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Brymen BM789

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Neutrion:

--- Quote from: 2N3055 on September 13, 2021, 03:27:40 pm ---

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 that is all getting into non important detail. Instrument shouldn't be used outside it's specified range. Good practice is to measure in Volts range all the time and only jump to mV range when we know there won't be hidden problems.

--- End quote ---

So if I understand you right, the 869s also does this or something similar? What does it show on the AC mV range if you feed in a similar 30V p-p squarewave?

Again I don't want to use the meter above its specified range (1 V p-p offset included) all what I would like, that the meter shows me that it overranged. Like it shows it correctly on DC mV or AC+DC mV.

Othervise when one measures around 5-600mV AC(correctly, because that is still within its range), and something goes wrong, and let's say 230V appears, but one won't have any clue about that. Because the meter shows almost exactly the same values.
But even a 30 V extra can be quiet problematic of course.

My question about the meter was also about the possibilities of showing "OL" above the 1V p-p level, and not whether it would be possible to measure mV AC on top of some dc offset above the specified 1V p-p treshold.

2N3055:

--- Quote from: Neutrion on September 13, 2021, 07:39:27 pm ---
--- Quote from: 2N3055 on September 13, 2021, 03:27:40 pm ---

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 that is all getting into non important detail. Instrument shouldn't be used outside it's specified range. Good practice is to measure in Volts range all the time and only jump to mV range when we know there won't be hidden problems.

--- End quote ---

So if I understand you right, the 869s also does this or something similar? What does it show on the AC mV range if you feed in a similar 30V p-p squarewave?

Again I don't want to use the meter above its specified range (1 V p-p offset included) all what I would like, that the meter shows me that it overranged. Like it shows it correctly on DC mV or AC+DC mV.

Othervise when one measures around 5-600mV AC(correctly, because that is still within its range), and something goes wrong, and let's say 230V appears, but one won't have any clue about that. Because the meter shows almost exactly the same values.
But even a 30 V extra can be quiet problematic of course.

My question about the meter was also about the possibilities of showing "OL" above the 1V p-p level, and not whether it would be possible to measure mV AC on top of some dc offset above the specified 1V p-p treshold.

--- End quote ---

Problem is that by pushing 30V in mV range meter circuitry is overloaded in such a way that not even OVL detection works anymore.
OVL is simply coming from ADC overrange register. If preamps are overloaded in such a way that that they are producing something inside ADC input range, meter (ADC) "doesn't see" overrange. That is simple fact based on fact that you need amplifier in front of ADC for sensitive range.

To circumvent around that, a separate, parallel circuitry could be added that would supervise input and monitor for overload. That circuit could be even used for autoranging.. But that would just add to the price. It is responsibility of the user not to connect meter to 10000V in millivolt mode.
It is your responsibility to know the 230V cannot appear from nowhere. Not only because of meter (it will survive) but because of you. You have to have clue. Or big badaboom...

You should be using old moving coil meter for 6 months and you would understand me well.
On those, OVL indicator was smoke coming from it...

Don't get me wrong, it would be nice if they could make completely foolproof meter.. But with good measurement practice that is not necessary.
Also good measurement practice takes into account your safety. Which always comes first.

bdunham7:

--- Quote from: 2N3055 on September 13, 2021, 10:19:42 pm ---OVL is simply coming from ADC overrange register. If preamps are overloaded in such a way that that they are producing something inside ADC input range, meter (ADC) "doesn't see" overrange. That is simple fact based on fact that you need amplifier in front of ADC for sensitive range.

To circumvent around that, a separate, parallel circuitry could be added that would supervise input and monitor for overload. That circuit could be even used for autoranging.. But that would just add to the price. It is responsibility of the user not to connect meter to 10000V in millivolt mode.

