UNI-T UT61E does not show zero in AC Voltage range with shorted test leads - Page 1
Forum section: electronics / testgear

author: sky666
author_rank: Contributor
timestamp: October 26, 2019, 12:18:34 pm
content: Hi,My UT61E do not show zero in ACV or ACmV ranges with shorten test leads (auto or manual range).The last digits always show a value that drops but never reaches zero and never responds to shortening leads.The value appears to be transmitted to any next range.Examples of values at one point in time across all ranges (ACV):0.0052V0.052V0.52V5.2VIs this normal for True RMS multimeter?It's kind of weird. As if it's not a true value, but the same change is bound to the last 2 numbers on the display in any range.For example, when the multimeter is set to AC range, it starts with a value of 15 (0.0015V; 0.015V; 0.15V depending on the range) and progressively decreases (like discharging). In 1 minute it can drop to 0.5 or 1 (in 750v range is 0.5V or 1V, in other range is 0.005V or 0.01V). Shortening the test leads has absolutely no effect on the value at any time.The measurement values ​​are identical if the measurement starts at a value of 15 or 2 for example.However if REL. button is pressed the measured value drops (231VAC without relative button, 213VAC with relative button).Too bad I can't find a stabilized source of lower AC voltage for testing...

author: tunk
author_rank: Super Contributor
timestamp: October 26, 2019, 02:39:40 pm
content: Don't think that's normal. I think I would have opened a dispute (take a photo of the 5.2V reading).

author: Uncle Bob
author_rank: Newbie
timestamp: October 26, 2019, 02:58:21 pm
content: I have the EU version of the 61E. I bought it from ELFA DISTRELEC here in Sweden a couple of years ago. It behaves like yours as it keeps the least significant digits when I switch range. At least it reaches zero but it takes several minutes.... Interesting feature anyway.

author: sky666
author_rank: Contributor
timestamp: October 26, 2019, 03:26:25 pm
content: This is an interesting statement from Fluke, but the deviation is not very clear.Digital Multimeter Frequently Asked QuestionsQ: Why does my multimeter not go to zero in the ac volts mode?A: If it is a true-rms ac multimeter, such as the 17X or 8X, it will not zero and should not be zeroed. Most of these true-rms type multimeters have a minimum ac voltage and current level specification for both the ac volts and ac current modes, typically specified from 3% to 5% of range. When a signal is applied above the specified minimum level, the meter will meet its accuracy specifications. You will find this specification in the specification section in the rear of the user manual.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 26, 2019, 04:53:52 pm
content: Quote from: sky666 on October 26, 2019, 12:18:34 pm"...ranges with shorten test leads..." Shorted?Like my gold standard Fluke 87V, I assume it will take minutes to become stable. Then again, it's very possible I am not following what you are asking. Rather than examples, if there is a specific case you would like me to run, just post your procedure. Maybe post a short video of it. The more details about your setup, the better. Then I can try and replicate your results.I have UT61E, but it is highly modified. However I don't believe any of these changes I've made would effect the AC voltage with a shorted input. If I short the inputs and select ACV or ACmV it will zero out.

author: sky666
author_rank: Contributor
timestamp: October 26, 2019, 06:42:04 pm
content: Hello,I appreciate you taking the time to answer my questions. In order to clarify the situation, I have attached pictures of my tests. I apologize for how rusticaly I have connected the contacts, but i think it will do.

author: Performa01
author_rank: Super Contributor
timestamp: October 26, 2019, 07:22:57 pm
content: Quote from: sky666 on October 26, 2019, 03:26:25 pmRead the answer from Fluke again and try to understand what they are saying.Analog True RMS converters (as used in the vast majority of meters) do have an (unspecified and variable by manufacturing tolerances) offset. Above that offset, the meter will work and the error will decrease to the specified accuracy once the signal level reaches some 5% of the range. Below it will just be unusable.Data sheets usually specify an additional error for the area of 1%to 5% of full scale. Below 1% of the range, operation is simply unspecified.I have a huge collection of TRMS DMMs, up to fancy Keithley 2001 instruments. None of them is really usable below 1% of the range.Most meters show the actual offset without a signal, but some are cheating and just read zero if no signal above the offset is detected. This might look good, but doesn't change the fact that you cannot measure low signals. And if it is shown (as the vast majority of lab instruments do) you at least get an idea what the lowest value is that you can measure in that range at all, albeit with some additional error as explained before.Trying to zero the instrument by means of the Rel function will introduce an error for all the otherwise valid measurements above 5% of the range.

