How to Check a Thermistor

Written by doug hewitt
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How to Check a Thermistor
(Wikimedia Commons)

A thermistor is an electrical component that consists of two wires a casing in which the wires are connected to two materials, typically metals, that have different electrical characteristics at varying temperatures. The thermistor, therefore, has a resistance that varies with temperature. Household appliances that need to monitor temperature, such as a dryer, use thermistors to determine the amount of heat to supply and when the heat present matches the temperature setting. By knowing how to check a thermistor, you can often troubleshoot an appliance. And because thermistors are relatively inexpensive, especially when compared for example to a heating element, checking a thermistor can save money.

Skill level:
Moderately Easy

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Things you need

  • Screwdriver
  • Ohmmeter
  • Ohmmeter probes
  • Hair dryer

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Instructions

  1. 1

    Remove the thermistor from the circuit in which it is used. For a household appliance, such as a dryer, this can typically be achieved by unscrewing the terminals to which the thermistor wires are connected. Other thermistors can have plugs which can simply be unplugged.

  2. 2

    Plug in your ohmmeter probes to your ohmmeter and turn it on. Most ohmmeters have automatic rangefinders, but if you need to select a range, start with the highest range and then adjust the range downward after making your resistance measurement.

  3. 3

    Connect the ohmmeter probes to the thermistor. If you get a resistance reading, the thermistor has an extremely high probability that it is not defective. Most thermistors that are defective will be open and show infinite resistance.

  4. 4

    Turn on the blow dryer and direct the air flow at the thermistor. The resistance measurement on the ohmmeter display should decrease. A typical thermistor will decrease from about 10,000 ohms to about 1,000 ohms, although these values can vary with the thermistor model.

  5. 5

    Disconnect the thermistor from the ohmmeter probes and turn off the ohmmeter.

Tips and warnings

  • Use a small plastic bowl when working on household appliances to keep small parts in so you don't lose them.
  • Household AC voltage is very dangerous and can cause electrical shock.

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