Brymen BM789

[floobydust]

AN8008 ACV bugs: Input 90mVAC on the mV AC range, no DC offset (autoranging enabled 9.999mV and 99.99mV ranges). Displays: 90.00 this is OK.
As you increase a +ve or -ve DC offset, the reading suddenly drops to say 15.00 then additional DC offset leads to display in a loop showing: 40.30, 0.000, 00.00, OL, and constantly repeating.
If you manual range to 99.99mVAC, you get a correct reading 90.00mVAC but this drops and even goes to 00.00mVAC with increasing offset. With no DC offset it returns to the correct reading.
If you now enable autoranging, the multimeter is lost and reads as if on manual range, but no longer does the autorange cycling and can read silly values.
You have to switch to another function or reboot to stop this.

Mentioning this as it is a test of the OL and autorange algorithms that commonly have bugs.

[joeqsmith]

... most of the meters I tried have the same issue: Fluke 101, 107, 17B+, Uni-T UT61e, Agillent U1252A, BM235, BM689S. Fluke 189, 89IV and Agillent U1282A seem to work fine. I don't care much either, but it's good to know. ...

I'm sure I could find a few in the my mix that would handle the ACmV fine but I would guess I could find other problems with them.   I'll typically gloss over what I consider fringe cases. 

Now if you make a point to advertise your meter has a frequency counter that can read up to 220MHz.  You're so proud of it, that you place it right on the box.  And I happen to watch Daves review and see your bright idea was to protect the input with a couple of series PTCs that any EE is going to know they will act like capacitors and you didn't limit the current through them.  Well,  I'm not going to let that go.  It's not a fringe case when you claimed it would do something.   I'm going to push it until it works, or goes up in flames.       

[bdunham7]

I can see the headlines tomorrow:
Electronics Engineer Exposes Major Design Flaw In Hundreds of Multimeters
Proves That Most DMMs Cannot Be Used To Measure Ripple And Noise In PSUs

...but trusty old Fluke 87V gets the job done once again and Fluke sells another million units!   

[bdunham7]

most of the meters I tried have the same issue: Fluke 101, 107, 17B+, Uni-T UT61e, Agillent U1252A, BM235, BM689S.

Do you mean they all displayed wrong readings or did they display an overload indicator?

[Caliaxy]

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..

You got it wrong. Like BM789, Fluke 87V does have an AC 600mV range (and a DC 600mV range), but they are not on the same rotary dial position. The AC 600mV is on the AC V position, which starts auto-ranging from 600mV up, whereas the DC 600mV range has its own rotary dial position, all for itself. The DC V mode starts auto-ranging from 6V up. All AC V ranges (including 600mV) are AC coupled, so the DC offset is irrelevant, unlike in BM789.
...

No, 2N3055 got it right, the AC V position on the Fluke 87V has 6000 count resolution, whereas the separate BM789 ACmV position has 60,000 count resolution. If you use the Auto position on the BM789 it will autorange exactly the same as the Fluke 87V.

That means the Brymen has a high resolution, separate dial position for manual range 600mV ACmv which you can choose to use for high resolution ACV measurements - exactly as 2N3055 wrote. And yes, they should add in the BM789 User Manual a warning that this range requires thinking... 

In my post #24 above (replying to 2N3055's post #21) I was joking (half seriously) that Fluke 87V probably avoided the problem under discussion here by not having a dedicated AC mV position on the dial at all (and moving it under the AC V mode, which is AC coupled). After reading his reply #79 I worried that my reply #24 misled him into believing that Fluke 87V doesn’t have an AC mV range at all (which is not true but might have been an interpretation of what I wrote in #24), so I tried to clarify. That’s the reason of my reply #88, which you (wrongly) disagree with.

#88 wasn’t intended to be malicious. I’m not sure why you tend to see everything this way. 

[Caliaxy]

most of the meters I tried have the same issue: Fluke 101, 107, 17B+, Uni-T UT61e, Agillent U1252A, BM235, BM689S.

Do you mean they all displayed wrong readings or did they display an overload indicator?

They all display meaningless numbers instead of OL, exactly like BM789. Forgot to (re)mention GW121.

[joeqsmith]

I would hate to think I left a few of you with the idea the having a meter show zero volts while in it's ACmV mode is unique to Brymen's new BM789,  here is another one for the UNI-T fan boys.   

Of course, switch it over to tri-display mode and we get a feel for what's really going on.   Lucky for me, the battery was charged this time when I went to use it.       Again, this meter has been damaged, repaired and then the PCB was modified but I doubt any of that would have an effect on these measurements. 