author: CDaniel
author_rank: Frequent Contributor
timestamp: October 26, 2019, 07:29:55 pm
content: As I see it is about the same value 50 counts on all ranges ... the true-rms chip AD737 has a pot for zeroing the output with no input , maybe it is not adjusted or something is wrong in that area . Anyway this should be repairable , but you should recalibrate it after ...

author: joeqsmith
author_rank: Super Contributor
timestamp: October 27, 2019, 12:47:24 am
content: Ok, you are basically doing the same test. Mine will zero out on all ranges after a few minutes.Quote from: CDaniel on October 26, 2019, 07:29:55 pmI've seen several posts of people claiming how the UT61E will drift. I haven't seen that with my meter. It could be a simple tweak if it's that important to the OP.

author: mjkuwp
author_rank: Unknown
timestamp: October 27, 2019, 02:25:15 am
content: My UT61E will move to 0 AC volts in all the ranges even if it starts with some value as I switch to that function. I'm not doing anything fancy with the shorting, just have the regular leads plugged in and the tips touching. Could there be differences due to environment? I've just done this test at my home and there isn't much equipment around that could cause any emitted radiations.

author: CDaniel
author_rank: Frequent Contributor
timestamp: October 27, 2019, 05:46:28 am
content: I don't want to disappoint anybody or state the obvious , this is a cheap meter and you can expect this sort of problems . A meter with adjustable pots is a joke in our days ...It is allmost certain that pot I was talking about for nulling the true-rms converter output , it is in the schematic so you should find it ... and not turning the one for the gain, anyway the accuracy is in doubt and should be checked

author: indman
author_rank: Super Contributor
timestamp: October 28, 2019, 08:14:31 am
content: Quote from: sky666 on October 26, 2019, 06:42:04 pmThese photos clearly suggest that the device is faulty.Require the seller to return the full amount or send a new serviceable device. Only so - nothing needs to be opened and regulated(adjustable) until the dispute is resolved!

author: Fungus
author_rank: Super Contributor
timestamp: October 28, 2019, 09:21:22 am
content: Quote from: indman on October 28, 2019, 08:14:31 amNo they don't.It's a 0.05% offset on a meter that's specified as 0.8% accurate andcalibratedadjusted with little potentiometers.Nothing to worry about, just wrong expectations.

author: indman
author_rank: Super Contributor
timestamp: October 28, 2019, 10:19:55 am
content: Here is a quote from the manual on the UT61E -"A residual reading of 10 digits with test leads shorted, will not affect stated."I see 50 digits on the display at short inputs and I consider it a fault. You can count it as you please.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 28, 2019, 11:22:23 am
content: I assume its 10 counts max as well based on that note.

author: indman
author_rank: Super Contributor
timestamp: October 28, 2019, 12:30:44 pm
content: In the photo the readings on the display of my UT61E in AC mode with an open input, with a short entrance by short wires and the last with a short entrance by complete probes. All readings calmed down after about 1-2 minutes.That 's the only way I think a serviceable device should react. I didn 't configure or adjust anything in the circuit for AC mode. I added only a display backlight and made the autopower-off mode.My collection has many multimeters of Chinese industry and I consider UT61E one of the best in the price category $50-60!

author: sky666
author_rank: Contributor
timestamp: October 30, 2019, 07:29:12 pm
content: Quote from: Fungus on October 28, 2019, 09:21:22 amI think it's a 0.7% offset (750V Range). Maybe I'm wrong...

