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Capacitor wiring questions

Discussion in 'General Electronics' started by Cotharyus, Feb 3, 2004.

  1. Cotharyus

    Cotharyus Guest

    I need some help with wiring on capacitors. I need a cap or group of
    caps capable of delivering 50,000uf at 100V DC. I'm trying to sort
    out what type and how many caps to use, and how to wire them. If
    memory serves, voltage is doubled and capacitance halfed if wired in
    series, and in parrallel I forget what happens - it seems like you
    have to be careful charging them though, can anyone refresh my memory
    and/or provide some pointers? Thanks.
     
  2. Quark Ng

    Quark Ng Guest


    I would buy 125V caps and wire them up in parallel. The math is much
    easier this way. When you wire them up in parallel, the capacitance
    values just add.

    Also, I am afraid that if you try to charge caps connected in series,
    not all the caps will be charged to an equal voltage - causing over
    voltage of some caps - causing explosions.

    Choose 125V caps so that you have some margin of error to give you
    breathing room.

    -Quark
     
  3. JeffM

    JeffM Guest

    I need a...group of caps capable of delivering 50,000uf at 100V DC
    Well, "deliver" is not the word, but Quark got it right anyway.

    I'd be interested in what exactly you're doing
    especially how you're going to charge it to 100V.
    I'm betting you're going to get that wrong.
     
  4. Cotharyus

    Cotharyus Guest

    I'm building a "battry zapper" for NiMH rechargable cells. Units that
    do this are commercially available, however, all of them appear to
    use military surplus parts (of which there are none right now) so you
    can't buy one right now - I see no reason why someone with enough
    sense to check and make sure they're doing it right couldn't build
    one. I've also been given the name of a local EE, who appears to be
    very difficult to get hold of...thank you for your help so far
    everyone.

    Quark - 125V caps (of lower capacitance?) wired in parallel? so if I
    had 5 10,000uF caps rated at 125V I could wire them in parallel to
    produce 50,000uF at 125V, and then only charge the caps to 100V? That
    makes sense. I knew somewhere in my past I'd heard something funny
    about charging caps wired in series or parallel, but I couldn't
    remember exactly what it was. Thanks again.
     
  5. Quark Ng

    Quark Ng Guest

    Yes, wire five 10mF 125VDC caps in parallel is what you want. Make
    sure that you don't charge them up to over 110VDC, or you'll risk
    heating up the cap and making it explode.

    And just to be a geek, let me explain why you don't want caps in
    series.

    Say you have a circuit

    A B C
    100V---(SWITCH)-----|C1|-----|C2|----|C3|----|> GND

    Start with the switch open. Assume that all caps are discharged and
    points A, B, and C are at the same potential as GND.

    When you close the switch, point A will almost instantaneously reach
    100V. The only thing charging capacitor C2 is the leakage current of
    C1. The leakage current is probably VERY small. So the potential at
    point B would be near zero for a while. And the potential at point C
    is near zero too.

    This means the voltage across C1 is about 100V! So if C1 is not rated
    for 100V (let's say it's rated for 50V) then it will be over-charged.
    This will cause the dielectric to breakdown and allow large leakage
    currents. So you're essentially shorting 100V to point B (which is
    about GND for the moment) through C1.

    Bye bye C1! :)

    -Quark
     
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