Keithley DMM6500 strange beahavor measuring transformer resistance

[ct1bxt]

Hi Guys,

When I measure the resistance of a transformer, tried with several, my DMM6500 does not show the correct value and when on auto scale does not change it.
With low inductance it shows the correct value.
As you can see per annex pictures the value should be 34 ohm and it shows 0.000003 Mohm.
If i change the scale to 100 ohm the multimeter stays overflow.
I´m using the last firmware.
Does anyone can simulate this issue?
Just send a request to Keithley support to see if they are aware of this problem

Regards
Rodrigo

[tooki]

As you can see per annex pictures the value should be 34 ohm and it shows 0.000003 Mohm.
If i change the scale to 100 ohm the multimeter stays overflow.

No, it shows 0.00003Mohm, which is... 30 ohms.

Maybe the Keithley doesn’t use a continuous current to measure resistance in the 100 ohm range. If it’s using pulses, then the inductive nature of the transformer could very much mess that up.

[bdunham7]

Hmmm.  What does it read if you manually select the 1k range?  Can you measure the voltage across the transformer with another meter when you are trying to get a reading in the 100R range?

[2N3055]

Use scope to look for the waveforms over DUT while measuring in 100 Ohm range. It probably uses pulses or even reverses current flow..
To measure DC resistance inductances (trafos, motors) there are special ohmmeter (or modes) for that purpose only. They use pure DC and use more current...
You need to read manual for DMM6500 how to disable offset null on measurement (if you can) or simply shoot known DC current through the winding and measure voltage drop.

But, mind you, Brymen has shown you quite good measurement. Do you need better than that? Value of resistance of that winding will vary with temperature quite a bit...
Or you just wanted to make sure why your DMM6500 is showing weird results?

[MegaVolt]

Check what value is in the parameter "offset compensation" should be OFF.

[ct1bxt]

Check what value is in the parameter "offset compensation" should be OFF.

Hi,

it´s off.

Tks

[ct1bxt]

Hmmm.  What does it read if you manually select the 1k range?  Can you measure the voltage across the transformer with another meter when you are trying to get a reading in the 100R range?

Hi,

there are some pulses, each one for different range as you can see by the annex pictures.
When measure resistances there are no pulses
It only happens with inductances, this one as 2,2H.
The lower frequency is 1 Kohm range and the other is 100 ohm.
Regards
Rodrigo

[bdunham7]

It looks to me like the inductance is making the current source circuit in the Keithley become unstable.  Three random thoughts:

1) I don't know if this makes sense--I haven't thought it through--but can you disable AutoZero?

2) Do you have a resistor in the 10-50 ohms range that you can measure and then put in series with the transformer?  Perhaps reducing Q will settle it down?

3) Have you tried 4W ohms?

[ct1bxt]

Use scope to look for the waveforms over DUT while measuring in 100 Ohm range. It probably uses pulses or even reverses current flow..
To measure DC resistance inductances (trafos, motors) there are special ohmmeter (or modes) for that purpose only. They use pure DC and use more current...
You need to read manual for DMM6500 how to disable offset null on measurement (if you can) or simply shoot known DC current through the winding and measure voltage drop.

But, mind you, Brymen has shown you quite good measurement. Do you need better than that? Value of resistance of that winding will vary with temperature quite a bit...
Or you just wanted to make sure why your DMM6500 is showing weird results?

Hi,

There some pulses on all ranges. I can see this only with indutancies. You can see up the pictures.
Yes, I have a brymen and also have an old fluke 189. But as every one, i presume, some times forget to switch power off the multimeter and the batteries go off. Also the angle of view is much better.
I checked on ebay and other sites for a second hand one, but they are very expensive. So I bought this one. 1K euro multimeter is crazy? Yes ah ah. But is the most used instrument !!
For a such quality multimeter and expensive it should NOT have this kind of problems!!

Rodrigo

[ct1bxt]

It looks to me like the inductance is making the current source circuit in the Keithley become unstable.  Three random thoughts:

1) I don't know if this makes sense--I haven't thought it through--but can you disable AutoZero?

