UT139C AC Volts BUG! - Page 1
Forum section: electronics / testgear

author: mos6502
author_rank: Frequent Contributor
timestamp: February 15, 2016, 02:24:11 pm
content: I'm doing some measurements on a 13.7V fixed voltage linear power supply using my UT139C multimeter. The supply is working fine. Here's a schematic:http://www.produktinfo.conrad.com/datenblaetter/500000-524999/510491-sp-01-en-Power_Supply_4_6A.pdfI measure (AC volts) the AC coming out of the transformer (between GND and the anode of D3/D4) and the meter shows 17V unloaded and 16V with a 4.5A load. This is plausible.I measure the (DC volts) the voltage at the main filter cap (C3). 24.4V unloaded and 18.4V with a 4.5A load. Plausible as well.Now it gets interesting. I measure (AC volts) the voltage at the main filter cap (C3) with a 4.5A load. The meter shows 0.000V. Huh? There should be a significant ripple voltage present.Also, if I measure (AC volts) the output of the PSU, it shows 0.2V. This seems somewhat implausible. I switch the meter to AC millivolts and measure again: 0.000V. On DC volts, the output of the PSU measures 13.70V and does not drop even one digit when connecting the 4.5A load.Anyone got an explanation?

author: mos6502
author_rank: Frequent Contributor
timestamp: February 15, 2016, 06:53:18 pm
content: I did some more testing. If I put a small cap in series with the meter, it seems to display the AC voltage correctly (1.7Vrms ripple voltage at the filter cap with a 4.5A load).I had a similar issue before:https://www.eevblog.com/forum/testgear/uni-t-ut139c-hz-measurement-defective/msg736044It seems like there is some kind of defect that makes the meter not work with AC signals that have a DC offset. Works fine in all other modes, though. I'm sending it back. Still have to decide if I want a replacement or a refund ...

author: mos6502
author_rank: Frequent Contributor
timestamp: February 26, 2016, 06:35:59 pm
content: OK, so I received my brand new replacement UT139C. Guess. Freaking. What. Same issue! WHAT THE ACTUAL F*CK!After some more experimenting, I found that if I set the range manually to 60V or 600V, it displays the correct value (1.8V, confirmed with Fluke 77). But on the 6V range, or on autorange, it displays 0.000V! This is a bug! Like, in a big way! What a piece of shit! I wasted so much time on this crap!God damn. I got so angry that I just bought a Fluke 87V. From this day on, I will never ever use anything else than Fluke.

author: Fungus
author_rank: Super Contributor
timestamp: February 26, 2016, 08:38:22 pm
content: Quote from: mos6502 on February 26, 2016, 06:35:59 pmIf you have a Fluke 77 and can afford to buy a Fluke 87V out of spite then why are you bothering with anything made by Uni-T?It seems like every time the name "Unit-T" comes up on these forums it's just to say more bad things about them.Apart from the Uni-T current clamp...people say good things about that.Edit: And the UT61E is really good value if you need data logging. (Got to get that one in before the Unit-T fans read this...)

author: mos6502
author_rank: Frequent Contributor
timestamp: February 26, 2016, 09:06:52 pm
content: Quote from: Fungus on February 26, 2016, 08:38:22 pmIt's an original series Fluke 77 that I had been gifted after it had been used in a power plant for a few decades. It looks like it's been through hell, but it's working fine (after I resoldered the input jacks). That alone should've told me something. But it's missing a couple features I'd like and the worst thing is that the beeper is dead slow.So, I checked out the spreadsheet, watched a bunch of reviews for the UT139C and thought, "Hey, that doesn't seem too bad. With all that technological progress, they should be able to build a decent little meter for 40 bucks, right? Right?"Quote from: Fungus on February 26, 2016, 08:38:22 pmNo, thanks, I'm cured. I've realized, UNI-T meters are like a box of chocolates. You never know what you're gonna get. Also, some of the chocolates are little turds.

author: hli
author_rank: Frequent Contributor
timestamp: February 26, 2016, 09:10:57 pm
content: Probably the RMS-to-DC converter is AC-coupled. I don't have the UT139C schematics, but on the UT61E schematics one can see that the AD737 input is AC coupled. (and it shows the same behaviour)

author: mos6502
author_rank: Frequent Contributor
timestamp: February 26, 2016, 09:16:57 pm
content: Quote from: hli on February 26, 2016, 09:10:57 pmCan you elaborate? If the meter is set to AC, the signal should be AC coupled, period. Any DC component should have no effect. Also, why does it measure correctly on the 60V and 600V ranges, but shows 0.000 on the 6V range and on autorange?

