Testing a Thermistor

[jameslondon81]

What is the best way in your opinions to test them?

I test them with resistance, with power off and lead(s) removed - and look for a stable reading (when cold) within calibration range as per chart - is there a better way to test for a faulty thermistor?

 

[petit_pablo]

Testing with a MM is the best way although they need testing both hot and cold.

 

[Boilerman2]

The problem with thermistors is that although they meter out OK at one temperature they go unstable as the temperature rises which makes an acurate test very difficult! IMHO

 

[jameslondon81]

thankls for your help

what's the best way to test/check a thermistor when hot? Any good ways in your opinions?

 

[adlplumbing]

just make sure the one your testing is not in the combustion chamber

 

[Agile]

There are two tests which can/should be done.

A static resistance measurement with the device isolated repeated at at least two temperatures, typically at 20 C and 80 C to ensure it is correctly responding to changes in temperature.

However, that test does not always identify faults. The most common fault after just being open circuit is for it to only go open circuit at a higher temperature.

The boiler control circuit applies a voltage through a series resistance and so its possible to measure that voltage as the boiler is operating. That will indicate if the thermister is not correctly responding. As the temperature increases the voltage across the thermistor will fall. Sudden changes in the voltage indicates a possible thermistor fault.

Tony

 

[jameslondon81]

Thanks for your help I appreciate it a lot.

Here's another one for you experienced breakdown guys:

what are the most common thermistor faults you find - I have had hot water from a combi running cold, hot water running hot then cold etc and temperatures getting so hot and staying too hot