Products > Test Equipment

Brymen BM789

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bdunham7:

--- Quote from: floobydust on September 18, 2021, 11:19:36 pm ---For the Brymen and other's using the same chip, you wouldn't have a DC blocking cap. It can't go after the divider chain because well, it's inside the DMM IC. Outside the IC means it's on the hot side side of the divider chain, so a large expensive HV film cap plus another rotary switch contact is required, which costs more.

--- End quote ---


--- Quote from: 2N3055 on September 18, 2021, 11:17:23 pm ---I keep repeating. Yes it will. AC volts is AC coupled.

--- End quote ---

I'm sorry if this seems repetitive or if I've missed something, but I'm not clear on what 'AC coupled' may mean in this case, I haven't seen anyone actually apply the test that I mentioned and in teardowns, I have not seen the HV blocking cap that I would expect.  So perhaps I'm not clear on some aspect of how the meter works.  I'm willing to believe 2N3055 is correct and I'm wrong, but I would like to know where I'm wrong.

First, I'll assume that like most of these, the AC input is actually not totally AC coupled and will in fact have a measured 10-11M input resistance when in the AC mode and measured with another meter.  If that isn't true, then that's where I'm wrong.

Second, I'll assume that indeed my observation that there is no 1kV HV blocking cap is correct and there is not in fact one lurking in there somewhere.  Again, if there is one, that's where I'm wrong.

Third, if the first two are true, then I'll assume that the AC coupling is added somewhere after the voltage divider and if that is a capacitor, it is a smaller, lower voltage variety which is only exposed to the voltage after the divider...but those dividers don't really work in such a way as to make it easy to shove a capacitor in there, do they?   So what does that look like?   And on the 6.0000V range, what would such a blocking capacitor be exposed to if the input voltage was 5VAC + 600VDC?

Just to be thorough, I tested my F116 with 5VAC + 100 to 600VDC.  Each step up resulted in a brief 'OL', then back to a 5VAC reading.  I took it apart and there appears to be a rectangular capacitor C1 near the switch that could plausibly be a blocking capacitor, although it is a bit smaller than others I've seen. 

Caliaxy:

--- Quote from: 2N3055 on September 18, 2021, 11:17:23 pm ---
--- Quote from: bdunham7 on September 18, 2021, 11:04:50 pm ---I don't have any of the meters like the 789 to test, otherwise I would check myself--but has anyone confirmed that the DC overload issue is strictly confined to the dedicated mVAC range?  So the meters will all show 1VAC with 900VDC bias on the 6.0000V (or whatever is appropriate) range?

--- End quote ---
I keep repeating. Yes it will. AC volts is AC coupled.

--- End quote ---

Right – the issue solely occurs on the "mV" position, in AC mode. The rotary dial "mV" position has 6 modes you can circle through by pressing the yellow soft “Select” button:

DC mV
AC mv
AC+DC mV (a combined RMS value)
frequency
duty cycle
dBm*

Interestingly, the “AC mV” position is the only one in which the meter can’t figure it’s overloaded (in the presence of a DC bias) and displays a false value. “DC mV” and “AC+DC mV” show, correctly “OL”.

As circling through these 6 modes doesn’t require moving the rotary dial, there is hope that the issue could be, in theory, fixed in software (by “fixing” I mean having the meter display “OL”, as it correctly does in "AC+DC mV" mode**). It won’t help the current owners, because the firmware is not user upgreadable, but still (it will stop people complaining about this issue and bring this thread to an end; not sure about the latter :)).

*dBm also doesn’t show “OL” when it should, but that’s just a calculated value based on whatever the meter thinks it measures in AC.
**haven't checked extensively (and won't...), but the AC+DC mV mode seems to take longer to display OL when AC+DC overloaded, and it sometimes displays a first wrong value which is quickly updated to OL, suggesting that it does some extra measurements/calculations. Whatever it does, maybe it could be done in AC mV mode as well?

floobydust:
Just look at the schematics, and for the big 1,000V film cap at the input to the divider string. Looking at a few:
34401a AC-coupled C301 0.22uF 400V max DC bias.
34410a AC-coupled C400 0.22uF 400V max DC bias.
3478a  AC-coupled C301 0.15uF 630V
Agilent U1273a AC-coupled C30 0.1uF 630V
Fluke 101 AC-coupled 0.01uF 1,000V C1
Fluke 189 AC-coupled 0.01uF 1,000V
Fluke 29/79II, III Fluke 87 old; AC-coupled 0.01uF 1,000V C3
Fluke 87V; AC-coupled 0.022uF 1,000V C1
Fluke 867; AC-coupled 0.1uF 1,000V C31
Fluke 27 AC-coupled 0.022uF 1,000V C3
Fluke 10/11/12 AC-coupled 0.01uF 1,000V C14
Fluke 17B DC-coupled
AN8008 DC-coupled
UT-61e DC-coupled

For the hardware to have a DC-blocking cap, if the CMOS switches and mV AC op-amp and true-RMS converter are all in the DMM IC, then you simply can't AC-couple post-divider without something getting swamped. Like the ANENG AN8008, Brymen 789, 121GW, Fluke 17B, UT61e. These all can get overloaded from the DC.
The issue is if these detect and display it.

Note the true-RMS DSP also loses the DC component, as well as an RMS converter IC i.e. AD637. So this is a second (downstream) location in the chain where the DC component is removed. BUT everything ahead of this can get saturated- unless there's a blocking cap at the input of the divider chain which the more expensive multimeters incorporate.

joeqsmith:

--- Quote from: bdunham7 on September 18, 2021, 11:04:50 pm ---
--- Quote from: joeqsmith on September 15, 2021, 03:34:14 am ---I would hate to think I left a few of you with the idea the having a meter show zero volts while in it's ACmV mode is unique to Brymen's new BM789,  here is another one for the UNI-T fan boys.   

Of course, switch it over to tri-display mode and we get a feel for what's really going on. 

--- End quote ---

I don't have any of the meters like the 789 to test, otherwise I would check myself--but has anyone confirmed that the DC overload issue is strictly confined to the dedicated mVAC range?  So the meters will all show 1VAC with 900VDC bias on the 6.0000V (or whatever is appropriate) range?

--- End quote ---

So we want to overdrive the ACV input with high DC offset?  So we put about 2KVAC into the meter and adjust the offset by a KVDC?   Seems like fun.  I'll sit back and watch this one. 

If you're interested in more details on the 189, I took a quick stab at tracing out this area.   I also made an attempt to look at the current into a short. 

bdunham7:

--- Quote from: floobydust on September 19, 2021, 01:13:27 am ---Just look at the schematics,

--- End quote ---

I have or have seen the schematics for the first 8 you listed, but I don't have and don't recall the details of any DC-coupled models.  If you have a 10M input resistor and can live with a 30-40Hz cutoff, the capacitor can be pretty small.


--- Quote ---For the hardware to have a DC-blocking cap, if the CMOS switches and mV AC op-amp and true-RMS converter are all in the DMM IC, then you simply can't AC-couple post-divider without something getting swamped. Like the ANENG AN8008, Brymen 789, 121GW, Fluke 17B, UT61e. These all can get overloaded from the DC.
The issue is if these detect and display it.

--- End quote ---

They don't in mVAC, but I'm wondering if/how they cope with the issue in the higher ACV ranges.  And if they indeed use a pre-divider HV cap, then not incorporating that in front of the mVAC range has to be for other reasons. 

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