Products > Test Equipment
Brymen 869s DC offset issue when reading ACmV
<< < (3/13) > >>
bdunham7:

--- Quote from: 2N3055 on January 21, 2022, 11:30:38 pm ---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.

--- End quote ---

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_:

--- Quote from: bdunham7 on January 22, 2022, 01:13:29 am ---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.

--- End quote ---

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:

--- Quote from: AnalogEngineer on January 23, 2022, 03:38:13 am ---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.

--- End quote ---

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.


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

--- End quote ---

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.
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod