Brymen BM789

[Fungus]

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.

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

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. 

Yep. This is a massive bangperbuck meter.

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]

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.   

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

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?

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]

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.

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

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

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.

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. 

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.

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! 

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]

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.

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...

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?

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.

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

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]

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

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.

[joeqsmith]

Sudden unforeseen  levels of voltages are also real life issues, which you don't want, but it just happens.

 
Don't I know it.  Again, it's why I started making reviews showing something besides unboxing them, talking about my years as being a expert in flipping burgers to validate my opinions and then giving them all five stars.   I'm looking for products that survive, not that can make an accurate measurement while being exposed to these various tests.   

But we are arguing here about whether it is the intended use of the meter: No it is not (except if one wanted to buy the meter for the mentioned ripple measurement.)
But the real question in the whole topic is whether the missing "OL" indication is solveable in FW.
According to Dave possibly not, 2N3055 provided a possible technical description why not, Joe seems to agree?.

Sorry, I wasn't aware we were arguing.   I would have no way to know if ANY meter would show over range under all conditions when set to their ACmV function.  Again, it's not something I would normally do and if people agree with me or not is of little concern.   

My collection of DMMs is mostly limited to ones that survived my tests.    Others may offer a broader spectrum to avoid adding bias.  I have little doubt if anyone wanted to try it we would see some problems.   I doubt you would need to run them up to the 30V you mention. 

[AndrewBCN]

The meter is unable to read AC ripple on a DC bias in the mVAC range, such as a very common PSU ripple measurement.  How is that not a limitation?

As for the value of the information, I don't know why you think it has zero value.  It certainly is instructive about how to avoid a particular issue in using the meter.  Of course, as I posted, it's in the manual too, but who reads those?

Personally I don't think a multimeter is the best tool to observe and measure PSU ripple and noise (and transients), I believe a DSO provides a lot more information.

In any case, as you noted, it's right there in the BM789 manual: in the ACmV (600mV) and AC+DCmV ranges, peak value including DC offset must be under 1000mV.

So perhaps people who are trying to find a fault in a multimeter should read the manual first ?
 

[joeqsmith]

As I understand it, for the OP it all comes down to if the over range indicator works under ALL conditions with the meters set specifically to their ACmV function which may not always be found in the manual.  I think this is why they are suggesting someone try running a test.   Again, for me the outcome isn't a big deal one way or the other.  I can believe we would find some cases. 

The problem I see when looking for these fringe cases is the OP has not provided any constraints.  I suspect it's more their general curiosity than anything.   Maybe limit it to the 40-500Hz they mentioned and keep it within +/-2 Volts so everyone could join in the fun? 

[AndrewBCN]

I have one suggestion for the firmware engineers at Brymen: please program the BM789's MCU to display the following characters when the ACmV or AC+DCmV ranges are selected and an input peak voltage including DC offset > 1000mV is detected:

RTFM

Honestly, why didn't they think of that? 

[bdunham7]

Personally I don't think a multimeter is the best tool to observe and measure PSU ripple and noise (and transients), I believe a DSO provides a lot more information.

Clearly this DMM is not the best tool for that, but there are many others that do just fine.  Whether a DSO is necessary or better depends on what you are doing.  What about a quick go/no-go test when a DSO isn't handy?  What if the power supply ripple is below the noise floor of the DSO?  For me personally, this would be a routine test and I would  typically only break out the scope if my DMM indicates a ripple issue or there is some other strange problem.

So perhaps people who are trying to find a fault in a multimeter should read the manual first ?

A documented limitation is still a limitation. 

[joeqsmith]

I think the OPs concern is that meters may read what appears to be a normal level.   They just want to know that they are out of range without using the ACV or any other mode.  Again, just my understanding as I boil these few pages down. 

I can believe we could find meters that would show 0.000 mVAC with volts applied.   

[joeqsmith]

So perhaps people who are trying to find a fault in a multimeter should read the manual first ?

A documented limitation is still a limitation.

It seems like its an undocumented limitation we are after.   

[bdunham7]

The problem I see when looking for these fringe cases is the OP has not provided any constraints. 

I agree, and didn't realize until he posted it that he doesn't have an actual AWG. That makes his test hard to replicate reliably.  I'm sure either you or I can make any meter display a "wrong" reading fairly easily.  A good challenge might be to come up with a signal that 'good' meters get wrong but the Harbor Freight free one reads correctly.

[bdunham7]

It seems like its an undocumented limitation we are after.

