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Capacitors What to pick?

Discussion in 'General Electronics Discussion' started by MadMechanic, Oct 11, 2012.

  1. MadMechanic

    MadMechanic

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    Aug 28, 2012
    Hello All,

    I am selecting a capacitor for smoothing and buffering rectified DC voltage fluctuation, and looking at different types of capacitors I have questions. Digikey.com has many to choose from and I was mainly looking at aluminum electrolytic capacitors. I noticed that the electrolytic ones have a lifetime rating where other types do not list a lifetime. Most electrolytic capacitors show around 2000 hours @ given temp. although they do go higher.

    The question being: is there any advantage to using another type of capacitor than electrolytic for what I need it for? Do these other materials have a longer life? some of them do not list a (lifetime) rating.

    I know what values I need, but Digikey has ceramic, mica, tantalum, film, silicon, etc..
    Too many options :eek::confused:
     
  2. BobK

    BobK

    7,599
    1,641
    Jan 5, 2010
    Most of those types do not come in 1000s of uF, which is what you are going to need. Typically, electrolytics are used.

    Bob
     
  3. MadMechanic

    MadMechanic

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    Aug 28, 2012
    That makes sense. I started looking at some of the others and some are surface mount etc... They even have an Electric Double Layer, Supercap that is 5000F! I'm sure it costs thousands, but only rated at 2.5V. What would that be used for?
     
    Last edited: Oct 11, 2012
  4. davenn

    davenn Moderator

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    Sep 5, 2009
    amongst other things, supercaps are often used as backup for memory etc do the same thing as a rechargable or a lithium button battery but last much longer

    just keep in mind for your electrolytic cap for smoothing...
    to have at least 1000uF per amp of current the PSU will be able to supply
    so for a say 5 Amp supply a 6800uF or 10,000uF cap wouldnt be out of place

    and a voltage rating of preferably twice that of the supply rail ....12VDC use a 25VDc rated cap


    Dave
     
  5. MadMechanic

    MadMechanic

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    Aug 28, 2012
    Interesting, I had never seen that much capacitance. I am looking at producing 12V 1.5A max I think, so I am going to use something like a 100,000uF 25V cap, I found one in that range with a lifetime of 7000 hours. Not bad! -Thanks again for the info
     
    Last edited by a moderator: Oct 13, 2012
  6. rob_croxford

    rob_croxford

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    Aug 3, 2010
    Just as a side note... the temperature rating states that it will have a life time of X hours at X temperature. For every 10 degree drop in temperature the lifespan doubles.
     
  7. shrtrnd

    shrtrnd

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    453
    Jan 15, 2010
    I don't have specs to back me up, just experience repairing equipment.
    Electrolytic caps are usually used for power supply smoothing because they're relatively
    electrcially forgiving about power fluctuations (within their ratings). The reason they have
    life-ratings is because they have a wet-electrolyte. Over time it dries out, and it dries
    out faster if it's under high heat. If you read the specs, some are more expensive, that
    have longer life-cycle ratings.
    The mica's, tantalum, ceramic, and poly types are pretty-much solid material and have
    much greater life expectancies (they DO have life-cycle ratings). (There IS a wet-tantalum
    type available which is very expensive now, but most tantalums are solid)
    I see a lot of non-electrolytic caps in low-voltage power supply boards, but I don't
    remember ever seeing a high voltage power supply that didn't use electrolytics to
    smooth the output ripple. I'm supposing there is a very valid reason for that.
    I could Google the why's of electrolytics in power supplies, but so can you. I'm just
    giving you a general explaination of what I've seen in use.
     
  8. davenn

    davenn Moderator

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    Sep 5, 2009
    a 100,000uF is so torally an overkill by many many magnitudes
    and the physical size of a 100,000uF is HUGE!!

    use a 4700uF or 6800uF is all you need


    Dave
     
  9. MadMechanic

    MadMechanic

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    Aug 28, 2012
    This is good to know, Thanks!
     
  10. MadMechanic

    MadMechanic

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    Aug 28, 2012
    Thanks, this does make sense
     
  11. MadMechanic

    MadMechanic

    48
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    Aug 28, 2012
    True, but this whole project is overkill and overbuilt :D. I found one about 2 inches long which is not bad, should fit fine. I thought about using a smaller cap and a ni-cad battery, but I may just one huge capacitor instead, unless I can find a battery I like.
     
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