--- End quote ---

I think some systems do use as separate path for the autorange to make it faster, but it is filtered.  You'd need to tap into it earlier for the overload indicator.  Some meters are better than others at dealing with this--try a 50Hz 1% duty cycle 0-5V square wave if you want a brutal test.  (CF = 10) Even my 8846A didn't range correctly on AC with that one.  However, the issue with the OPs example is that it seems to be a pretty basic example that I wouldn't expect it to not perform well at--but there it is in the specs.  A crest factor of only 1.6 might not be enough to measure some modified sine wave inverter outputs or triac circuits, among other things.

joeqsmith:

--- Quote ---1.If I use the mV Scale and feed in a squarewave with 50% duty cycle without(or very minimal like 100-200mV) zero crossing ...
--- End quote ---

I started to read your original post but the was not sure with all the missing details.  mV (rms, p-p, average, DC).  I wonder is scale DC, AC, AC+DC.   Want me to try and guess?    What does zero crossing mean to you?  Maybe use terms like offset.   

I have no problems running what ever test cases you want but you need to be VERY clear about what you want to see.   Even a scope shot would help.   


****
So, the BM789 is set to ACmV autorange.  You inject a 40Hz to 500Hz, 50% duty squarewave with no offset and 100mVp-p (+/-50mV) and the meter reads fine.  You then increase the amplitude to 1Vp-p and your meter displays 650mV.  You then continue to drive it up to 30Vp-p and the meter continues to display 650mV.   Then you flip the leads (even though there is no offset and they waveform is symmetrical) and the meter starts to read correctly.   You have verified the wavefrom you are injecting by other means (scope) and know it to be symmetrical.   

Then you go onto to talk about  " I assume it depends on which polarity the squarevawe has."  which now I am thinking it is not a symmetrical waveform but rather switching from 0 to some peak value.       Then later you mention  " (1 V p-p offset included) "    :-//

I thought if I waited and reread your post I could make sense of it.   My guess is you are adding an offset in which case, I bet you could make the meter do all sorts of strange things and I can believe that flipping the leads would make a difference.   A bit of an odd use case but I won't judge. 

Maybe come at it from another direction and post your actual requirements.  It seems you can't use a blocking cap which is typical and you don't want to have to get into the practice of starting in the high ranges.   So, I would say it sounds like you bought the wrong tool for the job. 
So maybe start with the requirements and someone could maybe recommend something. 
 

Neutrion:

--- Quote from: 2N3055 on September 13, 2021, 10:19:42 pm ---
You should be using old moving coil meter for 6 months and you would understand me well.
On those, OVL indicator was smoke coming from it...

Don't get me wrong, it would be nice if they could make completely foolproof meter.. But with good measurement practice that is not necessary.
Also good measurement practice takes into account your safety. Which always comes first.

--- End quote ---

Obviously I don't use the meter with starting on the millivolt scale, but things can happen in life (at least with me) which we don't foresee so I would be rather on the safe side when measuring something unknown, and be sure that the values around and above 500mV AC are also corect, and I don't have to worry that the reality is something entirely different. With a dual display, as I said it might won't be an issue as you could see on the DC or AC+DC value, that you are out of range but with the single display this might pose a problem.  So this is just safety, like the fuses in your meter, which are also not supposed to be blown- at least regulary.

Joe:

Without zero crossing I meant an offset, so it is an assymetrical squarewave. Meter scale is at AC mV.

An example:Assymetrical squarewave duty cycle 50% p-p 5,28V average 2,52V max 5,12V min -160mV rms 3,55 V
AC V scale correct display: 2,5-2,7 V(STdev on Siglent) which is correct, mV scale: 650mV
Will add a scopeshot if needed later on.
At 1V p-p the meter measures about correctly(not 650mV) so I am not talking about it not meeting the specs, things start to happen if I go above the limit.

I don't have a signal gen, so I can't just test it with different offset levels. But within the 1V p-p including offset there seems to be no problem with the measurement. So the specifications are seemingly met.
I can not tell you what it does if I feed in the same signal without offset and raise it above the 1V p-p level, but it seems, that at least if I go above the specified limit with an assymetrical signal (which again I in a normal case obviously don't want to do, but could happen during a measurement) the AC mV scale does not show that it overranged, but stays like that at around 650mV and changes only minimally.
If I flip the leads it shows "OL" what should be the correct behaviour.
So with changed polarity I meant that the sqarewave is assymetrical signal starts almost at zero, and goes up to above 1V, positive, so the meter gets the signal with a given polarity, but if I flip the leads the meter gets the signal with the opposite polarity.

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