What is a Potentiometer : Construction & Its Working

The Potentiometer is an electric instrument used to measure the EMF (electromotive force) of a given cell, the internal resistance of a cell. And also it is used to compare the EMFs of different cells. It can also use as a variable resistor in most of applications. 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. Although their most obvious use must be for volume controls on radios and other electronic equipment used for audio.

Potentiometer Pin Out

The pin diagram of the Trimpot potentiometer is shown below. These potentiometers are available in different shapes and include three leads. These components can be placed on a breadboard easily for easy prototyping. This potentiometer includes a knob over it and it is used to change its value by changing it.

Pin1 (Fixed End): The connection of this fixed end1 can be done to one finish of the resistive path

Pin2 (Variable End): The connection of this variable end can be done by connecting it to the wiper so that it provides variable voltage

Pin3 (Fixed End): The connection of this another fixed end can be done by connecting it to other finish of the resistive path

How to Select a Potentiometer?

The potentiometer is also called a POT or variable resistor. These are used to provide a variable resistance by just changing the knob on the potentiometer. The classification of this can be done based on two important parameters like Resistance (R-ohms) & Power rating (P-Watts).

The potentiometer resistance otherwise its value mainly decides how much resistance it gives to the current flow. When the resistor value is high then the less value of current will flow. Some of the potentiometers are 500Ω, 1K ohm, 2K ohm, 5K ohm, 10K ohm, 22K ohm, 47K ohm, 50K ohm, 100K ohm, 220K ohm, 470K ohm, 500K ohm, 1M.

The classification of resistors mainly depends on how much current it allows to flow through it, which is known as power rating. The power rating of a potentiometer is 0.3W & therefore it can be used simply for low-current circuits.

There are still several kinds of potentiometers and their selection mainly depends on certain necessities like the following.

• The necessities of the Structure
• The Resistance Change Characteristics
• Choose the kind of potentiometer based on the necessities of Use
• Choose the parameters based on the necessities of the circuit

Construction and Working Principle

The potentiometer consists of a long resistive wire L made up of magnum or with constantan and a battery of known EMF V. This voltage is called driver cell voltage. Connect the two ends of the resistive wire L to the battery terminals as shown below; let us assume this is a primary circuit arrangement.

One terminal of another cell (whose EMF E is to be measured) is at one end of the primary circuit and another end of the cell terminal is connected to any point on the resistive wire through a galvanometer G. Now let us assume this arrangement is a secondary circuit. The arrangement of the potentiometer as shown below.

The basic working principle of this is based on the fact that the fall of the potential across any portion of the wire is directly proportional to the length of the wire, provided wire has a uniform cross-sectional area and the constant current flowing through it.“When there is no potential difference between any two nodes there is electric current will flow”.

Now the potentiometer wire is actually a wire with high resistivity (ῥ) with uniform cross-sectional area A. Thus, throughout the wire, it has uniform resistance. Now this potentiometer terminal connected to the cell of high EMF V (neglecting its internal resistance) called driver cell or the voltage source. Let the current through the potentiometer is I and R is the total resistance of the potentiometer.

Then by Ohms law V=IR

We know that R= ῥL/A

Thus, V= I ῥL/A

As ῥ and A are always constant and current I is kept constant by a rheostat.

So L ῥ/A=K (constant)

Thus, V= KL. Now suppose a cell E of lower EMF than the driver cell is put in the circuit as shown above. Say it has EMF E. Now in the potentiometer wire say at length x the potentiometer has become E.

E= L ῥx/A=Kx

When this cell be put in the circuit as shown above figure with a jokey connected to the corresponding length (x), there will be no flow of current through the galvanometer because when the potential difference is equal to zero, no current will flow through it.

So the galvanometer G show null detection. Then the length (x) is called the length of the null point. Now by knowing the constant K and the length x. We can find the unknown EMF.

E= L ῥx/A=Kx

Secondly, EMF of two cells may also be compared, let the first cell of EMF E1 given a null point at a length= L1 and the second cell of EMF E2 show a null point at length= L2

Then,

E1/E2= L1/L2

Why is Potentiometer is Chosen Over Voltmeter?

When we use Voltmeter, current flows through the circuit, and because of the internal resistance of the cell, always terminal potential will be less than the actual cell potential. In this circuit, when the potential difference is balanced (using a Galvanometer null detection), no current flows in the circuit, so the terminal potential will be equal to the actual cell potential. So we can understand that the Voltmeter measures the terminal potential of a cell, but this measures actual cell potential. The schematic symbols of this are shown below.

Types of Potentiometers

A potentiometer is also commonly known as pot. These potentiometers have three terminal connections. One terminal connected to a sliding contact called a wiper and the other two terminals is connected to a fixed resistance track. The wiper can be moved along the resistive track either by use of a linear sliding control or a rotary “wiper” contact. Both rotary and linear controls have the same basic operation.

