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Brymen BM789

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bdunham7:

--- Quote from: 2N3055 on September 14, 2021, 07:45:53 pm ---As I said, in a perfect world, it would be nice that if you connect 1000V to mV input, it should not damage it, and then meter should, in a pleasant voice say: "bip. Dear operator..  please make a note that you connected meter to 1000V on a millivolt range. While I was designed to withstand such overload indefinitely, and your safety is not jeopardized, I am unable to measure voltage correctly. Please change input range to volts. Thank you for your cooperation....bip".

Are you kidding me? :palm:

--- End quote ---

Every single one of my DMMs does exactly that, more or less, AFAIK--including some very, very old models.  The newer ones with CAT ratings can take that 1000V on any range--ohms, diode check, etc.  I believe it is a requirement.  As far as there being a requirement for an overload indicator, I'll defer to others since I don't have any written standards to consult.  Input protection circuitry is not that hard to make and in any case, is a well known quantity.

Caliaxy:

--- Quote from: AndrewBCN on September 14, 2021, 07:01:21 pm ---
--- Quote from: Caliaxy on September 14, 2021, 06:42:46 pm ---...
In every single other mode (DC mV included), the meter displays “.OL” when overloaded, hence the reasonable expectation to do so in AC mV as well.
...

--- End quote ---
Unfortunately the "reasonable expectation" argument doesn't work here, for a variety of reasons, but I suggest you read the excellent post by 2N3055, here:

https://www.eevblog.com/forum/testgear/brymen-bm789/msg3686746/#msg3686746

Again, you and the OP are assuming that this was an oversight by the Brymen engineering team, or a bug in the firmware of the BM789. I don't think that's the case at all.

And as has been stated many times: if one so much as spends 45 seconds to read the User Manual and use the DMM accordingly, this issue will never come up.

--- End quote ---

I did read that post (and all the other posts in this thread). I find it excellent and educative (like most 2N3055's posts) but a little bit exaggerated (the part with 10000V on mV scale...). I didn't say it's a bug or an oversight, I said it is a design fail, which does not infer that Brymen are (or are not) aware of it.

Maybe "design fail" is too harsh. What would this be then? A price-motivated decision? It would be great if a good meter from a reputable brand would not display false information when overloaded. Grateful to learn that it might. It might not be easy to do it right. If this is understandable in lower cost meters, in Brymen's case, even their flagship model (BM869S) behaves the same way. But their flagship meter is also positioned at a very competitive price point... In A brands, the cheaper meters seem to behave similarly but at least the higher end ones behave correctly. Maybe this is the difference between A brands and B brands, and these are the kind of trade offs one should be aware of.

Again, no drama here, we can all live with this. I don't expect things to be perfect. The fact that you need to press a button 6 times to switch from AC mV to DC mV bothers me more than the DC offset issue raised here, but I can live with that too if I understand that it saves on the cost (and, of course, I'm free to choose to pay the difference if I want better).

Out of curiosity: what's your meter? Does it behave the same way? If yes, were you aware of it before this thread?

Neutrion:

--- Quote from: Fungus on September 14, 2021, 07:31:35 pm ---
Anything they couldn't cram in there? They documented it. What else could they do?

It might be possible to make it say 0L, we'll see. I'd bet heavily against them being able to make it measure correctly though.

--- End quote ---
I don't want it to read correctly above the specified limit, only to show "OL" if overloaded.

But it is kind of amazing to see people who never ever measure anyting else than what they expect, this would be my goal as well to get to this level.
But for those who are not perfect yet, an example again:
User starts measuring correctly on the V AC scale,  to see what is the level of the signal like.
Meter shows 200 mV AC, so user proceeds to the mV scale because he needs  better accuracy.
(Up until this point I think we all agree that he is doing right...)
Than he sees the signal raising to 600mV. And there he can not be sure whether it is 600mV or maybe 325V because something went wrong.
I will do some tests to see what is the highest reading which one can trust. But it depends both on frequeny and duty cycle. Possibly also on waveform, but I can not easily test anything else than a squarewave.

If we could find out whether there is any limit up to which the meter could be trusted, than it means that it could be set as overrange limit in the FW as well. But than we would lose counts.
But I hope to get a more elegant solution.

bdunham7:

--- Quote from: Neutrion on September 14, 2021, 09:03:03 pm ---But I hope to get a more elegant solution.

