What is a Potentiometer? Working Principle, Types, Applications

The Potentiometer (also known as a pot or potmeter)  is a three-terminal resistor having either sliding or rotating contact that forms an adjustable voltage divider. In order to use the potentiometer as a rheostat or variable resistor, it should have only two terminals with one end and the wiper.

Working Principle of POT

The potentiometer consists of L which is a long resistive wire and a battery of known EMF V whose voltage is known as driver cell voltage. Assume a primary circuit arrangement by connecting the two ends of L to the battery terminals. One end of the primary circuit is connected to the cell whose EMF E is to be measured and the other end is connected to galvanometer G. This circuit is assumed to be a secondary circuit.

The resistor has the uniform electrical resistance per unit length throughout its length.
Hence, the voltage drop per unit length of the resistor is equal throughout its length. Suppose, by adjusting the rheostat we get v volt voltage drop appearing per unit length of the resistor.

Now, the positive terminal of a standard cell is connected to point A on the resistor and the negative terminal of the same is connected with a galvanometer. The other end of the galvanometer is in contact with the resistor via a sliding contact as shown in the figure above. By adjusting this sliding end, a point like B is found where there is no current through the galvanometer, hence no deflection in the galvanometer.

That means, emf of the standard cell is just balanced by the voltage appearing in the resistor across points A and B. Now if the distance between points A and B is L, then we can write emf of standard cell E = Lv volt.

This is how a potentiometer measures the voltage between two points (here between A and B) without taking any current component from the circuit. This is the specialty of a potentiometer, it can measure voltage most accurately.

The working principle depends on the potential across any portion of the wire which is directly proportional to the length of the wire that has a uniform cross-sectional area and current flow is constant.

Following is the derivation of used to explain the potentiometer working principle:

V=IR (Ohm’s law)

Where,

I: current

R: total resistance

V: voltage

R=ρLA V=IρLA

Where,

⍴: resistivity

A: cross-sectional area

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

LρA=K V=KL E=LρxA=Kx

Where,

x: length of potentiometer wire

E: cell with Lower EMF

K: constant

The galvanometer G has null detection as the potential difference is equal to zero and there is no flow of current. So, x is the length of the null point. Unknown EMF can be found by knowing x and K.

E=LρxA=Kx

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

E1E2=L1L2

Types of POT

There are two main types of potentiometers:

  • Rotary potentiometer
  • Linear potentiometer

Although the basic constructional features of these potentiometers vary, the working principle of both of these types of potentiometers is the same.

Note that these are types of DC potentiometers – the types of AC potentiometers are slightly different.

Rotary Potentiometers

The rotary type potentiometers are used mainly for obtaining adjustable supply voltage to a part of electronic circuits and electrical circuits. The volume controller of a radio transistor is a popular example of a rotary potentiometer where the rotary knob of the potentiometer controls the supply to the amplifier.

This type of potentiometer has two terminal contacts between which a uniform resistance is placed in a semi-circular pattern. The device also has a middle terminal which is connected to the resistance through a sliding contact attached with a rotary knob. By rotating the knob one can move the sliding contact on the semi-circular resistance. The voltage is taken between a resistance end contact and the sliding contact. The potentiometer is also named as the POT in short. POT is also used in substation battery chargers to adjust the charging voltage of a battery. There are many more uses of rotary type potentiometer where smooth voltage control is required.

Linear Potentiometers

The linear potentiometer is basically the same but the only difference is that here instead of rotary movement the sliding contact gets moved on the resistor linearly. Here two ends of a straight resistor are connected across the source voltage. A sliding contact can be slide on the resistor through a track attached along with the resistor. The terminal connected to the sliding is connected to one end of the output circuit and one of the terminals of the resistor is connected to the other end of the output circuit.

This type of potentiometer is mainly used to measure the voltage across a branch of a circuit, for measuring the internal resistance of a battery cell, for comparing a battery cell with a standard cell and in our daily life, it is commonly used in the equalizer of music and sound mixing systems.

Application of POT

  • Audio control: Both linear, and rotary potentiometers, are used to control audio equipment for changing the loudness and other audio related signals.
  • Television: They are used to control the picture brightness, colour response and contrast.
  • Motion control: In order to create a closed-loop control, potentiometers are used as position feedback devices known as a servomechanism.
  • Transducers: As these give large output signals, they find applications in designing of displacement transducers.

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