How to test a thermistor

Summary: Learn about thermistors and how to check them with a meter.

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).

First we will discuss the PTC type thermistor. PTC thermistors are made out of ceramic or polymer. The resistance of the thermistor will increase when the temperature at it increases.

Some typical uses for the positive thermistor are current limiting devices, circuit protection, and in some cases can replace fuses. They can 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.

Warning: Use the safest means possible when applying heat to the thermistor. DO NOT BURN YOURSELF. The easiest way to test a thermistor is to turn a stove top on, or carefully use a lighter (at a safe distance) to safely heat the thermistor. Hook your ohmmeter up to the leads and watch for the resistance to drop/raise, if it does not then the thermistor is probably defective.

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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 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.

The testing of a negative thermistor is the same procedure as in the PTC thermistor, but now you want the resistance to decrease with the increase of temperature.

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