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.

 

[mikegas]

most are 11k ohms at room temp

 

[diamondgas]

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.

I think you may be a bit out with your figures GrahamM. You're more likely to get 1000ohms hot rising upwards the colder the water 🙂

 

[Billy Bob Bob]

get your self some connectors off old combis you rip out its then easy to test thermistors i always test on ohms scale as said 11k cold is what most run at but there are exceptions to get used to testing thermistors get a old connector and a known working thermistor take a reading from cold in ohms,then one after holding it in your hand for 5 minuites then dip it in some hottish water now compare the readings and you have a idea of how a typical thermistor behaves

 

[REDSAW]

This may be of value if you can see it?.

 

[GrahamM]

most are 11k ohms at room temp

A lot may be around the 11k ohms mark at room temp. A lot are not! What is your room temp?

For example (based on the manufacturer figures I have to hand from recent checks):
Worc. 24CDi, 280RSF, CDi(older) are all around 25K ohms at 25°C
Greenstar 15k ohms @ 20°C
Alpha 240/280 is 14k ohms @ 25°C
Pott. Performa, Promax HE 10k ohms @ 25°C
Most vokeras are around the 11k ohms @ 20°C

As I mentioned the MI's/Manufacturer should be consulted for accurate figures.

I think you may be a bit out with your figures GrahamM. You're more likely to get 1000ohms hot rising upwards the colder the water 🙂

Oops, I missed out the 'k' in front of my 'ohms'.


And just to clarify, the resistance decreases (number increases) on an NTC resistor with a rise in temperature.

 

[mikegas]

Be handy if there was a thermistor app with all the values for different boilers :smile5:

 

[AWheating]

Be handy if there was a thermistor app with all the values for different boilers :smile5:

write one and ill buy it 😉