Author Topic: Brymen BM789  (Read 68865 times)

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Offline bdunham7

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Re: Brymen BM789
« Reply #150 on: September 19, 2021, 01:31:05 am »
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. 

No, not quite that high!  I want to use the lowest 'AC coupled' non-mVAC range with an appropriate in-range AC signal and add DC bias much larger than the range but not outside the meters stated limitations.  So, Fluke 116, 5VAC on 6.000VAC range with 600VDC bias--works fine.  Brymen 789 on 6.0000VAC range with 5VAC + 1kV DC bias (or whatever is handy), works ???.  And is there an HV cap in there somewhere?
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline Caliaxy

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Re: Brymen BM789
« Reply #151 on: September 19, 2021, 01:59:25 am »
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.
The DC V mode (which allows combined DC+AC rms measurments) and the mV mode (AC, DC and AC+DC) on BM789 present, indeed a 10-11 MOhms input resistance (as measured by 189). The plain AC V mode shows high impedance (OL, above what 189 can measure).
 

Offline joeqsmith

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Re: Brymen BM789
« Reply #152 on: September 19, 2021, 02:41:56 am »
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. 

No, not quite that high!  I want to use the lowest 'AC coupled' non-mVAC range with an appropriate in-range AC signal and add DC bias much larger than the range but not outside the meters stated limitations.  So, Fluke 116, 5VAC on 6.000VAC range with 600VDC bias--works fine.  Brymen 789 on 6.0000VAC range with 5VAC + 1kV DC bias (or whatever is handy), works ???.  And is there an HV cap in there somewhere?

Where's the fun in that?  As I previously mentioned, a kV is a bit too risky (not the meter but TE).   But, if you will settle for something a bit lower, shown is 6.5VAC in the lowest ACV range (manual) with a -600VDC offset and again at 700VDC. 
 
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Offline AndrewBCN

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Re: Brymen BM789
« Reply #153 on: September 19, 2021, 02:56:14 am »
...
Yeah it's kind of misunderstanding or better say my prediction where that discussion would end up, not by you and me but maybe somebody else.

We cannot know whether Brymen did not include it because it was easy and lazy thing to do or there was intention and reason for it. Designer of the meter would know that, but we can only speculate. And in order to speculate on a level of "educated guess" instead of "some punters are crapshooting ideas" a reverse engineering could be done on meter inputs, and then you could see if decision to DC couple it had some simplification benefits (like less contacts on switch used, or simplification of layout, or whatever). It might have been because switch routing was simpler and provided more isolation distance or whatever.. Now I'm just throwing random ideas and that is useless..

Once reverse engineering of schematics was done, and alternative version that include capacitor was posited, there is a thin line between that and people that start cutting the board and adding capacitor into what is now a improvisation that outside looks  like a CAT IV meter and inside is a death trap..
I don't want to be complicit in something like that, and I personally don't care why. It is what it is, I use external cap if I need it, or lately just use MTX3293 if I need AC coupled mV. That one has the capacitor. But, mostly it is not an issue. I already spent too much time on it, and that only because I realized it was topic that was unknown to many, to my surprise. So it was good deed to spread the word, we also spoke a bit about good measurement practice (also good topic) and that's it.

If I ever decide to design my own meter design (hypothetically speaking, no intention to do so..) I would then think about it in more detail. I have no interest for it now.

Thanks for clearing that up, 2N3055 and I perfectly understand and agree with your position and Joe's about avoiding and not recommending modifications to the input circuitry of any instrument rated for high voltages. I fully subscribe to that.

I guess the discussion in this thread was sufficiently fruitful in the sense that it is clear (to me at least) that among the best practices for DMM usage, shorting the leads before and after any measurement can help avoid some kinds of accidents. And there is no need for further discussion of the input stages of various DMMs or positing about what the exact intentions of Brymen engineers were when they decided to make the input stage of the ACmV range of the BM789 DC-coupled - as you pointed out, we'll probably never know for sure.
 

