Potentiometer: Definition, Types, Applications and Principle

Potentiometer: The potentiometer is an important topic of Physics, and students need to understand the basic concepts related to this topic. The potentiometer is an instrument designed for measuring the unknown voltage by comparing it with the known voltage. In other words, it is the three-terminal device used for measuring the potential differences by manually varying the resistances. The known voltage is drawn by the cell or any other supply source.

The name “potentiometer” is a combination of Potential Difference and Metering, which came from the early days of electronics development. It was thought then that adjusting large wire-wound resistive coils metered or measured out a set amount of potential difference, making it a type of voltage-metering device. Students can find all the details related to the potentiometer, including the applications, types and principle of potentiometer, along with the diagram and the symbolic representation in this article.

Potentiometer Definition

The definition of a potentiometer will help students to understand the difficult concepts associated with the topic. A potentiometer is a device used to measure the potential difference in a circuit. The potential difference between two points in a circuit is the amount of work done to bring a charge from the first point to the second. When there is a potential difference in a circuit, the current flows through the circuit. It is a three-terminal resistor that acts as a voltage divider, and if only two terminals are used, it functions like a variable resistor or a rheostat.

The potentiometer is an electric instrument that is used to measure the EMF (electromotive force) of a given cell and also the cell’s internal resistance. Furthermore, it is used to compare EMFs of different cells. It can also be used as a variable resistor in most applications.

The potentiometer uses the comparative method as it is more accurate when compared to the deflection approach. So, it is Majorly used in places where higher accuracy is required or where no current flows from the source under test, unlike voltmeter, which draws some current from the circuit and hence, voltmeter gives less accurate results.

Symbolic Representation of Potentiometer

There are two symbols that are generally used for the purpose of representing a potentiometer. One is more prevalent in America, and the other is considered an international standard symbol for potentiometers. The American standard contains zigzag lines with three terminals placed between two straight lines. The international standard includes a rectangular box with three terminals placed between two straight lines.

Potentiometer Working Principle

The working principle of potentiometer is based on the fact that the drop in potential across any piece of a wire is precisely proportional to the wire’s length, assuming the wire has a uniform cross-sectional area and a steady current running through it. Thus, there will be no electric current flow when there is no potential difference between any two nodes.

To understand the potentiometer working principle, consider the following case:

Two batteries with the same EMF (E) are connected through a galvanometer. The positive terminals of batteries are connected through a galvanometer, and the negative terminals of the two batteries are connected. Since both the batteries have the same EMF, the galvanometer will show a null deflection, indicating no current flowing through the circuit. This is precisely how a potentiometer works.

Potentiometer Circuit Diagram

the potentiometer circuit diagram consists of:

1. Galvanometer: The device used for detecting the presence of a small current or for measuring its magnitude.
2. Ammeter: It is an Electronic Device or an Instrument used to measure the electric current flowing through an Electric Circuit.
3. Jockey: This is a pointed metal contact used to provide exact contact on a wire potentiometer or a Wheatstone bridge.
4. Wire: Generally, alloys like constantan or manganin are used as potentiometer wire because they possess high specific resistance and low-temperature coefficient.
5. Rheostat: It is a variable resistor that is used to control current. These can vary the resistance in a circuit without interruption.

Construction of Potentiometer

The potentiometer consists of a long resistive wire \(L\) made up of magnum or constantan and a battery of known EMF \(V.\) This voltage is called driver cell voltage. The two ends of the resistive wire \(L\) are connected to the battery terminals as shown below; this is assumed to be the primary circuit arrangement. One of the terminals of another cell (whose EMF \(E\) is to be measured) is at one end of the primary circuit. Another end of the cell terminal is connected to any point on the resistive wire through a galvanometer \(G\). This arrangement forms the secondary circuit. The arrangement of the potentiometer is shown below:

The potentiometer wire generally has a high resistivity \(\left(\rho  \right)\) and uniform cross-sectional area. Let it be \(A\). Thus, resistance would be uniform throughout the wire. This potentiometer terminal is connected to the cell of high EMF \(V\) (neglecting its internal resistance) called the driver cell or the voltage source. So let the current in the primary circuit be and be the total resistance of the potentiometer wire.

