Thermistors are resistors that change resistance depending on the temperature at the area around the thermistor.
There are two types of common thermistors, the PTC (Positive Temperature Coefficient), and the NTC (Negative Temperature Coefficient).
PTC type thermistor- PTC thermistors are made out of ceramic or polymer. The resistance of the thermistor will increase when the temperature increases. Some typical uses for the positive thermistor are current limiting devices, circuit protection, and in some cases they can replace fuses. PTC thermistors can also be used to shut down a circuit or system if overheating occurs. As the heat rises, the thermistor will sense it and increase resistance to a level that will allow very little to no voltage and current through.
Different types of thermistors
Test PTC Thermistor
Step 1- Heat the thermistor.
Warning: Use the safest means possible when applying heat to the thermistor. DO NOT BURN YOURSELF. The safest way to heat a thermistor would be to boil water and use the steam well above the pot as a heat source.
Step 2- Measure the resistance while heated.
Connect the ohmmeter up to the thermistor leads and watch for the resistance to increase with heat applied. If the measured resistance does not change with heat, then the thermistor is most likely defective.
NTC thermistor- The negative type thermistor (NTC) is also made of ceramic or polymer. It works the opposite as the PTC in that the resistance will decrease with temperature increase. It can be used as an inrush current limiting devices such as on power supplies. Also a great deal of car manufacturers use them to monitor fluid temps such as engine coolant and engine/transmission oil. Modern thermostats also use them to change the resistance and turn on/off an air conditioner.
Test NTC Thermistor
The testing of a negative thermistor is the same procedure as in the PTC thermistor, only now the resistance should decrease with the increase of temperature. See testing steps under PTC area.
Thermistor schematic symbol: