful in manufacturing electronic components that provide a way of adjusting circuits to obtain the correct outputs.

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Characteristics of Potentiometer

Some significant characteristics of Potentiometer are as follows:

  • As the potentiometers work on the comparative method rather than the deflection method for determining the unknown voltages, these are very accurate.

  • Potentiometers measure null or balance point that doesn't require the power for measurement 

  • As no current flows through pots when they are balanced, their working is free from the source resistance.

 

Potentiometer Working Principle

The basic potentiometer working principle is based on the fact that the potential across any piece of the wire is directly proportional to the length of the wire, which has a uniform cross-sectional area and the constant current flowing through it. In the case of no potential difference amid any two notes, the electric current will flow. The working principle of pot can be understood and explained through the below circuit, where L is the long resistive wire. The circuit also includes a battery of known EMF (electromotive force) and V, voltage named as driver cell voltage. Now, by connecting battery terminals to the two ends of L, assume a primary circuit arrangement. One end of this circuit is connected to a galvanometer (G) and other to the cell whose electromotive force (E) is to be measured. The new circuit formed now is considered to be a secondary circuit. 

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Below is the derivation to explain the working principle of Potentiometer:

According to Ohm's law, 

V = IR

Where I is current, R is total resistance, and V is voltage.   

Now, R = ρL/A and V = I (pL/A)

Where ρ is resistivity and A is the cross-sectional area

With ⍴ and A constant, I is also constant for a rheostat. Hence, 

Lρ/A = K

V = KL

E = Lρx/A =Kx

Where x: length of Potentiometer wire, E: cell with Lower EMF, and K: constant.

As the potential difference is zero (0) and there's no flow of current, the galvanometer has null detection. So, x is considered as the length of the null point. Ultimately, the unknown electromotive force can be found by using K and x, as shown below:

E = Lρx/A = Kx

Since the EMF here includes two cells, let L1 and L2 be the null point length of the first cell and second cell with EMF E1 and EMF E2 accordingly. So,

E1/E2 = L1/L2

 

Types of Potentiometer

Although the basic working principle and construction of all potentiometers are the same, they differ based on the geometry of moving terminals. Most of the potentiometers have a wiper that rotates on an arc-shaped resistive material. Nonetheless, in some other types of potentiometers, the wiper slides linearly on a straight resistive strip. Based on the concept of the resistive strip, the potentiometers are of two types: Rotary and Linear.

 

1. Rotary Potentiometer: This sort of pot has a wiper that rotates across two terminals for varying the resistance of the potentiometer. Depending upon the number of times one can turn or rotate the wiper, rotary potentiometers can be classified in the below categories:

  • Single Turn: The wiper takes only a single turn and often rotates just 3/4th of the complete turn. 

  • Multi Turn: These potentiometers can make multiple rotations such as 5, 10, or more. 

  • Concentric Pot: Here, two pots are adjusted together by using concentric shafts.

  • Servo Pot: This is a motorized pot and used to adjust or control a servo motor automatically.

 

2. Linear Potentiometer: Also known as slider, fader, or slide pot, these are potentiometers in which the wiper slides on a straight resistive strip. These can further be classified in the following categories:

  • Slide Pot: Slide potentiometers are the high-quality pots mostly made from conductive plastics.

  • Dual Slide Pot: This sort of pot is the calibration of two slide pots in parallel. 

  • Multi turn Pot: This type of potentiometer is constructed by using a spindle, which actuates the slider 

  • Motorized Fader: The resistance and movement of the wiper in this pot are controlled by a motor.

 

Application of Potentiometer

A potentiometer operates as a voltage divider, and therefore, has numerous applications. Some of the applications of pots are as follows:

 

Audio Control: Both rotary and linear Potentiometers are used to control audio devices for changing and controlling the loudness and other audio related signals.

 

Television: In televisions, the pots are used to control the brightness, colour, and contrast of the picture.

 

Motion Control: Pots are also used as servomechanisms, the position feedback devices used to create a closed-loop control.  

 

Transducers: Due to the aspect of giving large output signals, pots find applications in designing of displacement transducers.