Brymen 869s DC offset issue when reading ACmV

[AnalogEngineer]

Howdy,

I recently got a Brymen 869s in the hopes we could find a lower cost alternative for the Fluke 289 / 189 we use on the shop floor.  We need at least 20,000 counts and dBV or dBm reading so the choices are slim since few lower cost non Fluke meters include dB readings.

There appears to be a show stopper bug and I wonder if anyone can confirm or knows a work around solution. I suspect the bug is the same causing this issue, but since that is an old thread I decided to start a new one.
https://www.eevblog.com/forum/testgear/brymen-multimeters-fault/

What it appears is that when using the AC mV reading the meter is not AC coupled. So if there is a DC offset that is more than the maximum DCmV scale the AC mV reading will not work and you get nonsense readings.  This also seems to affect frequency and AC / DC dual readings as well.  In my case I am reading a 100mV signal riding on top of a 7.5VDC offset. My Fluke 289 handles this perfectly for all measurements as does my ancient Keithly and Tektronix models. The Brymen 869 cannot and I have to go to the AC Volt reading (with less accuracy due to small voltage). If I add a >0.22uF capacitor in series then it works but takes a 5-10 seconds to settle down to an accurate reading (due I assume to the meters input impedance). Smaller capacitors induce error in the value and larger increase settling time.

Any ideas? This seems like such a huge fail on the meters part basically making it unusable to me in our application.
Anyone want to recommend a sub $300 meter with >=19,999 counts and dBV or dBm reading capability?

[Fungus]

[mqsaharan]

Anyone want to recommend a sub $300 meter with >=19,999 counts and dBV or dBm reading capability?

I am not sure about the price in your country but may I suggest UT181A. It is another blatant attempt to copy Fluke 287 externally and functionally like some others. Even though its manual does not state measurements in dBv or dBm, its VAC menu does include both. Please check out the following link for Joe Smith's video at 17:50 minutes mark



It has AC coupling cap in it. And I can only hope it will also have AC coupling on mVAC range because usually the meters that have an independent mVAC range are AC coupled in that range unlike the ones that share one dial position for both DC and AC mV function.
I hope some other member who owns one will confirm AC coupling as well as dBV and dBm capability on mVAC range for UT181.

[Fungus]

I am not sure about the price in your country but may I suggest UT181A. It is another blatant attempt to copy Fluke 287 externally and functionally like some others. Even though its manual does not state measurements in dBv or dBm, its VAC menu does include both. Please check out the following link for Joe Smith's video at 17:50 minutes mark

It's a good meter but the the fancy screen means the battery doesn't last very long and you can't use it while it's charging becasue it charges through the input jacks. This can make it very inconvenient to use.

(plus it's a LIPO battery so it won't even hold a charge a few years from now)

joe says the battery lasts about 40 hours in a new meter:

https://www.eevblog.com/forum/testgear/uni-t-ut181a-pictures/msg3327088/#msg3327088

[AnalogEngineer]

I am not sure about the price in your country but may I suggest UT181A.

Thank you. I had not considered that before. I wonder how stable it is.
I have two Uni-T ut61e meters now and they both drift like crazy and don't stay in calibration more than a few months. This is a known complaint with Uni-T. Even my cheap Anengs don't do this.  I wonder if this meter suffers the same issue. I also wonder if the UT61e+ has fixed this. Otherwise it seems to be a great meter. The 61E actually meets all my needs except the dB reading so worth considering.

Someone else mentioned to me the Vici VICHY VC8145  which seems possible.  Alas all this trying other brands is making me a bigger Fluke fanboy. They are all so close, but I keep seeing small details that does make the fluke much nicer. I just can't afford them for every tech at the shop. Sigh.

[mqsaharan]

If you are a hobbyist and like to tinker/fight with your equipment in addition to the project you are working on, try getting 8060A. Don't take me wrong for "tinker/fight" remarks. I got one and even though it took me some time to make it work properly it is all set now and working great. If you can get one with 4xxxxxx or above serial number in original condition, hopefully its electrolytic capacitors have not leaked and you only need to replace them. It is 20k count, TRMS and has dBm and dBV measurement capability. And unlike most DMMs out there it shows dBm and dBV for both AC and DC. dBm and dBV requires a little bit of effort on operators part (check the attached manual pages) but it is a very good quality DMM with low price tag. Only downside for some is its manual ranging. Please check out the following two links by modemhead regarding its capacitor replacement:
http://mrmodemhead.com/blog/fluke-8060a-repair/
http://mrmodemhead.com/blog/ibm-8060aaa-fluke-8060a-refurbish/

For a bench top, VC8145 is not a bad choice, again from a hobbyist perspective. Please take a look at its review by HKJ for more information: https://lygte-info.dk/review/DMMVici VC8145 UK.html

The biggest downside of UT181A is its battery as Fungus has pointed out. To me the other one is its screen. But if you are buying it for bench use you can make it work. As far as the drifting problem goes, I doubt it drifts like 61E. There are quite a few members on this forum who own it and so far no one has reported such a problem with it.