Yes, the lack of an overload indication even at higher voltages seems odd, which is why I had speculated that perhaps the clamping circuit was breaking down too early.  But that's just rampant speculation that needs to be tested.

[Caliaxy]

I have one suggestion for the firmware engineers at Brymen: please program the BM789's MCU to display the following characters when the ACmV or AC+DCmV ranges are selected and an input peak voltage including DC offset > 1000mV is detected:

RTFM

Honestly, why didn't they think of that? 

If I understand correctly, the chief complain here is not that the meter cannot measure outside its published specs (DC offset > 1V in AC mV mode), but that it displays erroneous values when this happens (as opposed to "OL"), tricking the unsuspecting user (other people, of course, not us, because now we know it).

BTW, the AC+DC mV mode (a press of a button away from AC mV) displays OL correctly under these conditions. Then you need to press the yellow button 5 more times to return to the AC mV mode (which is why I'm not in love with Brymen...)

[joeqsmith]

A good challenge might be to come up with a signal that 'good' meters get wrong but the Harbor Freight free one reads correctly.

If you want to play around with it a bit and propose something, I would gladly attempt to follow along.   I think the simpler your setup, the better.   Using an Arb seems reasonable and allows automation.   If you have a way to make a video of it all running, even better.   

If I understand correctly, the chief complain here is not that the meter cannot measure outside its published specs (DC offset > 1V in AC mV mode), but that it displays erroneous values when this happens (as opposed to "OL"), tricking the unsuspecting user (other people, of course, not us, because now we know it).

I believe this is what the OP is after.

Don't get me wrong.  I make some pretty boneheaded mistakes but I just don't want the meter to be damaged as a result.

[Neutrion]

I have one suggestion for the firmware engineers at Brymen: please program the BM789's MCU to display the following characters when the ACmV or AC+DCmV ranges are selected and an input peak voltage including DC offset > 1000mV is detected:

RTFM

Honestly, why didn't they think of that? 

RTFM is also an option, but I still vote for OL.

I think the OPs concern is that meters may read what appears to be a normal level.   They just want to know that they are out of range without using the ACV or any other mode.  Again, just my understanding as I boil these few pages down. 

Exactly. And you can say "he". Even without any accurate measurement.

[AndrewBCN]

I have one suggestion for the firmware engineers at Brymen: please program the BM789's MCU to display the following characters when the ACmV or AC+DCmV ranges are selected and an input peak voltage including DC offset > 1000mV is detected:

RTFM

Honestly, why didn't they think of that? 

If I understand correctly, the chief complain here is not that the meter cannot measure outside its published specs (DC offset > 1V in AC mV mode), but that it displays erroneous values when this happens (as opposed to "OL"), tricking the unsuspecting user (other people, of course, not us, because now we know it).
...

And again, perhaps the unsuspecting user should read the fine manual, which I did even though I don't own a BM789, and it took all of 45 seconds to find the specs.
Here: http://www.brymen.com/images/ProductsList/BM780_List/BM789-5-manual-print1-r7.pdf

Pages 26 and 27, if you can only spend 15 seconds to look for the ACmV and AC+DCmV ranges specs.

...
A documented limitation is still a limitation. 

You can call it a documented limitation if you want, but - as you yourself noted - the BM789 ACmV range is DC coupled and even though I have an extremely limited understanding of DMM circuitry, even I can understand that this range was *deliberately* not designed to measure ripple and noise from a PSU. Of course I am assuming that Brymen engineers know what they are doing, but that seems like a reasonable assumption.

[bdunham7]

You can call it a documented limitation if you want, but - as you yourself noted - the BM789 ACmV range is DC coupled and even though I have an extremely limited understanding of DMM circuitry, even I can understand that this range was *deliberately* not designed to measure ripple and noise from a PSU. Of course I am assuming that Brymen engineers know what they are doing, but that seems like a reasonable assumption.

I'm not sure if we have a linguistic hangup here over the term 'limitation' or what, but I don't see the dispute.  If they deliberately design it knowing that it can't do something as a result, then that is a limitation.  If they put it in the manual, then it is documented.  Neither of these terms is inherently pejorative, simply descriptive.  Whether you consider the characteristic (limitation) itself to be a bad thing is up to you.

[Neutrion]

And again, perhaps the unsuspecting user should read the fine manual, which I did even though I don't own a BM789, and it took all of 45 seconds to find the specs.
Here: http://www.brymen.com/images/ProductsList/BM780_List/BM789-5-manual-print1-r7.pdf

Pages 26 and 27, if you can only spend 15 seconds to look for the ACmV and AC+DCmV ranges specs.

