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Maximum discharge current of capacitors.

Discussion in 'Electronic Components' started by Bill J., Jun 12, 2006.

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  1. Bill J.

    Bill J. Guest

    How do I find out the maximum discharge current of a capacitor before I
    order it?

    Is it somehow related to the maximum ripple amps by some formula?
     
  2. Tim Williams

    Tim Williams Guest

    What kind? Pulse-rated caps usually have the ratings in the datasheet (e.g.
    Digikey).
    Perhaps.

    Tim
     
  3. Bill J.

    Bill J. Guest

    Searches for the phrase "pulse rated" at mouser and digikey yielded no
    results relating to capacitors. I am building a power supply which
    delivers high power short duration pulses with a long charge time
    between pulses. Hence, I need to know the maximum amperage each
    capacitor can deliver so I can determine the number of capacitors
    needed in parallel to deliver the required power.
     
  4. Without knowing the voltages, and currents involved, it is hard to give a
    general answer. However from your description, a search for photo flash
    capacitors, may point in the right direction. These are designed to be low
    loss, and survive very high instantaneous discharge currents, with a
    relatively lower charge rate.

    Best Wishes
     
  5. Pooh Bear

    Pooh Bear Guest

    Certain types of caps have pulse ratings. Polypropylene for example usually have
    max dV/dt specified and you can get 'pulse' versions of electrolytics for things
    like flashguns too.

    Graham
     
  6. Bill J.

    Bill J. Guest

    I just want to be able to know how fast all of the ordinary
    electrolytics in the Mouser, etc catalogs can be discharged so I can
    choose among them.
     
  7. Pooh Bear

    Pooh Bear Guest

    It may be inadvisable to use an 'ordinary' electrolytic for pulse use. You're unlikely
    to find this info, it's not how they're intended to be used.

    What's the application anyway ?

    Graham
     
  8. Bill J.

    Bill J. Guest

    How can a designer determine what current a given capacitor in a
    catalog can deliver, then? There must be a way. It is not as though max
    discharge current isn't an important figure, after all.

    IS there a connection with max ripple amps?
     
  9. I think so. Low ESR (Equivalent Series Resistance) caps have lower ripple,
    usually. That lower resistance means they respond faster to changes on the
    input. I guess what matters is how deeply you want to discharge them. If
    you can afford to risk needing to replace them just look for low ESR
    standard electrolytics, and base your calculations on the given resistance
    value for them. If you also pick high temperature types, you'll probably
    improve the chances that they'll last a while. Some people suggest not
    expecting electrolytics to go beyond 2 to 5 years anyway, but I bet they
    will.
     
  10. Pooh Bear

    Pooh Bear Guest

    It isn't for most ppl. In fact I've never seen it specced. There is in fact no probable
    *maximum* . It's just a trade-off with component life.
    Probably inasmuch that esr is involved.

    Graham
     
  11. Pooh Bear

    Pooh Bear Guest

    You mean higher ripple current. Ripple *voltage* in a circuit is determined
    primarily by C alone..
    No it doesn't. It means they heat up less from I^2*ESR.
    Or flashgun tyes.
    Electrolytic life depends entirely on how abused they are ( notably temperature
    and delta T ). You can kill one in under a year or it can last for decades.

    Graham
     
  12. Bill J.

    Bill J. Guest

    The power supply I'll be building will be used to power a coil gun or
    another magnetic pulse generator. So I should just look for capacitors
    with a low enough internal resistance to fully discharge fast enough
    for my application?
     
  13. Pooh Bear

    Pooh Bear Guest

    The esr is unlikely to be a problem discharging the cap ! If it is you have a far larger
    problem. You might care to consider *adding* some R in the circuit to limit peak current though,
    it might make the device more reliable. OTOH a coil will have inductance so that's likely to be
    the determining factor in both peak current and time to discharge.

    Graham
     
  14. Wdino

    Wdino Guest

    You can't.
    It seems that you want the maximum (ie short circuit current).
    This is determine by the voltage that the capacitor is charged to and
    the ESR.
    Max I = Voltage/ ESR

    So order those that have the lowest ESR in their specs for a given
    voltage rating.
     
  15. But not all capacitors are suited to extreme high currents without
    rupture or capacitor degradation. So it is important to also know
    whether more than one or just a few such full pulse can be achieved.

    Peter Dettmann
     
  16. Ben Bradley

    Ben Bradley Guest

    Responding in another old thread:

    I'm pretty sure there IS a maximum pulse current you can do without
    damaging the cap, and it's less than you would be doing in this
    application (less than the max voltage divided by ESR). The ripple
    current rating is for limiting heat (full-time ripple current across
    ESR causes the cap to heat up), and so it stands to reason that a
    pulse rating (which is very intermittent) would be substantially
    higher. But since "ordinary" electrolytics are rarely used in such
    high-pulse-current applications, manufacturers don't take the time to
    test and characterize them for such a rating. Xenon flash is such an
    app which uses special caps designed for it, but they may not be
    available in the capacitance/voltage rating you want for your app.
    From my reading (ISTR at the site below, and maybe the quarter
    shrinker site), even "pulse rated" capacitors are rated for a fixed
    number of pulses and don't work as well after having given many
    high-current discharges.

    Sign up on http://www.4hv.org - they've used "regular" caps for
    these applications. ISTR reading that electrolytics' life is reduced,
    but are still usable for this. That's a great site for this stuff
    (coil guns and similar things), it appears to have a high
    concentration of people with a lot of knowledge in this area.
    Be sure to ask the right questions in the right forum. Even though
    your app for such a cap is a coil gun (which is discussed in The
    "Electromagnetic Projectile Accelerators" forum), you're asking about
    an electronic component (capacitor) so this goes in the "General
    Science and Electronics" forum. I actually didn't make this mistake
    myself, I learned it by lurking and seeing someone else get corrected
    on it. I think the site makes this a little less clear than it should
    be, perhaps I should mention something about it over there.
     
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