And for 61E drifting problem, watch the following video from Joe.

[_Wim_]

If I add a >0.22uF capacitor in series then it works but takes a 5-10 seconds to settle down to an accurate reading (due I assume to the meters input impedance). Smaller capacitors induce error in the value and larger increase settling time.

The BM869s has an input impedance of 10MOhm in AC-mode, so using a 0.222µF cap creates a high pass filter with a -3db point of 0.072Hz. This makes sense that this takes around 10 seconds to settle. I do not know what frequency the AC signal is you are trying to measure, but I expect the cap can be much smaller without any significant loss in accuracy.

With this online tool (http://sim.okawa-denshi.jp/en/CRtool.php) you can easily calculate the filter, and the step response will give you an idea about the settling time.

[2N3055]

If I add a >0.22uF capacitor in series then it works but takes a 5-10 seconds to settle down to an accurate reading (due I assume to the meters input impedance). Smaller capacitors induce error in the value and larger increase settling time.

The BM869s has an input impedance of 10MOhm in AC-mode, so using a 0.222µF cap creates a high pass filter with a -3db point of 0.072Hz. This makes sense that this takes around 10 seconds to settle. I do not know what frequency the AC signal is you are trying to measure, but I expect the cap can be much smaller without any significant loss in accuracy.

With this online tool (http://sim.okawa-denshi.jp/en/CRtool.php) you can easily calculate the filter, and the step response will give you an idea about the settling time.

10nF is just fine, unless you think meter can measure RMS of signals less than 5 Hz...
Even 4,7nF would be just OK for 10-15Hz and up.

[David Hess]

My Tektronix bench meters use a 0.1 microfarad AC coupling capacitor before the 10 megohm decade divider.  My Tektronix DMM916 handheld meter solves the problem for the millivolt range by only supporting DC.

Some meters include a warning in their documentation that high DC offsets will cause ranging when making AC measurements because they rely on their decade divider to bring the DC offset of the AC measurement into range.

[Caliaxy]

Anyone want to recommend a sub $300 meter with >=19,999 counts and dBV or dBm reading capability?

A Fluke 187/189 should be cheaper than $300 (in US, at least), if you are OK with buying used. Agilent U1272A should do it as well.

[bdunham7]

10nF is just fine, unless you think meter can measure RMS of signals less than 5 Hz...
Even 4,7nF would be just OK for 10-15Hz and up.

Unfortunately that neglects the input capacitance of the meter.  Any blocking capacitor must be much larger than the typical 100pF or so of input capacitance or you'll have an error.  10nF would cause the reading to be about 1% low and so on. 

[_Wim_]

Unfortunately that neglects the input capacitance of the meter.  Any blocking capacitor must be much larger than the typical 100pF or so of input capacitance or you'll have an error.  10nF would cause the reading to be about 1% low and so on.

10nF is indeed may be a little too low, but this would also speed up the current response time x22 which is probably also not needed. A response time around 1 sec seems acceptable (to me), so a 22nF  cap would achieve this. Taking into account the 80pF input capacitance on the 500mV range, this would create a 0.3% error which does not degrade too much the specified accuracy.   

[_Wim_]

For reference the Fluke 289 specs. This probably also just has a series cap somewhere between 22nF and 100nF (do not now the response time of the Fluke). I should thus be possible to create identical performance with the Brymen 869s


Edit: added the AC+DC specs for the 289, as this is really what the Brymen does.

[AnalogEngineer]

Just a couple points of clarity and thanx to people.
I still haven't had anyone confirm this is really a 869 design error. It seems strange that it is since other meters handle this fine.
The 0.22 cap was derived by testing. Any smaller and the error was too great for a 500Hz test signal and our calibration procedure.  Any larger was unneeded. No idea why but that is the reality.  I get it shouldn't need to be that big but that's the measurement I got.

I agree a 189 or 187 may be a good choice, but they seem to go for a lot and they are all getting pretty old.
I need something I can reliably get several of for the folks on the manufacturing floor so would rather not putz with old stuff.

As to the 61E comment. My issue with Uni-T is not temperature drift. It's time drift. Take a Ut-61E and wait 6 months and it will be way out of calibration, unacceptably so. That doesn't help me trust Uni-T.  Even the cheap Anegs and Harbor Freight meters hold calibration. Not so much Uni-T.

[bdunham7]

I still haven't had anyone confirm this is really a 869 design error. It seems strange that it is since other meters handle this fine.

There was a long discussion previously on this exact subject, I don't remember which thread.  There are many other meters that cannot be used to measure ripple on the mVAC range, including some Fluke meters like the 113-117 models, so that can be a bug or a feature or just something that isn't included.  The real issue, IMO, are the cases where you overload the mVAC range with DC but you just get a wrong reading rather than an OL indication of some sort.  There was no consensus IIRC, but IMO that is not an acceptable result from a professional tool.

I need something I can reliably get several of for the folks on the manufacturing floor so would rather not putz with old stuff.

