Keithley DMM6500 strange beahavor measuring transformer resistance

[2N3055]

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

Yes thank you, didn't see that, sorry! 
That probably means current source is unstable with inductive load. Try connecting capacitor in parallel to see what happens ( also with scope connected.).

[bdunham7]

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

Since the 100R test current is 1mA, there should be 34mV DC.  Your scope is AC coupled and shows what appears to be an envelope of a higher-frequency signal.  If you're willing to take the time, I'd be interested in seeing what that signal looks like expanded out to at least 1ms/div and perhaps 100us/div and 10us/div, and perhaps single-shot, all DC coupled.  I really couldn't say what is going on here--it does appear to me that in addition to being unstable, it is probing the DUT with some sort of intentional, repeated signal.  I hope that Keithley engineer isn't too scared to come back and visit.

[Bravo]

Apologies for my bold statement. It should of had "when measuring inductance" included in the sentence. 
The DMM6500 Reference manual  explains resistance measurement on Page 4.69.  On the 1 ohm to 1M ohm ranges it uses constant current measurement, & on 10 M ohm & 100M ohm it uses the ratiometric  method.

[tautech]

Apologies for my bold statement.

LOL. Nothing like the ones you told me after having to send it back to the factory for a dead battery and getting it back after months unchanged !
Abbreviated they were:   
 

[Kleinstein]

Mesurung highly inductive DUTs is always a problem in the ohms mode. Ideally a meter that already has a capacitance mode should be able to tell if the DUT is a resistor, capacitor or inductor. So from the HW side it should be possible to also measure at least higher value inductance, even if with a rather large uncertainty.  If needed the resitance mode could adjust in a way that it can tolerate inductance better - even though this would likely mean a slower reading. It is much better to get an approximate inductance and warning about increased uncertainty / lower speed than just a resistance reading with incresed error, but no warning.

The problem with high inductance would effect other meters too - at least in the mode with offset compensation, that switches the test current.

[jonpaul]

Bonjour, WOW overkill...Like  using a .357 to kill a fly....


we   test transformers and inductors on a bridge set to DCR with 4 terminal leads if needed.

For rough measurements, an analog VOM like Simpson 260 or any hand-held DVM will have DC and not pulses.

Suggest the  Chinese Red DVM at Harbor Freight $5.

Good enough for transformer DCR!

Enjoy,

Jon

[bdunham7]

Mesurung highly inductive DUTs is always a problem in the ohms mode.

The problem with high inductance would effect other meters too - at least in the mode with offset compensation, that switches the test current.

Are there any other DMMs known to have this issue?  I've tried over a dozen, including some old ones that use the series comparison method, none have an issue even with 7H.  I don't have a larger inductor handy.

[bdunham7]

Suggest the  Chinese Red DVM at Harbor Freight $5.

Good enough for transformer DCR!

Maybe Keithley should include a D830 in the box just for a second opinion when their $1K+ DMM is wrong. 

"Our fancy, delicate meter sometimes makes some silly errors while attempting to use the latest technology for those ultra precise readings you crave.  Please use the secondary meter to verify that it hasn't completely screwed up."

[MegaVolt]

If offset compensation is disabled, then there is one more option that can interrupt the current with some frequency: AutoZero. It might be worth turning it off, too.

[leighcorrigall]

Sorry to hear about the bad news. My Keithley DMM6500 spent more time in the shop than I got to use it. It failed 3 times and each time it needed a new board. Get a reliable DMM if you can get a return on your instrument. Tektronix made Keithleys are no good in my opinion.

[ct1bxt]

If offset compensation is disabled, then there is one more option that can interrupt the current with some frequency: AutoZero. It might be worth turning it off, too.

Sorry for delay.
Same values with AutoZero OFF

Rodrigo

[ct1bxt]

Sorry to hear about the bad news. My Keithley DMM6500 spent more time in the shop than I got to use it. It failed 3 times and each time it needed a new board. Get a reliable DMM if you can get a return on your instrument. Tektronix made Keithleys are no good in my opinion.

Hi,

not very happy to hear that.
The DMM is almost new, 2 months.

Regards
Rodrigo

[ct1bxt]

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

Since the 100R test current is 1mA, there should be 34mV DC.  Your scope is AC coupled and shows what appears to be an envelope of a higher-frequency signal.  If you're willing to take the time, I'd be interested in seeing what that signal looks like expanded out to at least 1ms/div and perhaps 100us/div and 10us/div, and perhaps single-shot, all DC coupled.  I really couldn't say what is going on here--it does appear to me that in addition to being unstable, it is probing the DUT with some sort of intentional, repeated signal.  I hope that Keithley engineer isn't too scared to come back and visit.