2) Do you have a resistor in the 10-50 ohms range that you can measure and then put in series with the transformer?  Perhaps reducing Q will settle it down?

3) Have you tried 4W ohms?

Hi,

1- Yes, i tried that and same results.

2- Tested with 33 ohm serial resistor, same results.

3- Have checked yesterday and is the same as 2W.

tks
Rodrigo

[Kleinstein]

For the current source a moderate series resistance does not help very much with stability.  It would need quite a bit of series resistance or some RC in parallel to the DUT to stabilize the current source. An inductive load is indeed the critical case for a current source circuit. It is not at all simple to make the current source very accurate, reasonable fast to get offset compensated Ohms to be reasonable fast and stable with all inductive DUTs. It is kind of chose 2 of the three.

To measure the resistance on a transformer it is usually a good idea to short out one winding (of cause not the one just measured) to reduce the inductance / add damping.

There are reports about other meters also having problems with transformers and similar highly inductive DUTs, esepcially with auto ranging.

[ct1bxt]

It looks to me like the inductance is making the current source circuit in the Keithley become unstable.  Three random thoughts:


2) Do you have a resistor in the 10-50 ohms range that you can measure and then put in series with the transformer?  Perhaps reducing Q will settle it down?

Update

2- Parallel resistor 33 ohm with transformer, low Q, the multimeter work as he should. Show 16, 713 ohm

[bdunham7]

Use scope to look for the waveforms over DUT while measuring in 100 Ohm range. It probably uses pulses or even reverses current flow..
To measure DC resistance inductances (trafos, motors) there are special ohmmeter (or modes) for that purpose only. They use pure DC and use more current...
Or you just wanted to make sure why your DMM6500 is showing weird results?

There are milliohm meters and other devices that use much higher test currents, both AC and DC.  However, I've never seen a DMM that used anything other than a constant DC signal, either a constant current source measuring voltage drop across the DUT, a constant current source through the DUT and a parallel resistor (for high-resistance ranges) or a constant voltage across the DUT and a reference resistor connected in series.  Keithley simply specifies a test current of 1mA for the 100R and 1k ranges and I'd be quite surprised if it was designed to be anything other than a 1mA constant DC source.  Of course that isn't what this DMM6500 is doing at the moment, and I certainly would want to know why if it were mine.

Just as a sanity test, I grabbed a similar transformer and measured 2.05H inductance, then checked it with the three most convenient devices I had that have good lo-ohms capability and they all measured 39.55-ish ohms without much fuss.  The OP's meter is behaving strangely, I think.

[bdunham7]

For the current source a moderate series resistance does not help very much with stability.

True, but I wanted to see if it was an edge case where it was just barely unstable with an unusually large high-Q inductor.  Apparently not, it seems pretty unstable in general. I expected a parallel resistor to stabilize it--in fact the 10M range that he reported as stable uses a parallel 10M resistor, so maybe that's the reason that range worked.

There are reports about other meters also having problems with transformers and similar highly inductive DUTs, esepcially with auto ranging.

Well, with a 2.05H  39.5R 56VA 230VAC transformer primary, a Fluke 8846A in AUTO, a Fluke 289 (in Lo-Ohms and AUTO) and a BK 886 LCR meter all had no issues or instability.  I can try more, but I would expect them all to work properly.  It wouldn't be uncommon to test transformer primaries for open circuits and you might not want to go the trouble of disconnecting or shorting everything on the secondary side.  I would classify this as a bug, or at least a quirk that the user should know about.

[2N3055]

Use scope to look for the waveforms over DUT while measuring in 100 Ohm range. It probably uses pulses or even reverses current flow..
To measure DC resistance inductances (trafos, motors) there are special ohmmeter (or modes) for that purpose only. They use pure DC and use more current...
Or you just wanted to make sure why your DMM6500 is showing weird results?