Offline bdunham7

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Re: Brymen BM789
« Reply #154 on: September 19, 2021, 02:57:21 am »
The DC V mode (which allows combined DC+AC rms measurments) and the mV mode (AC, DC and AC+DC) on BM789 present, indeed a 10-11 MOhms input resistance (as measured by 189). The plain AC V mode shows high impedance (OL, above what 189 can measure).

Where's the fun in that?  As I previously mentioned, a kV is a bit too risky (not the meter but TE).   But, if you will settle for something a bit lower, shown is 6.5VAC in the lowest ACV range (manual) with a -600VDC offset and again at 700VDC.

Well, that was easy.  Looks like I'm wrong on the first count and there is an HV capacitor right in front somewhere and it does just what it should.  So the decision vis-a-vis the mVAC range, like the F116, isn't over the cost of a capacitor.

Of course it means that the BM789 also stores device-destroying energy in the AC mode!  :)
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline floobydust

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Re: Brymen BM789
« Reply #155 on: September 19, 2021, 04:08:26 am »
[...] it is clear (to me at least) that among the best practices for DMM usage, shorting the leads before and after any measurement can help avoid some kinds of accidents.
NO!!! This is wrong, where did you pick this up from? If the equipment has a charged up DC-blocking cap, it will discharge into the front-end and kill it.
 

Offline AndrewBCN

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Re: Brymen BM789
« Reply #156 on: September 19, 2021, 04:47:23 am »
[...] it is clear (to me at least) that among the best practices for DMM usage, shorting the leads before and after any measurement can help avoid some kinds of accidents.
NO!!! This is wrong, where did you pick this up from? If the equipment has a charged up DC-blocking cap, it will discharge into the front-end and kill it.

 ???

How would a cap that is part of the front-end discharge into the front-end when the leads are shorted?

Joe, we need another video!!!!  :-DD
 

Offline floobydust

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Re: Brymen BM789
« Reply #157 on: September 19, 2021, 04:51:33 am »
Just look at the schematics,

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.

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.

For the mV range, the input attenuator is not employed and (DC-coupled) the DMM IC gets the same voltage input as the test leads, almost directly (post-PTC) connected to the IC. There is a 10MEG load switched in.
There is next a mux and op-amp or PGA because you need a gain of ~4-10x before the ADC to have a decent noise floor. 141mV in, X10 and you've got 1.41V going to the op-amp then ADC.
What happens with say 1V DC on 100mVAC ? The amp is saturated or even phase-inverts and you can get bogus readings. Unless you are constantly checking (in firmware) for this, which is what we should be checking for.
For AC-coupled equipment, discharging the blocking cap into the front-end, say 200V and a few hundred K seems to be at most a few mA but JFET, CMOS mux etc. don't always survive. It seems to be the high voltage damaging the semi. Some gear does not have much for series resistance or a huge cap and the discharge hits hard.
 

Offline Fungus

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Re: Brymen BM789
« Reply #158 on: September 19, 2021, 06:27:13 am »
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. 

Have you painted your 189?
 

Offline Fungus

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Re: Brymen BM789
« Reply #159 on: September 19, 2021, 06:27:49 am »
NO!!! This is wrong, where did you pick this up from? If the equipment has a charged up DC-blocking cap, it will discharge into the front-end and kill it.

Doesn't it have input protection for that?  :popcorn:
 

Offline 2N3055

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Re: Brymen BM789
« Reply #160 on: September 19, 2021, 06:32:37 am »
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.

I keep repeating. Yes it will. AC volts is AC coupled.

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.


Well if you put, like I suggested (also repetitively) other meter in set in ohms to input of the meter in question, you will see if input impedance going high, to isolation range. Also, you can take a meter in question, and exactly what was said: you charge meter input wit, say 12V to be safe and discharge it into scope probe set to safe range. If you see exponential decay, there you are you have capacitance.

Also, there is only one way to do it, really. So AC coupled, in this context, means serial capacitor...
« Last Edit: September 19, 2021, 07:20:27 am by 2N3055 »
 

Offline joeqsmith

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Re: Brymen BM789
« Reply #161 on: September 19, 2021, 02:06:23 pm »
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. 