Derivation of the working condition of a Potentiometer

By Ohm’s law \(V = IR\)

Resistance of the wire can be given as \(R = \frac{{\rho L}}{A}\)
Thus, from the above two equations, \(V = I\frac{{\rho L}}{A}\)
As \(\rho \) and \(A\) are always constant and current, \(I\) is kept constant by a rheostat.
This gives \(L\frac{\rho }{A} = K\) (constant)
Thus, \(V = KL\)

Now suppose a cell of lower EMF than the driver cell is put in the circuit, as shown above, say it has EMF \(E\)
Then, \(E = L\frac{{\rho x}}{A} = Kx\)
Where \(\left(x \right)\) is the length of a part of potentiometer wire.

When this cell is put in the circuit as shown above figure with a jokey connected to the corresponding length \(\left(x \right)\), there will be no current flow through the galvanometer because when the potential difference is equal to zero. So the galvanometer \(G\) shows null detection. The length \(\left(x \right)\) is called the length of the null point. Now by knowing the constant \(K\) and the length \(\left(x \right)\), we can find the unknown EMF.

Secondly, EMF of two cells may also be compared, let the first cell of EMF \(E_1\) given a null point at a length \(L_1\) and the second cell of EMF \(E_2\) show a null point at length \(L_2\)

Then,

\(\frac{{{E_1}}}{{{E_2}}} = \frac{{{L_1}}}{{{L_2}}}\)

Characteristics of Potentiometer

The sensitivity of a potentiometer implies the slightest voltage difference that the given potentiometer can measure. The potential gradient is the decrease in potential per unit length. It is calculated as \(V/L,\) where \(V\) is the potential difference between two points and L is the distance between them. The longer the wire, the lesser the potential gradient and the greater the sensitivity of the potentiometer. So, the sensitivity of the potentiometer is inversely proportional to the voltage.

Types of Potentiometer

Following are the two main types of potentiometer we use in Electronics:

A. Rotary potentiometer (the most common type) varies their resistive value due to an angular movement. For example, rotating a knob or dial attached to the shaft causes the internal wiper to sweep around a curved resistive element. The most common use of a rotary potentiometer is the volume control pot.

The rotary potentiometer can further be classified based on the number of rotations of the wiper into:

1. Single-turn pot: The wiper can make a single rotation of approximately \(270\) degrees or \(3/4\) of a complete turn.
2. Multi-turn pot: These can make multiple rotations (mostly \(5,10,\) or \(20\)) for increased precision. They are constructed either with a wiper that follows a spiral or helix form or a worm gear.
3. Dual-gang pot: It consists of two potentiometers combined on the same shaft, enabling the parallel setting of two channels.
4. Concentric pot:  It consists of two potentiometers, where each pot is individually adjusted using concentric shafts.
5. Servo pot: It is a motorized potentiometer that can be automatically adjusted by a servo motor.

B. Linear Potentiometer: A linear potentiometer is a type of position sensor. They are used to measure displacement along a single axis, either up and down or left and right. These are the potentiometers in which the wiper moves along a linear path. These are often called: slider, slide pot, or fader.

There are several types of linear potentiometers. Some of them are:

1. Slide pot: Single linear slider is used for audio applications. High-quality faders are often constructed from conductive plastic.
2. Dual-Slide Pot: It consists of a single slider controlling two potentiometers in parallel.
3. Multi-turn Slide: It is constructed from a spindle that actuates a linear potentiometer wiper.
4. Motorized fader: Fader, which can be automatically adjusted by a servo motor.

