Checking a thermistor

[Backboiler]

Hi, I'm fairly inexperienced at fault finding but familiar with the operation of a multimeter. Will someone let me know the correct way to test a thermistor please? i.e doI test it while the appliance is on? Do I test for ohms or voltage?

Thanks

 

[mikegas]

test on ohms but you need to know the temp the thermister is at to know the correct value, resistance should decrease as heat is applied to a ntc thermister

 

[Backboiler]

Thanks for the quick reply Mike. Do I test it with the appliance running and the wires connected to the thermistor?

 

[diamondgas]

Hi Backboiler

Thermistor works on sending different resistances to the circuit board which in tern interprets the resistance as temperature reading and adjusts the amount of heat supplied accordingly

I believe most thermistors are NTC (Negatively Temperature Coefficient) That is the colder the temp the higher the resistant!

So therefore the only beneficial test is resistance! If they are OTT then they'll never tell the PCB to reduce the gas supply and overheat, and likewise if they're over sensitive they'll shut the gas off prematurely

 

[Backboiler]

Thank you for the reply Diamondgas. Can the thermistor be tested in situ?

 

[mikegas]

Thanks for the quick reply Mike. Do I test it with the appliance running and the wires connected to the thermistor?

always test resistance with power off, test from wire connectors at pcb end first with wires connected to the thermister, then test at the thermister with wires disconnected, this way your checking both the wires and thermister

 

[mikegas]

or thermistor :dunce2:

 

[Backboiler]

Thanks guys

 

[diamondgas]

Thank you for the reply Diamondgas. Can the thermistor be tested in situ?

Simple answer is 'YES' practical answers is 'it depends on there accessibility"

Always test the component (thermistor) disconnected, otherwise how can you prove you are not checking resistance through alternative avenues?

 

[GrahamM]

Power off and disconnect connectors. Better to start from cold. Boilers all have varying resistance readings at approx. temps.
In general you will be looking at a reading of 20-30 ohms at room temp depending on boiler. At full temp anything from 1-3 ohms.
The MI's sometimes give you the figures.