Thermistor, a word formed by combining thermal with resistor, refers to a device whose electrical resistance, or ability to conduct electricity, is controlled by temperature. Thermistors come in two varieties; NTC, negative thermal coefficient, and PTC, positive thermal coefficient, sometimes called posisitors.

The resistance of NTC thermistors decreases proportionally with increases in temperature. They are most commonly made from the oxides of metals such as manganese, cobalt, nickel and copper. The metals are oxidized through a chemical reaction, ground to a fine powder, then compressed and subject to very high heat. Some NTC thermistors are crystallized from semiconducting material such as silicon and germanium.

Electrical circuitry is colder at startup than after running for a length of time. NTC thermistors are used to take advantage of this to protect the circuitry from the surge in electrical flow that accompanies startup. Because the resistance of NTC thermistors varies gradually with temperature, they are also used as temperature measuring devices.

PTC thermistors have increasing resistance with increasing temperature. They are generally made by introducing small quantities of semiconducting material into a polycrystalline ceramic. When temperature reaches a critical point, the semiconducting material forms a barrier to the flow of electricity and resistance climbs very quickly. Unlike the gradual changes in NTC thermistors, PTCs act more like on-off switches. The temperature at which this occurs can be varied by adjusting the composition of the thermistor.

Another type of PTC thermistor consists of a slice of plastic with carbon grains embedded in it. When the plastic is cool, the carbon grains are close enough to each other to form a conductive path. Plastic expands when as it warms; at a certain temperature, it will have expanded enough to push the carbon grains apart and break the conductive path.

This on-off behavior of PTC thermistors is useful in situations where equipment can be damaged by easily definable events. For example, they can be used to protect the windings in transformers and electrical motors from excessive heat.