None of the asian design multimeters I can find are using a DC-blocking cap, except the BM869s.
BM789 - I don't see a DC blocking cap in any PC board photos.
Look close and you may notice the part designator for the larger component shows R. I believe this is one of the shunts but to be clear, I have not spent any time trying to trace it out.
I'll follow traces on C1, for DC-block it needs to connect to the rotary switchwhich is miles awayit does as S1A1, S2A1 but not shorted out for the majority of functions like DCV, ohms etc.
So can I can get the BM789 to read 0 mVAC with a signal applied as I suggested. You bet. Do I care, other than for a small bit of entertainment, not at all. I could run others but will wait and see what bdunham7 comes up with.
It is really interesting that this issue surprises even experienced engineers. Might there are tonns of equipment which are getting damaged all the time because of this regulary, just people are not avare of the cause. Wonder how long they keep the charge.
I don't find it interesting if experienced engineers would be surprised by this.
Thing that would come to my mind would be more like: they should know better....
So can I can get the BM789 to read 0 mVAC with a signal applied as I suggested. You bet. Do I care, other than for a small bit of entertainment, not at all. I could run others but will wait and see what bdunham7 comes up with.What was the exact waveform here?
And regarding the AC coupled mode:
Similar to the ipact wrench, or cordless drill tests where they are getting attached to eachother to see which is the stronger, we might see multimeter fights as well in the future!
Hands on hearts:
How many people knew about this and took precautions before reading this thread?
So can I can get the BM789 to read 0 mVAC with a signal applied as I suggested. You bet. Do I care, other than for a small bit of entertainment, not at all. I could run others but will wait and see what bdunham7 comes up with.What was the exact waveform here?
I have no way to know exact. From attached, it was roughly a 0.649 VRMS sine with a 2.84VDC bias.
I had no idea, but than I am also not an experienced engineer. I think with a lots of motors everywhere using inverters, this problem will pose more problems in the future than before.
... so I was wondering whether the sinewave might got too high frequency.
I had no idea, but than I am also not an experienced engineer. I think with a lots of motors everywhere using inverters, this problem will pose more problems in the future than before.Well, I don't think measuring milivolts and measuring VFD connected equipment have anything in common.
... so I was wondering whether the sinewave might got too high frequency.
All of those were done using a 60Hz sinewave.
I had no idea, but than I am also not an experienced engineer. I think with a lots of motors everywhere using inverters, this problem will pose more problems in the future than before.Well, I don't think measuring milivolts and measuring VFD connected equipment have anything in common.
I've never really given it any thought. When I use an external block, out of habit I will discharge it. When working above 24V I tend to take more precautions.
Much of the time when I need a meter, I am not using the AC modes. Normally I'm not working on a mix of technologies at the same time where the internal block would even come into play. If I am troubleshooting power electronics, I normally start with it unpowered. When I get to where I am starting to bring it up, I am using a scope. For logic, I'm using PC/JTAG, scope, LA.
So, I doubt I will go out of my way to change any habits but it's certainly something to be aware of.
Now, could Dave's 121GW diode test take out a sensitive device?
But than also designing your own thing, and knowing all the guts of it (I suppose you Joe mostly design stuff) makes you to to take different measures than starting to measure around in something which you have not much idea about, maybe not even a schematics is aviable. And there could be also some hazardous failure modes, which in theory should not happen but they do sometimes.
Can any of you also bring the 869 to show 0 Volts with a dc offset? Though there with the dual display less of an issue.
But does the 869 correctly shows "OL" for AC in this scenario? Did you try to switch polarity?
With the 869 it makes sense to have everything on the same switch position, but with the single display if there is no way to show overrange they certainly could put in a separate AC mV position.
I think we have a language barrier here.
BM869S has 2 different positions for DC and AC mV, but both are nevertheless still DC coupled .
It does show OVL if you apply larger AC voltage. If you combine it with DC, that combined goes over range it also behaves funny.
For instance 1V P-P 400HZ square wave, will show correct 500mV RMS. If I add more than 0.6 V DC to it (offset) it will start showing wrong numbers. Funny enough, I can add -2,3V offset before it starts making problems. So it is not symmetric.
But than also designing your own thing, and knowing all the guts of it (I suppose you Joe mostly design stuff) makes you to to take different measures than starting to measure around in something which you have not much idea about, maybe not even a schematics is aviable. And there could be also some hazardous failure modes, which in theory should not happen but they do sometimes.
Can any of you also bring the 869 to show 0 Volts with a dc offset? Though there with the dual display less of an issue.
Can any of you also bring the 869 to show 0 Volts with a dc offset? Though there with the dual display less of an issue.Does it really matter? I saw it as more a gimmick than anything useful.
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But generally it matters with the 789 because first I thought, that below a given displayed mV AC level I could feel myself "safe" but it seems that it is not the case.
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