UT139C AC Volts BUG!

[mos6502]

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/

Correct 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.

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

Again, PLEASE don't confuse the True RMS AC+DC discussion with this. Two completely different things.

[Wytnucls]

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.

[Wytnucls]

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.

[joeqsmith]

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

Again, PLEASE don't confuse the True RMS AC+DC discussion with this. Two completely different things.

It was just a comment.  I do find it strange you would buy a meter without it. 

[mos6502]

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

Again, PLEASE don't confuse the True RMS AC+DC discussion with this. Two completely different things.

It was just a comment.  I do find it strange you would buy a meter without it. 

What 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.

[electr_peter]

I do not know why main poster is challenged this much about his/her reasonable expectations in 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! Fail

In 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/she knew in advance that 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 to trust equipment to show reality, not some nonsense numbers. UNI-T meters clearly lack this trust for a reason.

[joeqsmith]

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

Again, PLEASE don't confuse the True RMS AC+DC discussion with this. Two completely different things.

It was just a comment.  I do find it strange you would buy a meter without it. 

What 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.

You are correct.  I just don't like carrying a calculator when the meter can do the math with ease.   

[glicos]

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??

[Wytnucls]

Probably intended. It probably did not use any true RMS or ADC method to measure AC voltage. They rely on polarity commutating.

The meter uses the Dream Tech International (Hong Kong) Delta/Sigma ADC, with true RMS converter, 3V DTM0660L.

Datasheet (China):
http://doc.wendoc.com/sfff2f1781c5f878a6cdadf2c.html

It 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/