Hi,

Sorry for delay.
100us  DC coupling with AutoZero OFF and ON.

Regards
Rodrigo

[Kleinstein]

The oscillation is quite fast (some 25 kHz) and as it looks in the 2nd picture not very stable in frequency. This could be instability in the current source but maybe also some picup from a SMPS. 25 kHz suggstes that it does no need a really large transformer / high inductance to cause trouble.

In AZ or 4 wire mode the voltmeter input is switched to the input / low side. The loading from the voltage input could be enough to stabilze the current source.

If it is really the instablity of the current source, this would be a kind of bug that needs fixing. It may not be easy, as a change to the current source would also effect the capacitance measurement. Still this would be a point Keithley should fix. Chances are this is not an individual unit, but more like a design flaw.   

[Hydron]

I'll put mine (well, the loan unit I have) on the scope with a few different transformers connected and have a look too. I know it definitely switches the current source level in 4-wire mode at 10Hz or so (needed to do offset compensation) which could cause some obvious issues measuring a mains-freq transformer, but it sounds like this would be an issue in 2-wire mode as well, which is more concerning.

[ct1bxt]

Bonjour, WOW overkill...Like  using a .357 to kill a fly....


we   test transformers and inductors on a bridge set to DCR with 4 terminal leads if needed.

For rough measurements, an analog VOM like Simpson 260 or any hand-held DVM will have DC and not pulses.

Suggest the  Chinese Red DVM at Harbor Freight $5.

Good enough for transformer DCR!

Enjoy,

Jon

Hi,

I have several DMM and also LCR.
For doing what what you propose I need to have all of them working at the same time!!

Rodrigo

[macaba]

With regards to auto-ranging not ranging down to the appropriate range - I've seen this on DC voltage measurements.
The cause: the auto-ranging circuitry is measuring the signal from the high bandwidth input amplifier (high bandwidth to support the digitize modes), so in my case it's picking up mains noise that has a larger magnitude than my DC voltage measurement.

[ct1bxt]

Hi,

After exchanging emails with Keithley support, a senior technician finally responded.
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.
Several forum members have tested it with other DMMs and none have this problem. Even other Keithley models.
Another small problem that DMM has is the transformer. In quiet environments and with the DMM turned off, the hum of the transformer can be clearly heard.
Its quality doesn't seem the best to me and its location inside the box doesn't help much either.

Rodrigo

[bdunham7]

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

Could you repeat, as best you can, the reason that he gave or his description of their measurement technique?

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? 

[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.

[2N3055]

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.

[Kleinstein]

Getting the current source (with multiple ranges) stable with all inductive loads is not easy and may result in a relatively slow settling. Because of the faster digitizing mode they may not want this. Also the offest compensated ohms mode would need more settling time.

It depends on the inductor - not sure other DMMs are really stable with all inductors, it just looks like more tolerant. A normal (classical iron core) transformer is still not a very low loss inductor in the relevant frequency range.
Better tolerance to inductors may be a point in a future hardware update.

With an inductor there is a chance to create high voltages when the current is turned off (e.g. change of range). So this may also test the protection circuit.

[KedasProbe]

You should not use auto range otherwise you will get check pulses even if it holds on the the same range.
I did some checking 100ohm isn't working on my 67 Ohm coil, but 1kOhm range works.

Did you try the diode current source?
I tried 10mA without problem but I don't have a very big inductance to test it.
(if you need to do a lot of these you can use the linear conversion to multiply by 100 to get ohm)

You can also just put an 10V DC on it and measure the current. (assuming that is ok)
If it doesn't have to be super accurate just divide the two numbers displayed on your power supply.

[AVGresponding]

You should not use auto range otherwise you will get check pulses even if it holds on the the same range.
I did some checking 100ohm isn't working on my 67 Ohm coil, but 1komh ranges works.

Did you try the diode current source?
I tried 10mA without problem but I don't have a very big inductance to test it.

You can also just put an 10V DC on it and measure the current. (assuming that is ok)
If it doesn't have to be super accurate just divide the two numbers displayed on your power supply.

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.

[Kleinstein]

A way to judge the winding temperature is too look at the resistance. So the copper resistance can be used as a sensor. AFAIK this is the suggested method to determine the nominal power of a transformer: the maximum loss is set by the maximum allowed relativ increase of winding resistands corresponding to the temperature the isolation is supposed to be rated for. In this case one wants a reasonable good accuracy for the resistance and may very well need a 4 wire restance measurement.

It is true that many checks on a transformer are just simpel go / no good (e.g. the continuity beeper).  It is still nice if the meter also works in this case. At least the problem is more like an unexpected overflow and not a completely wrong reading with no warning.