Thermal means temperature. In this resistor, the resistance varies with temperature. If heat increases, the resistance decreases and vice versa. This is used for measurement and control purposes.

The main types of thermistors are NTC and PTC.

·        NTC is Negative Temperature Coefficient and in such devices, the resistance decreases as the temperature increases. These are used to protect the devices from over-voltage conditions.

·        PTC is Positive Temperature Coefficient and in such devices, the resistance increases as the temperature increases. These are used to protect the devices from over current conditions.

The following figure shows an NTC thermistor, along with its symbol.