author: wraper
author_rank: Unknown
timestamp: February 26, 2016, 09:32:13 pm
content: Quote from: mos6502 on February 26, 2016, 06:35:59 pmIMO intended "feature", not a bug.

author: wraper
author_rank: Unknown
timestamp: February 26, 2016, 09:34:26 pm
content: Quote from: mos6502 on February 26, 2016, 09:16:57 pmprobably it cannot measure AC if DC offset is higher than the range itself.

author: IanB
author_rank: Super Contributor
timestamp: February 26, 2016, 09:36:27 pm
content: If you want to measure AC volts with a DC offset, your meter should have an AC+DC capability. Does your meter have an AC+DC measurement option?

author: ModemHead
author_rank: Frequent Contributor
timestamp: February 26, 2016, 09:52:56 pm
content: Your meter is having problems responding to AC that is riding on a significant DC offset. This is not uncommon for some DMM designs, especially those that are based on a single DMM-on-a-chip.I think the problem stems from omitting the expensive high voltage AC coupling cap before the voltage divider, like you might find in a premium meter. Instead a lower-rated coupling cap is placed after the scaling occurs and also after the signal has passed through an internal buffering amp. If the DC component of the input is too high, it can saturate the buffer amp and then the AC component will be lost or distorted.

author: mos6502
author_rank: Frequent Contributor
timestamp: February 26, 2016, 11:05:39 pm
content: Quote from: IanB on February 26, 2016, 09:36:27 pmNo, AC+DC only means that it will measure the RMS value of the entire waveform, not just the AC part. The problem with the UT139C is that it can't measure AC volts if a DC component is present.Quote from: ModemHead on February 26, 2016, 09:52:56 pmThat makes sense and explains everything. That's a horrible design making the meter much less useful. Means that you can't just e.g. measure the ripple on a supply voltage (a very common task at least for me). That wouldn't be too bad if it displayed O.L. or some kind of error message. But the meter will essentially lie to you and display 0.000. Yeah, you can do the measurement by putting your own cap in series - no thanks.I wonder, are there oscilloscopes that show this kind of (mis)behavior?

author: mos6502
author_rank: Frequent Contributor
timestamp: February 26, 2016, 11:15:16 pm
content: Quote from: wraper on February 26, 2016, 09:32:13 pmIf the signal is out of range, it should be displaying O.L., not 0.000. Also, it should be switching to the correct range when in autorange mode, which it doesn't. In my opinion this is a bug.

author: IanB
author_rank: Super Contributor
timestamp: February 26, 2016, 11:24:17 pm
content: Quote from: mos6502 on February 26, 2016, 11:05:39 pmAs I understand it, the AC+DC range will separately (and accurately) show both the AC component and DC component of a measured signal.It seems from your complaints you are asking a cheap meter to measure something it is not specified to do. When you buy low cost tools you should not expect them to perform the same as more expensive ones.

author: mos6502
author_rank: Frequent Contributor
timestamp: February 26, 2016, 11:49:49 pm
content: Quote from: IanB on February 26, 2016, 11:24:17 pmAC+DC can refer either to AC+DC TRMS (which means the DC component gets factored into the reading) or AC+DC dual display, where a meter would display the AC and DC values simultaneously. Neither applies here. We're talking about the ability or inability of a meter to measure ACV of a signal that has a DC bias. There is no word for this. This isn't a thing. I'm pretty sure not many people here even knew that there are meters that are thrown off by a DC bias. I haven't heard it being mentioned in any multimeter reviews, Dave's or otherwise.Quote from: IanB on February 26, 2016, 11:24:17 pmSorry, but it seems to me you have no clue what this thread is even about. You think I should've just bought a dual display meter like the Fluke 89. That's not what we're talking about here.

author: Macbeth
author_rank: Super Contributor
timestamp: February 26, 2016, 11:55:30 pm
content: I've never even considered any DMM would display the AC ripple on a DC source. I guess it's just automatic to me that a DMM doesn't do that (unless its a specific feature). I come from the AVO 8 days of analogue meters though. I would use an oscilloscope to measure ripple.Good to see there are meters that do AC+DC readings though, whether dual display, TRMS, etc. I guess you need to read the specification for unusual functions like that.

author: IanB
author_rank: Super Contributor
timestamp: February 26, 2016, 11:59:16 pm
content: Quote from: mos6502 on February 26, 2016, 11:49:49 pmNow you are just being rude.As you have said yourself you are asking a cheap meter to read an AC signal with a large DC bias. Your cheap meter is unable to do this where more expensive meters are able to do so. When products are built down to a price, some features get removed or reduced. You have just found an example of that.You will notice that nobody in this thread has agreed with you. That tells you something, surely?

