Measuring heated bed resistance

wilriker

OK, this is actually straight forward - or should be.
Take a multimeter, switch it to resistance measuring (set it to 200 Ohms or less), use one lead connected to positive the other to negative (I know that for a resistor like a heated bed there is not really positive or negative).

Now, what I would expect to happen is that my multimeter takes a moment and then shows a stable reading of the resistance. Instead my multimeter starts somewhere around 0.2 Ohm and slowly but constantly increases the measured resistance by 0.1 Ohm.
I stopped "measuring" when it got somewhere around 4-5 Ohms while still not stabilizing (~5 minutes). Reattaching the probes some moments later showed an even higher reading of 6+ Ohms from the moment of connection.

Does this mean my multimeter is broken (I assume that actually because measuring resistance seemed never to have worked correctly) or am I just to stupid to get this right? (It's OK to answer "yes" to the last question )

deckingman

@wilriker Quick test - connect the leads together. It should read zero (or within 0.1 Ohm). Something at the back of my ageing brain tells me that what you describe can happen when the battery needs changing (but that could just be dementia setting in )

dc42

Did you measure that with the bed heater disconnected from the Duet? If yes then get a new multimeter.

jamesadoty

That is weird. I use a good quality Fluke meter and haven't run across that.

The ohms function introduces voltage in to the circuit it's testing. Could it be high enough voltage to attempt to warm the heated bed and cause the impedance change?

I haven't done much research on this but do know ohm meters inject voltage in to the parts being tested.

Just thinking out loud.

gavatron3000

Yeah if it's a cheapy multimeter it's likely it has a lower internal resistance than a high quality one. This affects the readings, higher quality meters are far better at reading very low resistances.

wilriker

@dc42 said in Measuring heated bed resistance:

Did you measure that with the bed heater disconnected from the Duet? If yes then get a new multimeter.

I'm gonna get a new multimeter.