The most common form of the potentiometer is the single turn rotary potentiometer. This type of potentiometer is often used in audio volume control (logarithmic taper) as well as many other applications. Different materials are used to construct potentiometers, including carbon composition, cermet, conductive plastic, and metal film.

Rotary Potentiometers

These are the most common type of potentiometers, where the wiper moves along a circular path. These potentiometers are mainly used to get a changeable voltage supply to a fraction of circuits. The best example of this rotary potentiometer is a radio transistor’s volume controller where the rotating knob controls the current supply toward the amplifier.

This kind of potentiometer includes two terminal contacts where a consistent resistance can be located in a semi-circular model. And also it includes a terminal in the middle that is allied to the resistance using a sliding contact that is connected through a rotating knob. The sliding contact can be turned by turning the knob over the half-circular resistance. The voltage of this can be obtained among the two contacts of resistance & the sliding. These potentiometers are used wherever level voltage control is necessary.

Linear Potentiometers

In these types of Potentiometers, the wiper moves along a linear path. Also known as slide pot, slider, or fader. This potentiometer is similar to the rotary-type but in this potentiometer, the sliding contact simply rotated on the resistor linearly. The connection of the resistor’s two terminals is connected across the voltage source. A sliding contact on the resistor can be moved using a path that is connected through the resistor.

The terminal of the resistor is connected toward the sliding which is connected to one finish of the circuit’s output & another terminal is connected to the other finish of the circuit’s output. This kind of potentiometer is mostly used to calculate the voltage in a circuit. It is used to measure the battery cell’s internal resistance and also used in the mixing systems of sound & music equalizer.

Mechanical Potentiometer

There are different kinds of potentiometers available in the market, in that mechanical types are used for controlling manually to change the resistance as well as the output of the device. However, a digital potentiometer is used to change its resistance automatically based on the given state. This type of potentiometer works accurately like a potentiometer and its resistance can be changed through digital communication such as SPI, I2C rather than turning the knob directly.

These potentiometers are called POT due to its POT shaped structure. It includes three terminals like i/p, o/p, and GND along with a knob on its pinnacle. This knob works like control to control the resistance by rotating it in the two directions like clockwise otherwise anticlockwise.

The main drawback of digital potentiometers is that they are simply influenced by different environmental factors such as dirt, dust, moisture, etc. To overcome these disadvantages, digital Potentiometers (digiPOT) were implemented. These potentiometers can work in environments like dust, dirt, moisture without altering its operation.

Digital Potentiometer

Digital potentiometers are also called as digiPOTs or variable resistors which is used to control analog signals using microcontrollers. These types of potentiometers give an o/p resistance that is changeable depending on digital inputs. Sometimes, these are also called RDACs (resistive digital-to-analog converters). The controlling of this digipot can be done by digital signals rather than through mechanical movement.

Each step on the resistor ladder includes one switch which is connected to the o/p terminal of the digital potentiometer. The ratio of the resistance in the potentiometer can be determined through the chosen step over the ladder. Generally, these steps are indicated with a bit value, for instance. 8-bits are equal to 256 steps.

This potentiometer utilizes digital protocols such as I²C otherwise SPI Bus (Serial Peripheral Interface) for signaling. Most of these potentiometers utilize simply volatile memory so that they did not remember their place once they are powered down and their final place may be stored through the FPGA or microcontroller to which they are connected.

Characteristics

The characteristics of a potentiometer include the following.

• It is extremely accurate as it works on the evaluating technique rather than the technique of deflection to determine the unidentified voltages.
• It determines the balance point otherwise null which does not need power for the dimension.
• The potentiometer working is free from the resistance of source as there is no flow of current throughout the potentiometer as it is balanced.
• The main characteristics of this potentiometer are resolution, taper, the marking codes & hop on/hop off resistance

Potentiometer Sensitivity

The potentiometer sensitivity can be defined as the least potential variation that is calculated with the help of a potentiometer. Its sensitivity mainly depends upon the potential gradient value (K). When the potential gradient value is low, the potential difference that a potentiometer can calculate is smaller, and then the potentiometer sensitivity is more.

So, for a given potential dissimilarity, the potentiometer sensitivity can increase through the increase in the potentiometer’s length. The potentiometer sensitivity can also be increased for the following reasons.

• By increasing the potentiometer length
• By decreased the flow of current within the circuit through a rheostat
• Both techniques will assist in reducing the value of the potential gradient & increasing the resistivity.

Difference between Potentiometer and Voltmeter

The main differences between potentiometer and voltmeter are discussed in the comparison table.

Potentiometer	Voltmeter
The resistance of the potentiometer is high & endless	The resistance of the voltmeter is high & limited
Potentiometer doesn’t draw the current from the emf source	Voltmeter draws a little current from the source of emf
The potential disparity can be calculated when it is equivalent to the definite potential difference	Potential difference can be measured when it is less than the definite potential difference
Its sensitivity is high	Its sensitivity is low
It measures simply emf otherwise potential difference	It is a flexible device
It depends on the zero deflection technique	It depends on the deflection technique
It is used to measure emf	It is used to measure the circuit’s terminal voltage

Rheostat vs Potentiometer

The main differences between rheostat and potentiometer are discussed in the comparison table.

