Products > Test Equipment
Brymen BM789
joeqsmith:
3055, funny post... I want that James Bond meter. Keep in mind that the OP just expects it to display over range, not draw the bath water for them.
--- Quote from: bdunham7 on September 14, 2021, 08:42:59 pm ---
--- Quote from: 2N3055 on September 14, 2021, 07:45:53 pm ---As I said, in a perfect world, it would be nice that if you connect 1000V to mV input, it should not damage it, and then meter should, in a pleasant voice say: "bip. Dear operator.. please make a note that you connected meter to 1000V on a millivolt range. While I was designed to withstand such overload indefinitely, and your safety is not jeopardized, I am unable to measure voltage correctly. Please change input range to volts. Thank you for your cooperation....bip".
Are you kidding me? :palm:
--- End quote ---
Every single one of my DMMs does exactly that, more or less, AFAIK--including some very, very old models. The newer ones with CAT ratings can take that 1000V on any range--ohms, diode check, etc. I believe it is a requirement. As far as there being a requirement for an overload indicator, I'll defer to others since I don't have any written standards to consult. Input protection circuitry is not that hard to make and in any case, is a well known quantity.
--- End quote ---
Ok, so no luck on your end. All your meters behave as expected while sweeping the frequency, amplitude and offset. Not at all what I would expect but I can try some of my meters and see how they behave.
Just a friendly reminder that you may remember one of the members here damaging a 121GW PCB. Dave suggesting that the lower voltages could not damage the meter the way that was shown. So I setup an experiment using a DC voltage with enough current that the with the low voltage clamps engaged, it would not load down the supply. 100s of mA lethal stuff. :-DD Basically the idea was if the switch was rotated with the input live, we could draw an arc and have enough current to burn the board.
Be careful.
floobydust:
16.1 REASONABLY FORESEEABLE MISUSE
"No HAZARDS shall arise if adjustments, knobs, or other software-based or hardware-based controls are set in a way not intended, and not described in the instructions. Other possible cases of REASONABLY FORESEEABLE MISUSE that are not addressed by specific requirements in this standard shall be addressed by RISK assessment (see Clause 17). "
But the certifiers can screw up too and just re-test the function that was being tested - not all the others, so side effect failures can sneak through.
If a DMM doesn't display a hazard when it's expected, i.e. on ACV and not functioning- that's unsafe. After a transient test for example, it's expected to still display HV if present and you are intending to measure that. The owner's manual can give coverage for many misuse scenarios.
The Brymen IC is familiar to us, study the PCB a bit. For these chips, true-RMS is calculated by a tiny DSP section, which removes the DC component from the result.
So A/D reading that module's output will never know if there is a DC overload unless you switch the mux and look pre-RMS for that A/D clipping.
I can't see any rectifier though, the op-amp if used can also saturate and you have no way of knowing like in the 121GW. The PCB shows provisions for an entire IC/subsection maybe for True-RMS (four tantalums).
A firmware challenge is correctly looking at the quantity to auto-range (can't on a fixed 100mV range...) so it can display OL, can get missed.
bdunham7:
--- Quote from: 2N3055 on September 14, 2021, 09:25:06 pm ---So here is a brain teaser : in your own words it will survive in any range including Ohms, right. And what will meter set on ohms show when you connect it to say 400V?
It will keep showing Open on the screen. Whoops....
--- End quote ---
Actually in that case they say 'OVERLOAD' or OL. Which is exactly what they are seeing--voltage above their range limit. I don't expect them to do more than tell me that they can't get a reading, what I don't want to see is a normal-looking but incorrect reading.
--- Quote ---That is exactly why meters for this "I have no clue what to expect" thing do exist. On my BM525 you have auto mode. It will automatically detect AC, DC or Ohms/continuity.
In a single switch position.
--- End quote ---
I'm not after that. I'm only asking that the meter do one of two things--give me a reasonably correct answer or say "I don't know". The only fail is when it gives me a plausible but completely wrong answer. I'm sure that you could probably make any meter fail this at some point, so it just comes down to what one thinks is reasonable and what other similar meters are capable of.
--- Quote ---And no AC mV range.. That is how Fluke solved that problem with F87V.
I prefer to have it, even if it requires thinking when using..
--- End quote ---
That's not true, the F87V has a 600 mVAC range, it is just doesn't have a separate switch position for it.
--- Quote ---When you guys find perfect meter let me know. I am interested, but in meantime I'll work with what I have.
--- End quote ---
OK, it's the Fluke 189. Which is why they go on eBay for three times the price of a new BM789! :)
2N3055:
--- Quote from: Neutrion on September 14, 2021, 09:03:03 pm ---
--- Quote from: Fungus on September 14, 2021, 07:31:35 pm ---
Anything they couldn't cram in there? They documented it. What else could they do?