author: Lightages
author_rank: Unknown
timestamp: February 27, 2016, 02:25:47 am
content: I just tested a number of my meters, including the UT139C. Some handle big DC offsets better, some without any problems, and some like the UT139C don't tolerate it very much at all. The difference I think is the input being AC coupled or not during AC measurements. The UT171A also has this problem. If it occurs, then the easiest work around is to use a capacitor in series with the leads to measure AC components of DC voltages. it is an inconvenience but that is what you get with some meters.

author: electr_peter
author_rank: Unknown
timestamp: February 27, 2016, 02:26:24 pm
content: As others mentioned, it all comes down to cost. This DMM apparently for cost cutting reasons does not have all parts required to properly measure AC voltage in the presence of DC bias (at least in some ranges). So UT139C can still measure AC voltage (with no DC bias) or DC voltage (with no AC or little AC on top), but not in AC+DC situation. Probably same goes for current ranges as well.More expensive meters do have AC coupling capacitors which allow to measure AC voltage correctly up to some DC bias limit. Oscilloscope AC/DC coupling modes are in principle the same concepts as with DMMs. But do not put >1kV DC bias on AC coupling mode either way, caps will not be rated for that.I have noticed that the simplest/cheapest/crappiest DMMs cannot correctly show DC voltage input on AC range. That is, if you feed positive DC voltage to AC input, it shows ~double DC voltage reading. With negative polarity it shows zero. This happens because simple peak hold circuit is used with no AC coupling. Beware.Solution - study your test gear capabilities before making measurements in the field. Otherwise your measurements will be misleading/incorrect at best if you are not aware of such subtleties. Personally I think that DMM that cannot measure simple AC with DC bias or fail with DC on AC range is not worth to be called proper DMM.

author: Wytnucls
author_rank: Super Contributor
timestamp: February 27, 2016, 02:34:33 pm
content: How does the Brymen 235 behave with a DC offset?

author: Wytnucls
author_rank: Super Contributor
timestamp: February 27, 2016, 04:45:50 pm
content: Perhaps relevant to the 139C:From the Fluke 87V manual:For better accuracy when measuring the dc offset of an acvoltage, measure the ac voltage first. Note the ac voltagerange, then manually select a dc voltage range equal to orhigher than the ac range. This procedure improves theaccuracy of the dc measurement by ensuring that theinput protection circuits are not activated.KeySight has the same message, word for word, in their manuals.

author: mos6502
author_rank: Frequent Contributor
timestamp: February 27, 2016, 06:59:35 pm
content: The 87V has arrived.It shouldn't be a big surprise that it works as expected (as did its older brother), showing the correct value of the ripple voltage. Yes, it costs 10 times more than the UNI-T (*spit*), but it's at least 100 times as good, so really, it's a much better value.Quote from: electr_peter on February 27, 2016, 02:26:24 pmI agree, especially with that last sentence. Often I measure stuff with my multimeter just for shits & giggles, to see if I can find new ways to test and measure stuff. But this one stumped me, because I have never heard this being discussed, ever.Quote from: Wytnucls on February 27, 2016, 04:45:50 pmThat isveryinteresting and actually relevant to my interests. But, I don't think it applies to the UNI-T (*spit*) issue, as that only happens in AC mode.Just found this:http://www.keysight.com/main/editorial.jspx?ckey=458371&nid=-11143.0.00&id=458371No mention of the DC offset posing a problem.As electr_peter explained, this is likely an issue with cheap/crappy DMMs. Anyone who reviews a multimeter in the future should test for this.

author: Lightages
author_rank: Unknown
timestamp: February 27, 2016, 07:50:48 pm
content: I am glad this limitation came up. I don't think that there are any specifications regarding the limitation you have found. I could be wrong but I don't recall seeing any.

author: Wytnucls
author_rank: Super Contributor
timestamp: February 27, 2016, 08:11:27 pm
content: DC offset was discussed before, in relation to the UT-61E:https://www.eevblog.com/forum/chat/how-can-i-test-if-a-dmm-is-reading-in-true-rms/

author: joeqsmith
author_rank: Super Contributor
timestamp: February 27, 2016, 08:29:44 pm
content: Strange you would buy a meter without AC+DC if you are working with it.

