Thermistors are of two types, NTC (negative temperature coefficient) and PTC (positive temperature coefficient types). As their name indicates the resistance of an NTC thermistor will decrease with temperature and the resistance of a PTC thermistor will increase with temperature. Both PTC as well as NTC thermistors can be roughly checked by using an analogue multimeter.
Keep the analogue multimeter in resistance mode. Connect the multimeter terminals to the thermistor leads. Polarity is not an issue here. Now heat the thermistor by moving your heated soldering iron tip to it. Now you can see the multimeter reading smoothly increases or decreases depending on whether the thermistor under test is PTC or NTC. This will happen only for a healthy thermistor.
For a faulty thermistor, following observations are possible.
- The change in reading will not be smooth or there will not be any change.
- For a short thermistor the meter reading will be always zero.
- For an open thermistor the meter reading will be always infinity.
This is only a rough test. For a perfect check up; you need some way to measure the temperature and the corresponding resistance reading must be according to the thermistor’s temperature-resistance characteristics provided by the manufacturer.