# Potentiometer | Variable Resistor | It’s Important applications

## Content

• What is a potentiometer(electric pot)?
• What does a potentiometer do?
• How does the potentiometer work?
• How does a pot work as a voltage divider?
• Types of potentiometer
• Examples: 1k resistor pot, 10k pot and 100k pot
• What is a potentiometer symbol?
• Variable resistor symbol?
• What is the wiper on a potentiometer?
• What is a potentiometer used?
• Explain the differences between rheostat and potentiometer?

## What is potentiometer?

### Potentiometer Definition:

“A potentiometer is an electrical device that changes the resistance value to control the flow of current and also measures emf of a cell.”

A potentiometer, also known as ‘pot‘ is a passive and three-terminal device. Though pot and variable resistors (rheostats) seem to be the same device, they differ in their connections within a circuit. It is an electrical device rather an electronic device.

## What does a potentiometer do?

A pot limits the current flow by providing resistance value. That means it can increase or decrease the current of a circuit. It also works as an adjustable voltage divider. Based on this functionality, a pot can measure the electrical emf also.

## Examples: 1k resistor potentiometer, 10k potentiometer& 100k potentiometer

The ‘k’ represents kiloohms. The numeric value tells the value of resistance. 1k means that the pot will provide resistance up to 1000 ohm. 10k & 100k means it will provide ten times and 100 times more resistance than 1k, respectively. The lesser the resistance value, the more the current drawn by that pot. Similarly, a 500k pot means it has a resistance value between 0 to 500 kiloohm.

## How does the potentiometer work?

Potentiometers have some basic working principles. A pot has two terminals as input (marked as red and green in the figure). The input voltage is applied – across the resistor. Then the output voltage is measured. It comes out as the difference between the fixed and moving contact. The wiper plays a vital role here. While optimizing the output voltage- as per the need, the wiper needs to be moved- along the resistive element. Moving the slider helps to balance the galvanometer in case of measuring the emf of a cell. Now it acts as a voltage divider as it continuously produces variable voltage. Based on this concept, a pot measures electrical emf.

## How does a potentiometer work as a voltage divider?

When the pot’s slider is moved- to the right, that causes a fall in resistance, fall in resistance further causes a small voltage drop. After that, if the wiper is moved- to the left, the resistance value eventually gets increased. Noe, there is also a voltage drop, but this time it is more than the previous case. So we can conclude that the output voltage has a direct relationship with the position of the wiper. The voltage drop value is calculated -by subtracting from the source voltage.

## Types of Potentiometers

Based on shape, there are mainly two types

They are –

• A. Linear pot.
• B. Rotary pot.

## A. Linear potentiometer:

• In this type of pot, the slider moves linearly. Some different types are –

## Slider potentiometer or slide pot:

• If the wiper moves, in left-right or up-down direction, to adjust the pot, then it is slide pot. Slide pots find its application in audio, where it is known as faders.

• Dual slide pot: If a single slider controls two pots at a time, then it is a dual-slide pot. It also finds application in audio controlling.
• Motorized slide pot:  If a servo motor controls a slide pot’s slider, the pot is called a motorized slide pot or motorized fader. It has applications in audio control, where automatic control is required.
• B. Rotary Pot : In this type of pot, the slider moves circularly. Some different types are –
• Single – turn pot: In a rotary pot, if it takes a single turn to control the pot, this type of rotary pot is known as a single turn pot. It takes approximately 3π / 2 degrees.
• Multi-turn pot: This type of pot requires multiple rotations of the slider. It generally takes 5-6 turns. It provides high precision and controls, that is why it has application in calibration circuits.

## What is the symbol of the potentiometer?

The symbol of a pot is a standard resistor symbol with an arrow. Note that a variable resistor or rheostat symbol is also a resistance symbol with an added arrow, but the arrow’s position differentiates the devices.

## What is a potentiometer used?

The pot has found its application in many electrical circuitries. Let’s discuss some of them –

• Variable resistor: One of the major applications of a pot is a variable resistor or adjustable resistor. A Pot can change both current and resistance of a circuit.
• EMF measurement: These are capable of measuring emf, as discussed earlier.  It has a voltage dividing property, which helps to find out the electrical emf. It is the fundamental- device to implement – voltmeter, ammeter, and watt-meter. This also compares the emf of two different cells.
• Controlling Audio: One of the modern- uses of the pot is audio control. The main component is – rotary pot. It attenuates noises, changes frequency, sharpness (intensity), loudness, etc. The audio taper pot is one of its variety.
• Transducers: It can measure both the linear and rotary displacement. It converts the linear or angular displacement into voltage. The movable body is connected- with a wiper. As the position of the slider changes, a change in resistance and voltage also occurs. That voltage change gives the displacement of the body.
• Television: A pot can control the color intensity, brightness, color saturation of a television.

