Keithley DMM6500 strange beahavor measuring transformer resistance

[bdunham7]

You could easily also included a precisely known resistor in series to limit the current using this method.

I must admit I am a little confused as to why you would use a high-end DMM to provide what is effectively a go/no-go measurement on a transformer.

He did, it didn't help in series but it worked in parallel.  (Replies 9, 11)

I use my 'high-end' service bench meter to measure anything and everything and I expect it to do it quickly and accurately without any hiccups.  That is why it is on my bench.  Whether I need all 6.5 digits is beside the point--after all these meters have continuity beepers, right?

[ct1bxt]

He explained the technique used to measure resistances and assumed that the meter had problems measuring resistances on coils.

It seems that the solution will be to buy another DMM from another brand!!!
It is a pity that a DMM of this quality presents this type of problem.

Did they suggest that solution?  It is always disappointing when precision test gear doesn't measure up, but there are always strong points, weak points and necessary compromises.  I just wonder what happened--is this the result of a compromise in respect to another performance parameter, cost, or something else?

1- He asked me not to copy and paste his email, which I intend to.
Basically explained that DMM uses current sources to transform resistance into voltage. This technique can introduce oscillations in inductive loads.
I know that. I think other manufacturers do too and don't have this kind of problem.

2-I suggested this solution, buy another brand.
I understand the compromise situation. However this is a basic measure. I've already tested it with some relays and the problem persists although with Auto Zero OFF the multimeter works better.

EVERY current measurement includes using current source and measuring voltage drop..
It's just their implementation of current source is not stable with inductive load.

They just have BAD implementation... It is normal to assume people will measure resistance of all kinds of inductive things...

If Rigol DM3068 can do it right so can they.
At this point, with all the software problems over the years, and this, I'm really glad I didn't buy DM6500 to "upgrade" my DM3068.

100% agree.
The Dmm6500 is very fast, no doubt, but then present these problems that in my opinion, as I said here, is unacceptable for an equipment of this price and supposed quality !!
Keep your RIGOL!!!
It's not that quick to measure, true, but at least it measures the resistance of any coil without a problem.
A touch display always looks good on the bench. But that's it!!!!

[AVGresponding]

You could easily also included a precisely known resistor in series to limit the current using this method.

I must admit I am a little confused as to why you would use a high-end DMM to provide what is effectively a go/no-go measurement on a transformer.

He did, it didn't help in series but it worked in parallel.  (Replies 9, 11)

I use my 'high-end' service bench meter to measure anything and everything and I expect it to do it quickly and accurately without any hiccups.  That is why it is on my bench.  Whether I need all 6.5 digits is beside the point--after all these meters have continuity beepers, right?

You misunderstood me. I was referring to the method of connecting it to a 10V supply and measuring the current to determine the resistance.

[Caliaxy]

No tool can do it all. In this case, DMM6550 clearly fails measuring R in inductive loads. My old Fluke 8842A doesn’t have any issue with that.

However, it’s not as bad as it seems. It looks to me like the issue is mostly related to autoranging, rather than to the measurement itself. Once set in manual range, the measurement seems quite accurate (according to my quick tests).

I measured the primary coil of a mains transformer, checked as follows:

L = 2.2 H @ 100 Hz (DE-5000)

DCR = 63.19 ohms (Fluke 87V)
DCR = 63.10 ohms (Brymen 789)

R = 63.104 ohms (old Fluke 8842A, autorange, with no issues)

DMM6550 measured as follows:

R = 00.00006 M (autorange) (actually ranging from 00.00004 M to 00.00007 M)
R = 00.00006 M (10M manual range)
R = 0.000067 M (1M manual range)
R= 00.06318 K (100K manual range)
R = 0.063183 K (10K manual range)
R = 0.063173 K (1K manual range)
Overflow (100 manual range and under)

The most interesting things happened when I checked the signal with an oscilloscope.

In autorange, DMM6550 shows some huge (40Vpp) spikes, which miraculously disappear in manual range.

Autorange:







Manual range:



In manual range, the magnified signal looks very similar with the on from Fluke 8842:
Edit: LED lamp artifact (Thanks, Kleinstein).

Might be wishful thinking, but what is the chance this is just another fixable software bug?

[Kleinstein]

There is not much chance that the software can do much about the oscillation when actually in a resitance mode. The software could reduce the constant switching between ranges when in auto-ranging and maybe give a warning, to suggest going to manual (still tricky as a variable reading may also be due to a changing resustance by the user doing things).

For the oscillation, when in the actuall current driving mode, there is not much the SW can do. This would need a HW fix (like a changed capacitor value, maybe added RC or such a thing).

Getting nearly the same signal for the Keithly and Fluke meter is a bit strage. Maybe there is not oscillation, but just a pick up of some EMI from the environment. In this case the DMM may be be at fault at all. It could be just a LED lamp or notebook supply, or maybe the scope to produce such a signal. So maybe check with just the coild at the scope, without the meter.

[Caliaxy]

Getting nearly the same signal for the Keithly and Fluke meter is a bit strage. Maybe there is not oscillation, but just a pick up of some EMI from the environment. In this case the DMM may be be at fault at all. It could be just a LED lamp or notebook supply, or maybe the scope to produce such a signal. So maybe check with just the coild at the scope, without the meter.

You're right. Darn LED lamp...

[ct1bxt]

There is not much chance that the software can do much about the oscillation when actually in a resitance mode. The software could reduce the constant switching between ranges when in auto-ranging and maybe give a warning, to suggest going to manual (still tricky as a variable reading may also be due to a changing resustance by the user doing things).

For the oscillation, when in the actuall current driving mode, there is not much the SW can do. This would need a HW fix (like a changed capacitor value, maybe added RC or such a thing).

Getting nearly the same signal for the Keithly and Fluke meter is a bit strage. Maybe there is not oscillation, but just a pick up of some EMI from the environment. In this case the DMM may be be at fault at all. It could be just a LED lamp or notebook supply, or maybe the scope to produce such a signal. So maybe check with just the coild at the scope, without the meter.

you are right. It looks more like a HW problem than a SW one.
It was good that keithley assumed they have this problem with the DMM6500 !

Rodrigo

[KedasProbe]

The auto range check pulses are the smaller wider bumps on your scope images between your (noise) spikes. It's like a relative short sin burst every 42-43ms if I remember correctly.

[ct1bxt]

Hi,

a new video showing the issue, not mine.
Here: