Testing Thermistors

A thermistor is a tool that measures resistance changes based on the temperature it is exposed too. These components are highly accurate and can be found in many different industrial applications. There are two chief variations of thermistors:  the positive temperature coefficient thermistor and the negative temperature coefficient. Although the two different versions both measure resistance, they have different resistance reactions to temperature variations.  Every new thermistor should be thoroughly tested before it is utilized in an industrial application. Both types of thermistors have different methods of testing and the steps below will provide a guideline for testing NTC or PTC thermistors.

NTC, or negative temperature coefficient thermistors, show a resistance decrease when the temperature rises. When testing NTC thermistors, it is vital to look for this reaction; if resistance does not lower as the temperature rises, the thermistor is faulty. Before applying any heat, make sure to check the rated value of the thermistor. This characteristic is measured in ohms, and the thermistors initial resistance value should be close to the rated resistance value. For example, if you are testing a 5KΩ NTC thermistor in a relatively warm room, it should have a read out of any where from 4.0-5KΩ,. This test helps to establish that the thermistor is in working condition and registering near its stated resistance value. If the number is far off, then it should never be used in an application.

The next test is to add heat to the thermistor. To do so, you can use any item or device that radiates heat: space heater, hair dryer, or radiator. Remember, for negative temperature coefficient thermistors, resistance is supposed to decrease as heat increases, so the same logic should apply when you are testing the NTC device.  When you apply heat directly to the body of the thermistor, the resistance should significantly decrease.  If this behavior does not happen, you have most likely encountered a faulty thermistor.
The steps for testing a PTC, or positive temperature coefficient thermistor, are exactly the same as testing NTC thermistor. The major difference between the two is that the resistance should increase steadily as heat is applied to the device.  When a PTC thermistor registers at room temperature, the thermistor should read out a very low resistance rating.

It is always wise to consult Sensor Scientific for more information on testing, use, and troubleshooting.