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| Brymen BM789 |
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| floobydust:
--- Quote from: AndrewBCN on September 19, 2021, 02:56:14 am ---[...] it is clear (to me at least) that among the best practices for DMM usage, shorting the leads before and after any measurement can help avoid some kinds of accidents. --- End quote --- NO!!! This is wrong, where did you pick this up from? If the equipment has a charged up DC-blocking cap, it will discharge into the front-end and kill it. |
| AndrewBCN:
--- Quote from: floobydust on September 19, 2021, 04:08:26 am --- --- Quote from: AndrewBCN on September 19, 2021, 02:56:14 am ---[...] it is clear (to me at least) that among the best practices for DMM usage, shorting the leads before and after any measurement can help avoid some kinds of accidents. --- End quote --- NO!!! This is wrong, where did you pick this up from? If the equipment has a charged up DC-blocking cap, it will discharge into the front-end and kill it. --- End quote --- ??? How would a cap that is part of the front-end discharge into the front-end when the leads are shorted? Joe, we need another video!!!! :-DD |
| floobydust:
--- Quote from: bdunham7 on September 19, 2021, 01:26:37 am --- --- Quote from: floobydust on September 19, 2021, 01:13:27 am ---Just look at the schematics, --- End quote --- I have or have seen the schematics for the first 8 you listed, but I don't have and don't recall the details of any DC-coupled models. If you have a 10M input resistor and can live with a 30-40Hz cutoff, the capacitor can be pretty small. --- Quote ---For the hardware to have a DC-blocking cap, if the CMOS switches and mV AC op-amp and true-RMS converter are all in the DMM IC, then you simply can't AC-couple post-divider without something getting swamped. Like the ANENG AN8008, Brymen 789, 121GW, Fluke 17B, UT61e. These all can get overloaded from the DC. The issue is if these detect and display it. --- End quote --- They don't in mVAC, but I'm wondering if/how they cope with the issue in the higher ACV ranges. And if they indeed use a pre-divider HV cap, then not incorporating that in front of the mVAC range has to be for other reasons. --- End quote --- For the mV range, the input attenuator is not employed and (DC-coupled) the DMM IC gets the same voltage input as the test leads, almost directly (post-PTC) connected to the IC. There is a 10MEG load switched in. There is next a mux and op-amp or PGA because you need a gain of ~4-10x before the ADC to have a decent noise floor. 141mV in, X10 and you've got 1.41V going to the op-amp then ADC. What happens with say 1V DC on 100mVAC ? The amp is saturated or even phase-inverts and you can get bogus readings. Unless you are constantly checking (in firmware) for this, which is what we should be checking for. For AC-coupled equipment, discharging the blocking cap into the front-end, say 200V and a few hundred K seems to be at most a few mA but JFET, CMOS mux etc. don't always survive. It seems to be the high voltage damaging the semi. Some gear does not have much for series resistance or a huge cap and the discharge hits hard. |
| Fungus:
--- Quote from: joeqsmith on September 19, 2021, 01:20:47 am ---If you're interested in more details on the 189, I took a quick stab at tracing out this area. I also made an attempt to look at the current into a short. --- End quote --- Have you painted your 189? |
| Fungus:
--- Quote from: floobydust on September 19, 2021, 04:08:26 am ---NO!!! This is wrong, where did you pick this up from? If the equipment has a charged up DC-blocking cap, it will discharge into the front-end and kill it. --- End quote --- Doesn't it have input protection for that? :popcorn: |
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