Nobody ever got fired for buying Fluke, right?  The 289 can be fairly criticized--it is expensive, big, cumbersome to use according to some and it eats batteries like popcorn.  But for some uses it seems to be only current production tool I know of that works, other than a bench meter which would have another set of issues.  There may be alternatives that will perform to your expectations, but unless you get lucky, they won't be all that cheap anyway.

[mqsaharan]

I agree a 189 or 187 may be a good choice, but they seem to go for a lot and they are all getting pretty old.
I need something I can reliably get several of for the folks on the manufacturing floor so would rather not putz with old stuff.

If you are using it on a production floor, like bdunham7 said, I would also suggest you to use a quality instrument like 287. But if budget is the issue then you might have to compromise and use a cheaper quality instrument. But you'll have to keep it in check which sometimes is extra headache.
With AC coupled dB measurement requirement, your choices are limited in handheld DMM catagory. Mostly expensive models cover your requirement. If you glance at dB column of the DMM spreadsheet, you'll find many multimeters supporting dB function. But it is difficult to confirm which ones have AC coupled inputs.
https://www.eevblog.com/forum/testgear/multimeter-spreadsheet/

As to the 61E comment. My issue with Uni-T is not temperature drift. It's time drift. Take a Ut-61E and wait 6 months and it will be way out of calibration, unacceptably so. That doesn't help me trust Uni-T.  Even the cheap Anegs and Harbor Freight meters hold calibration. Not so much Uni-T.

The video I linked in my last post is a no nonsense video regarding drift. In that video you'll learn about the drift of UT61E, BM869s and UT181A along with a few others. If I remember correctly, a few years back when 61E's drifting problem was kind of a hot topic, 61E's drift boils down to temperature cycling. Then again, I don't remember that discussion. I didn't take notes as I don't own one.

[Fungus]

I still haven't had anyone confirm this is really a 869 design error. It seems strange that it is since other meters handle this fine.

There was a long discussion previously on this exact subject, I don't remember which thread. z with old stuff.

There was a lso a discussion of how that AC coupling capacitor can be charged up to hundreds of volts then used to zap other electronics. 

Nobody ever got fired for buying Fluke, right?  The 289 can be fairly criticized--it is expensive, big, cumbersome to use according to some and it eats batteries like popcorn.  But for some uses it seems to be only current production tool I know o
f that works, other than a bench meter which would have another set of issues.  There may be alternatives that will perform to your expectations, but unless you get lucky, they won't be all that cheap anyway.

Yep. The Hioki DT4282 can do it, too, but it's not much cheaper than the Fluke. You may just have to bite the bullet.

[_Wim_]

The 0.22 cap was derived by testing. Any smaller and the error was too great for a 500Hz test signal and our calibration procedure.  Any larger was unneeded. No idea why but that is the reality.  I get it shouldn't need to be that big but that's the measurement I got.

What accuracy where you aiming for in the calibration procedure? Can you describe the test you performed? From your first post I would presume a 500Hz 100mVac signal riding on top of 7.5Vdc and then checking the deviation?

I still haven't had anyone confirm this is really a 869 design error. It seems strange that it is since other meters handle this fine.

I do not think this is a design error they omitted the series cap, but this is related to the fact the BM869s has dual display capability on mVac mode where it can measure Vac and Vdc simultaneously (which many meters can't do). It would have been nice of course to have a dedicated AC-mode (with series cap), as this would allow them to calibrate out the capacitive divider formed by the series and parallel input capacitance. 

Related to settling time, I would expect other meters to also to have a relatively low settling time with your test signal, as otherwise they would have too much error when measuring low frequency AC signals. As stated in a previous post, the Tektronix bench DMM also uses a 100nF series cap, which would only half your settling time, and maybe the 289 uses something similar?

[_Wim_]

The real issue, IMO, are the cases where you overload the mVAC range with DC but you just get a wrong reading rather than an OL indication of some sort.  There was no consensus IIRC, but IMO that is not an acceptable result from a professional tool.

I agree that this could be handled better. I wonder if this could be related to the fact that the OP does not achieve the expected accuracy? Many he could try manual ranging and see if the accuracy improves on a high range for his test signal.

[Fungus]

I wonder if this could be related to the fact that the OP does not achieve the expected accuracy? Many he could try manual ranging and see if the accuracy improves on a high range for his test signal.

A BM869S can easily read a 100mV AC signal with 7.5V DC offset in the normal AC+DC range.

I think the problem is that he wants it in dBm

[joeqsmith]

And for 61E drifting problem, watch the following video from Joe.

As to the 61E comment. My issue with Uni-T is not temperature drift. It's time drift. Take a Ut-61E and wait 6 months and it will be way out of calibration, unacceptably so. That doesn't help me trust Uni-T.  Even the cheap Anegs and Harbor Freight meters hold calibration. Not so much Uni-T.

I continue to monitor the long term drift after making those modifications.   It appears it has been almost a year since the I last checked.   

[Martin72]

The 289 can be fairly criticized--it is expensive, big, cumbersome to use according to some and it eats batteries like popcorn.

 

So damn true....

