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Capacitor rating question

Discussion in 'Misc Electronics' started by Ed Chilada, May 28, 2008.

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  1. Ed Chilada

    Ed Chilada Guest

    I have a failed PSU where a rather smelly capacitor appears to be the
    problem. It's rated 1200uf/16v but my local Maplin doesn't stock it.
    However they do stock 1000uf/16v and 2200uf/16v. Since I don't really
    know what the figures mean, does anyone else know whether either of
    these are likely to work? I dunno whether the 2200 means it's overkill
    for the application but it would work, so long as I can get it to fit
    or something.

  2. Pete Wilcox

    Pete Wilcox Guest

    The 2200uF would be the better choice, but you would probably get away
    with using either value. Electrolytics are usually something like +50/-20
    per cent tolerance rated, so there can be a large difference between
    what's in the can and what's written on it.

  3. Gerard Bok

    Gerard Bok Guest

    That all depends on the type of PSU !
    (Given the off-standard value of 1200 uF, you are likely talking
    about a SMPS.)

    In a SMPS (Switched mode PSU) like in a PC or in modern low
    weight wall chargers, you cannot change 1200 uF into 2200 uF nor
    into anything else. Even more: you cannot replace a 1200 uF
    capacitor by just any 1200 uF capacitor as it then probably needs
    to be a special low ESR type :)

    On the other hand: if it is a classic PSU (one with a rather
    heavy transformer), it is OK to replace your 1200 uF capacitor by
    2200 uF. If the new one fits the available space, that is.
    Or even replace it by 1000 uF.
  4. Jamie

    Jamie Guest

    the 2200 will work fine how ever, ESR could be factor.
    try to get a low ESR type. It will specify low ESR in the
    parts search."
  5. Pete Wilcox

    Pete Wilcox Guest

    Woops. I stand corrected. Have to admit, when I sent my original reply I
    was just thinking in terms of a smoothing capacitor in a bog-standard
    linear PSU. If it's a switcher, that's a whole different ball-game.

  6. Eeyore

    Eeyore Guest

    Will either of those values fit OK into the vailable space ?

    If the 1200uF failed it may mean it was under-rated for the job and a
    2200uF might well be the better choice, but as ever it's never 'quite' as
    simple as that. Simply a better brand of capacitor may provide as much
    improvement as you require. You can't go far wrong with Panasonic caps
    btw. If this was some cheap Chinese piece of kit it's unlikely the
    original cap would remotely match a Panasonic part for 'ripple current'
    which is probably what's killed it. Is the cap case 'bulged' too ?

    One other thing, is this a swichmode PSU or 50/60Hz with a traditional AC
    transformer ? Switchmode supplies require 'special' low-ESR caps designed
    for the job. Don't fit a 'standard' cap into a switchmode supply.

  7. Ed Chilada

    Ed Chilada Guest

    Cheers for the help everyone. Yeah it was from a slimline PC's PSU
    which is switched mode. I'll look into getting the proper replacement
    part although I'm actually still wrangling with the warranty and/or
    house insurance, but if it turned out to be a cheap fix I thought I
    could avoid the hassle. Thanks again.
  8. Gerard Bok

    Gerard Bok Guest

    If an electrolytic capacitor in a PSU leaks, it's probably a
    production problem (on the part of the manufacturer that produced
    the capacitor).
    If a capacitor explodes, it's most probably because of some other
    fault. (E.g. short circuit in a rectifier.)

    If a capacitor exploded, you are facing several problems.
    - liquid from inside the capacitor is splattered all over the
    surrounding PCB. Very hard to clean. And very conductive stuff.
    - the capacitor exploded for a reason (see above)
    - a good replacement is often very hard to get,

    So my advice would be: replace the PSU!

    (Which, by the way, is allways the best advice for a failing
    SMPS. Many enthousiasts don't realise that a quick and cheap
    'fix' may cause them dearly. As insurers are very keen to trace
    home fires back to 'inadequate modification' of electronic
  9. Eeyore

    Eeyore Guest

    I think most modern electrolytes are water soluble. Use warm water but onbiously
    avoid soaking any adjacent components.

    How old is your PSU btw ? You may be a victim of the 'bad caps' problem of some
    years back.

    Certain 'off-brand' caps are especially noted for the problem.

  10. Gerard Bok

    Gerard Bok Guest

    No. The plates have a very rough surface, increasing their
    surface area. The electrolyte follows those curves on the surface
    and acts as the second plate.
    A thin layer of oxyde, covering the plate forms the insulator.
    (That's why they sometimes only withstand some 10 volts :)

    What you are looking at is (partially) 'boiled' electroyte, which
    may not have the same characteristics as the original one.
    And the aggressive nature of the stuff may have affected the
    surrounding materials.
  11. Eeyore

    Eeyore Guest

    Depends where it went. If 'under' some components, it's best to remove those and
    clean thoroughly.

    It's slightly conductive. Electrolytics caps are very imperfect devices. All
    electrolytes have some conductivity.

  12. Nemo

    Nemo Guest

    I like the description of the capacitor:
    I shall have to define smell in my next parts list. "C3: 10uF, 16V, a
    redolent fruity little device with a hint of chives."

    I have designed several SMPS's and in every case it was OK to increase
    the capacitance, in fact you generally made the caps as big as you could
    afford, to reduce ripple levels. So I would think 2200uF is better,
    though as someone said, you want a "low ESR" type if possible (to stop
    it getting too warm from ripple currents heating it). I don't understand
    why Mr Bok advised against changing the capacitance, but perhaps it is
    different for multi output PSU's (I did simple non isolated step down or
    step up types). I'd be interested to learn more on this subject.

    Nor do I understand why it is bad to wash the board thoroughly with
    water. Many PCB's are washed with water to get rid of acidic flux
    remnants after assembly, and provided you dry it thoroughly afterwards
    (ie on a radiator or something, fairly rapidly, so nothing rusts) what
    is going to get damaged by water? Obviously you don't want water on a
    PCB when it's got mains voltages on it, but there's no reason not to use
    water for cleaning it.
  13. Eeyore

    Eeyore Guest

    It will change the loop response and subsequently the stability of the
    feedback loop.

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