[AndrewBCN]

No, 2N3055 got it right, the AC V position on the Fluke 87V has 6000 count resolution, whereas the separate BM789 ACmV position has 60,000 count resolution. If you use the Auto position on the BM789 it will autorange exactly the same as the Fluke 87V.

That's preposterous.  The bottom 10% of a 60,000 count range is not equivalent to a full 6000 count range, especially when you are talking about TRMS.  And the 87V has a 'hi-res' mode, but I don't have one so I can't tell you the details of that.  But it's OK to be silly if you like--this is EEVBlog after all.

I don't have a 87V either but I can read (the online user manual): the Hi-Res mode does not work with the ACV ranges.
So, as I wrote, if you set the BM789 to Autorange ACV position, it will autorange exactly the same as the 87V. The main difference with the 87V in that respect (ACV measurements) is that the BM789 provides an extra, separate, high-resolution, dc-coupled ACmV manual range, which as 2N3055 noted, requires a little bit of thinking to be used.

As for being "silly", I am sorry to say but that's one way to describe this entire thread. So I guess a little bit of humor is not entirely out of place. 

[2N3055]

I go to bed, and children in other time zones get in a fight right away..

Look, people. This topic is chock full of emotions and many hurried responses without doing homework.
So let's sumarize:

- Meters that do AC+DC type of measurements (combined) do that by keeping front end DC coupled.
- Meters SHOULD AC couple in AC only measurements so you could measure 20mV AC riding on top of 100V
- For some reason Brymen doesn't do that on AC mV range. That is NOT good way to do it.
- Because of that, if you plan to measure 20mV AC riding on top of 100V using mV range, on Brymen you have to use external capacitor. Like Joe said, just throw it in a box, if you do it on a regular basis.
- There are other meters that do the same thing as Brymen and also those that have AC mV range properly AC coupled. Make note that there is no instrument available that can measure in single AC+DC measurement (single or dual display) that can do mixed scales for AC and DC.
- Would I like that Brymen did it differently, so it also AC couples when AC only mV? Yes, of course I would. That is pretty much only thing they didn't do perfectly. On my BM869 I miss that and fact that it doesn't remember preferences between powerups.
- Is it a problem in practice? It might be annoying but not a problem if you know it. Would it be better if they did AC coupling? Yes.
- Overload indicator gets confused by DC offset overdrive only. AC overdrive without offset works perfectly. It will show OVL.
- Whole problem is caused by DC offset overloading input stages.

All of this applies only to dedicated mV AC measurements. Volt ranges properly AC couple and work perfectly.
At all times meter is safe and won't be damaged. Measurements won't be valid if operated outside operating envelope.

That is summary.

[AndrewBCN]

Let's summarize (again):
1. The OP found a way to "fool" his shiny new Brymen BM789 when set to the DC-coupled high-resolution ACmV range. This basically involves saturating the input circuitry such that the DMM cannot measure the (small) AC component because of a (large) DC bias. Under these conditions the DMM neither detects an overload condition nor provides a correct measurement of the (small) AC component.
2. joeqsmith has demonstrated that a number of different DMMs from different brands can be "fooled" using a similar method.

I think the lesson here, if there is one, is that using a DMM  (any model from any brand) to get correct measurements always involves a minimum of thinking - as 2N3055 reminded us. As well as spending a few seconds to read and understand the User Manual.

Edit: Sorry 2N3055, we seem to have posted our summaries almost simultaneously. I defer to yours which is much better than mine.

Edit2: I can think of a number of reasons why Brymen did not include a series capacitor in the ACmV range front end, and similarly for other DMMs from other brands. But that would be a matter for (reasoned) discussion in another thread, if you ever want to. I for one would be very interested in your opinion.

[joeqsmith]

...
- Meters SHOULD AC couple in AC only measurements so you could measure 20mV AC riding on top of 100V
...
- Because of that, if you plan to measure 20mV AC riding on top of 100V using mV range, on Brymen you have to use external capacitor. Like

Typical meter will have a your 1k + PTC and clamp at over a kV.   The mV function could very well clamp at much lower.  UT61E for example, with  mV selected, the signal goes through PTC1 and it then clamped by Q1,8.

https://electronicshelponline.blogspot.com/2015/10/uni-ut61e-digital-multi-tester-schematic.html

The block should handle this and I would expect to see a 1kV part in use.  You should be able to measure your 20mV on top a 1kVDC.

But then again, I like that low frequency response that I can only get with DC coupling.....  Ah the choices we have. 

***
Looks like Fluke knows they need a 1kV part in there, even on their low end meters.   Looking at 300mVAC (wanted to show the 2nd meter reading something) riding on top of a 750VDC.    I'm sure a kV would be possible but I'm a bit concerned with my setup.   