author: mos6502
author_rank: Frequent Contributor
timestamp: February 27, 2016, 08:34:28 pm
content: Quote from: Wytnucls on February 27, 2016, 08:11:27 pmCorrect me if I'm wrong, but I believe that was a different issue. What's being discussed in that thread is that most meters don't give the True RMS value in DC mode. I.e., the RMS converters is disabled in DC mode. That whole True RMS AC vs. True RMS AC+DC discussion.However, this is moot when the input is AC coupled. The meter should not see any DC offset in AC mode, just like an oscilloscope. The problem with UT139C is that it doesn't really use AC coupling in AC mode, or the AC coupling isn't working correctly.Quote from: joeqsmith on February 27, 2016, 08:29:44 pmAgain, PLEASE don't confuse the True RMS AC+DC discussion with this. Two completely different things.

author: Wytnucls
author_rank: Super Contributor
timestamp: February 27, 2016, 08:48:54 pm
content: While being mostly about true RMS, it has relevance. In AC mode, the UT61E measures the true RMS value of the AC waveform. The measurement value shouldn't change with a DC offset, being AC coupled, but it does.

author: Wytnucls
author_rank: Super Contributor
timestamp: February 27, 2016, 08:55:47 pm
content: A meter like the true RMS (AC+DC) Fluke 185, can display both the DC and AC values at the same time, with no ambiguity.

author: joeqsmith
author_rank: Super Contributor
timestamp: February 27, 2016, 09:29:20 pm
content: Quote from: mos6502 on February 27, 2016, 08:34:28 pmIt was just a comment. I do find it strange you would buy a meter without it.

author: mos6502
author_rank: Frequent Contributor
timestamp: February 27, 2016, 09:55:06 pm
content: Quote from: joeqsmith on February 27, 2016, 09:29:20 pmWhat would you need it for? Usually, you're more interested what the AC and DC components are separately. If you ever needed it, to get the AC+DC RMS value, you simply square each, add them and take the square root.

author: electr_peter
author_rank: Unknown
timestamp: February 27, 2016, 10:53:09 pm
content: I do not know why main poster is challenged this much about his/herreasonable expectationsin DMM capabilities. He/she simply asks for DMM to work well with very simple and standard waveforms, nothing fancy or extreme. And crappy UNI-T DMM simply fails to deliver. TRMS AC only or TRMS AC+DC discussion has nothing to do with this at all.I agree that UNI-T brand is not the best and very low end, but this case (AC measurement with DC offset) DMM could perform much better, really. This and other case, where UNI-T choose to save penny and make DMM much worse for no good reason, do no put any trust for UNI-T branded DMMs.Have you seen any equipment repair instructional videos from, say, 80s and 90s?Many of these videos included segments where section with linear regulator powered by rectified AC is checked with DMM. DMM is put in DC mode, probes are put on main capacitor before linear regulator to check DC level. Then meter is changed to AC mode to check for ripple (with 50/60Hz or 100/120Hz frequency). Very typical and straightforward operation, no high frequencies are involved, simply DC or AC at 50/60/100/120Hz only.And UNI-T UT139C falls on it's face right here, in very simple situation! FailIn the field, one typically does not know in advance what kind of voltages are present on the lines. That is why one brings DMM - to get information about voltages and frequencies.In OP case, he/sheknew in advancethat DC had low frequency AC ripple, yet could not measure it properly! What about cases where he/she does not know what to expect in advance? One has totrustequipment to show reality, not some nonsense numbers. UNI-T meters clearly lack this trust for a reason.

author: joeqsmith
author_rank: Super Contributor
timestamp: February 28, 2016, 12:09:10 am
content: Quote from: mos6502 on February 27, 2016, 09:55:06 pmYou are correct. I just don't like carrying a calculator when the meter can do the math with ease.

author: glicos
author_rank: Regular Contributor
timestamp: March 24, 2016, 02:52:12 am
content: I havent recieve my UT-139C yet and will test this if i have the same result as the thread starter. I'm just thinking, what if we use the VFC function of UT139c to measure AC signal with DC offset?? What would the reading be??

author: Wytnucls
author_rank: Super Contributor
timestamp: March 25, 2016, 10:56:27 pm
content: Quote from: blueskull on February 26, 2016, 08:18:48 pmThe meter uses the Dream Tech International (Hong Kong) Delta/Sigma ADC, with true RMS converter, 3V DTM0660L.Datasheet (China):http://doc.wendoc.com/sfff2f1781c5f878a6cdadf2c.htmlIt is possible to enable RS232 comms, according to these posts:http://www.kerrywong.com/2016/03/19/hacking-dtm0660l-based-multimeters/https://www.eevblog.com/forum/testgear/hacking-dtm0660l-based-multimeters/