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Idea for safely discharging capacitors.

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  • Idea for safely discharging capacitors.

    You guys probably already know this but......I set my Fluke 116 meter to "Auto-V Lo Z" and touch both ends of the capacitor. This way I can actually watch the voltage value drop to zero.

    As you know low impedance mode uses something like 3k ohms to put a slight load on item measured in order to eliminate ghost (induced) voltages. Regular voltage mode is high impedance of around 10 Megohms which will take forever to drain capacitors.
    Last edited by Axtman; 11-10-2020, 05:38 AM.

  • #2
    Not too sure that is what the AutoV is intended for.
    I thought it was intended for measuring 'ghost' voltages.
    To drain an E Cap hook up a 1k resistor to either end.
    To measure the E Cap you need a cap tester.

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    • #3
      I wonder if the meter is able to stand power surges of 100W or more.
      - Own Opinions Only -

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      • #4
        "Auto-V" function on the Fluke meter will determine if you are measuring DC voltage or AC voltage and display accordingly without turning a dial or pushing a button.

        Fluke meters are extremely robust and really designed for professional electricians working in commercial / industrial applications. Their meters are filled with MOVs, PTCs, HRC fuses, etc. so I have full confidence they can take a good surge.

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        • #5
          What's cheaper? A clip lead with a 5 or 10W 1K ohm resistor (or... choose your own value more or less) wired into it & covered with heat shrink? OR your valuable Fluke meter? I would not hazard roasting the Fluke nor any other meter by using it to discharge capacitors. But, that's me.
          This isn't the future I signed up for.

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          • #6
            But how will you know that you discharged the capacitors completely?

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            • #7
              Originally posted by Axtman View Post
              But how will you know that you discharged the capacitors completely?
              Simply by connecting any high resistance DCV meter across the 1k resistor. This will make sure that the discharge current runs through the resistor and not through the meter.
              - Own Opinions Only -

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              • #8
                The problem here is that Fluke are silent of the matter of limitations. But consider this. LoZ mode is intended to take measurements on high impedance circuits i.e limited currents as that is when ghost voltages can be seen. Second, voltage testing is usually an intermittent operation as you connect the probes for a quick test.

                I would not use the meter like this without express approval from Fluke.
                Experience is something you get, just after you really needed it.

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                • #9
                  Never heard of "ghost voltage" before. And there doesn't seem to be a good german translation. From what I've googled it seems to be a capacitively coupled (very high source impedance) AC voltage.

                  If so, there can't be a DC ghost voltage - except if the term includes voltage produced by resistive leakage. That would probably better called stray or leakage voltage.
                  Last edited by Helmholtz; 11-10-2020, 08:11 PM.
                  - Own Opinions Only -

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                  • #10
                    Originally posted by Helmholtz View Post
                    Never heard of "ghost voltage" before. And there doesn't seem to be a good german translation. From what I've googled it seems to be a capacitively coupled (very high source impedance) AC voltage.

                    If so, there can't be a DC ghost voltage - except if the term includes voltage produced by resistive leakage. That would probably better called stray or leakage voltage.
                    "Spurious" or "leakage" would be close. You'll know one when you see one......


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                    Experience is something you get, just after you really needed it.

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                    • #11
                      If you can measure and/or feel it, it's real - not a ghost.
                      - Own Opinions Only -

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                      • #12
                        I've been using an old set of test probes, cut down to around 8 inches each, soldered onto a Dale 100 ohm 50W Power Resistor with shrink tubing over the solder tabs. Every so many years, I have to repair the solder joint when one breaks, but they do a great job discharging caps. If it's a high value cap, I'll tap it a few times to take it down to 0V. There's always Dielectric Absorption on Electrolytics, so there will be a few volts creep back up, once they've been discharged. I don't work in super high voltage area normally. I do have a higher value discharge probe for those occasions.

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                        Logic is an organized way of going wrong with confidence

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                        • #13
                          I watch a lot of youtube and saw where Dlab would put a meter across a cap then jump the leads at the meter with a plug in resistor at the banana connections at the meter. I thought it was genius. You only need some leads that accept banana style stacking and boom, hands free discharge while watching the voltage fall.

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                          • #14
                            Like others have said, the meter is intended for higher Z circuits and not (necessarily) a higher current load. I would avoid doing that unless I didn't care about the meter.

                            Sort of like nevets thingy I have a 1k resistor between a pair of alligator clips. I like the alligator clips because they're hands free once connected. In less than a minute you can safely test with your meter and there's no risk to it. I'm not a repair tech and I don't have a "shop". Just a bench at home. I only have one nice meter and I don't want to toast it. I hate using the cheapies I have on hand to get by if I'm in between better meters. And, of course, as has surely been observed by many here, the moment you subject a cheapie to any abuse it goes phut. So why do that also. I'm in for using a purpose made tool for the job of draining voltages.
                            "Take two placebos, works twice as well." Enzo

                            "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                            "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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                            • #15
                              I use the LoZ settings on my Fluke 87V and Brymen BM235 for discharging caps regularly, though I also have a discharge resistor box with a 10k 10W and 1kV pulse rating with two pairs of banana sockets so I can monitor voltage simultaneously (the discharge box is mostly a demonstrator to show people a cheap, safe option, but I always use the LoZ setting personally).
                              Meters with a LoZ setting have a PTC thermistor as the load, so they're self-limiting to a degree. The time constant for any filter cap you're likely to see should be short enough to avoid exceeding overload ratings. Of course I wouldn't recommend discharging several amps in a row without allowing time for the PTC to cool, but how often does that possibility arise? And yes, they'll be damaged by sustained high voltage DC, but so long as the amp is unpowered and the meter is appropriately CAT rated, this is quite a safe method, and the worst that can happen is the PTC is roasted and you loose the LoZ function, the normal HiZ settings will not be affected.
                              Last edited by Greg Robinson; 11-11-2020, 07:16 AM.

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