Gotta say, I still like the 189.  Just a nice, basic meter.  Sure it's a relic and could use a few improvement but I bought one for the sole purpose to transient test.  While it had been badly abused before I got it, it took everything I threw at it.   

[2N3055]

Let's summarize (again):
1. The OP found a way to "fool" his shiny new Brymen BM789 when set to the DC-coupled high-resolution ACmV range. This basically involves saturating the input circuitry such that the DMM cannot measure the (small) AC component because of a (large) DC bias. Under these conditions the DMM neither detects an overload condition nor provides a correct measurement of the (small) AC component.
2. joeqsmith has demonstrated that a number of different DMMs from different brands can be "fooled" using a similar method.

I think the lesson here, if there is one, is that using a DMM  (any model from any brand) to get correct measurements always involves a minimum of thinking - as 2N3055 reminded us. As well as spending a few seconds to read and understand the User Manual.

Edit: Sorry 2N3055, we seem to have posted our summaries almost simultaneously. I defer to yours which is much better than mine.

Edit2: I can think of a number of reasons why Brymen did not include a series capacitor in the ACmV range front end, and similarly for other DMMs from other brands. But that would be a matter for (reasoned) discussion in another thread, if you ever want to. I for one would be very interested in your opinion.

Thank you for the kind words.
I would not mind if you started that discussion, but in my opinion it would necessitate at least partial R.E of front end and switching circuit. Otherwise we would all have many ideas, but no confirmation. I guess Joe did quite some work on that already..
It might even be that meter could be bodged with a capacitor as an afterthought. But I would be very reluctant to do that to my meter and also to publish it on Internet. I have same opinion on that as Joe has about his "hardenings" of various meters. Maybe that is a Pandora box better be left unopened.
Best,

[2N3055]

...
- Meters SHOULD AC couple in AC only measurements so you could measure 20mV AC riding on top of 100V
...
- Because of that, if you plan to measure 20mV AC riding on top of 100V using mV range, on Brymen you have to use external capacitor. Like

Typical meter will have a your 1k + PTC and clamp at over a kV.   The mV function could very well clamp at much lower.  UT61E for example, with  mV selected, the signal goes through PTC1 and it then clamped by Q1,8.

https://electronicshelponline.blogspot.com/2015/10/uni-ut61e-digital-multi-tester-schematic.html

The block should handle this and I would expect to see a 1kV part in use.  You should be able to measure your 20mV on top a 1kVDC.

But then again, I like that low frequency response that I can only get with DC coupling.....  Ah the choices we have. 

***
Looks like Fluke knows they need a 1kV part in there, even on their low end meters.   Looking at 300mVAC (wanted to show the 2nd meter reading something) riding on top of a 750VDC.    I'm sure a kV would be possible but I'm a bit concerned with my setup.   

Gotta say, I still like the 189.  Just a nice, basic meter.  Sure it's a relic and could use a few improvement but I bought one for the sole purpose to transient test.  While it had been badly abused before I got it, it took everything I threw at it.

Well said.

If they would restart making F189, that one I would buy right away.
If I was lucky to have bought one before, when available, I would still have it and keep it.
But I have no appetite for lottery of buying used.

[Fungus]

Gotta say, I still like the 189.  Just a nice, basic meter.  Sure it's a relic and could use a few improvement but I bought one for the sole purpose to transient test.  While it had been badly abused before I got it, it took everything I threw at it.

If they would restart making F189, that one I would buy right away.

187 is even better - same meter but no pesky supercap to worry about.

[bdunham7]

it will autorange exactly the same as the 87V.

If you are going to make such arguments, at least be honest about it instead of acting like a marketing or political spin doctor.  What you are implying is that since the BM789 has 10X the counts (the almighty counts!) of the 87V, you can consider the 6.0000V range to be the usable equivalent of the 600.0mV range of the latter since they appear to have the same resolution (100uV).  But read footnote 1 of the excerpt that I posted.  The bottom 10% of the range is 'unspecified'.  Maybe if you characterize your meter you might imagine you are getting some useful information out of that range, but don't tell me that it is 'exactly the same' or even 'just as good'.

[joeqsmith]

187 is even better - same meter but no pesky supercap to worry about.

I have replaced the SC on all four meters.  No big deal. 

If they would restart making F189, that one I would buy right away.
If I was lucky to have bought one before, when available, I would still have it and keep it.
But I have no appetite for lottery of buying used.

Same.  Maybe a better display and handle the part obsolescence is all.   