author: sky666
author_rank: Contributor
timestamp: October 30, 2019, 08:18:43 pm
content: Thank you all for your help. I have already opened a dispute. I recorded two videos and some pictures. I wanted to be fair so I asked for a partial refund. Just got a generous offer from the seller with 0% refund and incomplete video that everything is fine with the instrument.Of course I didn't agree with the offer and will have to shoot another video. I'm starting to feel like a producer and a director together.By the way, the video is in Chinese...AliExpress

author: Sredni
author_rank: Frequent Contributor
timestamp: October 30, 2019, 08:26:35 pm
content: Just to be clear: the reading with shorted probes remain fixed after say 5 minutes (300 seconds)?Because in my case it takes about 2-3 minutes to go down to zero.

author: sky666
author_rank: Contributor
timestamp: October 30, 2019, 08:33:37 pm
content: The reading drops to about 2.5 - 2.6V (or 25-26 counts in auto range) after a few minutes and remains at that value. With shorted probes.

author: AudioFile
author_rank: Contributor
timestamp: October 31, 2019, 08:57:08 pm
content: Performa01 has identified the issue precisely. I thought it was well known that 'true RMS' meters need a certain minimum voltage to calculate and read properly.

author: Fungus
author_rank: Super Contributor
timestamp: October 31, 2019, 10:40:38 pm
content: Quote from: sky666 on October 30, 2019, 08:33:37 pmAnd, as noted, that's not a defect, it's the way these meters work in AC mode.

author: sky666
author_rank: Contributor
timestamp: October 31, 2019, 10:47:33 pm
content: If the factory declares a maximum value of 10 digits, it should be less than 10. What is the way these instruments work? With an undefined number of digits (leads shorted)?

author: joeqsmith
author_rank: Super Contributor
timestamp: October 31, 2019, 11:43:21 pm
content: Quote from: sky666 on October 31, 2019, 10:47:33 pmThat's the way I understand their specifications as well, 10 counts max with leads shorted. It seems clear enough.

author: Shock
author_rank: Super Contributor
timestamp: November 01, 2019, 12:36:13 am
content: Make sure you try new batteries as well. Seems fairly obvious if other members with the same model settle out and zeros it is most likely calibration if everything else has been checked (including other modes).Edit:If Indmans meter also settles on average from 25 to 0 (perhaps he can confirm) that looks suspiciously similar to how your meter is settling as well, but just reading high at the end. Meaning it's probably operating correctly but might just need tweaking. Still not your problem though.

author: Fungus
author_rank: Super Contributor
timestamp: November 01, 2019, 01:36:52 pm
content: Quote from: joeqsmith on October 28, 2019, 11:22:23 amI'm not sure if "will not affect stated" is the same as "max allowed", but that's just me.

author: maginnovision
author_rank: Super Contributor
timestamp: November 01, 2019, 08:56:00 pm
content: I'm not sure but I'm with fungus here. It's in the notes of specs it sounds like they're just saying that 10 digits doesn't affect anything. It's also not true rms under 10% of range so it may see some DC offset. Also doesn't specify under 45hz and you're looking at (ideally) 0. If it was 100 counts I'd see an issue but 25? I don't know. Ask uni-t maybe they'll tell you if that's ok or not.

author: rsjsouza
author_rank: Super Contributor
timestamp: November 02, 2019, 03:21:42 am
content: On my unmodified UT61E (non-GS version) it took 2 min to zero the meter in VAC and 2-1/2 min to zero it in mVAC.Just for comparison, the U1273A settles in 46 counts in VAC (4.6mV) and 30 counts in mVAC (30ųV).After 2 min the Brymen BM857 settles in 8 counts in VAC (800ųV) and 7 counts in mVAC (70ųV).After 2 min the U1282A settles in 4 counts in VAC (400ųV) and 1 count in mVAC (1ųV).After 2min the Fluke 87V zeros in VAC.After 2min the Fluke 189 settles in 31 counts in VAC (3.1mV) and 12 counts in mVAC (12ųV)All measurements made at 18°C.