--- End quote ---

People have mentioned an external blocking capacitor, but to me that just seems like an opportunity to short something out or get zapped.  After all, the reason I have CAT-rated meters, shielded test leaks, insulated alligator clips and so on is to avoid the accidental zapping of either hapless components or myself.  So here's an idea!

Some of Fluke's older but still produced meters lack the Lo-Z function.  This feature, also known as Ghost Voltage or Stray Voltage eliminator, works by lowering the input impedance of the meter with a parallel PTC so as to short out very high impedance voltage leakage that you don't want to measure.  So, Fluke has an add-on unit, with nicely shielded jacks and plugs, that can be inserted between the test leads and the meter to add this functionality.  Note the price!  ::)

https://www.fluke.com/en-us/product/accessories/adapters/fluke-sv225

Brymen (or someone) can introduce the same thing only put a 1kV blocking capacitor in the unit instead of a PTC.  They can call it the 'ripple master' or TrueAC or DC Block...or whatever.

2N3055:

--- Quote from: bdunham7 on September 14, 2021, 08:42:59 pm ---
--- Quote from: 2N3055 on September 14, 2021, 07:45:53 pm ---As I said, in a perfect world, it would be nice that if you connect 1000V to mV input, it should not damage it, and then meter should, in a pleasant voice say: "bip. Dear operator..  please make a note that you connected meter to 1000V on a millivolt range. While I was designed to withstand such overload indefinitely, and your safety is not jeopardized, I am unable to measure voltage correctly. Please change input range to volts. Thank you for your cooperation....bip".

Are you kidding me? :palm:

--- End quote ---

Every single one of my DMMs does exactly that, more or less, AFAIK--including some very, very old models.  The newer ones with CAT ratings can take that 1000V on any range--ohms, diode check, etc.  I believe it is a requirement.  As far as there being a requirement for an overload indicator, I'll defer to others since I don't have any written standards to consult.  Input protection circuitry is not that hard to make and in any case, is a well known quantity.

--- End quote ---

Ok, so your point is that it should survive.
Ok, valid point.
And it will, because it was tested it will. Survival is not in question here.

But, if the point is that it should survive,  and also show voltage is too high for the range and that there might be dangerous voltages and that you should change range?
On every range ??? :popcorn:

So here is a brain teaser :  in your own words it will survive in any range including Ohms, right.  And what will meter set on ohms show when you connect it to say 400V?
It will keep showing Open on the screen. Whoops....

That is exactly why meters for this "I have no clue what to expect" thing do exist. On my BM525 you have auto mode. It will automatically detect AC, DC or Ohms/continuity.
In a single switch position.

You want failsafe?
There are are also Duspol type voltage tester devices. I have BM75EU from that type. Guess what I use when poking around electric installations, power sockets, fuses and anything 240V AC ? It also has vibration warning, RCD tester, and works even when batteries die. LCD with precise measurements and resistance/continuity stops working, but voltage detection still works. That is regulatory requirement for that type of devices.. User safety first. Best accuracy 1.5% DC, 2.5% AC.

BM789 is a electronics meter, very, very nice one. High resolution and accuracy, lots of functions. It even has "fluke like" diode test, continuity with screen blinking, it will remember settings..
It is probably best meter in the class. Even if you don't look at the price.
But it is not failsafe device for field use, it is not explosion proof, it is not IP87...
It requires operator to operate it with intelligence, otherwise garbage in , garbage out.

Don't get me wrong. I do understand that completely failsafe device probably could be made.
You would end up with a device that would be a fancy version of that Duspol type device.
No current measurement. They are inherently unsafe. Slow auto ranging/switching (because now it has to not only range but also decide which one is it supposed to do)...No high bandwidth, for better RF resilience. etc etc..

And no AC mV range.. That is how Fluke solved that problem with F87V.
I prefer to have it, even if it requires thinking when using..

When you guys find perfect meter let me know. I am interested, but in meantime I'll work with what I have.
In a  way the instrument designer expects me to do it. That guarantees good results.

Whoever doesn't like that, I have a snarky Open Source zealots like comment : " If you don't like, it make your own!".

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