Broke Rigol 3068 current measurement range, but why? - Page 1
Forum section: electronics / testgear

author: taydin
author_rank: Frequent Contributor
timestamp: March 15, 2020, 12:12:57 pm
content: I tried to measure current at a transformer secondary that has about 1060 VAC using my Rigol DM3068. The expected current was in the microamps range (I was doing isolation testing). At the instant I applied voltage to the primary of the transformer, I saw a "matrix flowing screen" effect on the DM3068 screen. I immediately cut power to the transformer, but now the DM3068 current measurement range is shot (boty 200 mA range and 10 A range).I did the same measurement using my UT139C (in microamps range) and it did the measurement just fine (about 4.2 uA of current). It wasn't as accurate as I wanted to be, but the point is, the UT139C didn't die.So what could be the reason for the DM3068 to break in this scenario? I'm thinking that environmental capacitive coupling caused a high voltage to appear at the input of the DM3068, but the same type of coupling would be there for the UT130C also. So what do you guys thing could have caused this?I know that the DM3068 current range was working before. But now both 200 mA and 10 A range don't show any current at all.

author: wraper
author_rank: Unknown
timestamp: March 15, 2020, 12:20:51 pm
content: UT-139C is completely isolated from anything and with internal battery power. So it basically does not matter at what voltage it is floating, only voltage between terminals does matter. DM-3068 is not. Also bench meters in general are less robust. DM3068 has CAT I 1000V rating and CAT II is 300V only but UT-139C has CAT III 600V which is way more.

author: taydin
author_rank: Frequent Contributor
timestamp: March 15, 2020, 01:27:55 pm
content: Quote from: wraper on March 15, 2020, 12:20:51 pmYes the CAT ratings are different, but aren't CAT rating related to the handling of very high voltage spikes (6 kV, 8 kV etc)? There is no transient in my case, just the open secondary of a transformer.Also, while the DM3068 is not completely floating, it still is separated from the mains through its power supply transformer, isnt' it? Shouldn't that be enough isolation?

author: AVGresponding
author_rank: Super Contributor
timestamp: March 15, 2020, 02:11:20 pm
content: Does your DM 3068 have a class Y suppression cap in the power supply section..?

author: TurboTom
author_rank: Super Contributor
timestamp: March 15, 2020, 02:15:48 pm
content: Citation from Rigol's manual:"2) LO-ground protection limit: at most 500 Vpk (relative) is allowed to float at LO terminal with safety."I guess you exceeded this figure by at least a factor of two. Since there isn't a schematic avalaible for this instrument, figuring out what actually failed may be quite tedious... Sorry for not having better news for you!

author: wraper
author_rank: Unknown
timestamp: March 15, 2020, 02:18:40 pm
content: Quote from: ThickPhilM on March 15, 2020, 02:11:20 pmThat's irrelevant since analog section is powered from separate transformer winding and the rest of the circuit is isolated with AD ADUM digital isolators + optocoupler and grounded.

author: AVGresponding
author_rank: Super Contributor
timestamp: March 15, 2020, 02:26:12 pm
content: Quote from: wraper on March 15, 2020, 02:18:40 pmWell I haven't seen the schematics, but if the cap was connected to signal ground (I've seen crazier things) there could be consequences.Looks like TurboTom found the relevant info in the manual in any case.

author: taydin
author_rank: Frequent Contributor
timestamp: March 15, 2020, 03:02:17 pm
content: Quote from: ThickPhilM on March 15, 2020, 02:11:20 pmI didn't open the unit yet. But I will check.

author: taydin
author_rank: Frequent Contributor
timestamp: March 15, 2020, 03:07:34 pm
content: Quote from: TurboTom on March 15, 2020, 02:15:48 pmTrying to understand what this means ... So the LO terminal can be AT MOST 500 Vpk above mains earth?If so, does that mean, if I had connected one end of the transformer secondary to mains earth, the DM3068 would NOT HAVE gotten damaged?

author: taydin
author_rank: Frequent Contributor
timestamp: March 15, 2020, 03:10:53 pm
content: Not sure if this is relevant, but when I attached one probe lead of my UT139C to one side of the secondary, with the other probe lead floating, I was seeing about 300 Vrms voltage. So whatever capacitive coupling the transformer has to earth, it was allowing this much voltage to be present.Attached is the test setup. I was basically verifying whether a pair of probes would withstand 1000 VAC, and what leakage current was present.

author: wraper
author_rank: Unknown
timestamp: March 15, 2020, 03:23:30 pm
content: Quote from: ThickPhilM on March 15, 2020, 02:26:12 pmMultimeter shouldn't have that since it will make it a very crappy meter. IMHO you don't know how bench multimeters are made. And there is no need to begin with since it has usual transformer, not SMPS.

author: taydin
author_rank: Frequent Contributor
timestamp: March 15, 2020, 03:29:30 pm
content: Here are a few extra pieces of information:I have measured the insulation resistance of the transformer secondary to its mounting bracket. It is in the order of 0.5 TΩ so there is practically no leakage.Then I have measured one end of the secondary with respect to mains earth. It is around 190 Vrms. (This step up transformer is the last thing in the chain of a UPS, an isolation transformer, and a variac. I'm using the variac to adjust the secondary voltage (0 to 5000 VAC)

author: AVGresponding
author_rank: Super Contributor
timestamp: March 15, 2020, 05:06:29 pm
content: Quote from: wraper on March 15, 2020, 03:23:30 pmI know how multimetersshouldbe made.I've poked about enough in the ones I own to know.I've already admitted I don't know howthismultimeter was made.