author: Shock
author_rank: Super Contributor
timestamp: November 02, 2019, 04:44:54 am
content: That's so far 3 Uni-T 61E models showing the same settling to zero. I tested a good and flat battery on my Fluke 112 and found battery capacity does actually affect it's ability to zero properly.Fluke 87V settles to zeroFluke 112 settles to zeroFluke 117 cheats and zeros automaticallyFluke 87 (old and crusty) got to about 9and various Fluke 70 series (none of which are true rms) all zero

author: Per Hansson
author_rank: Unknown
timestamp: November 02, 2019, 11:49:38 am
content: It takes my TUV / GS rated UT61E ca 2 minutes to settle in VAC mode to 2 counts with probes connected together, if I give it another minute it goes down to 1 count.In mVAC it takes 1 minute to 5 counts and 1.5 minutes to 0 counts.I have just yesterdaymodded itwith a LT1790BCS6-1.25 voltage reference, but I do not think it makes any difference.Also input protection transistors Q1 & Q8 arestill missing, but likewise I do not think that makes any difference either for this test.

author: Fungus
author_rank: Super Contributor
timestamp: November 02, 2019, 01:12:54 pm
content: Quote from: rsjsouza on November 02, 2019, 03:21:42 amQuote from: Per Hansson on November 02, 2019, 11:49:38 amThe elephant in the room is thatnobodywaits two (or three) minutes for the reading to "settle" when measuring AC voltage.In real life we just connect the probes and look at the number.Edit: Also ... this Uni-T is only specified at 0.8% accuracy so an extra digit or two down there is a red herring.

author: rsjsouza
author_rank: Super Contributor
timestamp: November 02, 2019, 01:18:34 pm
content: Yes. The absolute zero is a non-issue for the most part of our measurements. We usually work in approximations.

author: Fungus
author_rank: Super Contributor
timestamp: November 02, 2019, 04:00:59 pm
content: Quote from: rsjsouza on November 02, 2019, 01:18:34 pmWe also buy 5% resistors and use "5V" USB power supplies that measure 5.1V.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 02, 2019, 04:46:52 pm
content: Quote from: Fungus on November 02, 2019, 01:12:54 pmThe note seems to be a specific test case to determine if the meter is working properly. I'm not sure how long my 61E takes to settle within 10 counts but the datasheet makes no reference to time.

author: Fungus
author_rank: Super Contributor
timestamp: November 02, 2019, 05:08:05 pm
content: Quote from: joeqsmith on November 02, 2019, 04:46:52 pmThat, too. The leads may be shorted but there's obviously a high impedance somewhere in the circuit if a capacitor is taking a couple of minutes to get close to zero. It may be that the capacitor will fully discharge if you left it overnight.Is that impedance there during actual measurements?I just did an informal test where I shorted my Brymen BM857s leads together and waited for the display to go to 0.0001. I connected it to an AC source and let it settle for a minute then repeated both steps.The second time I connected it to the AC source it gave the same reading in less than five seconds. It seems the capacitor charges a lot faster when there's a real signal applied to it.Another thing to test is touching the leads with your hands when it's in AC mode with nothing connected. I can make the readings go all over the place when I do it. The tiny capacitance between my hands is swamping any error due to residual electrons in the system.

author: Shock
author_rank: Super Contributor
timestamp: November 02, 2019, 05:14:03 pm
content: When I tested on my Flukes it was a very logarithmic curve, so if that is the same behavior on the 61E if it doesn't move the last 20 or so counts in a few minutes (but others 61E do reach zero). The meter is probably out of cal, but as I mentioned I'd at test it fully first and put a new battery in, I wouldn't assume I'm correct, but evidence points that way.In fact if it was me I would zero cal the AC if everything checks out.Mentioned on this websiteVR4 seems to be a zeroing adjustment specifically for AC. At your own risk with the proper tool! It's a lot easier when you have the gear to ensure you can recover from mistakes.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 02, 2019, 05:34:12 pm
content: We certainly can dream up all sorts of tests to run but in this particular case, the designers seem to have made it clear that we short the leads and make sure its 10 counts or less.I would imagine if we let it settle for 10 years once the batteries have long leaked out, so no digits are displayed that may constitute less than 10 counts. I don't think that was the designer's intent.I guess we could short the supplied leads together without twisting them and have them them routed to an AC source large enough to exceed the 10 counts as well but the may assume the user has enough common sense that it wouldn't be tested that way.Quote from: Fungus on November 02, 2019, 05:08:05 pmMy guess is yes. If you have been following the thread where we are looking to measure 100uV, I was asked to try this with the Fluke 87V using the high res mode. This clip gives you some idea how long the meter takes to settle. I wouldn't use this meter for this measurement, not only because of of slow it is but it's just not very accurate. Obviously at higher levels, the 87V is going to settle faster.Quote from: Shock on November 02, 2019, 05:14:03 pm