Have you painted your 189?

This was that meter that I bought to transient test.  You will find my posts on painting it here:

https://www.eevblog.com/forum/testgear/painting-over-rubber/msg1827791/#msg1827791

Over the years, the paint peeled off and you are seeing the primer.  The rubberized paints just don't seem to adhere well in this application.   

Offline joeqsmith

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Re: Brymen BM789
« Reply #162 on: September 19, 2021, 02:28:27 pm »
[...] it is clear (to me at least) that among the best practices for DMM usage, shorting the leads before and after any measurement can help avoid some kinds of accidents.
NO!!! This is wrong, where did you pick this up from? If the equipment has a charged up DC-blocking cap, it will discharge into the front-end and kill it.

 ???

How would a cap that is part of the front-end discharge into the front-end when the leads are shorted?

Joe, we need another video!!!!  :-DD

The last video shows me shorting the leads after applying a kV to meter in an attempt to look at the current.   Maybe that's how they design them in Canada.  floobydust would need to provide a brand/model for one that can be damaged as they suggest.      If I have the meter, I would certainly be willing to attempt to replicate their findings.     

Offline joeqsmith

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Re: Brymen BM789
« Reply #163 on: September 19, 2021, 02:47:57 pm »
Well, that was easy.  ...

Of course it means that the BM789 also stores device-destroying energy in the AC mode!  :)

I could run it up higher if you wanted but would want to make a few changed to my setup just to play it safe.   

Yes, after applying a DC voltage to the 789 while it is in it's ACV mode, I can then discharge the meter to the LED bank and have it light.  I would suggest it stores potentially device-distroying energy.   The parasitics alone may be enough to damage something sensitive.  If you watched that last video, that was basically my point.  This isn't a static problem in the sense that everything is at steady state.   Measuring the DC resistance has little to do with the potential peak currents.   

Offline Fungus

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Re: Brymen BM789
« Reply #164 on: September 19, 2021, 02:48:32 pm »
The last video shows me shorting the leads after applying a kV to meter in an attempt to look at the current.   Maybe that's how they design them in Canada.  floobydust would need to provide a brand/model for one that can be damaged as they suggest.      If I have the meter, I would certainly be willing to attempt to replicate their findings.   

Surely the stress of discharging it is no worse than the stress of charging it.

(assuming a low source impedence for charging).
 

Offline joeqsmith

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Re: Brymen BM789
« Reply #165 on: September 19, 2021, 03:17:15 pm »
The last video shows me shorting the leads after applying a kV to meter in an attempt to look at the current.   Maybe that's how they design them in Canada.  floobydust would need to provide a brand/model for one that can be damaged as they suggest.      If I have the meter, I would certainly be willing to attempt to replicate their findings.   

Surely the stress of discharging it is no worse than the stress of charging it.

(assuming a low source impedence for charging).

Considering the attached schematic, assuming V+ is 12V, D is a silicon diode,  R is 10kohms, C is 10uF and all components are perfect (no parasitics,  switch resistances are zero when closed and infinite when open), under what condition will the current through the capacitor reach it's highest peak current?

A) when charging
B) when discharging
C) A and C
D) non of the above
E) what's a diode?

Offline Fungus

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Re: Brymen BM789
« Reply #166 on: September 19, 2021, 03:46:48 pm »
Considering the attached schematic, assuming V+ is 12V, D is a silicon diode,  R is 10kohms, C is 10uF and all components are perfect (no parasitics,  switch resistances are zero when closed and infinite when open), under what condition will the current through the capacitor reach it's highest peak current?

Does current flow through a capacitor?  >:D

A) when charging
B) when discharging
C) A and C
D) non of the above
E) what's a diode?

Assuming you meant the peak current flowing at point "I" in the circuit, I'm gonna go with (A)
 

Offline joeqsmith

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Re: Brymen BM789
« Reply #167 on: September 19, 2021, 04:02:10 pm »
Considering the attached schematic, assuming V+ is 12V, D is a silicon diode,  R is 10kohms, C is 10uF and all components are perfect (no parasitics,  switch resistances are zero when closed and infinite when open), under what condition will the current through the capacitor reach it's highest peak current?

