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Thermistors

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With conventional resistors we would normally require resistance to remain the same over a wide range of temperatures (i.e., α should be zero). On the other hand, there are applications in which we could use the effect of varying resistance to detect a temperature change. Components that allow us to do this are known as thermistors. The resistance of a thermistor changes markedly with temperature and these components are widely used in temperature sensing and temperature compensating applications.

Two basic types of thermistor are available, according to whether their resistance increases or decreases with temperature. These are respectively known as positive temperature coefficient (PTC) and negative temperature coefficient (NTC).

Typical NTC thermistors have resistance's which vary from a few hundred (or thousand) ohms at 25°C to a few tens (or hundreds) of ohms at 100°C. PTC thermistors, on the other hand, usually have a resistance-temperature characteristic which remains substantially flat (typically at around 100Ω) over the range 0°C to around 75°C. Above this, and at a critical temperature (usually in the range 80°C to 120°C) their resistance rises very rapidly to values of up to, and beyond, 10kΩ.

A typical application of PTC thermistors is over-current protection. Provided the current passing through the thermistor remains below the threshold current, the effects of self heating will remain negligible and the resistance of the thermistor will remain low (i.e., approximately the same as the resistance quoted at 25°C). Under fault conditions, the current exceeds the threshold value by a considerable margin and the thermistor starts to self-heat. The resistance then increases rapidly and, as a consequence, the current falls to the rest value. Typical values of threshold and rest currents are 200mA and 8mA respectively for a device which exhibits a nominal resistance of 25Ω at 25°C.

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