author: Fungus
author_rank: Super Contributor
timestamp: November 02, 2019, 05:54:23 pm
content: Quote from: joeqsmith on November 02, 2019, 05:34:12 pmEmpirically: My Brymen seems to go towards a stable reading of an AC voltagemuchfaster than it goes towards zero with shorted leads.(it takes about five minutes to reach a display of 1 count and another four to reach zero)

author: Fungus
author_rank: Super Contributor
timestamp: November 02, 2019, 06:00:50 pm
content: Quote from: Shock on November 02, 2019, 05:14:03 pmSo it seems like it's just badlycalib^Wadjusted, not faulty. Maybe it's just anoffset voltageon an op-amp that hasn't been trimmed properly or something like that.Quote from that page: "i always knew the UNI-T stock calibration is rather sucky"

author: CDaniel
author_rank: Frequent Contributor
timestamp: November 02, 2019, 07:08:45 pm
content: The true-rms chip is responsable ( usualy cheap AD737 which is the slowest ) ... if you read the theory . It has a settling time dependent on the component values used , so implementation varies . The averaging capacitor is electrolytic , so it takes time to discharge . Also it has a nulling pot , if not adjusted right this situation will happen , when a number of counts will remain as an offset on oll AC ranges

author: Shock
author_rank: Super Contributor
timestamp: November 02, 2019, 10:37:41 pm
content: Quote from: Fungus on November 02, 2019, 06:00:50 pmVR4 looks like some kind of bias/offset for the True RMS DC converter. I would want to send it back or at least be compensated, a dollar a digit sounds fair enough doesn't it?https://www.analog.com/media/en/technical-documentation/data-sheets/AD737.pdf

author: Fungus
author_rank: Super Contributor
timestamp: November 03, 2019, 07:42:09 am
content: Quote from: Shock on November 02, 2019, 10:37:41 pmIt definitely looks fixable, it probably has no effect on real-life readings, but yeah... it's not as it should be.

author: slingshot
author_rank: Newbie
timestamp: September 25, 2025, 09:10:47 pm
content: Just found this old topic...my good old UT61E behaves similarly, but with much more bias. Also doesn't measure correctly in a big percentage of the range. Most of the times it starts with a very big value at the selected range, and decreasing slowly. I was curious, and the schematics is available, I started an investigation.I suspect that the AD737 is somewhat involved. Seems the idea is to output an AC voltage on ADO (pin 31) of the ES51922, and receive the True RMS value in DC at ACVH (pin 29).So I measured voltages, and seems it's true. When the multimeter measures correctly, 29 mV AC on ADO is 29 mV DC on ACVH. Depending on the range, it's translated into 29V, 290V, etc...However when there's a large bias, and the multimeter cannot measure less voltage then it currently displays, the values are greatly differ, especially at the lower area of the selected range.I already replaced the AD737, but no success. Next I'll try replacing the capacitors around the true RMS chip, and will see what happens. If it doesn't help, then maybe some big capacity built up in the ACVH input, which is charged up?. Probably it can be tested by disconnecting the AD737 output from the DMM chip?I wonder if the original problem is solved, or just replaced the multimeter?

author: slingshot
author_rank: Newbie
timestamp: October 03, 2025, 01:36:36 pm
content: So, if someone is interested...Seems the fairly slow settling to 0 is normal, just look at the AD737 datasheet. Probably there's some transient which puts some voltage to the ADO pin when the multimeter is turned on and/or changing ranges.If it doesn't settle to 0 after some time, just turn VR4, until it'll show 0. I found zeroing is the best to do at the last (1000V) range. My meter still doesn't go to exact 0, but settles in ~1.1V at 1000V range, which I can accept.I can see the transient where the meter is settled at 1000V, then I change range to the smallest one, and usually it starts to count upwards a little. Then this value can be seen at the larger ranges (which is 0.02V at the smallest range is 20V at 1000V range).