## What is the difference between rheostat and a potentiometer?

There is a misconception that a rheostat and a potentiometer are the same things, but there are some differences. Let us discuss some of them –

## Some frequently asked questions on Potentiometers

### 1. What is the resistive element composed of a potentiometer?

The resistive element is the cause that a pot can offer resistance. Generally, Graphite is the material for making resistive elements. Sometimes they are also made of carbon materials, resistive wires, ceramic metal mixtures, etc.

### 2. What is a digital potentiometer?

A digital pot is a digital device. It performs the same task as an analog pot does. It has found application in microcontroller electronics.

### 3. What is a logarithmic potentiometer?

A logarithmic pot changes its resistance value logarithmically. It comes under the non-linear type.

### 4. What are the parts of a potentiometer?

A typical pot consists of – two fixed terminals and a moving terminal. It also has a resistive element. By using the two fixed terminals, potentiometers take the input. The other part is a wiper or slider.

### 5. Does a potentiometer reduce voltage?

No, a pot doesn’t change the voltage of the circuit. It only controls resistance.

### 6. What is a potentiometer knob?

A pot knob is a holder for the slider of a rotary pot. By rotating the knob, the resistance changes.

### 7. How to compare – the emf of two cells using a pot?

EMF or electromotive-force is a parameter of measuring energy. It is the reason behind the flow of current in a circuit. The potential difference between two points is referred- to as electromotive-force. Its unit is volt.

The mathematical formula is – e = E / Q, where q is the charge, and E is the energy. By using a pot, we can find the emf of a cell. We need to find out the balancing length, where the galvanometer deflection is nearer to null. The potential fall along the length l is the measure of the emf. E is proportional to l.

We can write,

E ∝ l

or, E = K * l , K =Constant

or, E / l = k ———- (i)

Now relation of E1 and l1 with E2 and l2 can be written using equation (i) –

E1 = k * l1

or, E1 / l1 = k ——— (ii)

E2 = k * l2

or, E2 / l2 = k ———(iii)

From (ii) and (iii) we can write –

E1 / l1 = E2 /l2 = k

or, E1 / l1 = E2 / l2

### 8. A cell with internal resistance 1 ohm and emf 5 volts balances on a potentiometer wire at a length of 1.25 meters. The driving cell has an emf of 50 volts. If a 1-ohm wire connects the balance point and the battery, then the balance point will shift.

(Assuming that balancing length is measured from the higher potential side of pot wire.)

• A. 1.25 meters towards the right
• B. 1.25 meters towards the left
• C. 2.5 meters towards the left
• D. None of the above

At first, the balanced length is 1.25 m. Let’s consider it as l1.

Now a wire of 1-ohm resistance connects the balance point and the cell.

We know that E = k * l

Here, l = l1 and E = 5v

k * l1 = 5                – (i)

Now the current through resistance is = (5/2) A = 2.5 A

By adding up the resistance of 1 ohm, the equivalent resistance comes as = 1+1 = 2 ohm.

Hence the E value for the later case becomes 2.5 v.

k * l2 = 2.5             – (ii)

We know that –

E1/E2 = l2/l1

from equations (i) and (ii), we find out –

5/2.5 = l2/l1

putting l1 in the equation,

l2 = 0.5 * l1

or, l2 = 0.5 * 1.25

or, l2 = 0.625 m

So, the balance point shifts 0.625m towards the left.

The correct answer is the option – D. None of the above.

### 9. A potentiometer is better than a voltmeter for measuring emf of a cell. Why?

When we balance a cell against a pot wire, There is no current through the cell. The emf is measured then. Now, when we use a voltmeter to measure the emf for the cell, there is a small current which flows through the cell. Thus, we get only the terminal potential.

### 10. How can you increase the accuracy of the potentiometer?

The accuracy of a pot can be increased by maximizing the length of the wire up to a certain limit.

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