Keithley DMM6500 strange beahavor measuring transformer resistance - Page 1
Forum section: electronics / testgear

author: ct1bxt
author_rank: Contributor
timestamp: June 30, 2021, 12:56:43 am
content: 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 problemRegardsRodrigo

author: tooki
author_rank: Super Contributor
timestamp: June 30, 2021, 02:01:03 am
content: Quote from: ct1bxt on June 30, 2021, 12:56:43 amNo, 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.

author: bdunham7
author_rank: Super Contributor
timestamp: June 30, 2021, 02:12:09 am
content: 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?

author: 2N3055
author_rank: Super Contributor
timestamp: June 30, 2021, 06:16:43 am
content: 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?

author: MegaVolt
author_rank: Frequent Contributor
timestamp: June 30, 2021, 07:59:01 am
content: Check what value is in the parameter "offset compensation" should be OFF.

author: ct1bxt
author_rank: Contributor
timestamp: June 30, 2021, 01:04:42 pm
content: Quote from: MegaVolt on June 30, 2021, 07:59:01 amHi,it´s off.Tks

author: ct1bxt
author_rank: Contributor
timestamp: June 30, 2021, 04:33:30 pm
content: Quote from: bdunham7 on June 30, 2021, 02:12:09 amHi,there are some pulses, each one for different range as you can see by the annex pictures.When measure resistances there are no pulsesIt only happens with inductances, this one as 2,2H.The lower frequency is 1 Kohm range and the other is 100 ohm.RegardsRodrigo

author: bdunham7
author_rank: Super Contributor
timestamp: June 30, 2021, 04:48:28 pm
content: 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?

author: ct1bxt
author_rank: Contributor
timestamp: June 30, 2021, 04:57:54 pm
content: Quote from: 2N3055 on June 30, 2021, 06:16:43 amHi,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

author: ct1bxt
author_rank: Contributor
timestamp: June 30, 2021, 05:06:39 pm
content: Quote from: bdunham7 on June 30, 2021, 04:48:28 pmHi,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.tksRodrigo

author: Kleinstein
author_rank: Super Contributor
timestamp: June 30, 2021, 05:14:01 pm
content: 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.

author: ct1bxt
author_rank: Contributor
timestamp: June 30, 2021, 05:15:09 pm
content: Quote from: bdunham7 on June 30, 2021, 04:48:28 pmUpdate2- Parallel resistor 33 ohm with transformer, low Q, the multimeter work as he should. Show 16, 713 ohm

author: bdunham7
author_rank: Super Contributor
timestamp: June 30, 2021, 05:18:48 pm
content: Quote from: 2N3055 on June 30, 2021, 06:16:43 amThere 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.

author: bdunham7
author_rank: Super Contributor
timestamp: June 30, 2021, 05:34:48 pm
content: Quote from: Kleinstein on June 30, 2021, 05:14:01 pmTrue, 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.QuoteWell, 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.

author: 2N3055
author_rank: Super Contributor
timestamp: June 30, 2021, 08:46:59 pm
content: Quote from: bdunham7 on June 30, 2021, 05:18:48 pmWell, 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....

author: bdunham7
author_rank: Super Contributor
timestamp: June 30, 2021, 10:16:05 pm
content: Quote from: 2N3055 on June 30, 2021, 08:46:59 pmI 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.

author: ct1bxt
author_rank: Contributor
timestamp: July 01, 2021, 02:41:21 am
content: 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

author: Bravo
author_rank: Contributor
timestamp: July 01, 2021, 12:47:17 pm
content: 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.

author: Bravo
author_rank: Contributor
timestamp: July 02, 2021, 04:26:48 am
content: 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 OhmsThe 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.

author: tooki
author_rank: Super Contributor
timestamp: July 02, 2021, 05:35:42 am
content: Quote from: Bravo on July 02, 2021, 04:26:48 amQuite 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.

author: 2N3055
author_rank: Super Contributor
timestamp: July 02, 2021, 09:04:18 am
content: Quote from: Bravo on July 02, 2021, 04:26:48 amAnd 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.

author: bdunham7
author_rank: Super Contributor
timestamp: July 02, 2021, 02:22:51 pm
content: Quote from: tooki on July 02, 2021, 05:35:42 amI 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?

author: ct1bxt
author_rank: Contributor
timestamp: July 02, 2021, 10:42:00 pm
content: Quote from: 2N3055 on July 02, 2021, 09:04:18 am1- 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

author: 2N3055
author_rank: Super Contributor
timestamp: July 02, 2021, 10:50:09 pm
content: Quote from: ct1bxt on July 02, 2021, 10:42:00 pmRodrigo,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.

author: ct1bxt
author_rank: Contributor
timestamp: July 02, 2021, 10:57:33 pm
content: On reply #6 you can view what is on test leads.

author: 2N3055
author_rank: Super Contributor
timestamp: July 02, 2021, 11:38:27 pm
content: Quote from: ct1bxt on July 02, 2021, 10:57:33 pmYes 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.).

author: bdunham7
author_rank: Super Contributor
timestamp: July 02, 2021, 11:56:22 pm
content: Quote from: ct1bxt on July 02, 2021, 10:57:33 pmSince 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.

author: Bravo
author_rank: Contributor
timestamp: July 03, 2021, 08:39:06 am
content: 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.