It might be possible to make it say 0L, we'll see. I'd bet heavily against them being able to make it measure correctly though.
--- End quote ---
I don't want it to read correctly above the specified limit, only to show "OL" if overloaded.
But it is kind of amazing to see people who never ever measure anyting else than what they expect, this would be my goal as well to get to this level.
But for those who are not perfect yet, an example again:
User starts measuring correctly on the V AC scale, to see what is the level of the signal like.
Meter shows 200 mV AC, so user proceeds to the mV scale because he needs better accuracy.
(Up until this point I think we all agree that he is doing right...)
Than he sees the signal raising to 600mV. And there he can not be sure whether it is 600mV or maybe 325V because something went wrong.
I will do some tests to see what is the highest reading which one can trust. But it depends both on frequeny and duty cycle. Possibly also on waveform, but I can not easily test anything else than a squarewave.
If we could find out whether there is any limit up to which the meter could be trusted, than it means that it could be set as overrange limit in the FW as well. But than we would lose counts.
But I hope to get a more elegant solution.
--- End quote ---
Do you work on thermionic valve amplifiers?
That is, for instance, place where you can have 100mV AC combined with 300V. On audio amplifiers you can have 10mV riding on 80V...
But most of those high DC voltages are static working point related, i.e. don't change much or at all.
Most of them might move a bit with thermal bias circuits doing their stuff.
I cannot find scenario where it suddenly goes from 100mVAC+0 DC to 100mVAC+50V DC.
If that happens during testing, that means you switched to mV too soon, before you really figured out what voltages you have here.. Back to the square one..
Honestly, I understand you would like it to be smarter than it is. I have nothing against it. It's just I really rarely see that kind of scenario. Because of measurement practice that was beaten into me many, many years ago with moving coil, manual range meters. One wrong move, bum, you don't have a meter anymore.. I know todays meters survive pretty much anything, and that you can switch between functions without disconnecting them etc.. But I still don't do it. And I am rewarded with a fact that i don't see those fringe cases where meters behave weirdly.
I'm not saying you're wrong, but pointing out that you CANNOT trust meter blindly EVER. Measure 3 times, cut once.
While we are talking real world scenarios, when, for instance, repairing something, my first troubleshooting start with meter in volts (autorange) and checking power busses. If they look kinda OK, I fire up scope. And check those power busses again with a scope to see if they are clean.
Meters are utterly useless to see actual SIGNALS. They come in handy when I first see, with a scope, there is a clean DC voltage on a point and IF I want to see precisely what it is.
To add insult to the injury, most of the time scope measurements will do good enough job, and it can measure dozens of parameters simultaneously.
Meter is more used for continuity, resistance, and when more accuracy is needed. Which also is a moot point when you have 16bit 0.25% accurate scope..
My method is to poke around circuit to understand it. That way there aren't much surprises.
Every repair job is actually reverse engineering. That or you simply stab in a dark and change random components, or just change them in a bulk.
When you're designing your own circuits, there should not be such unexpected surprises. Otherwise you're doing something wrong.
If there is a scenario where I suspect that (because of the circuit) there can be malfunction that can inject some weird stuff, i connect two meters, one measuring main parameter, other one monitoring for error conditions. Or watch it with a scope. Truth is you should always go back and forth and repeat measurement, varying methods, until you're sure you're measuring the right thing.
Best,
bdunham7:
--- Quote from: joeqsmith on September 14, 2021, 09:39:33 pm ---Ok, so no luck on your end. All your meters behave as expected while sweeping the frequency, amplitude and offset. Not at all what I would expect but I can try some of my meters and see how they behave.
--- End quote ---
No, I'm just referring to their ability to withstand without exploding. Getting any of them to read 'wrong' is easy with a combination of high crest factor and offset. I even had my 8846A reading what should be a DC signal as the wrong polarity, until I turned on the analog filter and it started behaving. Finding a signal that baffles something like the F189 but not the the Harbor Freight one is the trick. I'm down to my last HF meter anyway....
Edit: We would have to agree on what the second meter should be so that any results can be replicated. So far, the 8846A has easily been beaten "Harbor Freight freebie outperforms Fluke's best bench meter!" because it starts by default with the analog filter off. A 0-1V 5% 1kHz test signal, which should read ~50mV, is 15.4mV on the 8846A, 46.4mV on the HF. The 46.4mV is more or less correct because if you turn the analog filter on or use a 189/289, that's what you get. I was going to suggest the 189 as the target because you have one and they're well regarded, but I'm finding it very difficult to fool.
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