UT139C AC Volts BUG!

[mos6502]

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.pdf

I 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?

[mos6502]

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/msg736044

It 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 ...

[mos6502]

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.

[Fungus]

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.

If 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...)

[mos6502]

If 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'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?"

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...)

No, 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.

[hli]

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)

[mos6502]

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)

Can 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?

[wraper]

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!

IMO intended "feature", not a bug.

[wraper]

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)

Can 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?

probably it cannot measure AC if DC offset is higher than the range itself.

[IanB]

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?

[ModemHead]

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.

[mos6502]

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?

No, 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.

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.

That 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?

[mos6502]

IMO intended "feature", not a bug.

probably it cannot measure AC if DC offset is higher than the range itself.

If 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.

[IanB]

No, 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.

As 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.

[mos6502]

As I understand it, the AC+DC range will separately (and accurately) show both the AC component and DC component of a measured signal.

AC+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.

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.

Sorry, 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.

[Macbeth]

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.

[IanB]

Sorry, 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.

Now 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?

[Lightages]

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.

[electr_peter]

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.

[Wytnucls]

How does the Brymen 235 behave with a DC offset?

[Wytnucls]

Perhaps relevant to the 139C:

From the Fluke 87V manual:
For better accuracy when measuring the dc offset of an ac
voltage, measure the ac voltage first. Note the ac voltage
range, then manually select a dc voltage range equal to or
higher than the ac range. This procedure improves the
accuracy of the dc measurement by ensuring that the
input protection circuits are not activated.

KeySight has the same message, word for word, in their manuals.

[mos6502]

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.

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.

I 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.

Perhaps relevant to the 139C:

From the Fluke 87V manual:
For better accuracy when measuring the dc offset of an ac
voltage, measure the ac voltage first. Note the ac voltage
range, then manually select a dc voltage range equal to or
higher than the ac range. This procedure improves the
accuracy of the dc measurement by ensuring that the
input protection circuits are not activated.

KeySight has the same message, word for word, in their manuals.

That is very interesting 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=458371

No 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.

[Lightages]

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.

[Wytnucls]

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/

[joeqsmith]

Strange you would buy a meter without AC+DC if you are working with it.