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| Brymen BM789 |
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| joeqsmith:
--- Quote from: joeqsmith on September 19, 2021, 02:47:57 pm --- --- Quote from: bdunham7 on September 19, 2021, 02:57:21 am ---Well, that was easy. ... Of course it means that the BM789 also stores device-destroying energy in the AC mode! :) --- End quote --- I could run it up higher if you wanted but would want to make a few changed to my setup just to play it safe. Yes, after applying a DC voltage to the 789 while it is in it's ACV mode, I can then discharge the meter to the LED bank and have it light. I would suggest it stores potentially device-distroying energy. The parasitics alone may be enough to damage something sensitive. If you watched that last video, that was basically my point. This isn't a static problem in the sense that everything is at steady state. Measuring the DC resistance has little to do with the potential peak currents. --- End quote --- Shown with the 789 set to the ACV mode, again with it set to the lowest manual range. Fluke 189 will over range at 1100V. Shown at 1050VDC offset then again with 1200VDC. |
| joeqsmith:
--- Quote from: Fungus on September 19, 2021, 04:27:08 pm --- --- Quote from: joeqsmith on September 19, 2021, 04:02:10 pm ---For the meters, I would assume that the charge/discharge would be basically the same. Even on the 189, it looks like it follows two different paths. Still the difference is negligible. Still, it's common to find exceptions when making blanket statements. --- End quote --- It's hard to imagine a measuring device that would want the capacitor to discharge faster than it charges. It would go horribly wrong as the AC frequency increases. --- End quote --- Say you wanted to make an oscillator out of CMOS gates and have something other than a 50% duty cycle. Rather than a simple RC to set the timing, we could use this circuit to provide two different time constants. Maybe your measuring device has an RC network on the CPUs reset for a slow start but when the power is cycled, you want it to reset fast. It's not that uncommon. |
| Fungus:
--- Quote from: joeqsmith on September 19, 2021, 04:49:55 pm ---Maybe your measuring device has an RC network on the CPUs reset for a slow start but when the power is cycled, you want it to reset fast. It's not that uncommon. --- End quote --- That's a measurement device's reset circuit, not a measurement device's measuring circuit. |
| joeqsmith:
--- Quote from: Fungus on September 19, 2021, 05:06:49 pm --- --- Quote from: joeqsmith on September 19, 2021, 04:49:55 pm ---Maybe your measuring device has an RC network on the CPUs reset for a slow start but when the power is cycled, you want it to reset fast. It's not that uncommon. --- End quote --- That's a measurement device's reset circuit, not a measurement device's measuring circuit. --- End quote --- Your comment had broadened the scope. --- Quote ---It's hard to imagine a measuring device that would want the capacitor to discharge faster than it charges. --- End quote --- |
| floobydust:
--- Quote from: Fungus on September 19, 2021, 06:27:49 am --- --- 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: --- End quote --- Summarizing, the BM789s is DC-coupled as far as the hardware and DMM IC, and there's a HPF from the true-RMS DSP section if Brymen is using it verses the MCU doing the math with raw A/D samples. So I believe it can be saturated with DC on any ACV measurements if it can't auto-range up, and the F/W needs a check to see of if it lets you know this is happening. I looked at the usual dozen multimeter schematics, the ones incorporating a DC-blocking cap for ACV. Maybe others can contribute. None of the asian design multimeters I can find are using a DC-blocking cap, except the BM869s. The "protection circuit" is needed to clamp down to the IC level, the CMOS switches downstream of the HV protection MOV's. Interestingly, the AC path does not include anything like our usual diode-connected transistors etc. which end up on the DC/ohms path. AC protection is by large series resistance, 87V is 10MEG and 34401a is 1MEG so the cap discharge is low current, enough for an IC's substrate diodes to handle. But the voltage is still high for whatever you are probing, not sure what a mosfet does with HV on the gate at low current, for a long time. ESD is short duration and 10X and the voltage at 1/20 to 1/200 the capacitance. Interestingly ACV the 87V moves the 9.99MEG after the cap, so it's a 10MEG AC impedance and very high (leakage) DC resistance. So using a single-resistor HV probe with the DMM on ACV would charge to a clamped couple kV DC-component and not be a good experience. Oscilloscopes have larger blocking cap (up to 22X) and lower impedances for the bandwidth, some using relays/switches or maybe solid-state AC/DC coupling switching. The cap discharge is brutal there maybe 15mJ across the switch and goodbye JFET if you short the probe's input. 2235a rated 400VDC goes 0.022uF down to 1,000pF +ve discharge through 70R to the JFET, minus a 10X probe. It all depends on the moves you make while the cap has a HV charge. As far as providing a make/model/schematic a showman could use, awareness of the charged cap is more important than damaging something. I treat it like a grenade- a high risk measurement (AC ripple/signal on HV DC) across the board. |
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