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A thermistor is a “thermally sensitive resistor.” This is a semiconductor composed of metallic oxides such as manganese, nickel, cobalt, copper, iron, and titanium. Basic ceramics technology is utilized to fabricate thermistors in wafer, disk, bead, and other shapes. There are two basic types of thermistors, negative temperature coefficient (NTC) and positive temperature coefficient (PTC). NTC thermistors are much more commonly used than PTC thermistors. The resistance of NTC thermistors decreases with increasing temperature.
Thermistor applications are based on the resistance-temperature characteristic of a thermistor. NTC thermistors give a relatively large output (change of resistance) for a small temperature change. This output can be transmitted over a large distance. No compensation for ambient temperature is needed. The amount of change per °C is expressed by Beta value (material constant) or Alpha coefficient (resistance temperature coefficient). The larger Alpha or Beta the greater the change in resistance with temperature, and the temperature versus resistance curve is steeper.
The resistance versus temperature relationship is not linear. With increasing temperature the nonlinearity decreases. The Stainhart-Hart Equation expresses the relationship between resistance and temperature:
1/T = a + b + (lnR) + c(lnR)3
where T is temperature, R is resistance and a, b, c are coefficients derived from measurements. Thermistors are calibrated at three different temperatures, usually at 0°, 25°, and 70°C. This gives three different values of resistance.
Coefficients a, b, c are common for all three measurements, therefore their value can be calculated by solving three equations simultaneously. The a, b, c coefficients are important for an instrumentation setup. Resistance versus temperature Tables are published for thermistors with nominal values. Most temperature controllers have the ability to define the a, b, c setpoints when temperature and corresponding resistance are entered.
NTC thermistors are the most sensitive of all the temperature sensing elements. Small dimensions of wafer, bead, disc and chip thermistors result in a rapid response time. This is especially useful for control system feedback.
Interchangeability is another important feature. NTC thermistor interchangeability guarantees close tolerances (0.1 to 0.2 °C) in a certain temperature range, usually 70°C. Instruments and control systems do not have to be recalibrated when a thermistor of the same nominal value is replaced. The ceramic manufacturing process of NTC thermistors produces extremely hard and rugged sensors. NTC thermistors are able to handle mechanical and thermal shocks better than any other temperature measuring device.
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