A potentiometer is a simple electronic device that is made up of a resistive wire wrapped around an insulating core and a sliding contact. It can be driven by either a DC or an AC voltage source. The slider movement can be translational (linear), rotational, or a combination of both, and hence it is highly versatile in various applications.
Schematic Representation of a Potentiometer
A resistive potentiometer can be schematically represented so that its working mechanism can be illustrated. The overall distance between two terminals is denoted as Xt, and the resistance between two fixed terminals is constant. The resistance between the sliding point and one terminal, however, changes with the input displacement, which is denoted as X0.
If the excitation voltage is V, then the output voltage V0 can be expressed mathematically as:
Similarly, if the total resistance is Rt and the resistance between the sliding point and the endpoint is R0, then based on the voltage division rule, the output voltage is:
This equation shows that the output voltage of the potentiometer is proportional to the displacement.
The Loading Effect on Potentiometer
A potentiometer is usually connected to an amplifier or a measuring device (meter) for practical purposes. The input resistance of the device is a load on the potentiometer, which has an effect on the flow of current between the sliding point and the end terminal. This effect is referred to as the loading effect on a potentiometer.
When a meter of input resistance Rm is connected, and the resistance between the sliding point and the endpoint is still R0, the total effective resistance can be calculated as:
After loading, the effective voltage is:
This causes an error, which can be calculated as:
Knowing and minimizing the loading effect is important in the design of precise measurement systems with potentiometers.