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Brymen 869s DC offset issue when reading ACmV

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Brumby:

--- Quote from: Caliaxy on January 24, 2022, 07:53:19 pm ---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-).

--- End quote ---
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:

--- Quote from: joeqsmith on January 23, 2022, 04:08:02 pm ---
--- Quote from: mqsaharan on January 21, 2022, 05:19:25 am ---And for 61E drifting problem, watch the following video from Joe.

--- End quote ---


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

--- End quote ---

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.

--- End quote ---

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:

--- Quote from: bdunham7 on January 23, 2022, 04:08:11 am ---
--- 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. ...

--- End quote ---

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:

--- Quote from: joeqsmith on January 25, 2022, 03:02:04 am ---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...   

--- End quote ---

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:

--- Quote from: joeqsmith on January 25, 2022, 03:02:04 am ---Is the claim that the Fluke 189 is without a fringe case?   If so, seems like a challenge I could take on...   

--- End quote ---

Or any other Fluke.

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