The one on the right was given to me by a good friend.  I'm the third owner.  The one on the left cost me about $120 non-working as is for parts.   Bought it to transient test.  It turned out to be in very good of condition and felt it would upset the Fluke collectors if I put a classic meter into an early grave.     The last one I brought home was a similar story.  Bought it as non-working as is. It's case looked mint which is why I bought it.  I think $60 which seemed fair for good used case.  But when I opened it up,  it was mint inside as well.  I have no plans to buy another no matter how good the deal seems so I'll pass my 189 finders luck onto you. 

[bdunham7]

That is summary.

Looks right, just the facts.  Which I think is what the OP was looking for.  Nobody replicated his specific test, but it seems clear that his issues are primarily because of the design.  And until this thread I didn't know that the mVAC range on my own F116 is also DC-coupled, although none of the other issues affect it.

But I have no appetite for lottery of buying used.

And some have no appetite for the lottery of buying new! 

[Caliaxy]

I don't have a 87V either but I can read (the online user manual): the Hi-Res mode does not work with the ACV ranges.

Not true.

[joeqsmith]

Vintage 1980's Fluke 8506A thermal RMS meter I saved from scrap.  Its set to high accuracy mode and shown with an external blocking capacitor.    My new 87V in high res mode.  Signal is a little closer to the 20mV noise mentioned above, with 800VDC offset.

Eventually the MOVs would clamp the input and we limit the current but I wonder if these caps would survive....  I'll stick with my old habits of using an external block.   It does look a bit scary hanging out there though...   

[AndrewBCN]

@Joe

That external blocking capacitor looks like it could hold a lethal amount of energy!

The question is, on the DMMs that use an internal blocking capacitor, how do they discharge it before you go on to the next measurement?

[joeqsmith]

@Joe

That external blocking capacitor looks like it could hold a lethal amount of energy!

The question is, on the DMMs that use an internal blocking capacitor, how do they discharge it before you go on to the next measurement?

 
Assuming worse case,  1/2CV^2,  0.005uF @ 20KV is a Joule.   This is the upper end of Class 3.2b.  The part would normally be used with Class 3.1d (< 0.25J).    There's some small risks when experimenting but I'm not too concerned about discharges from this part.   

I don't see any need to discharge the internal block before changing measurements. What would be your concerns?  Seems like having the low voltage clamps engaged and connecting to a +1kVDC source, then instantly measuring -1kV would provide the most stress to the meter.    Well, then again, rotating the switch with a 1kVDC source my light it up pretty good.  Outside of my rebuttal videos for Dave,  I've never tried it for real.  I assume people making such measurements are well educated, use the proper tools and think before they act ....   

Then again, the beginner hooks their new meter to the output of a MOT.  Surely it can handle a couple kV as I saw this guy Joe put 6kV into his.     

DOE standard
https://www.standards.doe.gov/standards-documents/1000/1092-BHdbk-2013/@@images/file

Datasheet for part in question
https://www.plasticcapacitors.com/images/com_sellacious/products/attachments/57/OF200.pdf

[AndrewBCN]

...

The question is, on the DMMs that use an internal blocking capacitor, how do they discharge it before you go on to the next measurement?

...
I don't see any need to discharge the internal block before changing measurements. What would be your concerns?
...

Joe, thank you very much for your detailed and perfectly documented (as always) answers. My concern is not really with an internal blocking capacitor damaging the high impedance (10MOhm?) DMM input circuit, but what would happen in the following measurements, if it could damage the circuit under measurement (presumably at a different DC potential), in other words, if the blocking capacitor inside the DMM would discharge back through the leads into the circuit under measurement?

[joeqsmith]

Oh, now there's something I had not considered.      Say low voltage clamp is active, you make your kVDC noise measurement like shown, then immediately attach the probe to your 3.3V CMOS input....  10nF with a 2kohm source at a kV.... Interesting...

[Kleinstein]

The voltage in the DC blocking cap can add the external voltage. Especially bad when you change sign of the DC part.  This way I once killed the input stage of a scope:  first measure ripply (rather low) at a +150 V supply with AC coupling and a x 1 probe, and than the same with a negative -150 V. This was too much for a scope that showed 200 V max.    Afterwards you are smarter, but still a bit surprised that so little extra voltage from the internal cap can damage the input.

I think the AC input behind the coupling capacitor could be more like 1 M (maybe even 10 M in a handheld) input resistance and I don't expect a clamp between the resistor and cap.

[Fungus]

10nF with a 2kohm source at a kV.... Interesting...

Doesn't seem like it could damage much ... but your grill starter has killed a few meters that have some input protection on them (high speed clamps after a protective resistor).