I came across a weird issue I've never seen before which is baffling.
My scope is down for the count so I can't look at the waveforms coming out of the meters which may explain this.
I was measuring the primary resistance of a 10kVA transformer. I tried a Fluke 87 V. The meter freaked out and was showing negative OL and all sorts of garbage. With autorange on, it was unable to sort of measure the approximately 315 ohms. Manual ranging worked, part of the time.
Ok, time for a new battery. Still no good.
weird, the meter must be blown out. Next I tried the known good Fluke 73 series 2. Same problem. It can't autorange at all, but will sometimes read from 200 to 300 ohms if manually ranged, and if you let the meter sit for about 10 seconds.
Any other transformer or anything with resistance works can be read fine.
Now it's time for the HP 34401. No problem, the X1-X2 resistance in 315 ohms, in autorange or manual mode.
So is the resistance range on a DMM really some sort of AC signal?
I tried the same test with the H1-H2 terminals shorted out. Neither Fluke meter can autorange, but if set manually, they show about 600Ohms which then drops to about 310-320ish after a while.
The HP meter shows the same.
Without the scope, I'm guessing the ohms range is not really putting out a clean DC current, or there's some interference going on with the integrator in these meters and whatever the inductance is of this one transformer.
Has anybody come across this before? It's never crossed my mind to second guess ohm readings on a DMM as long as the DUT is off.