...
A documented limitation is still a limitation. 

You can call it a documented limitation if you want, but - as you yourself noted - the BM789 ACmV range is DC coupled and even though I have an extremely limited understanding of DMM circuitry, even I can understand that this range was *deliberately* not designed to measure ripple and noise from a PSU. Of course I am assuming that Brymen engineers know what they are doing, but that seems like a reasonable assumption.

Please RTFFP I could say:
"I know that I should use the higher ranges for signals like this, but when someone measures a lower level signal, and it raises up above the limit, there is no way of knowing it, you just stop seeing anything above this level, while the voltage might be skyhigh. I can not test it with higher voltages unfortunately, but would be nice if someone could do that.
So I am not vorrying about accuracy above the specified limit, but about the fact that it does not show overrange on the AC scale."

It is NOT documented however, that the meter can not show that it overranged on the AC scale under certain circumstances. (We still don't know whether this is also the case with simmetrical signals or not.)

[Caliaxy]

If I understand correctly, the chief complain here is not that the meter cannot measure outside its published specs (DC offset > 1V in AC mV mode), but that it displays erroneous values when this happens (as opposed to "OL"), tricking the unsuspecting user (other people, of course, not us, because now we know it).
...

And again, perhaps the unsuspecting user should read the fine manual, which I did even though I don't own a BM789, and it took all of 45 seconds to find the specs.
Here: http://www.brymen.com/images/ProductsList/BM780_List/BM789-5-manual-print1-r7.pdf

Thank you for the link. bdundham7 also posted a snapshot of the relevant page of the manual earlier in the thread.

Pages 26 and 27, if you can only spend 15 seconds to look for the ACmV and AC+DCmV ranges specs.

That line in the manual ("Signal peak absolute values, including DC bias, less than 1000mVpeak") does not infer that the meter would display seemingly valid but false values when overloaded. 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. Nothing more, nothing less.

Again, I don’t think it’s a big deal if you are aware of it. Many other meters on the market do the same thing.

[AndrewBCN]

...
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.
...

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.

[bdunham7]

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:
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.

We're all assuming stuff, 2N3055 included.  The 1V_pk limitation is documented.  The apparent lack of an overload indication under specific circumstances is not, so why it is that way is unknown.  As I said that could be a design issue that Brymen knows about, one they don't know about or a single-unit defect of the OPs meter. 

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.

The whole point of using a meter a good portion of the time is to measure an unknown (to a degree) signal.  If you are expecting ~500mV but the meter reads 650mV and the actual voltage is 30V--or 300V--that is at least a glitch. It might also just violate a standard somewhere, but I'm not sure about that.

[Fungus]

even I can understand that this range was *deliberately* not designed to measure ripple and noise from a PSU.

I wouldn't use that word for it. I doubr it was a design goal.

I'd say "knowingly". The Brymen engineers did what they could with the chipset and price point they had available.

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.

[2N3055]

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:
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.

We're all assuming stuff, 2N3055 included.  The 1V_pk limitation is documented.  The apparent lack of an overload indication under specific circumstances is not, so why it is that way is unknown.  As I said that could be a design issue that Brymen knows about, one they don't know about or a single-unit defect of the OPs meter. 

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.

The whole point of using a meter a good portion of the time is to measure an unknown (to a degree) signal.  If you are expecting ~500mV but the meter reads 650mV and the actual voltage is 30V--or 300V--that is at least a glitch. It might also just violate a standard somewhere, but I'm not sure about that.

Fact that you cannot pour hydrochloric acid in input sockets is not documented.
Fact that you cannot hit it with a AVe hammer is not documented.
Fact that you cannot plant daisies with it is not documented.
Bad, bad specs...tsk, tsk...

 
What is this, McDonalds ?  "You cannot pour hot coffee on babies" kind of thing?

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?

Meter allread has foolproof range designed to completely autorange form 0 to 1000V. It is autoranging volt range. That is the one you use when you don't know what are you measuring. mV range is manual specialized mode for precise measurement that you use only when you know it will be less than 600mV for sure...

That is how you use that meter. Period. Everything else is either lack of knowledge (which is fine, you learn and move on) or deliberate abuse of instrument. Which is also fine if you paid it with your own money. You might as well burn it, see if I care..
But don't expect meter to do anything you want. It is operating outside it's operating envelope. All bets are off.