Odd DVM readings -- seek help

On Wednesday, September 7, 2016 at 1:37:21 PM UTC+10, snipped-for-privacy@gmail.com wro te:

Every workplace is full of 50Hz/60Hz electromagetic field radiation, from t he mains wiring.

You are an aerial, and a high-input impedance digital voltmeter can detect to radiation you pick up vis-a-vis some nearby ground point.

Make sure that the two leads of your DVM are connected to points that share a common ground before you take the output seriously.

It's a bit more obvious if you wave oscilloscope probes around - you get to see the 50/60Hz trace on the screen.

I had the same problem once with an electron beam voltage tester. You had t o ground whatever it was you were testing to the same ground as the electro n microscope, and the guy that had just got the machine hadn't bothered to do that. I plugged in the ground connection to the right place, and the pro blem went away.

I'd written the bit of the manual that said you had to ground everything - in English - and clearly I hadn't made the point emphatically enough for a French engineer.

1. The readings you are getting with only one probe connected and you touching the other is because you are acting as antennna picking up the oscillator capacitively, and also ambient mains, AM broadcast etc etc. This is because the DMM has such a high input impedance.

The reading takes time to decay because internally the DMM has a filter capacitor after the rectifier.

1. The 20V reading you get on the AC range when connected across the 15V supply is possibly because your DMM is DC sensitive even on the AC range. You can test this by seeing how it responds to pure DC on the AC ranges and then try swapping test probes round. My guess is your DMM has a simple half wave (single diode) rectifier and although responsive to the peak voltage is calibrated to read the RMS equivalent assuming the input is sine, hence 15V reads as a bit over 20V.

To be sure your meter is only responding to AC you could connect a capacitor in series with it so no DC can pass.

Using an oscilloscope is very instructive and revealing. A lot can be learnt from even the simplest cheapest uncalibrated 'scope.

piglet

French engineer doesn't know you have to ground a meter ?

Maybe we better take their nuclear power plants away.

or try a Simpson 260

Thanks to each of you for pointing out the cause of the problem.