| Products > Test Equipment |
| Brymen BM789 |
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| 2N3055:
--- Quote from: AndrewBCN on September 18, 2021, 09:47:04 pm --- ??? It seems I got quoted? :o Actually 2N3055 this is what you wrote: --- Quote from: 2N3055 on September 15, 2021, 08:15:29 am ---... - Meters SHOULD AC couple in AC only measurements so you could measure 20mV AC riding on top of 100V - For some reason Brymen doesn't do that on AC mV range. That is NOT good way to do it. ... --- End quote --- I believe Brymen does not AC couple the ACmV range because they don't want to have to deal with the kind of accident that Joe has clearly demonstrated can happen if you add a blocking capacitor in your DMM input circuitry, as Fluke did with the 189. It's simpler, safer and cheaper for a DMM manufacturer to just add a note in the User Manual that the meter will not provide a correct reading when the offset + signal > 1000mV, and make that range DC coupled, as Brymen does with most or all of their DMMs. And of course any user that knows a little bit what she/he is doing is going to use an external blocking capacitor when needed and be careful about discharging said capacitor after completing the measurement. This is what I offered to discuss with you and others in a separate thread, but it seems you somehow misinterpreted my post as me wanting "to open Pandora's box". :-// --- End quote --- 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. |
| bdunham7:
--- Quote from: AndrewBCN on September 18, 2021, 09:47:04 pm ---I believe Brymen does not AC couple the ACmV range because they don't want to have to deal with the kind of accident that Joe has clearly demonstrated can happen if you add a blocking capacitor in your DMM input circuitry, as Fluke did with the 189. It's simpler, safer and cheaper for a DMM manufacturer to just add a note in the User Manual that the meter will not provide a correct reading when the offset + signal > 1000mV, and make that range DC coupled, as Brymen does with most or all of their DMMs. And of course any user that knows a little bit what she/he is doing is going to use an external blocking capacitor when needed and be careful about discharging said capacitor after completing the measurement. --- End quote --- I usually try to use logic and facts in these discussions, rather than characterization and hyperbole. However, in your case I'm going to make an exception. Your entire statement is an embarrassing fanboy apologist fantasy that exceeds, or a least meets, the highest levels of marketing spin that I've ever encountered. The simplest explanation is that Brymen added mVAC to a range that would otherwise only be mVDC and mVDC+AC because they could do it at virtually zero additional expense using only software. |
| bdunham7:
--- 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? |
| 2N3055:
--- 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 --- I keep repeating. Yes it will. AC volts is AC coupled. |
| floobydust:
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. The real issue is the DMM IC saturating if DC is present and it's not reliably indicated by the firmware. The op-amp in and out are available to the ADC, if the firmware looked. |
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