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| Brymen BM789 |
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| AndrewBCN:
Thank you for yet another great video, Joe. My takeaway from this thread, apart from the information that some DMMs use a blocking capacitor in their ACmV input circuit and others don't (in other words, some are AC-coupled and others are DC-coupled in their ACmV range): remember to always switch on my brains before I switch on the DMM to make a measurement. :-DMM |
| bdunham7:
--- Quote from: joeqsmith on September 17, 2021, 01:34:31 am ---Bleed and high impedance don't mix. Maybe change your habits to discharge the meter. All these years and still new tricks to learn. :-DD Using a sting of LEDs to look at the waveform as well as conducting the finger test.. --- End quote --- Interestingly I was easily able to replicate this and found that even if you switch to DC, the capacitor retains its charge. If, for example, you go to measure 500V but the meter is in mVAC, if you switch to DC and measure your 500VDC, if you then remove the test leads and switch back to mVAC, you get the jolt. |
| floobydust:
If the cap is on the high-side of the divider chain, it charged up to the DC component and the next action you take can cause damage. Moving the probe to a different circuit node, the cap discharges into it and any semi is usually killed due to the voltage. Moving the probe to GND or a low resistance point to GND discharges the cap into the test equipment front-end. There are many RMS-converters like AD636 etc. in the graveyard as this kills their input, even though the blocking cap is post-divider string. There is no protection at that node. With a scope, switching from AC to DC coupling shorts the blocking cap and the scope's switch makes a loud ka bang and arc. Tektronix includes a small series resistor. The cap seems to be 27nF-220nF depending on the low freq. response of the instrument. My practice is to touch the probe to a 1MEG resistor to GND and wait for the cap to discharge before proceeding. |
| Fungus:
Cool! I bet even Dave didn't know this. :popcorn: --- Quote from: joeqsmith on September 17, 2021, 01:34:31 am ---Using a sting of LEDs to look at the waveform --- End quote --- Is that the correct collective noun for LEDs? |
| joeqsmith:
--- Quote from: bdunham7 on September 17, 2021, 03:26:21 am --- --- Quote from: joeqsmith on September 17, 2021, 01:34:31 am ---Bleed and high impedance don't mix. Maybe change your habits to discharge the meter. All these years and still new tricks to learn. :-DD Using a sting of LEDs to look at the waveform as well as conducting the finger test.. --- End quote --- Interestingly I was easily able to replicate this and found that even if you switch to DC, the capacitor retains its charge. If, for example, you go to measure 500V but the meter is in mVAC, if you switch to DC and measure your 500VDC, if you then remove the test leads and switch back to mVAC, you get the jolt. --- End quote --- I'm not surprised that the cap would be isolated when the function is changed. It's too bad the manufactures no longer supply schematics. |
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