A thermistor is a device that has a resistance that changes when the temperature changes. The term 'thermistor' is a combination of the words 'thermal' and 'resistor'. 

There are two types of thermistor: 1) the positive temperature coefficient (PTC) thermistor (also known as posistor) and 2) the negative temperature coefficient (NTC) thermistor.

The basic linear relationship between the resistance and the temperature of a thermistor is governed by the following equation:  ΔR = kΔT where ΔR is the change in resistance, ΔT is the change in temperature, and k is the first-order temperature of coefficient of resistance. 

A PTC thermistor has a positive k while an NTC thermistor has a negative k.  Thus, the resistance of a PTC thermistor increases whenever the temperature increases, while the resistance of an NTC thermistor decreases as the temperature increases.

Not all thermistors exhibit a linear relationship between its resistance and temperature. Some thermistors are non-linear, i.e., they exhibit a different amount of change in resistance for each degree of change in temperature.

Figure 1.  Photo of thermistors (left) and the symbol for a thermistor (right)

PTC thermistors are usually manufactured by doping a polycrystalline ceramic with semiconductor materials. PTC thermistors can also be made by embedding a piece of plastic with carbon grains. NTC thermistors, on the other hand, are usually fabricated from oxides of manganese, nickel, cobalt, and copper. NTC thermistors may also be formed by crystallizing semiconductors such as silicon and germanium.

Thermistors have been employed in a variety of applications that include current limiters, temperature sensors, fault protection systems, and heat regulators.

See Also:   Resistance;  Passive Components;  More Articles