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.
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.
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?
...
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.
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.
[...] 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.
[...] 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.
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.QuoteFor 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.
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.
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.
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.
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?
[...] 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!!!!
Well, that was easy. ...
Of course it means that the BM789 also stores device-destroying energy in the AC mode!
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.
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?
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?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)
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.
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...
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.
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.
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.
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.
It's hard to imagine a measuring device that would want the capacitor to discharge faster than it charges.
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?