[2N3055]

I wonder if this could be related to the fact that the OP does not achieve the expected accuracy? Many he could try manual ranging and see if the accuracy improves on a high range for his test signal.

A BM869S can easily read a 100mV AC signal with 7.5V DC offset in the normal AC+DC range.

I think the problem is that he wants it in dBm

And it can easily do the dBm too, and at the levels OP asked for.
It won't be accurate down to 0.1 dBm, mind you... It will be inside 0.3-0.4 dBm of reading.

But if you ask me, it this is critical testing, for pretty much the price of F289 + a bit more you can get DMM6500 and do it right...
Precision testing is not a handheld job. Also, you might need some kind of record or printout of "Passed" label....

[bdunham7]

And it can easily do the dBm too, and at the levels OP asked for.
It won't be accurate down to 0.1 dBm, mind you... It will be inside 0.3-0.4 dBm of reading.

But if you ask me, it this is critical testing, for pretty much the price of F289 + a bit more you can get DMM6500 and do it right...
Precision testing is not a handheld job. Also, you might need some kind of record or printout of "Passed" label....

F289 will do that measurement (100mVAC @500Hz, -17.78dBm) 10X more accurately (+/- 0.02dBm) if that's what he needs.  The errors get down to the point where the typical 1M input impedance of a bench meter may affect the results more than the lesser precision of the F289, if the source is actually 600 ohms.

[joeqsmith]

.... But if you ask me, it this is critical testing, ....

...Precision testing is not a handheld job. ...

I would be concerned if I worked for a company who was having problems buying a few new handheld meters let alone trying to save the cost of their production testing.   

[J-R]

7.5VDC+100mVAC (RMS) 500Hz sine is definitely a tricky signal.  Out of my stash, the only DMMs that could actually provide those three fundamental values were:

Fluke 87V
Fluke 88V
Keysight 1233A
Sanwa PM300 (no mV range however)
Of course Fluke 287


Some notable "failures":

121GW
Brymen BM789
Of course Brymen BM869s

Like the BM869s, the BM789 displayed a garbage reading in mVAC mode.  No matter the amplitude of the AC component, it always displayed approximately 105mV.


The Keysight 1233A surprised me with the fact that while displaying the frequency, it uses the bar graph to display the AC voltage.  It's also very fast and pretty accurate (99.4mV).  It did lose the frequency measurement below about 30mV AC (RMS) (using the above stated signal).

The Fluke 87V & 88V were also quite accurate (100.17mV & 99.86mV).  They lost the frequency measurement below about 20mV AC (RMS).

No surprise I suppose, the Fluke 287 was the most accurate (100.02mV) and made it to 5mV AC (RMS) before dropping out.

[bdunham7]

7.5VDC+100mVAC (RMS) 500Hz sine is definitely a tricky signal.  Out of my stash, the only DMMs that could actually provide those three fundamental values were:

Just for fun I quickly tried every meter I could easily grab, with or without dBm/dB readings.

Meters with mVAC and dBm: Fluke 8846A, Fluke 289, Fluke 189, HP 403B w/ OPT 01 -- all read correctly and accurately.

Meters with mVAC but not dBm:  Fluke 8842A, Fluke 27, Fluke 8800A, Fluke 116 -- all read correctly except the F116 which displayed 'OL' in the mV range, but was exactly correct (0.100V) in the 6V range.  This is what the Brymen should be doing.

Meters without mVAC: Mastec/Centec (Harbor Freight) P37772, Fairchild 7000A, Simpson 270 (OUTPUT) --obviously all AC-coupled voltmeters should work fine here, and they do.  The Simpson 270 requires careful zeroing, using the special 2.5VAC scale and some squinting, but it is accurate to less than a needle-width.

The bottom line is that I can't find any meters in my stash that give me a wrong reading.  Something to at least be aware of and IMO, something that should be fixed if possible. 

[_Wim_]

Just for fun I quickly tried every meter I could easily grab, with or without dBm/dB readings.

Meters with mVAC and dBm: Fluke 8846A, Fluke 289, Fluke 189, HP 403B w/ OPT 01 -- all read correctly and accurately.

Meters with mVAC but not dBm:  Fluke 8842A, Fluke 27, Fluke 8800A, Fluke 116 -- all read correctly except the F116 which displayed 'OL' in the mV range, but was exactly correct (0.100V) in the 6V range.  This is what the Brymen should be doing.

Meters without mVAC: Mastec/Centec (Harbor Freight) P37772, Fairchild 7000A, Simpson 270 (OUTPUT) --obviously all AC-coupled voltmeters should work fine here, and they do.  The Simpson 270 requires careful zeroing, using the special 2.5VAC scale and some squinting, but it is accurate to less than a needle-width.

The bottom line is that I can't find any meters in my stash that give me a wrong reading.  Something to at least be aware of and IMO, something that should be fixed if possible.

Just did a quick test after reading your post, and indeed my other 2 multimeters (keithley 196 and gossen 22M) read the signal also correctly. The brymen 869s does read it correctly in Vac mode (0.0998Vac was indicated), but no frequency was detected in that mode.

