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Capacitor drift

Discussion in 'Electronic Basics' started by steamer, Apr 4, 2010.

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  1. steamer

    steamer Guest

    --Silly question time: do capacitors tend to drift in the same
    direction? And if so, which way? Just got stereo out of the shop and the
    repair guy said the problem was with the caps..
  2. Tom Biasi

    Tom Biasi Guest

    Depends on what is causing the drift.
    Temperature is one cause. Some drift positive, some drift negative.
    Why do you interpret "problem with the caps" as drift?
  3. Phil Allison

    Phil Allison Guest

    "steaming great idiot "
    ** Depends which way the wind blows - fuckwit.

    ** No fooling ??

    .... Phil
  4. Capacitors can fail. They lose their capacitance over time and electrolytic
    can drop significantly. This is probably what you are asking for as it makes
    the most sense. Capacitance can obvious locally change due to temperature
    fulgurations and other things but generally it will not cause a circuit to
    fail or generally even operate erratically(unless it is very poorly
    designed). But if it is old or uses cheap capacitors, specifically if
    electrolytics, then the device can fail in any number of ways.

    For audio stuff caps tend to be used for two primary purposes. One is power
    regulation and the other is audio filtering. The filtering caps generally
    rarely go bad and if they do it is results in a gradual decay of sound
    quality except when a catastrophic event occurs. The power regulation caps
    are generally what fail the most and can be due to over voltages and such.
    They are the large capacitors in the device. Generally they cost up to a few
    bucks each so if he charged you an arm and a leg then he's probably ripping
    you off. For example, it could simply have been a fuse and he said it was
    the caps.

    If they were replaced it should be someone easy to tell and generally there
    are just two to four of these(they will be the largest ones there). It is
    possible the power stage uses SMPS which reduces cap size and may make it
    harder to find the.

    BTW, a capacitor is basically a battery that works in a similar way but has
    opposite properties. A capacitor can be charged up or discharged very
    quickly(battery = slowly) and cannot old a charge as long(a battery can last
    quite some time). In fact, in some cases capacitors can be used to replace
    batteries(so called supercaps that can hold a ton of charge). Actually a
    battery is a type of capacitor that was designed to last longer but at the
    price of not being able to dump it's electrons as quick.

    Example, if you short out a capacitor it will "release" a large number of
    electrons in almost an instant. Say maybe something like 1 million amps in
    1ps(10^-9 seconds). 5ps later it maybe down to 10,000A. 1000ps later it's
    basically at 0.

    A batter OTH might be able to put out a few amps for several minutes which
    will slowly drop off.

    Of course I'm just trying to give you some idea as the specific numbers
    depend on the actual devices used.
  5. Ken

    Ken Guest

  6. Bob Eld

    Bob Eld Guest

    In general, no they do not change capacitance over time within their
    tolerance ratings. They all have a tolerance rating with some types are as
    bad as minus 82% meaning they can lose almost half of their capacitance with
    time and temperature but still be in range. Others are more tight in
    tolerance and can be expected to keep their values indefinitely.

    Of course any cap or electronic part can go bad and lose capacitance for any
    number of reasons.

    When a repair guys says the problem was with the caps, what does he mean?
    What caps? A certain cap? Or a whole bunch of them? We really need more
    information for a definitive answer. I would be wary if more than one cap
    was found bad. It could happen but you may have been scammed. Some repair
    guys just "shotgun" a problem replacing everything they can think of, good
    or bad, in hopes of hitting the problem..
  7. Phil Allison

    Phil Allison Guest

    "Bob Eld"

    ** Utter BOLLOCKS !!

    Even the poorest quality electros will not lose any capacitance until they
    reach the end of their useful life.

    The percentage tolerance band quoted is FROM NEW !!

    But ALL caps drift in value with temperature, some go up and some go down.

    Refer to maker's specs for the details.

    ..... Phil
  8. No, tolerance is a different thing. It's about how close to the stated
    value it is when it comes from the factory. For some applications, the
    value doens't matter much, so you can live with a wider tolerance, which
    at one point meant a cheaper capacitor (I'm not sure how that holds
    today), while other capacitors need to be very close to the needed value
    and thus you get a higher tolerance capacitor.

    Electrolytics generally have a wide tolerance, which seems to be a
    reflection of the manufacturing process, but then maybe when they are
    used most applications (filter and bypass capacitors, audio coupling
    capacitors) aren't fussy so nobody spends the money for higher tolerance
    electrolytics on a regular basis.

    Some types of capacitors are more likely than others to change value over
    enough time.

    There is also how a a capacitor reacts to temperature, again for a lot of
    purposes that doesn't matter since the value isn't critical, while for
    some very specific applications it matters a lot (in which case one picks
    capacitors that are will vary less with temperature, and even get
    capacitors that are specifically designed for temperature compensation.

  9. steamer

    steamer Guest

    --Thanks for the useful response! I would guess that the power
    regulation caps went south as there was a continued tripping of solenoids,
    disabling the speaker outputs. And yes, it's an older unit: maybe 35 yrs or
    so now. First failure of this sort so a decent MTBF IMO. The guy didn't
    charge as much as I had feared so I'm happy with the fix. Also happy to have
    learned something.
    Ack. I guess the real trick is knowing how to detect the fault in so
    complex a contraption. I took a peek under the hood B4 I took it to the shop
    and I was totally flummoxed. Not just circuit boards but lotsa wire wrapped
    stuff. Decided to let an expert work on it! :)
    --Ack; I built a small mass driver once upon a time; used about .5
    farad and burned out a few switches in the process, heh.
    Thanks again
  10. steamer

    steamer Guest

    --Thanks; saved the link. Most helpful!
  11. steamer

    steamer Guest

    --It's been a long time since I killfiled anyone but this bozo
    prompted action. Any more kooks I should look out for over here?
  12. steamer

    steamer Guest

    --Aha! Thanks for that; for some reason I had assumed they all fail
    in the same direction, so to speak.
    --Well I know it's one of several ways they fail; Just curious about
    what/where/when/why, so to speak, as I've heard of the problem from others
    messing with Stamp circuits, etc. I guess there are several failure modes;
    it's just a term I've encountered so curiosity got me thinking.
    Thanks for helpful response!
  13. Tom Biasi

    Tom Biasi Guest

    I wouldn't killfile Phil. He quite often gives intelligent accurate advice.
    From time to time he gets like you saw. (Some of us think it's a medication
    I always read his responses, if they are the wacky kind I just ignore it.

  14. Fred Abse

    Fred Abse Guest

    Phil is abrasive and abusive, but he generally knows what he's talking
    about, and has made many useful contributions to the group.

    Why he behaves as he does, I neither know nor care. I have quite a few
    bozos killfiled, but he is not one. Engineering skill overrides
    personality, as far as I am concerned.

    Leave him out of your killfile, and maybe you'll learn something you
    didn't already know,
  15. Phil Allison

    Phil Allison Guest

    "David Eather"

    ** Did you actually read the fuckwit OPs absurd question ??

    Calling him a "steaming great idiot" was being kind.

    ...... Phil
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