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Testing a capacitor

Discussion in 'General Electronics Discussion' started by bigone5500, Aug 27, 2014.

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  1. bigone5500

    bigone5500

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    Apr 9, 2014
    I have a 2700mfd 400vdc, 450 surge capacitor that I want to test. How can I do this? Would it be safe?
     
  2. Gryd3

    Gryd3

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    Jun 25, 2014
    What is it from? Are there any other markers on the can?

    This is a dangerous voltage, and the capacitor can provide a ridiculous amount of current.
     
  3. bigone5500

    bigone5500

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    Apr 9, 2014
    I'm sure they are good. They came from an Allen Bradley variable frequency drive. They are Philips 120640-7. I just wanted to know how I could test them. Being high voltage caps I was pretty sure it wasn't safe.

    tmp_5401-20140827_182920739583021.jpg
     
  4. Gryd3

    Gryd3

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    Jun 25, 2014
    They can be tested if safety is put first. A large charge can be dangerous.
    Considering they look like they have already been removed let's not assume they are safe just yet.
    You can grab a multi-meter, and set it to read voltage to test the leads to see if there is a charge. If possible, work with one hand and clip the test lead onto one of the posts. You never want to accidentally send voltage across your chest if you have faulty test probes and are holding one in each hand.
    Capacitors will self-discharge if left long enough, but this is not to be counted on when handling them. If you wish to speed this process up, a high wattage resistor can be used.
    If there is no charge, you can switch the meter to resistance. At first it will read like a short circuit of 0Ω, as the meter remains connected the resistance will slowly rise. If the resistance stays at 0, or it shows overload, open, or some other ridiculously large value, it may be bad.

    This will only test if the capacitor has an internal open or short circuit. This will not tell you if the capacity is still as advertised.

    When working with a cap like this, one of the dangerous parts is the voltage it is rated for because that will usually roughly translate into the expected full charge of the capacitor. So when taking it out, or accidentally touching it, it could deliver 300+ volts. Always use the not-so-common sense and think safety first.
     
  5. bigone5500

    bigone5500

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    Apr 9, 2014
    The caps don't have any charge on them. They were connected to 50 watt 15k resistors while in the equipment.

    I work around high voltages once in a while. I mainly deal with 480vac 3phase power anywhere from 10A to 30A mostly. I have watched training videos on arc flash that make the movie Saw seem like a carnival. Safety is always first.

    I was really hoping that there was a quick and easy method to test this thing.

    A side question: Could someone use these in their solar setup?
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Whatever you do, realise that when fully charged this capacitor can kill you.

    Ensure you know how to safely discharge it ("screwdriver" is not a valid option).

    After that, slowly charge it up to say 100V and ensure it holds the charge.

    Then continue the charge up to say 300V and ensure it holds the charge.

    Then discharge it and slowly charge it up to 400V and ensure it holds the charge.

    Then discharge it again.

    All connections to the cap should be made while the cap is discharged (this includes the connections to your multimeter).

    The flash circuit board from a disposable camera will slowly charge the cap to around 300V to 330V (wire this cap in instead of the smaller 220uF one it currently has. Also remove the flash tube. You'll blow it up if you try discharging this cap through that!
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Those 15k 50W resistors are great as a discharge tool. Connect these via a switch (a regular mains switch is OK since you won't be trying to turn it off with a voltage present!)

    Wire it across the cap and flick the switch on to start the discharge. Monitor this on the multimeter (which should be connected and powered up before you start to charge the cap. As much as possible, do things hands-free.
     
  8. bigone5500

    bigone5500

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    Apr 9, 2014
    I don't have access to high voltage DC. Even if I did, I would not bother with it. I don't want to chance anything. However, I did take my 1.5A battery maintainer and momentarily touch the leads to the cap. I connected my meter and it read 13.1v. It was losing .01 volts per second.
     
  9. bigone5500

    bigone5500

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    Apr 9, 2014
    I like alligator clips...they are my friend.
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    That is within the bounds of reasonable leakage.

    To really test it you need to charge it up to a higher voltage, but it's obviously working.

    For an indication of what even 12V across that capacitor is like, short it with a screwdriver. You may be surprised. This isn't a good thing for the cap, but at such a low voltage the stress isn't as great.

    The power rises with the square of the voltage, so fully charge the sparking would be over 1000 times more energetic!
     
  11. bigone5500

    bigone5500

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    Apr 9, 2014
    That is enough to make a person curse in 6 languages.
     
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