So indeed it seems the Brymens are more the exception than the rule, I did not expect that. 

[Neutrion]

7.5VDC+100mVAC (RMS) 500Hz sine is definitely a tricky signal.  Out of my stash, the only DMMs that could actually provide those three fundamental values were:

Fluke 87V
Fluke 88V
Keysight 1233A
Sanwa PM300 (no mV range however)
Of course Fluke 287


Some notable "failures":

121GW
Brymen BM789
Of course Brymen BM869s

Like the BM869s, the BM789 displayed a garbage reading in mVAC mode.  No matter the amplitude of the AC component, it always displayed approximately 105mV.


The Keysight 1233A surprised me with the fact that while displaying the frequency, it uses the bar graph to display the AC voltage.  It's also very fast and pretty accurate (99.4mV).  It did lose the frequency measurement below about 30mV AC (RMS) (using the above stated signal).

The Fluke 87V & 88V were also quite accurate (100.17mV & 99.86mV).  They lost the frequency measurement below about 20mV AC (RMS).

No surprise I suppose, the Fluke 287 was the most accurate (100.02mV) and made it to 5mV AC (RMS) before dropping out.

The 87V has no mV scale so if we are talking about the V AC scale the Brymens read also correct on the v AC scale.

[Fungus]

Like the BM869s, the BM789 displayed a garbage reading in mVAC mode.  No matter the amplitude of the AC component, it always displayed approximately 105mV.

What did it display in normal AC+DC mode?

[Caliaxy]

The 87V has no mV scale so if we are talking about the V AC scale the Brymens read also correct on the v AC scale.

That's actually the interesting thing we noticed while having this discussion in the BM789 thread.

Fluke 87v has a DC mV position on the dial (with a single 600mV range). There is no AC mV position on the rotary dial per se, but there is a 600mV AC range on the AC V position of the rotary dial. I suspect they did this precisely to avoid the issue under discussion here without adding an extra position on the dial (the alternative would have been having a common AC/DC mV position on the rotary dial and switch between AC and DC with a soft touch button).

[bdunham7]

The 87V has no mV scale so if we are talking about the V AC scale the Brymens read also correct on the v AC scale.

It actually does, in fact IIRC in Hi-res mode it has a 200.00mVAC range, so 10uV resolution. 

[Caliaxy]

A quick fix for the OP (other than a capacitor) would be to use a lab power supply to subtract the offset. Set the output of the power supply to the offset voltage (7.5V), connect (V-) to the GND of the DUT and use BM689S to measure the AC voltage between the measuring point and (V+) on the power supply (as opposed to GND or V-).

[Fungus]

The 87V has no mV scale so if we are talking about the V AC scale the Brymens read also correct on the v AC scale.

It actually does, in fact IIRC in Hi-res mode it has a 200.00mVAC range, so 10uV resolution.

So the OP can simply use AC range (or AC+DC range) with no problems?

[Neutrion]

The 87V has no mV scale so if we are talking about the V AC scale the Brymens read also correct on the v AC scale.

It actually does, in fact IIRC in Hi-res mode it has a 200.00mVAC range, so 10uV resolution.

Maybe this one goes to the  87V but it would be still interesting to compare the 87Vs 200,00mVAC range(0,7%+1d) with the 789s 6,0000VAC range   0,5%+30d in an actual test.

Did by the way anyone tested this scale of the 87V with a DC bias, and for overrange indication?

[bdunham7]

Did by the way anyone tested this scale of the 87V with a DC bias, and for overrange indication?

IIRC yes, but it is AC coupled so there are no surprises there and not a memorable event.

You mentioned in another thread that this issue was 'common' and that even many Flukes exhibit it.  I can't comment on any that I don't have to test, but I'm not seeing that at all.  What might be observed is that some of the Flukes, for example my F116, will appear at first to have an error when you apply a DC bias to an AC signal in the mVAC range.  This actually takes effect even at lower signal levels and is not due to overloading.  What these meters with DC-coupled mVAC ranges are doing is correctly displaying the TRMS AC+DC value, not just the AC component.  That isn't clear just by looking at the meter, so demerits points for that.  However, keeping that in mind, I still haven't managed to get any of them to display a 'wrong' reading.  It is correct or OL.

[J-R]

To clarify, I only tested some of my handheld DMMs and didn't provide the full list of failures.  I have a few more from Amprobe, UEi, etc. that I didn't mention.

There are plenty of handheld DMMs that can get away with showing some/most readings.  There are also plenty of tricks to work around the "issue".

But I consider it a fail if it can't correctly display all three of the signal properties, AC, DC & frequency.  This also goes for bench DMMs that don't support frequency.

I gave the Sanwa a pass but the others get a hard fail for not being able to show the frequency, even despite the fact they can display 100mVAC correctly in VAC mode.

[bdunham7]

But I consider it a fail if it can't correctly display all three of the signal properties, AC, DC & frequency.  This also goes for bench DMMs that don't support frequency.

I gave the Sanwa a pass but the others get a hard fail for not being able to show the frequency, even despite the fact they can display 100mVAC correctly in VAC mode.

