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| Brymen BM789 |
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| 2N3055:
--- Quote from: EEVblog on September 13, 2021, 12:47:05 pm --- --- Quote from: Neutrion on September 13, 2021, 11:35:09 am ---And the question is, will it be possible at all to change this in FW. --- End quote --- If it's related to the lack of a DC blocking cap in AC mV mode then there is likely nothing you can in firmware. Dual display meters have a physical DC measurment path a separate AC coupled path, and they get measured separately. --- End quote --- They still have same input attenuator.. BM869 has DC blocked AC path on AC volts.. But it is not blocked on mV range. Despite having two positions on switch, both AC mV and DC mV is DC coupled. So injecting 10V DC into it will confuse meter, despite being in AC mV mode. AC volt ranges are fine. I don't understand why they didn't DC block it on a dedicate AC mV position either.. On BM78X series they simply dispensed with a confusion and have single mV switch position that is DC coupled. |
| bdunham7:
--- Quote from: Neutrion on September 13, 2021, 11:35:09 am ---But do you have a meter which also has the same AC, AC+DC, DC range? Because AC+DC is OK and it overranges properly. --- End quote --- Yes, the Fluke 189 has AC, AC+DC and DC. It's a little different because the AC range is a separate switch position. --- Quote ---I already contacted Brymen through the seller as mentioned, and their ansver was, that this is OK because the meter works well WITHIN the specs. So I suppose all the BM789 should work like this, and possibly the 786 as well. I hope some other guys with 789s will show up. --- End quote --- Well first of all, if it is overrange, it should say so, not just give you a wrong number. Second, it happens in one direction, leading me to think there's a specific defect. Third, what you are asking it to do is not unreasonable and the correct result if it is properly measuring AC of a 0-1V square wave is 500mV--which is within the range. So why does it read 650mV?? |
| 2N3055:
--- Quote from: bdunham7 on September 13, 2021, 01:26:48 pm --- --- Quote from: Neutrion on September 13, 2021, 11:35:09 am ---But do you have a meter which also has the same AC, AC+DC, DC range? Because AC+DC is OK and it overranges properly. --- End quote --- Yes, the Fluke 189 has AC, AC+DC and DC. It's a little different because the AC range is a separate switch position. --- Quote ---I already contacted Brymen through the seller as mentioned, and their ansver was, that this is OK because the meter works well WITHIN the specs. So I suppose all the BM789 should work like this, and possibly the 786 as well. I hope some other guys with 789s will show up. --- End quote --- Well first of all, if it is overrange, it should say so, not just give you a wrong number. Second, it happens in one direction, leading me to think there's a specific defect. Third, what you are asking it to do is not unreasonable and the correct result if it is properly measuring AC of a 0-1V square wave is 500mV--which is within the range. So why does it read 650mV?? --- End quote --- He was measuring "more than 1V" with excursions below zero of 100-200 mV. That makes it 1.2-1.3V P-P and right at the saturation point. It had to simply show "OVL" and that is it. When I do the same to BM869S it will show "OVL". But because of DC path, I can inject DC into it and make it go crazy too, with specific values. But it behaves much better. |
| bdunham7:
--- Quote from: 2N3055 on September 13, 2021, 12:45:00 pm ---For instance, if I feed 300mV RMS 400Hz signal with 700mV offset I get propper 300mV AC reading. If I go higher offset than that, it starts showing bad AC amplitude, less than it is. Until DC offset reaches 8V when it shows overload. Any do you know why? Because you are (me in this case) severely overloading input amplifier with DC. --- End quote --- That seems to me to be a deficiency in design. Or at least one not meeting normal expectations. --- Quote ---There is this misunderstanding that meter can simultaneously measure 10 mV AC and 40V DC , or vice versa, because it has dual display. It cannot. Meter has ONE input attenuator and one ADC. If you set it for 100V it cannot be in position for 10mV. And vice versa. I personally don't know of any meters that do that. I wish it existed, I would gladly buy one. People that work on tube amplifiers would love it. --- End quote --- The old Fluke 867 Graphing Multimeter could do this, but I can't think of any others. Other than that, the answer to simultaneous DC and ripple measurement is two meters or a 6.5 digit DMM which can measure 10mV on the 100V scale. Or you can just switch back and forth. --- Quote ---If you do have 10mV AC riding on top of 10V DC, you can use capacitor. That will work well. If you have 150V AC riding on top of 1 mV DC, you're out of luck. You are asking meter to suppress difference in amplitude of 150000 times... Funny enough, meter might have more suppression than that at 50Hz.. But input might start smoking too... --- End quote --- There's a difference between normal mode rejection of AC in the DC mode, which seems an unavoidable limitation to me, and blocking of DC in a true AC mode (not AC+DC) which to me seems a normal and expected function of a meter. I can read the ripple (only) of a PSU by simply selecting mVAC and connecting the leads. Of course, the F116 fails this test, but it also tells me that it isn't true AC on the front panel and it indicates overload. I then have the option of moving the selector switch and using the 6V true AC mode, which fully blocks DC. I would be interested to see what happens on the other ranges of the Brymen because they appear to have adopted the same strategy of using a single switch position for all 3 functions and then selecting the mode with the button. |
| Caliaxy:
Well, if this is not a bug and it works as intended, it's a design fail. I replicated the issue on all the Brymen meters within my reach (235, 789 and 689S): AC mV shows nonsensical values in the presence of a high DC offset. The DC offset doesn't have to be huge. I tried 500mV AC on top of a 2V DC offset - something that one would likely encounter in normal everyday life. Other cheap(er) meters (Fluke 101 and 107) do the same (show nonsensical values rather than OL). Daves GW121 does it too. Other even cheaper ones (UT81E and Aneng 8009) don't, they correctly display OL (kudos to them). Fluke 189 (in AC mV) is not tricked by a high DC offset either. Fluke 87V doesn't have this issue at all because it doesn't have a separate AC mV position on the dial in the first place ;D I guess it's one of those situations of "know your meter's shortcomings" and "use it accordingly". Check for DC bias before measuring AC mV. No meter is perfect. Thanks for letting us know! |
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