There are milliohm meters and other devices that use much higher test currents, both AC and DC.  However, I've never seen a DMM that used anything other than a constant DC signal, either a constant current source measuring voltage drop across the DUT, a constant current source through the DUT and a parallel resistor (for high-resistance ranges) or a constant voltage across the DUT and a reference resistor connected in series.  Keithley simply specifies a test current of 1mA for the 100R and 1k ranges and I'd be quite surprised if it was designed to be anything other than a 1mA constant DC source.  Of course that isn't what this DMM6500 is doing at the moment, and I certainly would want to know why if it were mine.

Well, fact that you didn't seen it doesn't mean they don't exist. In fact, it is very common to have "offset compensation" that will either alternate polarity of current and then cancel out any residual or to simply switch measurement current ON/OFF and measure voltage with and without and cancel out residual that way.  Rigol DM3068 has it, Keysight 34465A/70A has it, Keithley DMM6500 has it... I'm sure many more have it.  On these mentioned, what it does is that current source is being switched between 2 different current levels and residual is nulled. To user that looks like squarewave current shape. That will induce nice spikes in large inductance, confusing the DMM....

[bdunham7]

Well, fact that you didn't seen it doesn't mean they don't exist. In fact, it is very common to have "offset compensation" that will either alternate polarity of current and then cancel out any residual or to simply switch measurement current ON/OFF and measure voltage with and without and cancel out residual that way.  Rigol DM3068 has it, Keysight 34465A/70A has it, Keithley DMM6500 has it... I'm sure many more have it.  On these mentioned, what it does is that current source is being switched between 2 different current levels and residual is nulled. To user that looks like squarewave current shape. That will induce nice spikes in large inductance, confusing the DMM....

I wasn't thinking of autozero or 'offset compensation', that probably isn't unusual--but I think reversing the polarity would be.  Any examples of that in a DMM? In any case, I would expect that to occur at a very slow rate if it was even continuous, not 25Hz--although, as you point out, I've not seen everything.  I have seen autozeroing perturbations though--that is a common enough issue. Since measuring the resistance of a large inductor is a common task that every meter I have appears to do just fine, I would consider it a design error, or at least a poor choice, if a situation like this was caused by overly rapid autozeroing or the like.  It appears in this case that the problem has nothing to do with that, however, since the OP reports that autozero and offset compensation are off and a plain resistor shows a steady source signal.

[ct1bxt]

I found that when I was check for continuity of a fluke 199c power supply.
That is a ordinary check i do whit transformer as every body do using DMM !!

Rodrigo

[Bravo]

I have one of these meters which i bought new from the local agent. It turned out to be faulty & was sent back to USA to be repaired & have the battery replaced. The battery was flat because it had been in stock for 18 months.  The warranty will only be honored if Tektronix USA replaces the battery !!!! The best part was it came back to the agent in Australia & the battery had not been replaced !!!!  So I had use of the demo one for a while until a new one came off the production line, & I asked for a full calibration certificate.  I have sent a link to Vicom Australia about issue. Hopefully they may provide some info, & at least confirm if the unit is faulty. I will do a test with mine tomorrow & see what happens.

[Bravo]

Today I measured 2 transformers with Brymen 869 & DMM6500. My DMM6500 was new in December.
Both units had their leads shorted & the reading nulled.
The first transformer was a 600 Ohm Audio transformer type VTX131-001 from Element 14. The DC spec is 68 Ohm +- 15%
The Brymen showed the resistance to be 63.1 Ohms on both Auto & Ohms range.  The DM6500 showed the resistance to be 0.06397K on Auto range, Over range on 100 Ohm range & 0.06403 K Ohm on the 1K range. 
The second transformer was a mains toroid transformer.
The Brymen showed the primary as 38.65 Ohms  & the secondary at 0.73 Ohms
The DMM6500 showed the primary as 39.02 Ohms on  100 Ohms range, 0.03902 K on the 1K range & 0.009xxxM on auto.
The DMM6500 showed secondary as 0.82238 Ohms on the 10 Ohms range, 0.8286 Ohms on the 100 Ohm range, 0.000829 K on the 1 K range & 0.000146 M  on Auto.