I don't see the point of 'failing' a device for lacking a specific feature that it doesn't purport to have.

[J-R]

But I consider it a fail if it can't correctly display all three of the signal properties, AC, DC & frequency.  This also goes for bench DMMs that don't support frequency.

I gave the Sanwa a pass but the others get a hard fail for not being able to show the frequency, even despite the fact they can display 100mVAC correctly in VAC mode.

I don't see the point of 'failing' a device for lacking a specific feature that it doesn't purport to have.

I was primarily referring to the handheld DMMs that DO claim to have that feature.  The bench DMMs that can't do frequency I am considering a fail for the OPs original request.

[bdunham7]

I was primarily referring to the handheld DMMs that DO claim to have that feature.  The bench DMMs that can't do frequency I am considering a fail for the OPs original request.

OK.  If you take the OP's stated needs to be AC-coupled mVAC, readout in dBm, 20,000 counts or more, frequency readout at ~100mVAC or lower and then you read in handheld and probably CAT-rated, that knocks out a lot of products.  If he doesn't need handheld and CAT III/IV ratings, then most modern bench meters will likely work. 

The only inexpensive (relatively) meter I can think of that may meet the OP's specific requirements in this case is the manual-ranging BK Precision 391A.  All the AC ranges are AC coupled, it has a 200mVAC range, the frequency range stated sensitivity is 50mV_rms and it is 20,000 counts. 

[Brumby]

A quick fix for the OP (other than a capacitor) would be to use a lab power supply to subtract the offset. Set the output of the power supply to the offset voltage (7.5V), connect (V-) to the GND of the DUT and use BM689S to measure the AC voltage between the measuring point and (V+) on the power supply (as opposed to GND or V-).

This idea also crossed my mind - but it does introduce some other concerns.

Not having any practical expertise in this area, here are my observations:

The PSU would, obviously, be floating, reasonably stable and have (ideally) zero ripple.  It wouldn't need to be precise - just close enough to the 7.5V offset that it brings the input to the DMM's ADC within range.

How practical it would be is another question.  Having extra leads floating around increase both mechanical and EMI risks.  There is then the time taken to set up such a rig - with the risk of errors doing so - plus the expense of purchasing another piece of gear.  Also, is there somewhere you could place such an item in the production floor environment?  Not to mention the fact that, if you are going to have some calibration history requirements, this PSU would need to be certified (as I understand it).


This sounds to me like a classic case of "Buy once, cry once".  As others have said I, too, would recommend the "bite the bullet" direction.  The pain will only exist at the initial point of expenditure - but if you go down the "cheaper" path, it will flare up repeatedly - costing you time, money and reputation - until you get to the point of wanting to rip your hair out.  In desperation, you eventually cave and fork out the funds to do it right then a month later you marvel at how smoothly things are running and kick yourself for not having done this in the first place.

This applies to ANY situation: Get the right tool for the job.  I daresay we ALL have had experience here - whether working on electronics, your car or any other task.

[joeqsmith]

And for 61E drifting problem, watch the following video from Joe.

As to the 61E comment. My issue with Uni-T is not temperature drift. It's time drift. Take a Ut-61E and wait 6 months and it will be way out of calibration, unacceptably so. That doesn't help me trust Uni-T.  Even the cheap Anegs and Harbor Freight meters hold calibration. Not so much Uni-T.

I continue to monitor the long term drift after making those modifications.   It appears it has been almost a year since the I last checked.

Pulled out the UT61E today to check it it's long term drift.  After six years of collecting data (not six months), I'm not seeing anything concerning.   Obviously I am not taking this data in a controlled environment but after compensating the reference, I doubt the bit the office changes plays into it too much.   

[joeqsmith]

I still haven't had anyone confirm this is really a 869 design error. It seems strange that it is since other meters handle this fine.

There was a long discussion previously on this exact subject, I don't remember which thread. ...

There's been several what I consider fringe topics like this.  My favorite so far was measuring a high value resistor right at the switch point.  Lots of drama on that one.  I attempted to replicate it with our 120VAC mains.  It took several twists of the leads around the lamp cord but I was able to replicate it.   Mixed AC/DC waveforms are a common problem for many meters I have looked at. 

I think this is the last time it came up:
https://www.eevblog.com/forum/testgear/brymen-bm789/msg3696892/#msg3696892

For the UNI-T UT181A fan boys.  What kind of meter reads zero volts when it's obvious there is a signal applied?  More than you may think!
https://www.eevblog.com/forum/testgear/brymen-bm789/msg3689206/#msg3689206 

Is the claim that the Fluke 189 is without a fringe case?   If so, seems like a challenge I could take on...     

[bdunham7]

There's been several what I consider fringe topics like this...

Is the claim that the Fluke 189 is without a fringe case?   If so, seems like a challenge I could take on...   

