Thermistors are solid-state devices that operate on the basis of change of electrical resistance with temperature and are available as negative temperature coefficient (NTC – resistance falls with rising temperature), or positive temperature coefficient (PTC – resistance increases with rising temperature). NTC is the most usual type for temperature measurement, and the rate of change of resistance with temperature is very much higher than that of a resistance thermometer, providing high sensitivity within a small temperature span. This makes thermistors very suitable for measuring temperatures around ambient and for medical applications.
The overall temperature range in which thermistors can be used is approximately –80°C to +400°C. ‘The characteristics are determined by the manufacturers, and a wide range of types are available. Unlike thermocouples and resistance thermometers, there are no universally accepted standards but certain values are widely used (e.g. 5K, 10K and 100K ohms at 25°C), and the choice of value will depend upon the operating temperature range. Apart from the over-temperature devices mentioned below, Testemp will only supply thermistor assemblies if the element is supplied free-issue or if its exact specification and source of supply are advised by the customer.
Thermistor leads can be extended with copper wire, and if the thermistor is appropriate for the temperature range the resistance of the leads will be negligible compared with that of the device itself. Good quality electrical connectors and terminal blocks can be used in thermistor circuits.
Some PTC thermistors undergo a sudden large increase of resistance (e.g. from 100 ohms to 10K ohms) at a certain temperature. These can be used with very simple circuitry to provide an over-temperature warning or safety trip. Assemblies using such devices are available from Testemp to switch at 80°C, 90°C and 120°C. The switching temperature must be specified at the time of order and cannot be changed.