Obviously the auto range readings of low restance on the DMM6500 are more or less useless.   

[tooki]

Obviously the auto range readings of low restance on the DMM6500 are more or less useless.   

Quite a bold claim, that a Keithley meter would be “useless” on precisely a feature Keithley is famous for.

I think this is more accurately characterized as bumping into a “gotcha” of a high performance device.

If manually ranged readings are always right, but auto aren’t when measuring a transformer, why do you think that might be? Think about it. What’s different about autoranging compared to manual?

I have a strong suspicion about it, and it has to do with a key feature of high end meters. And it means that normal resistors will not be affected.

[2N3055]

Today I measured 2 transformers with Brymen 869 & DMM6500. My DMM6500 was new in December.
Both units had their leads shorted & the reading nulled.
The first transformer was a 600 Ohm Audio transformer type VTX131-001 from Element 14. The DC spec is 68 Ohm +- 15%
The Brymen showed the resistance to be 63.1 Ohms on both Auto & Ohms range.  The DM6500 showed the resistance to be 0.06397K on Auto range, Over range on 100 Ohm range & 0.06403 K Ohm on the 1K range. 
The second transformer was a mains toroid transformer.
The Brymen showed the primary as 38.65 Ohms  & the secondary at 0.73 Ohms
The DMM6500 showed the primary as 39.02 Ohms on  100 Ohms range, 0.03902 K on the 1K range & 0.009xxxM on auto.
The DMM6500 showed secondary as 0.82238 Ohms on the 10 Ohms range, 0.8286 Ohms on the 100 Ohm range, 0.000829 K on the 1 K range & 0.000146 M  on Auto.

Obviously the auto range readings of low restance on the DMM6500 are more or less useless.   

And now try these 2 things:

1. When measuring primary resistance, short the secondary. Also , when measuring secondary, short the primary on both transformers. Measure all the same as before and write down.
2. Go into setup and disable "Offset compensation" (consult manual for details). Repeat measurements and write them down.

Then report back.

[bdunham7]

Quite a bold claim, that a Keithley meter would be “useless” on precisely a feature Keithley is famous for.

I think this is more accurately characterized as bumping into a “gotcha” of a high performance device.

If manually ranged readings are always right, but auto aren’t when measuring a transformer, why do you think that might be? Think about it. What’s different about autoranging compared to manual?

I have a strong suspicion about it, and it has to do with a key feature of high end meters. And it means that normal resistors will not be affected.

I have to disagree because the way I use my primary bench meter on a service bench, this would be a nearly fatal flaw.  All test equipment has quirks and flaws and I generally prefer to get to know these 'characteristics' so that I can get the best use out of a device, but sometimes manufacturers need to be called out when they screw up.  And this isn't (solely) an autorange issue, it clearly poses problems on the 100 ohm manual range.  The manual readings are not always right.

So lets say I have a mains transformer powered device that isn't working and seems completely dead.  The first thing I'll do is plug it into my iso-variac device, dial in 120V and check the current.  If there is no current draw, I might pop the top or check any external fuses and maybe test at the line cord plug for continuity.  This meter might show me a false open here.  So I would quite quickly zero in on the transformer and I might verify that I have 120VAC to the primary of the transformer.  On many devices, the secondaries and low voltage circuitry can be very difficult to get to, so the natural next step is to isolate the primary--often no work at all needed--and test it for continuity.  This meter shows me an open or some strange autoranging up into the megohm ranges, so I manually range down to 100R and get.....open!  So I conclude that the transformer is bad.  Oops!

The bottom line is that you can't trust the meter in certain (not uncommon) cases.  That makes it useless in any situation where one of those cases might apply.  The key for a service bench meter is how much confidence you have that it won't lie to you.  Nobody is perfect, but reading an ordinary power supply transformer as an open when it isn't is a pretty big error, one that could cost money.  More to the point, none of the other 8 or 9 meters I tested (with 2 and 7H inductors), including 3 or 4 that are roughly comparable to the DMM6500 in precision, made that error.  So how can I trust any of its DCR readings in unknown circuits?  In cases where it does have a stable, plausible reading--is it accurate?

