Electronics > Beginners

Odd multimeter reading


I have a new (ex-demo) fluke 87V meter, and was trying it out this evening with the output of a toroidal transformer on AC voltage mode. When the test leads were in contact with the secondary winding terminals on the transformer, all was well, and I got the reading I expected (25V AC RMS).

When I removed one test lead, the reading varied a bit, but was almost 20V AC still! I thought it must be some electromagnetic effect, or some capacitance effect, so I straightened my test leads as much as possible, and pulled them as far away from the transformer as possible, as well as pulling the meter away as far as possible, probes and leads as far apart as they could be, and still got the same reading. This is on a bare hardwood workbench top.

Also - and this was possibly dangerous, but I couldn't resist touching the floating meter probe (high input impedance, right?) Then the display showed 80 or so V AC. Yikes!

When nothing was connected to the probes, I got odd levels of AC voltage, too, 20mV or 50mV or so (don't quote me on the amount, but much more than I thought the noise floor would be, given the accuracy specs).

Any ideas? I'm not using fluke leads, but they are marked as CAT III and Cat IV, they are Ideal TL-104 leads to be precise.

It's working fine on DC volts, amps, ohm, temperature, etc.


The DMM represents a fairly high impedance, so it's probably just inductive pickup. Does reducing the loop area by twisting the leads or completely removing them reduce the voltage? Does it read close to 0 V when shorted? The spec does allow for a certain offset in AC mode due to non-linearity of the RMS converter, note that the accuracy spec only applies above 5% of full scale or so.

The 80 V when you touch it shows that you make a fairly good antenna at 50/60 Hz.

Hi Alm,

Thanks for putting my mind at rest, I thought it must be some kind of induction/magnetism issue, I thought to unwind my coil (the leads), but didn't think to wind them together into a twisted pair.

I'll try that this morning


PS: Happy chocolate day one and all!


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