Transducers/Sensors: Sample DeviceThermistor

• A thermistor is a temperature-sensing element composed of semiconductor material (typically a mix of metal oxides) that exhibits a large change in resistance in response to a small change in temperature

• Thermistors usually have negative temperature coefficients, the resistance of the thermistor decreases as the temperature increases

(Positive temperature coefficient thermistors also exist, they make good current limiting devices)

Transducers/Sensors: Sample DeviceThermistor

• Accuracy: Thermistors are one of the most accurate types of temperature sensors. Typical accuracy of ±0.2°C.

• Range: Thermistors are fairly limited in their temperature range, working only over a nominal range of 0°C to 100°C .

• Stability: Thermistors are chemically stable and not significantly affected by aging.

Transducers/Sensors: Sample DeviceThermistor

Transducers/Sensors: Sample DeviceThermistor

• Linearity: Thermistors are highly non-linear, following a law called the Steinhart-Hart equation

A, B and C are constants found experimentally by measuring R at three different temps and substituting back in the equationGraphs or tables of RvsT are usually used

Transducers/Sensors: Sample DeviceThermistor

Typical Thermistors

Transducers/Sensors: Sample DeviceThermistor

• Using a Thermistor to sense/measure temperature 1:

If the thermistor is part of a potential divider then the output signal will vary with temperature.

This simple circuit lacks precision, it’s often used in a set-point switch

Transducers/Sensors: Sample DeviceThermistor

Using a Thermistor to sense/measure temperature 2:The thermistor is one of the resistors in a Wheatstone Bridge – say R4.The output voltage is now related to temperature and will vary as the temperature varies.Calibration will enable temperature to be measured.