Electronics > Metrology

Bench DMM resistance drift. Rigol DM3058e


I've added the Rigol 5/12 digit DMM to my bench.  I haven't used a bench DMM for some time, since my EE labs in college.  My bench is in the basement and quite cool.  I've noticed that when I measure a typical 1/4 watt through-hole resistor on my handheld meter it is pretty steady.  However, when I measure a resistor with the bench DMM the resistance rises steadily for some time.    If I then hold the resistor between my fingers, the resistance steadily falls.   So, I'm hoping that I'm just not used to a meter with the resolution to pick up the thermal variation in the resistance caused by my warm fingers.

Is this typical in a cold lab or is my meter due for a trip to the shop?

It really depends on the changes you measure. A resistor, depending on the type of product, can be rated with a TC of say 200ppm/K, maybe even more. Standard metal film usually are 50ppm/K.
I have certainly seen that effect with 6.5 digit meters, even self-heating through the test currect of the meter. If you indicate the changes you see, one could better assess this.

As an example, an old carbon 1M resistor measured @ 1.020xx M cold @ ~65 degrees sitting on the bench.  After holding it in my warm fingers for a full minute the resistance is all the way down to 940 K.   Of course, this is going from fully cold to fully warm and all my meters are picking up the change - so I'm thinking it is a normal thermal coefficient.  Under typical circumstances I wouldn't be fully warming the resister but just handling it for a moment to hook up the meter.   In that case, the warming is slight and the resistance drift shows up on the bench meter but is too small to pick up on the handheld.

In any case, it is a little disconcerting hooking up a resistor to my new shiny meter only to see the value steadily changing.

You are aware of the temperature coefficient of a carbon resistor in general? And then particularly for high resistance values like 1M?

This can easily be 3000ppm/K = 0.3%/K. So every single K temperature change will affect the resistance value by some 3k. While this will show even on a 3 digit meter - but only in the last digit - it will affect the three last digits on a 5 digit meter.

Combining a precision bench meter with a resistor of lowest possible stability, i.e. carbon, leads to very striking effects as you'd described ;)


--- Quote ---You are aware of the temperature coefficient of a carbon resistor in general?
--- End quote ---

I am now!   I was aware that resistors had temperature coefficients but just didn't think it would be so dramatic.   I would have expected it to be significant on one of those boards with a brown spot around a smoking hot resistor but didn't expect it from a warm fingertip.   Sometimes I'm amazed anything electronic works at all.  :-) 

Thanks for the insight.


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