Products > Test Equipment

Brymen BM789

<< < (23/81) > >>

joeqsmith:

--- Quote from: 2N3055 on September 15, 2021, 08:15:29 am ---...
- 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

--- End quote ---

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:

--- Quote from: AndrewBCN on September 15, 2021, 08:33:31 am ---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.

--- End quote ---

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:

--- Quote from: joeqsmith on September 15, 2021, 11:23:21 am ---
--- Quote from: 2N3055 on September 15, 2021, 08:15:29 am ---...
- 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

--- End quote ---

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.

--- End quote ---

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:

--- Quote from: 2N3055 on September 15, 2021, 01:05:00 pm ---
--- Quote from: joeqsmith on September 15, 2021, 11:23:21 am ---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.

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

--- End quote ---

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

bdunham7:

--- Quote from: AndrewBCN on September 15, 2021, 07:50:01 am --- it will autorange exactly the same as the 87V.

--- End quote ---

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'.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod