About thermistors devices

Thermistors are manufactured from beads of semiconductor material prepared from oxides of the iron group of metals such as chromium, cobalt, iron, manganese and nickel. The resistance of such materials varies with temperature according to the following expression:

R = Ro exp[β(1/T 1/To)]

This relationship exhibits a large negative temperature coefficient (i.e. the resistance decreases as the temperature increases), and so is fundamentally different from the relationship for the resistance thermometer, which shows a positive temperature coefficient. The form of equation above is such that it is not possible to make a linear approximation to the curve over even a small temperature range, and hence the thermistor is very definitely a non-linear instrument.

The major advantages of thermistors are their relatively low cost and their small size. This size advantage means that the time constant of thermistors operated in sheaths is small. However, the size reduction also decreases its heat dissipation capability, and so makes the self-heating effect greater. In consequence, thermistors have to be operated at generally lower current levels than resistance thermometers and so the measurement sensitivity afforded is less. Like resistance thermometers, the resistance of a thermistor is usually measured with d.c. bridge.

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