Negative resistance displayed by 34465A when measuring DCR of an electromagnet

[I wanted a rude username]

[bdunham7]

Since it appears to work completely properly in manual ranges, unlike the Keithley DMM6500 issue, it's just the autoranging method--although the way it stabilizes seems odd.  Everyone wants fast autoranging, but there are drawbacks and you can always find corner cases.

[Kean]

Yes, this will be a side effect of the DUT inductance on the current source for resistance meesurement when in autoranging (and possibly other high ranges).
bdunham7 is referring to a recent discussion where a similar issue was noticed on the Tek DMM6500.
https://www.eevblog.com/forum/testgear/keithley-dmm6500-strange-beahavor-measuring-transformer-resistance/

[Kleinstein]

The problem is only in the auto scale mode, so a bit less critical than with the DMM6500. Still the meter should ideally regognize that there is a problem. The software should be able to detect that reading after switching the current is not stable and show a warning instead of the wrong result.

With high inductance DUTs there is a known principle problem when switching ohms ranges. So this should not be such a surprize and it would make sense to consider this case in the software to detect it.

[2N3055]

Yeah it is to be expected, and agree meter might be able to handle it better..
Range switching (current switching) seems to have quite fast edge and there is counter EMF...
I always wondered why manufacturers don't make autoranging parameters at least partially user settable. At least autoranging speed.
This problems with autoranging seem to be more often lately, because of the race to make autoranging as fast as possible.

A slight tangent if you allow me. I'm usually not a stickler for terminology but a note:
Keysight shows "+Overload" in Ohms mode with open condition.
It is not an overload.
R&S OTOH shows "OVER RANGE" which is much better term for Ohms mode with open condition.

When you're in voltage and current mode then it should "Overload", with a red background on the screen, even a beeper.
In ohms mode, it is simply "Over range"

[I wanted a rude username]

Followup video showing a different failure mode of the same Keysight 34465A when measuring actual inductors (higher ESR than the electromagnet). Adding capacitance in parallel seems to be a workaround.

[Circlotron]

Put a scope across the electromagnet and see if the test setup is oscillating.