Potentiometers are among the most commonly used devices in electronic circuits to achieve a variable voltage from a fixed voltage source. Potentiometers are simply three-terminal devices with manually adjustable resistance. They typically come in several types, but the rotating contact (or rotary) potentiometers are the most common.

As its name implies, a rotating contact potentiometer features a rotating contact (or a circular resistor material). These potentiometers vary their resistive value due to the angular movement of one of the terminals over the resistor material. But how does a rotating contact potentiometer work in a circuit, and what are its applications today?

Working principle of a rotating contact potentiometer

Consider a typical rotating contact potentiometer with three terminals (Figure 1). The two terminals (left and right) feature a resistive element (or material) placed between them in a semi-circular pattern. The third terminal (middle contact) is connected to the resistive element through a sliding contact (or wiper) attached to a rotating knob. This middle terminal is free to move along the semi-circular resistive element.Figure 1: Rotating contact potentiometer. Source: Zephyris/CC BY-SA 3.0Figure 1: Rotating contact potentiometer. Source: Zephyris/CC BY-SA 3.0

The electric resistance of a conductor is directly proportional to its length. Therefore, when the resistance between the outer terminals (or across the entire length of the resistive element) is measured, it gives the potentiometer's maximum (or rated) resistance. However, when the resistance between one of the outer terminals and the middle terminal is measured, one would obtain a resistance value according to the position of the middle terminal on the sliding contact.

This unique operation of rotating contact potentiometers makes them suitable as rheostats or voltage dividers, depending on how the terminals are connected.

Applications of rotating contact potentiometers

Potentiometer used as a rheostat

A rheostat is an adjustable resistor used to control the electrical current in a circuit. It can also be used in applications where a varying resistance value is required. In such applications, only the middle terminal and one of the outer terminals are used. Engineers can obtain different resistance values by simply turning (or moving) the wiper across the semi-circular resistive element.

Potentiometer used a voltage divider

A voltage divider is a simple circuit that turns a large voltage into a smaller one. This circuit features a voltage source connected across two resistors, as shown in Figure 2. It is used to produce an output voltage that is a fraction of the input (supply) voltage.

Figure 2: Voltage divider circuit of a potentiometer.Figure 2: Voltage divider circuit of a potentiometer.

Rotating contact potentiometers are commonly used as voltage dividers. For this application, all three potentiometer terminals are used: the input voltage is applied between the two outer terminals while the output voltage is taken across the middle terminal. By varying the position of the wiper across the sliding contact, engineers can vary the output voltage. The output voltage can be calculated from the following equation derived from Ohm’s law.

Rotary contact potentiometers are used to obtain adjustable supply voltage in frequency attenuation and volume control applications (or audio control devices).

Sensitivity of potentiometers

The sensitivity of a potentiometer is defined as the smallest potential difference that the potentiometer can measure. A potentiometer’s sensitivity depends on the value of its potential gradient (k), as shown in the equation below:

Where:

V = potential difference between two points

l = distance between the two points

Engineers can increase a potentiometer’s sensitivity by decreasing its potential gradient (k). For instance, increasing the length of the potentiometer’s resistive element causes the potential gradient to reduce. As a result, this increases the sensitivity of the potentiometer. Engineers can also increase a potentiometer’s sensitivity by reducing the flow of current through the circuit (which leads to a reduction in voltage).

Conclusion

Rotating contact potentiometers (or rotary position sensors) are essential devices used across many industries today, so long as they are correctly sized for an application. Therefore, engineers are advised to reach out to rotary position sensor suppliers to discuss their application needs.

To contact the author of this article, email engineering360editors@globalspec.com