author: tautech
author_rank: Super Contributor
timestamp: July 03, 2021, 08:51:30 am
content: Quote from: Bravo on July 03, 2021, 08:39:06 amLOL. 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:

author: Kleinstein
author_rank: Super Contributor
timestamp: July 03, 2021, 08:59:16 am
content: 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.

author: jonpaul
author_rank: Super Contributor
timestamp: July 03, 2021, 09:10:24 am
content: 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

author: bdunham7
author_rank: Super Contributor
timestamp: July 03, 2021, 03:23:19 pm
content: Quote from: Kleinstein on July 03, 2021, 08:59:16 amAre 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.

author: bdunham7
author_rank: Super Contributor
timestamp: July 03, 2021, 03:30:54 pm
content: Quote from: jonpaul on July 03, 2021, 09:10:24 amMaybe 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."

author: MegaVolt
author_rank: Frequent Contributor
timestamp: July 05, 2021, 08:02:44 am
content: 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.

author: leighcorrigall
author_rank: Frequent Contributor
timestamp: July 05, 2021, 06:53:13 pm
content: 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.

author: ct1bxt
author_rank: Contributor
timestamp: July 05, 2021, 10:46:15 pm
content: Quote from: MegaVolt on July 05, 2021, 08:02:44 amSorry for delay.Same values with AutoZero OFFRodrigo

author: ct1bxt
author_rank: Contributor
timestamp: July 05, 2021, 10:48:26 pm
content: Quote from: leighcorrigall on July 05, 2021, 06:53:13 pmHi,not very happy to hear that.The DMM is almost new, 2 months.RegardsRodrigo

author: ct1bxt
author_rank: Contributor
timestamp: July 05, 2021, 11:15:28 pm
content: Quote from: bdunham7 on July 02, 2021, 11:56:22 pmHi,Sorry for delay.100us DC coupling with AutoZero OFF and ON.RegardsRodrigo

author: Kleinstein
author_rank: Super Contributor
timestamp: July 06, 2021, 08:42:56 am
content: 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.

author: Hydron
author_rank: Super Contributor
timestamp: July 06, 2021, 03:36:10 pm
content: 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.

author: ct1bxt
author_rank: Contributor
timestamp: July 07, 2021, 11:14:54 pm
content: Quote from: jonpaul on July 03, 2021, 09:10:24 amHi,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

author: macaba
author_rank: Regular Contributor
timestamp: July 08, 2021, 09:46:16 am
content: 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.

author: ct1bxt
author_rank: Contributor
timestamp: July 25, 2021, 12:03:19 am
content: 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

author: bdunham7
author_rank: Super Contributor
timestamp: July 25, 2021, 12:24:01 am
content: Quote from: ct1bxt on July 25, 2021, 12:03:19 amCould you repeat, as best you can, the reason that he gave or his description of their measurement technique?QuoteDid 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?

author: ct1bxt
author_rank: Contributor
timestamp: July 25, 2021, 12:52:30 am
content: Quote from: bdunham7 on July 25, 2021, 12:24:01 am1- 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.

author: 2N3055
author_rank: Super Contributor
timestamp: July 25, 2021, 09:19:17 am
content: Quote from: ct1bxt on July 25, 2021, 12:52:30 amEVERY 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.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 25, 2021, 09:34:06 am
content: 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.

author: KedasProbe
author_rank: Frequent Contributor
timestamp: July 25, 2021, 10:32:22 am
content: 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.

author: AVGresponding
author_rank: Super Contributor
timestamp: July 25, 2021, 10:38:30 am
content: Quote from: KedasProbe on July 25, 2021, 10:32:22 amYou 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.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 25, 2021, 12:13:12 pm
content: 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.

author: bdunham7
author_rank: Super Contributor
timestamp: July 25, 2021, 01:46:22 pm
content: Quote from: AVGresponding on July 25, 2021, 10:38:30 amHe 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?

author: ct1bxt
author_rank: Contributor
timestamp: July 25, 2021, 04:14:04 pm
content: Quote from: 2N3055 on July 25, 2021, 09:19:17 am100% 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!!!!

author: AVGresponding
author_rank: Super Contributor
timestamp: July 25, 2021, 07:26:14 pm
content: Quote from: bdunham7 on July 25, 2021, 01:46:22 pmYou misunderstood me. I was referring to the method of connecting it to a 10V supply and measuring the current to determine the resistance.

author: Caliaxy
author_rank: Frequent Contributor
timestamp: July 25, 2021, 07:26:33 pm
content: 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?

author: Kleinstein
author_rank: Super Contributor
timestamp: July 25, 2021, 07:52:27 pm
content: 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.

author: Caliaxy
author_rank: Frequent Contributor
timestamp: July 25, 2021, 08:10:57 pm
content: Quote from: Kleinstein on July 25, 2021, 07:52:27 pmYou're right. Darn LED lamp...

author: ct1bxt
author_rank: Contributor
timestamp: July 25, 2021, 11:22:04 pm
content: Quote from: Kleinstein on July 25, 2021, 07:52:27 pmyou 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

author: KedasProbe
author_rank: Frequent Contributor
timestamp: July 26, 2021, 07:43:50 am
content: 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.

author: ct1bxt
author_rank: Contributor
timestamp: August 22, 2021, 04:00:25 am
content: Hi,a new video showing the issue, not mine.Here: