What is a Potentiometer
A potentiometer or POT is a transducer that is used for the conversion of displacement in to a corresponding potential difference. This displacement can be linear or rotational and consequently the potentiometer used can be classified as a Translational POT or a Rotary POT.
A potentiometer consists of a resistive element provided with a sliding contact known as a wiper. This resistive element usually consists of a thin wire of a metallic alloy wounded around an insulator core. An outside electrical supply is used to power the resistive element in order to create a voltage drop across it. Thus potentiometers are passive transducers.
To understand the working of a potentiometer, one must first know that the voltage drop across any resistive element is proportional to its resistance in accordance with ohm’s law. Elaborating further on it, the resistance of an element is directly proportional to its length considering the element to be uniform. So the potential drop measured across any resistive element will in fact be directly proportional to the length across which it is measured. This is the basic principle that governs the working of a POT.
So if we measure the potential difference between the positive terminal of the battery powering the resistive element and the free end of the sliding contact, this potential difference will increase linearly as the sliding contact would move away towards the end of the resistive element that is connected to the negative terminal of the battery thus giving an indication of the extent of displacement. The reverse will be true when the sliding contact would move towards the end connected to the positive terminal of the battery causing the potential difference to decrease linearly in this case.
The sensitivity of a potentiometer is then given as:-
Sensitivity = Potential Difference between the positive terminal of the battery and the free end of wiper
Displacement of the sliding contact from the end connected to the +ve battery terminal
There is another type of potentiometer that is used to measure both translational and rotational motion. In this the resistive element is in a helical shape and consequently these are known as helipots.
Potentiometers fall under the category of potential divider circuits as the sweeping contact divides the potential across the resistive element in to two parts. Also as stated above, in an ideal case the output potential difference measured will vary linearly with displacement but this is not the case in a realistic scenario as errors occurs in this linearity because of the loading effects of the instrument that is used to measure this potential difference.