Products > Test Equipment
Brymen BM789
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floobydust:
AN8008 ACV bugs: Input 90mVAC on the mV AC range, no DC offset (autoranging enabled 9.999mV and 99.99mV ranges). Displays: 90.00 this is OK.
As you increase a +ve or -ve DC offset, the reading suddenly drops to say 15.00 then additional DC offset leads to display in a loop showing: 40.30, 0.000, 00.00, OL, and constantly repeating.
If you manual range to 99.99mVAC, you get a correct reading 90.00mVAC but this drops and even goes to 00.00mVAC with increasing offset. With no DC offset it returns to the correct reading.
If you now enable autoranging, the multimeter is lost and reads as if on manual range, but no longer does the autorange cycling and can read silly values.
You have to switch to another function or reboot to stop this.

Mentioning this as it is a test of the OL and autorange algorithms that commonly have bugs.
joeqsmith:

--- Quote from: Caliaxy on September 15, 2021, 01:07:25 am ---... most of the meters I tried have the same issue: Fluke 101, 107, 17B+, Uni-T UT61e, Agillent U1252A, BM235, BM689S. Fluke 189, 89IV and Agillent U1282A seem to work fine. I don't care much either, but it's good to know. ...
--- End quote ---

I'm sure I could find a few in the my mix that would handle the ACmV fine but I would guess I could find other problems with them.   I'll typically gloss over what I consider fringe cases. 

Now if you make a point to advertise your meter has a frequency counter that can read up to 220MHz.  You're so proud of it, that you place it right on the box.  And I happen to watch Daves review and see your bright idea was to protect the input with a couple of series PTCs that any EE is going to know they will act like capacitors and you didn't limit the current through them.  Well,  I'm not going to let that go.  It's not a fringe case when you claimed it would do something.   I'm going to push it until it works, or goes up in flames.    :-DD   
bdunham7:

--- Quote from: AndrewBCN on September 15, 2021, 12:21:57 am ---I can see the headlines tomorrow:
Electronics Engineer Exposes Major Design Flaw In Hundreds of Multimeters
Proves That Most DMMs Cannot Be Used To Measure Ripple And Noise In PSUs

--- End quote ---

...but trusty old Fluke 87V gets the job done once again and Fluke sells another million units!   :-DD
bdunham7:

--- Quote from: Caliaxy on September 15, 2021, 01:07:25 am ---most of the meters I tried have the same issue: Fluke 101, 107, 17B+, Uni-T UT61e, Agillent U1252A, BM235, BM689S.

--- End quote ---

Do you mean they all displayed wrong readings or did they display an overload indicator?
Caliaxy:

--- Quote from: AndrewBCN on September 15, 2021, 12:00:23 am ---
--- Quote from: Caliaxy on September 14, 2021, 10:55:14 pm ---
--- Quote from: 2N3055 on September 14, 2021, 09:25:06 pm ---And no AC mV range.. That is how Fluke solved that problem with F87V.
I prefer to have it, even if it requires thinking when using..

--- End quote ---
You got it wrong. Like BM789, Fluke 87V does have an AC 600mV range (and a DC 600mV range), but they are not on the same rotary dial position. The AC 600mV is on the AC V position, which starts auto-ranging from 600mV up, whereas the DC 600mV range has its own rotary dial position, all for itself. The DC V mode starts auto-ranging from 6V up. All AC V ranges (including 600mV) are AC coupled, so the DC offset is irrelevant, unlike in BM789.
...

--- End quote ---
No, 2N3055 got it right, the AC V position on the Fluke 87V has 6000 count resolution, whereas the separate BM789 ACmV position has 60,000 count resolution. If you use the Auto position on the BM789 it will autorange exactly the same as the Fluke 87V.

That means the Brymen has a high resolution, separate dial position for manual range 600mV ACmv which you can choose to use for high resolution ACV measurements - exactly as 2N3055 wrote. And yes, they should add in the BM789 User Manual a warning that this range requires thinking...  :-DMM

--- End quote ---

In my post #24 above (replying to 2N3055's post #21) I was joking (half seriously) that Fluke 87V probably avoided the problem under discussion here by not having a dedicated AC mV position on the dial at all (and moving it under the AC V mode, which is AC coupled). After reading his reply #79 I worried that my reply #24 misled him into believing that Fluke 87V doesn’t have an AC mV range at all (which is not true but might have been an interpretation of what I wrote in #24), so I tried to clarify. That’s the reason of my reply #88, which you (wrongly) disagree with.

#88 wasn’t intended to be malicious. I’m not sure why you tend to see everything this way.  :-//
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