[ct1bxt]

Today I measured 2 transformers with Brymen 869 & DMM6500. My DMM6500 was new in December.
Both units had their leads shorted & the reading nulled.
The first transformer was a 600 Ohm Audio transformer type VTX131-001 from Element 14. The DC spec is 68 Ohm +- 15%
The Brymen showed the resistance to be 63.1 Ohms on both Auto & Ohms range.  The DM6500 showed the resistance to be 0.06397K on Auto range, Over range on 100 Ohm range & 0.06403 K Ohm on the 1K range. 
The second transformer was a mains toroid transformer.
The Brymen showed the primary as 38.65 Ohms  & the secondary at 0.73 Ohms
The DMM6500 showed the primary as 39.02 Ohms on  100 Ohms range, 0.03902 K on the 1K range & 0.009xxxM on auto.
The DMM6500 showed secondary as 0.82238 Ohms on the 10 Ohms range, 0.8286 Ohms on the 100 Ohm range, 0.000829 K on the 1 K range & 0.000146 M  on Auto.

Obviously the auto range readings of low restance on the DMM6500 are more or less useless.   

And now try these 2 things:

1. When measuring primary resistance, short the secondary. Also , when measuring secondary, short the primary on both transformers. Measure all the same as before and write down.
2. Go into setup and disable "Offset compensation" (consult manual for details). Repeat measurements and write them down.

Then report back.

1- With a short circuit on secondary ranges below, and including 100K, show OVERFLOW. Above that: 1M 0.000035, 10M .0000003 and 100M 000.0000.
     AUTO range, whit short circuit 0.000035 Mohm and no short 00.0003 Mohm.
    The offset compensation is OFF by deault and can´t change.


Rodrigo
   

[2N3055]

Today I measured 2 transformers with Brymen 869 & DMM6500. My DMM6500 was new in December.
Both units had their leads shorted & the reading nulled.
The first transformer was a 600 Ohm Audio transformer type VTX131-001 from Element 14. The DC spec is 68 Ohm +- 15%
The Brymen showed the resistance to be 63.1 Ohms on both Auto & Ohms range.  The DM6500 showed the resistance to be 0.06397K on Auto range, Over range on 100 Ohm range & 0.06403 K Ohm on the 1K range. 
The second transformer was a mains toroid transformer.
The Brymen showed the primary as 38.65 Ohms  & the secondary at 0.73 Ohms
The DMM6500 showed the primary as 39.02 Ohms on  100 Ohms range, 0.03902 K on the 1K range & 0.009xxxM on auto.
The DMM6500 showed secondary as 0.82238 Ohms on the 10 Ohms range, 0.8286 Ohms on the 100 Ohm range, 0.000829 K on the 1 K range & 0.000146 M  on Auto.

Obviously the auto range readings of low restance on the DMM6500 are more or less useless.   

And now try these 2 things:

1. When measuring primary resistance, short the secondary. Also , when measuring secondary, short the primary on both transformers. Measure all the same as before and write down.
2. Go into setup and disable "Offset compensation" (consult manual for details). Repeat measurements and write them down.

Then report back.

1- With a short circuit on secondary ranges below, and including 100K, show OVERFLOW. Above that: 1M 0.000035, 10M .0000003 and 100M 000.0000.
     AUTO range, whit short circuit 0.000035 Mohm and no short 00.0003 Mohm.
    The offset compensation is OFF by deault and can´t change.


Rodrigo
 

Rodrigo,

thank you for that, that is very useful information.. There might be a problem that current source in Keithley DMM6500 is unstable with inductive load... That should be checked with scope and manual ranging, to see if there is oscillations with current going trough trafo. In auto ranging, switching between ranges changes current which can create problems..
You could try to connect a 10uF elco in paralel with trafo and check again.

[ct1bxt]

On reply #6 you can view what is on test leads.