Potentiometer: Definition, Types, And Working Principle

What is a Potentiometer?

How Does a Potentiometer Work?

A potentiometer is a passive electronic component with two end terminals and a movable wiper. Applying an input voltage across the complete resistor produces an adjustable output between the wiper and either end terminal. The output follows the wiper position only when loading and wiper resistance are small enough for the required accuracy.

The source connects to the two end terminals. Moving the wiper changes the fraction of the resistive track between the selected end terminal and the output.

A rheostat uses the wiper and one end terminal as a two-terminal variable resistance, usually in series with a load.

A laboratory potentiometer can compare cell EMF and support calibration of an ammeter, voltmeter or wattmeter. Its working principle of a potentiometer is a null comparison: the unknown voltage is balanced against a known voltage drop until the detector current reaches zero.

At balance, the compared electric potential differences are equal, so no current flows through the galvanometer. This null condition is the working principle of potentiometer measurement.

Now let’s think about another circuit, where a battery is connected across a resistor via a switch and a rheostat as shown in the figure below.

The slide wire has uniform electrical resistance per unit length.
With constant current, its voltage gradient is uniform. If the adjusted gradient is v volts per unit length, a balance length L represents a voltage Lv.

Connect the positive terminal of a standard cell to point A on the resistor and the negative terminal to a galvanometer. The other end of the galvanometer contacts the resistor via a sliding contact. Adjusting this contact finds point B where no current flows through the galvanometer, causing no deflection.

At null, the standard-cell EMF equals the slide-wire drop between A and B. If that distance is L, then E = Lv volts.

At balance, the detector branch draws no current from the source under test. This reduces loading error, but final uncertainty still depends on the reference, voltage gradient, contact quality, thermal EMFs and balance resolution.

Potentiometer Types

Mechanical potentiometers are commonly grouped by wiper motion:

• Rotary potentiometer
• Linear potentiometer

Both types use a movable contact on a resistive element. Track taper, rotation or travel, wiper construction and power rating determine their behaviour.

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.

Digital Potentiometers

A digital potentiometer is an integrated resistor string or network with electronically selected taps. Many devices expose two end terminals and a wiper, while some are supplied only for two-terminal rheostat operation.

Digital potentiometers can calibrate systems, adjust offset voltage, tune filters, control screen brightness and set sound volume.

Mechanical potentiometers can wear, collect contamination and respond to vibration or humidity. Digital devices avoid moving contacts and support repeatable programmed settings, but they are not automatically more accurate. End-to-end tolerance, wiper resistance, code linearity, supply rails, current limits, bandwidth and distortion must suit the circuit.

Digital Potentiometer Circuit

The circuit of a digital potentiometer consists of two parts, first the resistive element along with electronic switches and second the control circuit of the wiper. The figure below shows both the part respectively.

The first part is an array of resistors, and each node is connected to a common point W, except the endpoints A and B, via a two-way electronic switch. The terminal W is the wiper terminal. Each of the switches is designed using CMOS technology and only one of the switches out of all is in ON state at any given time of the potentiometer operation.

The selected switch sets the tap position, and the number of taps sets nominal step resolution. A control register may use SPI, I²C, up/down inputs, push buttons or a digital encoder. In the illustrated control, UP and DOWN inputs move the wiper through discrete resistance steps.

Power-up position is device-specific. Some parts reset to midscale, while others can power up at another state or restore stored data. Certain digital potentiometers include non-volatile memory; others use volatile registers and lose the setting when power is removed.

For example, in the case of volume control of a device, we expect the device to remember the volume setting we used last even after we switch it on again. Hence a permanent type of memory such as EEPROM is suitable here. On the other hand for systems that recalibrates the output continuously and it is not necessary to restore previous value, a volatile memory is used.