Does current flow through a capacitor?  >:D

A) when charging
B) when discharging
C) A and C
D) non of the above
E) what's a diode?

Assuming you meant the peak current flowing at point "I" in the circuit, I'm gonna go with (A)
First hit on displacement current gives us wiki:
https://en.wikipedia.org/wiki/Displacement_current

For the meters, I would assume that the charge/discharge would be basically the same.  Even on the 189, it looks like it follows two different paths.  Still the difference is  negligible.   Still, it's common to find exceptions when making blanket statements. 

Offline Fungus

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Re: Brymen BM789
« Reply #168 on: September 19, 2021, 04:27:08 pm »
For the meters, I would assume that the charge/discharge would be basically the same.  Even on the 189, it looks like it follows two different paths.  Still the difference is  negligible.   Still, it's common to find exceptions when making blanket statements.

It's hard to imagine a measuring device that would want the capacitor to discharge faster than it charges. It would go horribly wrong as the AC frequency increases.
 

Offline bdunham7

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Re: Brymen BM789
« Reply #169 on: September 19, 2021, 04:30:10 pm »
Well if you put, like I suggested (also repetitively) other meter in set in ohms to input of the meter in question, you will see if input impedance going high, to isolation range. Also, you can take a meter in question, and exactly what was said: you charge meter input wit, say 12V to be safe and discharge it into scope probe set to safe range. If you see exponential decay, there you are you have capacitance.

Also, there is only one way to do it, really. So AC coupled, in this context, means serial capacitor...

I don't have this meter to test, so until Caliaxy and joeqsmith posted, I had no way of knowing what this result would be.

There is not only one way to do it, this I know because I have examples where it was not done in exactly the same way.  The F116, for example, passes the HV blocking test even on the lowest (non-mV) range of 6.000V.  However, its measured input resistance (DC) is 10M on all ranges.  This is an issue of some significance to me because I'm using HV divider probes to measure HV at times and I've always wanted better results.  Depending on the divider probe, you could end up charging a blocking cap to the full HVDC bias, which would be bad--so you would avoid using a truly AC-coupled range with an HV probe that didn't have a lower divider resistor.  Fortunately most of them DO have  this lower divider, but I did have one go open. 

So I'm still learning tidbits here and there that I haven't thought of.  The F116 is fairly new to me, and I hadn't considered the input characteristics beyond knowing that the AC input had a 10M straight resistance across it.  Only with this thread did I discover that the mVAC range is actually only mVAC+DC.  Now I know that the F116 also eliminates the hazards of a charged AC blocking capacitor, since the 10M across the front end will quickly discharge it.  I'm not saying that Fluke did this deliberately to prevent HVAC technicians from frying control circuitry, since that might be an " embarrassing fanboy apologist fantasy", but it is a characteristic.  Interestingly, they seem to have done this by using a 10M capacitive coupled input in parallel with a 10M resistor across the front end, giving the meter an effective input impedance of 5M on the AC ranges.  That does actually seem deliberate.   And it messes up my 80k-40 HV probe by 10% or so...
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline joeqsmith

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Re: Brymen BM789
« Reply #170 on: September 19, 2021, 04:43:33 pm »
Well, that was easy.  ...

Of course it means that the BM789 also stores device-destroying energy in the AC mode!  :)

I could run it up higher if you wanted but would want to make a few changed to my setup just to play it safe.   

Yes, after applying a DC voltage to the 789 while it is in it's ACV mode, I can then discharge the meter to the LED bank and have it light.  I would suggest it stores potentially device-distroying energy.   The parasitics alone may be enough to damage something sensitive.  If you watched that last video, that was basically my point.  This isn't a static problem in the sense that everything is at steady state.   Measuring the DC resistance has little to do with the potential peak currents.