Well, it is apparently a real use-case and not an unrealistic expectation.  The problem is fairly simple though--lack of AC coupling along with an external block not being an adequate fix.  We sort of expect AC ranges to work with DC bias, probably because it is easy enough to accomplish--and has been widely implemented.  However, if you take a DC reading with a large AC bias, most meters won't handle that very well beyond a certain point and we just accept that as the way things are.

I have not been able to find any unexpected fringe case for the F189. 

[Fungus]

Is the claim that the Fluke 189 is without a fringe case?   If so, seems like a challenge I could take on...   

Or any other Fluke.

[joeqsmith]

There's been several what I consider fringe topics like this...

Is the claim that the Fluke 189 is without a fringe case?   If so, seems like a challenge I could take on...   

Well, it is apparently a real use-case and not an unrealistic expectation.  The problem is fairly simple though--lack of AC coupling along with an external block not being an adequate fix.  We sort of expect AC ranges to work with DC bias, probably because it is easy enough to accomplish--and has been widely implemented.  However, if you take a DC reading with a large AC bias, most meters won't handle that very well beyond a certain point and we just accept that as the way things are.

I have not been able to find any unexpected fringe case for the F189.

All fringe cases are real use-cases.  Everyone who finds one needs it fixed.  Every meter with a fringe case is garbage.   Other threads covered it. 

[Neutrion]

Did by the way anyone tested this scale of the 87V with a DC bias, and for overrange indication?

IIRC yes, but it is AC coupled so there are no surprises there and not a memorable event.

You mentioned in another thread that this issue was 'common' and that even many Flukes exhibit it.  I can't comment on any that I don't have to test, but I'm not seeing that at all.  What might be observed is that some of the Flukes, for example my F116, will appear at first to have an error when you apply a DC bias to an AC signal in the mVAC range.  This actually takes effect even at lower signal levels and is not due to overloading.  What these meters with DC-coupled mVAC ranges are doing is correctly displaying the TRMS AC+DC value, not just the AC component.  That isn't clear just by looking at the meter, so demerits points for that.  However, keeping that in mind, I still haven't managed to get any of them to display a 'wrong' reading.  It is correct or OL.

In the BM789 topic, what I think you are pointing to,(and the other older topic which I linked in) it was mostly about AC+DCmV capable meters  which above a certain point of AC or DC bias, or overloading would read entirely wrong values. Mostly in case of overload.
That is what we were discussing 10 pages long last time. Unfortunately not even te experts from here were able to come up with an easy to implement Idea, how it could be solved. I suppose from the discussion there, that possibly some extra parallel sampling would be needed only to be able to flag the the overload situation. Or maybe some really smart analogue circuit?
To not to talk about reliable ACmv reading with unlimited DC bias on the AC+DCmv scale which seems also like a challenging issue.


And yes I remembered wrong, the 87V had it's wrong reading problem in Joe's test(bm789 video) on the DC scale with AC "bias", not the other way around.

[joeqsmith]

Is the claim that the Fluke 189 is without a fringe case?   If so, seems like a challenge I could take on...   

Or any other Fluke.

You mean like this:
https://www.eevblog.com/forum/testgear/brymen-bm789/msg3688948/#msg3688948

That thread had a lot of info for those wanting to read it but obviously it's much easier, maybe more fun to repeat it.   


Interesting if you read:
https://www.eevblog.com/forum/testgear/brymen-bm789/msg3696619/#msg3696619
and
https://www.eevblog.com/forum/testgear/brymen-bm789/msg3696679/#msg3696679

Go back a few pages:
https://www.eevblog.com/forum/testgear/brymen-bm789/msg3689695/#msg3689695
Specifically:

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

Fun with fringe cases.

[joeqsmith]

And yes I remembered wrong, the 87V had it's wrong reading problem in Joe's test(bm789 video) on the DC scale with AC "bias", not the other way around.

Of course I went after the gold standard of the industry.   

[Neutrion]

Maybe it's the colour.   However these days maybe even shining gold would be possible! On the other hand I have to admit that I like the yellow colour of the Fluke too, pity that Brymen chose the red. But it has some positive meaning in buddhist tradition if I remember right.

[David Hess]

Maybe it's the colour.   However these days maybe even shining gold would be possible! On the other hand I have to admit that I like the yellow colour of the Fluke too, pity that Brymen chose the red. But it has some positive meaning in buddhist tradition if I remember right.

Fluke's yellow is protected IP.  That was something they sued Tektronix over causing Tektronix to change the color of their meter holsters to deep blue.

[AVGresponding]

Maybe it's the colour.   However these days maybe even shining gold would be possible! On the other hand I have to admit that I like the yellow colour of the Fluke too, pity that Brymen chose the red. But it has some positive meaning in buddhist tradition if I remember right.

Fluke's yellow is protected IP.  That was something they sued Tektronix over causing Tektronix to change the color of their meter holsters to deep blue.

All the more ironic that they both ended up getting bought by Danaher. And then split off into Fortive.

[NickKUK]

Hi,
I have query about this as I'm on the lookout for a 5digit DMM that can handle up to 1kV (tube amps, solid state and other general electronics). I'm thinking that the BM869 would suit this requirement but reading the mVac threads, I'm in doubt.

