The great advantage of thermistors is their high sensitivity. They are used mostly for room temperature measurement up to moderately high temperatures. They are popular in research and in medical applications, e.g as electronic medical thermometers.
A thermistor is a thermally sensitive resistor that exhibits a change in electrical resistance with temperature. The resistance is measured by passing a small, measured direct current (dc) through it and measuring the resulting voltage drop.
There are two basics types of thermistor, PTC and NTC:
Thermistors are commercially available in a wide variety of shapes: bead, disc, chips and probes. The most common form is a bead with two wires attached. The bead diameter can range from about 0.5mm to 5 mm.
Temperature dependence of the resistance
The resistance of a CTN thermistor falls exponentially with increasing temperature. It is negative and non-linear, as expressed by the following relation:
where R0 is the nominal resistance measured at the absolute temperature T0 (often taken at 25 °C) and B is a constant for the particular thermistor material. The non-linear nature of the resistance may be undesirable and this can be offset by using two or more thermistors in series or parallel, packaged in a single device.
The temperature coefficient of resistance α, expressed in %/K (or %/°C), is defined as:
The coefficient α is positive for a PTC thermistor and negative for NTC thermistor.
The temperature can be determined from a thermistor using the Steinhart & Hart equation:
where R is the thermistors resistance in ohms, T the absolute temperature in K, and a, b and c are constants, normally supplied by the manufacturers but may be determined by calibration at three different temperatures and solving three simultaneous equations.
This equation is a close fit to practical devices but it does not always provide the precision required over the full temperature range. This can be corrected by fitting the Steinhart & Hart equation over a series of narrow temperature ranges and then 'splicing' these fits together to cover the required range.
Thermistor advantages and disadvantages
Because of their performance and moderate cost, thermistors are suitable for temperature measurement and control, temperature compensation, electronic, medical and many other applications.