Advantages of Digital Potentiometers

The advantages of digital potentiometers are:

• No mechanical contact wear
• Repeatable digital adjustment within specified errors
• Small size, multiple potentiometers can be packed on a single chip
• Characterised resistance drift and temperature coefficients
• Reduced sensitivity to vibration and wiper contamination
• No moving parts
• Some precision models provide calibrated resistance or ratio data
• Low control power, subject to terminal-current and package-dissipation limits

Disadvantages of Digital Potentiometers

The disadvantages of digital potentiometers are:

• Limited terminal voltage, wiper current, power dissipation and operating temperature.
• Parasitic switch and wiper capacitance limits the bandwidth of digital potentiometers. Datasheets commonly state the frequency at which the wiper output has 3 dB attenuation. The response resembles a low pass filter.
• The nonlinearity in the wiper resistance adds a harmonic distortion to the output signal. The total harmonic distortion, or THD, quantifies the degree to which the signal is degraded after crossing through the resistance.

Applications of Potentiometer

There are many different uses of a potentiometer. The three main applications of a potentiometer are:

1. Comparing the emf of a battery cell with a standard cell
2. Measuring the internal resistance of a battery cell
3. Measuring the voltage across a branch of a circuit

Comparing EMF of Battery Cells

To compare cell EMFs, connect the standard and unknown cells to the same slide wire through a two-way switch and galvanometer. Record the null-balance length L for the standard cell. Switch to the unknown cell and record its balance length L₁ without changing the wire current. If the standard EMF is E, the unknown EMF is E₁, and their ratio equals the ratio of balance lengths.

As the emf of the standard cell is known, hence emf of the unknown cell can easily be determined.

Measuring Internal Resistance of A Battery Cell

In this process, one battery is connected across the resistor of a potentiometer through a galvanometer as shown in the figure below. One resistance of known value (R) is connected across the battery through a switch. First, we keep the switch open and adjust the sliding contact of the potentiometer resistor to make the galvanometer current zero. Once the galvanometer shows zero deflection from its null point we take the position of the sliding contact tip on the resistor scale. Say this is L₁.

Close the switch so current flows through the cell and known resistance R. The cell’s terminal voltage is then below its open-circuit EMF because of internal resistance. Adjust the sliding contact on the potentiometer resistor until the galvanometer returns to zero, then record the new balance length L₂.

The internal resistance of the battery cell can be found out by using this below shown formula.

Where r is the internal resistance of the battery cell.

Measurement of Voltage by Potentiometer

The principle of measuring voltage across a branch of a circuit with help of a potentiometer is also simple. Here first we have to adjust the rheostat to adjust the current through the resistor so that it causes a specific voltage drop per unit length of the resistor. Now we have to connect one end of the branch to the beginning of the resistor and other end is connected to the sliding contact of the resistor through a galvanometer. Now we have to slide the sliding contact on the resistor until the galvanometer shows zero deflection. When the galvanometer comes to its null condition we have to take the reading of the position of the sliding contact tip on the resistor scale and accordingly we can find out the voltage across the branch of the circuit since we have already adjusted the voltage per unit length of the resistor.

Rheostat vs Potentiometer

A potentiometer uses three terminals as an adjustable voltage divider. A rheostat uses two terminals as a variable series resistance. The same physical component can sometimes serve either function, but current, power and failure-mode requirements differ.

In potentiometer mode, output is taken between the wiper and one end while the full track is connected across a source. In rheostat mode, the resistance between the wiper and one end is placed in series with the load. The rheostat connection should normally tie the unused end to the wiper so an intermittent wiper does not leave the circuit open.

Neither connection may exceed the track’s voltage or power rating, the wiper-current rating or the permitted environmental limits. Loading at the potentiometer output changes the ideal divider ratio.

Potentiometer Driver Cell

A slide-wire potentiometer needs a driver source to establish current and a stable voltage gradient along the wire. A series rheostat adjusts that current. The available slide-wire drop must exceed the voltage being balanced, while the chosen gradient must keep wire current, heating and resolution within their limits.

Potentiometer Sensitivity

For a slide-wire measurement potentiometer, sensitivity is the smallest voltage difference that produces a detectable change in balance. A longer wire or lower voltage gradient provides more balance length per volt. Contact resolution, gradient stability, detector sensitivity, reference uncertainty and thermal EMFs set the practical limit.