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capacitors in series

Discussion in 'Electronic Basics' started by Johan Wagener, May 3, 2004.

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  1. I need 75V 10,000uF capacitors for a four channel leach amplifier I am
    building. I will have two 750va toroids. Voltage rails will be + and
    - 58V dc

    I cant seem to find 75V caps. I did find 10,000uF 50V caps at a very
    cheap price thought. I am thinking - wiring two caps in series for

    Something just doesn't feel right about it. Please comment on this!
  2. Anders

    Anders Guest

    The effect will be 5,000uF !
  3. happyhobit

    happyhobit Guest

    2 in series = twice the voltage and half the capacitance.

    2 in series & 2 in parallel (4) = twice the voltage and the same capacitance.
  4. CFoley1064

    CFoley1064 Guest

    Subject: capacitors in series
    Hi, Johan. First, you'll need four of these caps to get one 10,000uF 100V cap
    (view in fixed font or M$ Notepad):

    4 X 10000uF Caps

    +#| +#|
    | #| #| |
    + o o -
    | +#| +#| |
    #| #|

    Figure series capacitance like you calculate parallel resistance, and figure
    parallel capacitance like you'd calculate series resistance. (Sounds crazy,
    but look it up!)

    Ideally, the four caps will have identical capacitance, and equivalent ESR
    (Equivalent Series Resistance), which is your main consideration in brute force
    filter caps. In fact, they'll be different, and you'll end up with one cap
    taking more DC voltage than the other. But also as a practical matter, you've
    got so much safety margin, you should be O.K.

    Since you have to buy 4 50V caps to equal one 100V cap, the cheapies might not
    look so economical.

    Good luck
  5. This is probably an AC application, yes? Don't use electrolytic caps
    unless they are always properly biased. If they are subjected to any
    significant voltage in the reverse direction (meaning more than a few
    volts), you can blow them up.

    If you put two electrolytic caps in series (plus side to minus side),
    and subject them to an AC voltage, the caps will leak in the reverse
    direction, get hot, and thus blow up. If somehow they don't manage to
    blow, they will still generate lots of distortion.

    If you put them + to +, then the junction will go way positive (since
    electrons can more easily flow from + to -), causing the junction to
    charge up to near your max AC voltage. Thus, at opposite points in the
    waveform, each cap will be subjected to the peak to peak AC voltage.
    This may work for a while until they form hot spots, start to leak,
    and maybe blow up.

    In either case, you don't really get what you want.

    Bob Monsen
  6. Rich Grise

    Rich Grise Guest

    In addition to what the others have said, put a resistor in parallel
    with each cap - say, 100k to 1 meg. This will help balance the voltage.
    Actually, if your supply design has a bleeder resistor across the cap,
    use two at half the value each.

    Good LUck!
  7. The effective series resistance will be doubled and the capacitance
    halved. Better to get the right capacitors.

    10,000 uf @ 80 VDC

    Or parallel 2 of the 5,600uf 80VDC units for about the same money but
    higher ripple current rating and lower ESR:

    Digikey has these.
  8. They will give in series a capacitance of 5000 uF! And more: leakeage
    between capacitors vary. When one leaks 10 time that of the other one,
    the best one will get most of the voltage (90%), so you must use some
    resistors across the caps to cancel that out!

    I have 8 nice elko's over here of Epcos: 33.000 uF, 100V. But I think
    it will be expensive to send them to South-Africa...


    Pieter Hoeben
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