Rheostat	Potentiometer
It has two terminals	It has three terminals
It has only a single turn	It has a single and multi-turn
It is connected in series through the Load	It is connected in parallel through the Load
It controls the current	It controls the voltage
It is linear simply	It is linear & logarithmic
The materials used to make the rheostat are carbon disc and metallic ribbon	The materials used to make the potentiometer is graphite
It is used for high power applications	It is used for low power applications

Measurement of Voltage by Potentiometer

The measurement of voltage can be done using a potentiometer in a circuit is a very simple concept. In the circuit, the rheostat must be adjusted and current flow through the resistor can be adjusted so that for each unit length of the resistor, an exact voltage can be dropped.

Now we have to fix one finish of the branch to the resistor beginning whereas the other end can be connected toward the resistor’s sliding contact using a galvanometer. So, now we have to move the sliding contact over the resistor till the galvanometer displays zero deflection. Once the galvanometer reaches its zero states then we have to note the position reading on the resistor scale & based on that we can discover the voltage in the circuit. For better understanding, we can adjust the voltage for each unit length of the resistor.

Advantages

The advantages of potentiometer include the following.

• There is no chance of getting errors because it uses the zero reflection method.
• The standardization can be done by using a normal cell directly
• It is used to measure small emf’s due to highly sensitive
• Based on the requirement, the potentiometer length can be increased to get accuracy.
• When the potentiometer is used in the circuit for measurement then it doesn’t draw any current.
• It is used to measure the inner resistance of a cell as well as compares the e.m.f. of two cells but by using a voltmeter, it is not possible.

Disadvantages

The disadvantages of potentiometer include the following.

• The potentiometer usage is not convenient
• The cross-section area of potentiometer wire should be consistent so that is not possible practically.
• While doing an experiment, the wire temperature should be stable but this is hard due to the current flow.
• The main drawback of this is, it needs a huge force to move their wiper or sliding contacts. There is erosion because of the movement of the wiper. So it decreases the transducer’s life
• Bandwidth is limited.

Potentiometer Driver Cell

The potentiometer is used to measure the voltage by evaluating the measuring voltage across the potentiometer’s resistance with voltage. So for the potentiometer operation, there should be a voltage source that is allied across the circuit of a potentiometer. A potentiometer can be operated by the voltage source which is provided by the cell is known as the driver cell.

This cell is used to deliver the current throughout the resistance of the potentiometer. The resistance & the current product of the potentiometer will provide a complete voltage of the device. So, this voltage can be adjusted to change the potentiometer’s sensitivity. Usually, this can be done by regulating the current throughout the resistance. A rheostat is connected with the driver cell in series.

The flow of current throughout the resistance can be controlled using a rheostat which is connected with the driver cell in series. So the driver cell voltage must be better as compared with the measured voltage.

Applications of Potentiometers

The applications of potentiometer include the following.

Potentiometer as a Voltage Divider

The potentiometer can be worked as a voltage divider to obtain a manually adjustable output voltage at the slider from a fixed input voltage applied across the two ends of the potentiometer. Now the load voltage across RL can be measured as

VL= R2RL. VS/(R1RL+R2RL+R1R2)

Audio Control

Sliding potentiometers, one of the most common uses for modern low-power potentiometers are as audio control devices. Both sliding pots (faders) and rotary potentiometers (knobs) are regularly used to frequency attenuation, adjust loudness, and for different characteristics of audio signals.

Television

Potentiometers were used to control picture brightness, contrast, and color response. A potentiometer was often used to adjust “vertical hold”, which affected the synchronization between the received picture signal and the receiver’s internal sweep circuit (a multi-vibrator).

Transducers

One of the most common applications is measuring displacement. To measure the displacement of the body, which is movable, is connected to the sliding element located on the potentiometer. As the body moves, the position of the slider also changes accordingly so the resistance between the fixed point and the slider changes. Due to this the voltage across these points also changes.

The change in resistance or the voltage is proportional to the change in the displacement of the body. Thus the voltage change indicates the displacement of the body. This can be used for the measurement of translational as well as rotational displacement. Since these potentiometers work on the principle of resistance, they are also called the resistive potentiometers. For example, the shaft rotation might represent an angle, and the voltage division ratio can be made proportional to the cosine of the angle.

Thus, this is all about an overview of what is a Potentiometer, pinout, its construction, different types, how to select, characteristics, differences, advantages, disadvantages, and its applications. We hope that you have got a better understanding of this information. Furthermore, any queries regarding this concept or electrical and electronics projects, please give your valuable suggestions by commenting in the comment section below. Here is a question for you, What is the function of a rotary potentiometer?