Products > Test Equipment
Brymen BM789
bdunham7:
--- Quote from: Fungus on September 14, 2021, 12:49:45 pm --- mV/mA ranges are designed for sensitivity, not to handle overloads.
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Typical designs of AC/mVAC ranges include blocking capacitors at some point to prevent any overload by a DC bias. This is how you would read ripple on a power supply, right? This meter is atypical, although documented in the specs, and would be unable to read power supply ripple in the mVAC range without an external blocking capacitor. This may not be a deal killer for most users, but it is a shortcoming that cannot be overcome with 'correct technique'. You have to add an external component or move up to a higher range and lose resolution. So now we know.
--- Quote ---You've done several similar tests on bigger scale, adding hundreds of volts of offset on the ACV scale. Many meters fail to handle it gracefully.
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That was simply a ranging issue. The vast majority of meters will not be able to handle over-range normal mode AC rejection, it's just an inherent limitation that can't be easily solved with one component. The Brymens can't handle over-range normal mode rejection either, but the autoranging appears to respond to the AC enough to move up to a higher range, at least under those test conditions. That's an advantage of certain types of fast autoranging circuits.
AndrewBCN:
--- Quote from: Fungus on September 14, 2021, 12:49:45 pm ---...
Which is fine! It's good to know the limitations of the tools we own, playing is a good way to find them.
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Except in this case it's not exactly a "limitation" of the BM789. The OP stumbled upon an edge case which is very similar to an operator error, where the BM789's autorange algorithm doesn't compute an appropriate response. The obvious "fix" for this issue is not to reprogram the MCU inside the multimeter (because there is an almost unlimited number of similar edge cases), but to operate the Brymen BM789 following standard good practices, which even a beginner can learn is less than 15 minutes.
Also note that the edge case / wrong operation of the BM789 that the OP stumbled upon is completely different from the methodical stress testing that joeqsmith performs on his DMMs. Joe simulates real-life accidents and situations that the meters are supposed to be able to cope with, within and sometimes beyond their specifications.
Joe stresses the hardware and points to flaws in the design and safety of DMMs, the OP just found an operator error that the programmers at Brymen didn't account for in their autorange algorithm.
I very much enjoy Joe's videos where he goes through various steps to test his DMMs, because what I see is a talented engineer making sure his tools are fit for the job, and this information is very valuable. An operator error that leads to a DMM incorrect display is anecdotal information of next to zero value.
Fungus:
--- Quote from: Neutrion on September 14, 2021, 02:00:12 pm ---I also really like the warm white backlight, which is uniqe.
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OT, but... Brymen do some orange backlights, too, eg. my BM857. :)
bdunham7:
--- Quote from: Neutrion on September 14, 2021, 01:53:54 pm ---So do you agree with the others, that there is no way of showing the overload under these circumstances?
Does this happen with every major brand with similar funcions?
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To be clear, since you are implying what 'others' agree on and I'm an other, I don't agree and think the meter should show an overload indication. Whether that's a design issue or a one-off defect with your unit, I don't know.
And no, this doesn't happen with most meter designs AFAIK, at least not the ones I have with an explicit mVAC function. I think it is a result of 'bangforbuckitis' or the manufacturer trying to add as many features as possible at a price point. Something has to give.
--- Quote ---If we are talking about best practice, with the 789 it seems, that if one reads anything on the mV AC range higher than 500mV one should not trust those numbers, and constantly doublecheck it on an other scale, or in the dc range as well. Just as a sidenote. If no FW update is possible for this.
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This shouldn't be necessarily be dealbreaker, its just another characteristic you have to learn to deal with--if your individual meter is not, in fact, defective. I think the easiest way to guard for it would be to always take your reading initially in mVAC+DC, then if it reads in range you can switch to mVAC and get the correct reading. This actually isn't a bad idea for any AC range on that meter, since it is easy to do with just two button pushes.
And, b/t/w, contrary to some earlier discussion, if this behavior is characteristic of all the units, I think issue of the reading hanging without an OL indication might well be resolvable in firmware.
joeqsmith:
--- Quote from: Neutrion on September 14, 2021, 01:53:54 pm ---
--- Quote from: joeqsmith on September 14, 2021, 12:28:07 pm ---Now that it is clear you are adding a DC offset, none of what you wrote surprises me. At first I thought it was the wrong tool for the job but not posting your requirements, it seems you're just playing around with your new toy. Most of the responses here seem correct.
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So do you agree with the others, that there is no way of showing the overload under these circumstances?
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I believe that given a set of requirements, a good group of EEs could design products to achieve them.
--- Quote ---Does this happen with every major brand with similar funcions?
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As I already mentioned, its not how I would normally use a meter. Another good example of what I would consider a fringe case, someone had posted how they were trying to measure a high value resistor that was right on the edge of the range switch point. The meter was picking of the AC mains and had problems. The tried a similar test using other meters and they did not have the problem. Something like that. In this case, I had the same meter and I think I was able to replicate it by twisting the probes with a lamp cord while trying to make the measurement. I think in that case, the company came up with a hardware solution but it's nothing I considered adding as I am not normally wrapping my leads around lamp cords.
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