Shown with the 789 set to the ACV mode, again with it set to the lowest manual range.   Fluke 189 will over range at 1100V.  Shown at 1050VDC offset then again with 1200VDC.   

Offline joeqsmith

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Re: Brymen BM789
« Reply #171 on: September 19, 2021, 04:49:55 pm »
For the meters, I would assume that the charge/discharge would be basically the same.  Even on the 189, it looks like it follows two different paths.  Still the difference is  negligible.   Still, it's common to find exceptions when making blanket statements.

It's hard to imagine a measuring device that would want the capacitor to discharge faster than it charges. It would go horribly wrong as the AC frequency increases.

Say you wanted to make an oscillator out of CMOS gates and have something other than a  50% duty cycle.  Rather than a simple RC to set the timing, we could use this circuit to provide two different time constants.   Maybe your measuring device has an RC network on the CPUs reset for a slow start but when the power is cycled, you want it to reset fast.   It's not that uncommon. 

Offline Fungus

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Re: Brymen BM789
« Reply #172 on: September 19, 2021, 05:06:49 pm »
Maybe your measuring device has an RC network on the CPUs reset for a slow start but when the power is cycled, you want it to reset fast.   It's not that uncommon.

That's a measurement device's reset circuit, not a measurement device's measuring circuit.
 

Offline joeqsmith

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Re: Brymen BM789
« Reply #173 on: September 19, 2021, 05:31:38 pm »
Maybe your measuring device has an RC network on the CPUs reset for a slow start but when the power is cycled, you want it to reset fast.   It's not that uncommon.

That's a measurement device's reset circuit, not a measurement device's measuring circuit.

Your comment had broadened the scope. 
Quote
It's hard to imagine a measuring device that would want the capacitor to discharge faster than it charges.

Offline floobydust

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Re: Brymen BM789
« Reply #174 on: September 20, 2021, 08:50:43 pm »
NO!!! This is wrong, where did you pick this up from? If the equipment has a charged up DC-blocking cap, it will discharge into the front-end and kill it.
Doesn't it have input protection for that?  :popcorn:

Summarizing, the BM789s is DC-coupled as far as the hardware and DMM IC, and there's a HPF from the true-RMS DSP section if Brymen is using it verses the MCU doing the math with raw A/D samples.
So I believe it can be saturated with DC on any ACV measurements if it can't auto-range up, and the F/W needs a check to see of if it lets you know this is happening.

I looked at the usual dozen multimeter schematics, the ones incorporating a DC-blocking cap for ACV. Maybe others can contribute.
None of the asian design multimeters I can find are using a DC-blocking cap, except the BM869s.
The "protection circuit" is needed to clamp down to the IC level, the CMOS switches downstream of the HV protection MOV's. Interestingly, the AC path does not include anything like our usual diode-connected transistors etc. which end up on the DC/ohms path. AC protection is by large series resistance, 87V is 10MEG and 34401a is 1MEG so the cap discharge is low current, enough for an IC's substrate diodes to handle. But the voltage is still high for whatever you are probing, not sure what a mosfet does with HV on the gate at low current, for a long time. ESD is short duration and 10X and the voltage at 1/20 to 1/200 the capacitance.

Interestingly ACV the 87V moves the 9.99MEG after the cap, so it's a 10MEG AC impedance and very high (leakage) DC resistance.
So using a single-resistor HV probe with the DMM on ACV would charge to a clamped couple kV DC-component and not be a good experience.

Oscilloscopes have larger blocking cap (up to 22X) and lower impedances for the bandwidth, some using relays/switches or maybe solid-state AC/DC coupling switching. The cap discharge is brutal there maybe 15mJ across the switch and goodbye JFET if you short the probe's input. 2235a rated 400VDC goes 0.022uF down to 1,000pF +ve discharge through 70R to the JFET, minus a 10X probe. It all depends on the moves you make while the cap has a HV charge.

As far as providing a make/model/schematic a showman could use, awareness of the charged cap is more important than damaging something. I treat it like a grenade- a high risk measurement (AC ripple/signal on HV DC) across the board.
 


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