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checking Chinese capacitors

Discussion in 'Troubleshooting and Repair' started by (*steve*), Mar 7, 2020.

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  1. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    I bought some cheap Chinese 630V capacitors and I wondered... How good are they?

    So I built something to check leakage current.

    The simple idea is to place a current meter in series with the capacitor and connect it to a high voltage power supply. But as you can imagine, that's probably not a great idea if the capacitor suddenly goes short circuit.

    So the simple solution is to use a current limited supply and/or protect the meter with some diodes


    But let's say you limit your power supply to 20mA and you're measuring leakage in fractions of a uA. This will protect the meter from high voltages, but not necessarily from high currents.


    So that's a solution, ats a bit of series resistance to limit the current. Using 10, 100, and 1k ohms will limit the current to 70, 7, or 0.7mA depending on the burden voltage of your meter.

    A quick check in my meter indicated the maximum readings would be closer to 50, 5, and 0.5 mA.

    The circuit also contains a series resistor from the power supply, but I'm using a current limited supply, so I omitted it.

    Here it is partially constructed


    I had to replace 2 of the terminal posts, so this is actually during this step. The other side shows the keying of the holes.


    The top plate is some white acrylic painted black and etched/cut. A similar MDF version was made to guide the pilot holes in the case

    Fully built it looks like this


    And put together

    IMG_20200307_163514_compress53.jpg IMG_20200307_163443_compress68.jpg

    In operation, the capacitor is connected across the top pair of terminals, the power supply to the next ones, and the meter between the lower left black post and the H, M, or L red post, depending on the amount of leakage you expect.

    I tested a small tantalum capacitor to destruction using a low voltage power supply, then used my 1kV variable supply to test some known good, and Chinesium capacitors.

    A known good 0.1uF 1200V cap showed a leakage near zero at 1000V. My meter in the uA range shows 0.1uA or 0.2uA when the leads are open, and maybe it spent more time on 0.2uA at 1kV.

    The Chinese caps measured pretty much the same up to about 700V -- I didn't go higher.

    A 10uF 450V cap showed about 50uA leakage at 450V. I've yet to test the Chinese electrolytics.

    This turns out to be a very simple and useful device.
  2. duke37


    Jan 9, 2011
    I use an analog meter in series with the capacitor and high voltage power supply. If the meter reads zero, then there is no leakage. The meter on an appropriate voltage range will limit the current.
    I have also used this method to measure the capability of hgh voltage diodes.
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    I have done similar things when using a multimeter, however, if you're using a power supply with any significant output capacitance and the device under test suddenly fails, you can expose your meter to a high current/voltage. Whilst your meter may survive this, is prefer not to risk the input protection, because that's likely to be more troublesome to repair. And if we're looking at measuring sub-picoamp currents, you'll be connecting a very sensitive input to this device, one simple DIY tool essentially connects the (tad expensive) up amp inputs almost directly to the current source.

    Obviously this device isn't set up to enable the measurement of sub-pA currents, although it doesn't show leakage anywhere near the 100pA resolution of the meter I chose at 1000V, I would be very surprised if it wasn't leaking at the resolution of a far more sensitive meter.

    Asking those lines, for very sensitive readings, a high resistance in series with the capacitor reduces currents from the voltage noise of the power supply. A series diode is a really good choice for this. The series diode is also good for protecting your power supply of it can't sink current (mine can), but adds an extra complexity of needing to add a method of discharging the capacitor.

    But it's a good testbed to do more development.
  4. 73's de Edd

    73's de Edd

    Aug 21, 2015
    OK last two photos, isn't the black-white-black layered, pebble finish panel materiel, the same that can be engraved for making badges. and isn't the white lettering medium being decal sheeets that you can laser/inkjet print onto in white, and then slip off onto your panel.
    AND then, if such was the case, did you use any final spray on clear overcoat?
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    The front panel is white acrylic painted black. The lettering is an engraved on the laser to remove the paint (and some of the acrylic).

    I originally made a version of this to mark the holes, and I was going to use another MDF layer as a quick and nasty front panel.

    However, once I made one or of MDF, I thought I might as well make a slightly nicer version. I didn't spray the acrylic very well, and I didn't bother to wait until it was properly set, hence some of the marks that are visible.

    I didn't bother to put on a protective coat because until the paint is completely dry, a clear layer can start dissolving the layer below it. And I was in too much of a hurry to use it.
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