Introduction

The most used component in an electrical circuit is the resistor. Its purpose is to regulate the flow of electrical current by inducing or removing resistance in the circuit. They are readily available in different sizes and shapes and can be classified as fixed or variable. A fixed resistor operates under a fixed value of resistance, while a variable resistor has a value that can be varied over a specified range. Variable resistors are further classified into linear and nonlinear variable resistors. The potentiometer is the most used variable resistor and, therefore, the most common. Here, we shall focus on the potentiometer. Let us jump into the content!

Defining Potentiometer

Also referred to as the POT. It is a three-terminal variable resistor with two fixed terminals and one variable terminal. The variable terminal can be either linear or rotary. The potentiometer resistance value can be varied with a specific value, from zero to any required value. In other words, it has a lower and upper limit.

Figure 1:The Potentiometer

Figure 2 The Potentiometer Symbol

The Working Principle of the Potentiometer

As introduced earlier, the potentiometer is made up of three terminals. Connection is done so that the two fixed terminals are connected to the resistive components while the variable terminal is connected to a wiper. Let us have a look at the simple circuit below.

Figure 3:Circuit Diagram of the Potentiometer

In the circuit diagram above, the terminals of the potentiometer are marked 1, 2, and 3. The supply voltage is connected to the potentiometer across terminals 1 and 3, where one is connected to the positive while three is connected to the negative. The terminal marked two is connected to the output, which is probably a wiper. Terminal two divides the resistance between terminals 1 and 3 into R1 and R2. From the figure, the wiper is the voltage across the resistance R2. The resistance increases with an increase in length, provided the material that makes up the resistor remains the same.

The circuit above can be equated to a voltage divider with the output voltage calculated using the equation;

Therefore, we need to adjust the resistor R2 value by sliding the potentiometer wiper towards its terminal 3 to change the output voltage. If the wiper is placed at terminal 1, the resistance remains zero, and the voltage across the wiper is equal to the supply voltage.

Let us look at the problem below to understand the working principle well.

Problem 1

 A 150 Ohms resistor labeled R1 is connected in series with another 50 Ohms resistor, R2, across a power supply of 10V, as shown in the figure below. Compute the total series resistance, circuit current, and the voltage drop through R2.

Figure 4: Potentiometer Problem 1 Circuit

Solution 1

Resistors being in series, total resistance R can be calculated as;

We can note that by changing the values of any of the resistors, the voltage across the resistors changes in the range of 0 to 10V provided that the total resistance of this circuit remains constant.

Potentiometer types

• Linear type: the wiper sides over a straight resistive strip.
• Side potentiometer: made up of a single resistive strip that offers the surface for the sliding of the wiper.
• Dual slide potentiometer; it is made up of calibrating two slide pots connected in parallel.
• Multi-turn potentiometer, made up of the spindle, which plays the role of actuating the slider where it turns 5, 10, or even 20 times for improved precision.
• Motorized fader: this has a wiper controlled by a motor

Figure 5:Types of Potentiometers

Potentiometer Important sizing Formulas

1.     The Potential gradient(k)

The formula can give this;

2.     The Emf/ Potential Difference Measurement

Let us take the balancing length for the emf, E0 of the cell to be I0 and that for the potential difference, V be I, then;

3.     Small Resistance Measurement

The potential difference across the resistor R is given by;

4.     The Cells Internal Resistance

Let the emf of the cell be E and V be the terminal voltage.

The internal resistance of the cell can be given by;

Potentiometer Calibration

First, you need to calculate the error, which is the difference between the actual and apparent value;

Come up with a calibration curve, which is the graph of error and apparent values.

Actual values are values of voltage or current measured by the potentiometer.

Applications of the Potentiometer

•  Audio control for controlling the loudness of the audio.
• Television for picture brightness control
• Motion control, where they are used as position feedback gadgets for creating the closed-loop control.
• Transducers such as the displacement transducer due to their large output signals delivery.

Summary

This article has enlightened the readers on the following;

• Defining the potentiometer as a three-terminal resistor.
• The potentiometer symbol.
• How the transformer can regulate the voltage or current that flows through the output load.
• How to calculate different parameters of the potentiometer to be able to carry out the sizing and selection in a better way.
• The different types of potentiometers that are available in the market.
• The parameters necessary for calibration of the potentiometer.
• Application of the potentiometer in different industries and appliances.