Potentiometer Uses

The instrument designed to measure the unknown voltage by comparing it with the known voltage is the potentiometer. There are various potentiometer applications. The following are the most common potentiometer uses:

1. Audio control: Low-power potentiometers, both slide, and rotary, are used to control audio equipment, changing loudness, frequency attenuation, and other characteristics of audio signals.
2. Television: Potentiometers were formerly used to control picture brightness, contrast, and colour response.
3. Motion Control: Potentiometers can be used as position feedback devices to create closed-loop control, such as in a servomechanism. This motion control method control is the simplest method of measuring the angle of displacement.
4. Transducers: Potentiometers are also very widely used as a part of displacement transducers because of the simplicity of construction and because they can give a significant output signal.
5. Potentiometers as tuners and calibrators: Pots can be used in a circuit to tune them to get the desired output. Also, during the calibrations of a device, a preset pot is often mounted on the circuit board. 
6. Potentiometers as measuring devices: The most typical application of a potentiometer is as a voltage measuring device. The name itself has that implication. It was first manufactured to measure and control the voltage.

Summary

The potentiometer is an electrical instrument generally used to measure the potential difference in a circuit. It can also be used to find the EMF of a given cell or compare the two cells’ EMF. Further, we can use it to find the internal resistance. It does not draw any current from the circuit, giving a more accurate reading than a voltmeter. It consists of a long wire (manganin or constantan) with a uniform area of cross-section. It works on the principle that the potential drop across a segment of a wire of uniform cross-section carrying a constant current is directly proportional to its length. 

1. To calculate unknown potential: Let \(K = V/L\) be the potential gradient of the given potentiometer, where \(V\) is the potential difference between two points and is the distance between two points. Then, \(K = I\rho /A.\)
2. To compare the EMF of two cells: Let the first cell of EMF \({E_1}\) given a null point at a length \({L_1}\) and the second cell of EMF \({E_2}\) show a null point at length \({L_2}.\) Then, \(\frac{{{E_1}}}{{{E_2}}} = \frac{{{L_1}}}{{{L_2}}}.\)
3. To find internal resistance of a cell: \(r = \frac{{R\left({L – l} \right)}}{l}\) where \(r\) is the unknown resistance.

Frequently Asked Questions

Frequently asked questions related to potentiometer are listed as follows:

Q.1: What does a potentiometer measure?
Ans: A potentiometer measures voltage or’ potential difference’ by comparing an unknown voltage with a known reference voltage.

Q.2: What is a potentiometer, and how does it work?
Ans: The Potentiometer is a three-terminal device used to measure the unknown potential difference between two points by comparing it with a known reference voltage.

Q.3: Is the potentiometer analogue or digital?
Ans: Potentiometers can be both analog and digital. An analog potentiometer is a three-terminal resistor in which the resistance is changed manually to control the flow of electric current. In a digital potentiometer, the resistance can be varied by digital communication. An analog pot is also known as a mechanical potentiometer, and a digital pot is known as an electronic potentiometer due to its construction.

Q.4: What are the types of potentiometer?
Ans: A potentiometer (also known as POT) is a variable resistor that contains three terminals. There are two main types of potentiometers: Linear and rotary.

Q.5. What are the potentiometer applications?
Ans: The potentiometer is used in radio and television (TV) receivers for volume control, tone control, and linearity control. The potentiometer is used in medical equipment. It is used in the wood processing machine.

Q.6: Which type of potentiometer is used for volume control?
Ans: A rotary potentiometer is used for volume control purposes.

Q.7: Why is a potentiometer preferred over a voltmeter?
Ans: The potentiometer does not draw any current from the circuit, while the voltmeter draws a small amount of current from the circuit. Thus, the potentiometer gives an accurate result.

Q.8: How can we increase the sensitivity of a potentiometer?
Ans: The sensitivity of the potentiometer can be increased by: Increasing the length of the potentiometer wire.

Q.9: Where can I find the study material on the topic of a potentiometer?
Ans: Embibe offers study materials for the potentiometer, and you can find the basic conceptual details related to potentiometers on this page itself.

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