Products > Test Equipment

Brymen BM789

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Fungus:

--- Quote from: Fungus on September 14, 2021, 02:39:47 pm ---
--- Quote from: bdunham7 on September 14, 2021, 02:28:35 pm ---To be clear, since you are implying what 'others' agree on and I'm an other, I don't agree and think the meter should show an overload indication.  Whether that's a design issue or a one-off defect with your unit, I don't know.

--- End quote ---

That assume the meter can detect the condition, which is an unknown.

--- End quote ---

Although... thinking about it... if it's displaying the number "650" on a 60,000 count meter then the ADC is probably seeing binary 11111111111111111111...111111 (however many bits there are in the ADC).

In that case it's firmware fixable. Over to Joe.  :P


--- Quote from: Fungus on September 14, 2021, 02:39:47 pm ---Yep. This is a massive bangperbuck meter.

--- End quote ---

The hardware limitation could be as simple as them running out of inputs on the ADC's input multiplexer. A separate, AC-coupled circuit would need another input. It might not be possible to do it even though capacitors are cheap.

joeqsmith:

--- Quote from: Fungus on September 14, 2021, 02:39:47 pm ---The way to find out would be to have Joe do a video on it. If Brymen fixes it and sends him another meter for his collection then we know it's a firmware issue.   :D

--- End quote ---

 :-DD 
There was another concern I had early on with the 78x and when I bought it up, the user had not read the manual.   :palm:     



--- Quote ---Adding a capacitor in series sounds like a simple fix but is it really that simple or would it make the meter less robust? Does the entire circuit get more complicated and need many more components than just a capacitor?
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I think my old analog meter from my youth had a capacitor built-in with a separate jack similar to that cheapo unit I looked at.  Note that the jack is marked output but you can clearly see the DC blocking cap in the schematic and we know what it's purpose is.   

https://youtu.be/_c7tD7UeXUg?t=1620

I've used DMMs to look at ripple but just add a blocking cap out of habit.  Not a lot of drama.

Neutrion:

--- Quote from: joeqsmith on September 14, 2021, 02:29:39 pm ---
--- Quote from: Neutrion on September 14, 2021, 01:53:54 pm ---
--- Quote from: joeqsmith on September 14, 2021, 12:28:07 pm ---Now that it is clear you are adding a DC offset,  none of what you wrote surprises me.     At first I thought it was the wrong tool for the job but not posting your requirements, it seems you're just playing around with your new toy.   Most of the responses here seem correct.

--- End quote ---

So do you agree with the others, that there is no way of showing the overload under these circumstances?

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I believe that given a set of requirements, a good group of EEs could design products to achieve them. 



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Ok I mean in this case with this meter, without(complicated, or "robustness lowering") HW mod.

But it is also interesting what are the general reqirements here.



--- Quote from: bdunham7 on September 14, 2021, 02:28:35 pm ---
To be clear, since you are implying what 'others' agree on and I'm an other, I don't agree and think the meter should show an overload indication.  Whether that's a design issue or a one-off defect with your unit, I don't know.

And no, this doesn't happen with most meter designs AFAIK, at least not the ones I have with an explicit mVAC function.  I think it is a result of 'bangforbuckitis' or the manufacturer trying to add as many features as possible at a price point.  Something has to give.

And, b/t/w, contrary to some earlier discussion, if this behavior is characteristic of all the units, I think issue of the reading hanging without an OL indication might well be resolvable in firmware. 

--- End quote ---

OK sorry, it is also good to know that I might not be alone and a complete idiot with my "requirement".

And you also think it is solvable in FW. Which would be nice.


--- Quote from: joeqsmith on September 14, 2021, 02:41:28 pm ---
Outside of any influence my small YT channel and posts on DMMs may have had on peoples purchasing decisions, I am not involved with marketing or sales.    I don't work for Brymen and really have no understanding of the hows and whys they or any other DMM company make the choices they do. 

Again,  I find meters that can display more than one parameter at a time allow me to work more efficient.   That's my personal choice.   As a consumer, it's good to have such choices to make! 

--- End quote ---

I thought you might could have a general idea -not necesseraly based on hard facts, so the category of oppinion which you seemingly don't like to have:) -  what kind of technical aspect could lay in the background for Brymen choosing the single display.
More processor needed, more complex circuitry other interesting stuff.
I mean generally only complexity and the mentioned possible industrial case speaks against the multi display.Maybe even you could like the meter more than the 869 as you mentioned.

bdunham7:

--- Quote from: Fungus on September 14, 2021, 02:32:10 pm ---Yes, but it's a 50,000 count meter so you should be able to read power supply ripple perfectly well on the normal ACV range.

--- End quote ---

50k counts helps, but you still have TRMS residual counts and residual count suppression to deal with, and it also depends on how low you want to measure the ripple.  If your spec is 5mV on a 20V supply and you have to read that on the 50V range...


--- Quote ---Adding a capacitor in series sounds like a simple fix but is it really that simple or would it make the meter less robust? Does the entire circuit get more complicated and need many more components than just a capacitor?

--- End quote ---

On the designs I've looked at, adding it early on is fairly simple.  Simply switching it in ahead of everything, giving the meter an infinite DC impedance, requires a HV capacitor but causes no issues.  After the voltage divider/gain stage is a bit trickier.  If you look at your 89IV,  you'll see that it has distinct switched mVAC and VAC ranges and those have the capacitor in front and the DC impedance of the input will be infinite-ish.  Thus you can read microvolts AC on top of a kilovolt DC bias.  The mVDC and VDC ranges, OTOH, have all the AC/AC+DC/DC functions like the BM789 we are discussing and those ranges will have some definite limitations, although I don't know what they are.  The BM789 just omits the switched AC ranges entirely.

It is possible to add the capacitor after the voltage divider, but if you have an early gain stage like you would in a mV range, it becomes difficult to put it anywhere but right in front.  This is why the F116 is able to have a 10M-ish input impedance on all ranges AC and DC, but the mV range actually is only mVAC+DC or mVDC.


--- Quote ---Yep. Joe gave the 87V a pass on the incorrect reading because it's quite atypical.

--- End quote ---

I don't know how atypical--I have an occasional need to measure and set a <5mV DC bias with 30VAC on top of it.  The reason I would give it a pass is because I pretty much give any meter a pass on autoranging issues.  Autoranging has so many compromises that need to be made that I don't think there ever is one 'right' solution.  My 8846A, for example--great meter in most ways, but the autoranging is so twitchy it is laughable.  Unless you actually need it to work, then it isn't funny, it's just utter crap.  On my old 8505A, OTOH, the autoranging is glacially slow, but as far as I can tell is never wrong.

Fungus:

--- Quote from: Neutrion on September 14, 2021, 03:09:32 pm ---And you also think it is solvable in FW. Which would be nice.

--- End quote ---

If might not be as simple as, "add another 'if () ...' statement", in the code editor.  :)

All multimeter chipsets have very limited space for firmware and most of them end up at 100% usage one way or another.

It might even be a thing they took out to make space for something else.

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