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Brymen BM789

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2N3055:

--- Quote from: Neutrion on September 13, 2021, 02:50:47 pm ---
But does the 869 correctly shows "OL" for AC in this scenario?  Did you try to switch polarity?

With the 869  it makes sense to have everything on the same switch position, but with the single display if there is no way to show overrange they certainly could put in a separate AC mV position.


--- End quote ---

I think we have a language barrier here.

BM869S has 2 different positions for DC and AC mV, but both are nevertheless still DC coupled .

It does show OVL if you apply larger AC voltage. If you combine it with DC, that combined goes over range it also behaves funny.

For instance 1V P-P 400HZ square wave, will show correct 500mV RMS. If I add more than 0.6 V DC to it (offset) it will start showing wrong numbers. Funny enough, I can add -2,3V offset before it starts making problems. So it is not symmetric.

But that is all getting into non important detail. Instrument shouldn't be used outside it's specified range. Good practice is to measure in Volts range all the time and only jump to mV range when we know there won't be hidden problems.

2N3055:

--- Quote from: Caliaxy on September 13, 2021, 03:04:33 pm ---
--- Quote from: 2N3055 on September 13, 2021, 02:08:33 pm ---
--- Quote from: Caliaxy on September 13, 2021, 01:52:54 pm ---Other even cheaper ones (UT81E and Aneng 8009) don't, they correctly display OL (kudos to them). Fluke 189 (in AC mV) is not tricked by a high DC offset either. Fluke 87V doesn't have this issue at all because it doesn't have a separate AC mV position on the dial in the first place  ;D

--- End quote ---
Are you sure about F87V ? Connect other meter in ohms mode and measure input resistance of F87V. Go through modes and ranges. Not only input resistance won't be constant 10 MOhm, but when it goes Hi impedance then it is AC coupled.

--- End quote ---

I only meant that Fluke 87V doesn't have an "AC mV" mode on the dial at all, only a "DC mV" one (which has a single 600 mV range). It does measures AC mV in "AC V" mode, which goes down to 600 mV range (and all AC ranges are AC coupled). The lowest DC V range in "DC V" mode is 6 V, hence the separate "DC mV" 600 mV DC mode.

Not sure why they did that - maybe it was their way of dodging the bullet. They must have figured the problem and this is how they avoided it. They don't call themselves Fluke for no reason   ;D

--- End quote ---

Doing AC mV right is not easy. And also, we keep forgetting, as much as people love F87V, it is not a meter for electronics. It is industrial electrician workhorse meter. That is what Fluke says too. For that audience, AC mV is not a thing.

bdunham7:

--- Quote from: Caliaxy on September 13, 2021, 03:04:33 pm ---he lowest DC V range in "DC V" mode is 6 V, hence the separate "DC mV" 600 mV DC mode.

Not sure why they did that

--- End quote ---

It would be interesting to see what its capability for normal mode AC rejection is in that mVDC range.

Caliaxy:

--- Quote from: Caliaxy on September 13, 2021, 01:52:54 pm ---Other even cheaper ones (UT81E and Aneng 8009) don't, they correctly display OL (kudos to them). Fluke 189 (in AC mV) is not tricked by a high DC offset either. Fluke 87V doesn't have this issue at all because it doesn't have a separate AC mV position on the dial in the first place  ;D

--- End quote ---

I'll have to take back the statement that UT81E doesn't have this issue. I just realized that 500 mVrms in my quick test was way over its AC mV range (200 mV or so), which might have contributed to its displaying OL. At 100 mVrms it also displays nonsense at DC offsets higher than 2V (but it seems a bit more tolerant to DC bias than the Brymens). Aneng 8009 might behave similarly (haven't rechecked, but its upper AC mV limit is only 100 mV or so...).

Fluke 189 works fine (and so does Keysight U1282A, if anyone wonders; U1252A doesn't).

bdunham7:
Well, I guess part of the answer is "RTFM"!  At least they've documented this issue.  I still think the OP may have an issue though, as I think it should indicate OL, but I'll await someone else chiming in with some tests.  The F116 and F189 don't have this limitation--I tried a 0-2V 5% duty cycle signal (CF~5) and they both showed approximately the expected readings. 

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