Introduction of Thermistors | Sensor & Industrial Instrumentation - Electronics and Communication Engineering (ECE) PDF Download

Thermistors

Thermistors are a class of metal oxide (semiconductor material) that typically has a high negative temperature coefficient of resistance. They also can be positive. Thermistors have high sensitivity, which can be up to a 10% change per degree Celsius, making it the most sensitive temperature element available, but thermistors also have very nonlinear characteristics. The typical response time is from 0.5 to 5 seconds, with an operating range typically from −50° to +300°C [3]. Devices are available with the temperature range extended to 500°C. Thermistors are low cost, and are manufactured in a wide range of shapes, sizes, and values. When in use, care has to be taken to minimize the effects of internal heating.
Thermistor materials have a temperature coefficient of resistance (α) given by:

where ∆R is the change in resistance due to a temperature change ∆T, and R_s is the material resistance at the reference temperature.
The nonlinear characteristics, make the device difficult to use as an accurate measuring device without compensation, but its sensitivity and low cost makes it useful in many applications. The device is normally used in a bridge circuit, and padded with a resistor to reduce its nonlinearity.