A Linear resistor is one whose resistance doesn’t vary with the flow of current through it. The current through it, will always be proportional to the voltage applied across it. Linear resistors are further classified as Fixed and Variable resistors.
Variable resistors are those whose values can be varied manually, according to the requirement. A particular value of resistance is chosen from a range of resistance values, with the help of a shaft connected. The symbol of a variable resistor is as shown below.
These resistors are better understood with the help of the classification we have. Variable resistors are further divided into Potentiometers, Rheostats and Trimmers.
A Potentiometer is simply called as a Pot. This is a three-terminal resistor having a shaft which slides or rotates. This shaft when operated forms an adjustable voltage divider. The following figure shows an image of a Potentiometer.
A potentiometer also measures the potential difference (voltage) in a circuit. A path of resistive material with resistance of low to high value is laid internally and a wiper is placed so that it connects the resistive material to the circuit. This is mostly used as a volume controller in TV sets and Music systems.
A Rheostat can be simply called as a Wire wound resistor. A Resistive wire is wound around an insulating ceramic core tightly. A Wiper slides over these windings. One connection is made to one end of the resistive wire and the second connection is made to the wiper or the sliding contact, to obtain the desired resistance.
The Rheostat is used to control current. These are mostly used in the speed control of heavy motors. The resistance obtained by these is in the order of kilo ohms. Rheostats are mostly available as single tube and double tube rheostats, as shown in the following figure.
As a variable resistance they are often used for tuning and calibration in circuits. Now-a-days, the usage of rheostats was replaced by switching electronic devices, as rheostats have lower efficiency.
Trimmer is both a variable resistor and a potentiometer (measures potential difference). This Trimmer Potentiometer is, in short called as Trim Pot. If these are used as variable resistors, then they are called as Preset Resistors.
These trim pots are of different types such as single turn or multi turn. These are small variable resistors used for tuning and calibration. Their life span is shorter than other variable resistors.