The scenario I specifically want is the ability to measure the top of a triode cascode (320Vdc+small Vac) or power output stage (400Vdc+ with a small mVac to large 100Vac waveform).

Short question - can the BM869 do that or not?

Now I could use a HV film cap and measure the AC, but would auto ranging Vdc up to 800Vdc with a 100-200Vac component work to measure the Vdc side with the BM869?

(I have a scope but I'm looking for DMM)

[2N3055]

Hi,
I have query about this as I'm on the lookout for a 5digit DMM that can handle up to 1kV (tube amps, solid state and other general electronics). I'm thinking that the BM869 would suit this requirement but reading the mVac threads, I'm in doubt.

The scenario I specifically want is the ability to measure the top of a triode cascode (320Vdc+small Vac) or power output stage (400Vdc+ with a small mVac to large 100Vac waveform).

Short question - can the BM869 do that or not?

Now I could use a HV film cap and measure the AC, but would auto ranging Vdc up to 800Vdc with a 100-200Vac component work to measure the Vdc side with the BM869?

(I have a scope but I'm looking for DMM)

Short version?

Only mV range is always DC coupled.

So measuring 4 mVAC  riding on 5V (or more) is a no go in mV mode. In mV mode any combination that produces more than +-500 mV P-P. is a no go.
Id you want to measure mV level AC ripple on top of 200V, you need to use capacitor, and will have to measure DC separately.

But if you are in V mode (AC, DC, AC+DC) all works fine. And you can easily measure 10VAC riding on 800V.

[NickKUK]

Short version?
Only mV range is always DC coupled.

So measuring 4 mVAC  riding on 5V (or more) is a no go in mV mode. In mV mode any combination that produces more than +-500 mV P-P. is a no go.
Id you want to measure mV level AC ripple on top of 200V, you need to use capacitor, and will have to measure DC separately.

But if you are in V mode (AC, DC, AC+DC) all works fine. And you can easily measure 10VAC riding on 800V.

Perfect (as perfect as any DMM can be!).

[Fungus]

But if you are in V mode (AC, DC, AC+DC) all works fine. And you can easily measure 10VAC riding on 800V.

And then zap the next component you touch using the 800V stored in the internal decoupling capacitor. 

[2N3055]

But if you are in V mode (AC, DC, AC+DC) all works fine. And you can easily measure 10VAC riding on 800V.

And then zap the next component you touch using the 800V stored in the internal decoupling capacitor. 

And how is that different than any DMM ever made that has AC mode..? And not in AC+DC mode...

[Fungus]

And how is that different than any DMM ever made that has AC mode..? And not in AC+DC mode...

Who said it was any different? I'm just making people aware.

Please relax... not everything is a personal attack.

[bdunham7]

And then zap the next component you touch using the 800V stored in the internal decoupling capacitor. 

Sure, if you leave the meter in the 1000VAC range and for some reason then immediately go and put the leads on some small, helpless ESD-sensitive CMOS IC....

[2N3055]

And how is that different than any DMM ever made that has AC mode..? And not in AC+DC mode...

Who said it was any different? I'm just making people aware.

Please relax... not everything is a personal attack.

Exactly. I agree with you on this 100%.
Just making people aware, and no, it is not personal attack.
I have absolutely no animosity towards you. Relax.
Friends ? 

[Kleinstein]

The AC coupling can do harm when charged to a high voltage. By nature it does not discharge very fast. If you are lucky there is some 10 M directly at the input side to ground and than maybe some 1.1 M as input impedance for the actual AC input. So discharge may take a few seconds. In some cases the resitor to ground at the input is missing and the charge can stay there longer.

It happend to me once with a scope. Look for ripple on to of +-150 V range supplies - because of the low volatage and relative low frequency with an 1:1 probe. The step from the +150 to -150 was than a bit too much for the scope input with specs for some 200 or 250 V peak. Chances are a charged capacitor of the scope or DMM could also damage sensitive parts - not just very sensitive ones.

[David Hess]

It happend to me once with a scope. Look for ripple on to of +-150 V range supplies - because of the low volatage and relative low frequency with an 1:1 probe. The step from the +150 to -150 was than a bit too much for the scope input with specs for some 200 or 250 V peak. Chances are a charged capacitor of the scope or DMM could also damage sensitive parts - not just very sensitive ones.

On the coupling switch, some older oscilloscopes use the Ground selection between between the AC and DC coupling options to precharge the AC coupling capacitor.  Very rarely they have a separate precharge position.

[NickKUK]

Just to follow up - I've had the Brymen 896s for a while now and it's great bit of kit. I like the leads have screw fitting for the little banana connectors too but also the plugs also fit my other HV probe kits with big croc clips so no fingers in shock range.

It's valid point on the cap discharge and it helps the inbuilt bleeds that take the B+ & B- plus the DMM back down to zero. Top of the all tube cascode + CCS (+320) to bottom (-320) means with 640Vdc in there I'm not sticking my fingers anywhere near it to reposition whilst in operation! By the time the system has bled down to preposition so has the DMM.