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.
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