| Products > Test Equipment |
| Brymen BM789 |
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| bdunham7:
--- Quote from: joeqsmith on September 19, 2021, 01:20:47 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. --- End quote --- 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? |
| Caliaxy:
--- Quote from: bdunham7 on September 19, 2021, 12:36:43 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. --- End quote --- 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). |
| joeqsmith:
--- Quote from: bdunham7 on September 19, 2021, 01:31:05 am --- --- Quote from: joeqsmith on September 19, 2021, 01:20:47 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. --- End quote --- 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? --- End quote --- 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. |
| AndrewBCN:
--- Quote from: 2N3055 on September 18, 2021, 10:18:24 pm ---... 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. --- End quote --- 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. |
| bdunham7:
--- Quote from: Caliaxy on September 19, 2021, 01:59:25 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). --- End quote --- --- Quote from: joeqsmith on September 19, 2021, 02:41:56 am ---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. --- End quote --- 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! :) |
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