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What happens when electrolytic capacitors 'dry out'?

Discussion in 'Electronic Basics' started by /dev/phaeton, Nov 9, 2007.

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  1. /dev/phaeton

    /dev/phaeton Guest

    I hear about this a lot when it comes to old amplifier equipment. "The
    caps were dried out in my 1971 Doumaflauuchi Quadraphonic HiFi Set so I
    had to replace them all".

    Does this mean that the dielectric has met the destructive end of a slow
    chemical reaction? Does it mean the capacitor package sprung a leak and
    released whatever 'moisture' it had in it, thus changing its properties?
    How long does it take for electrolytics to 'dry out'?

    Are there any preventative measures, such as powering up the device every
    few months to re-energize the plates?

    Once the caps are all dried up, what are the options?

    Replace them all with new?
    Remove them, bake them in an oven, reinstall them?
    Power up the circuit and leave it on in an 'idle state' for an extended
    period of time?


    Thanks, and sorry for the wordiness!

    -ph
     
  2. Tim Wescott

    Tim Wescott Guest

    The mechanism by which an electrolytic capacitor "dries out" is that the
    water in the electrolyte evaporates. This is just like the dregs in a
    bottle of beer drying out, only with quote marks (and without the mold).

    This "drying out" happens because all common* electrolytic capacitors
    aren't hermetically sealed, and the electrolyte _does_ have to stay wet.

    Electrolytic capacitors that don't dry out can lose their ability to
    withstand voltage, because the oxide layer on the aluminum gradually goes
    away**. When powering up equipment with high voltage electrolytics that
    have been unused for a long time the capacitors must be "reformed" -- do a
    web search for the procedure.

    * I know wet-slug Tantalums, as produced for aerospace applications, are
    sealed -- I know of no aluminum electrolytics that are so constructed.

    --
    Tim Wescott
    Control systems and communications consulting
    http://www.wescottdesign.com

    Need to learn how to apply control theory in your embedded system?
    "Applied Control Theory for Embedded Systems" by Tim Wescott
    Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
     
  3. Guest

    As the water in the electrolyte dries out the internal resistance (ESR
    - equivalent series resistance) of the capacitor increases and renders
    it useless.

    The following is a good explanation of what's happening:
    http://www.sencore.com/custsup/pdf/TT104.pdf
     
  4. Tom Biasi

    Tom Biasi Guest

    Replace all the electrolytic caps in old equipment before you even power up
    the unit.
    A bad cap can do more damage like fry coils etc.

    Tom
     
  5. phaeton

    phaeton Guest

    Thanks for all the help and PDFs, everyone.


    Quick question- Does use or lack of use have any effect on how soon
    caps dry out? I.e, should I power up all my amplification devices at
    least once a month or so as a preventative measure?

    thx

    -phaeton
     
  6. Phil Allison

    Phil Allison Guest

    "phaeton"

    ** Time and ambient temperature are the causes.

    But most electros last a couple of decades, just fine.



    ** Lack of use and storage in damp places *damages electronics*.
    Condensation forms on metal parts and initiates surface corrosion and rust
    spots. All connectors, pots ad switches are damaged by this.

    Electro caps can "depolarise" after a long period in storage, then overheat
    and fail when the item next turned on.

    Occasional use is best, plus storage in a warm, dry environment.




    ....... Phil
     

  7. Heat causes the seals do dry out and fail.

    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  8. HapticZ

    HapticZ Guest

    the elctrolyte inside between the foils that comprise the two "plates"
    actually has a liquid that dries out over time. they are usually sealed well
    enough to last 20-30 yrs, but beyond that it is very iffy and prone to
    catastrophic failure (short) or in some cases they act like no capacitor
    exists in their place. (open)

    some are actually "vented" to prevent bursting the can, (they make quite a
    mess of things when they do explode too!) if overvoltaged or heat makes
    them fail.

    my uncles old 1920's RCA radio (36 inch x 20 inch x 20 inch behemoth)
    actually had brilliant chrome plated steel covers over each can capacitor
    and tube.

    they just have a "lifetime" and will eventually fail. this is really old
    stuff u must remebr, way before xixtors and micropowered amps.
     
  9. Jamie

    Jamie Guest

    Yup, built in obsolescence. Just install a few near a
    heated area so that you can buy a new MB later on because
    every knows with in a short time, MS will have your MB
    hogged down any ways.

    What marketing tactics.
     

  10. Where would YOU put them? On the opposite corner of the MB? They
    are close to the CPU's voltage regulator and CPU, because the HAVE to
    be. Trace inductance and resistance would make the CPU unstable if it
    wasn't right next to the CPU.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  11. Marra

    Marra Guest

    The biggest problem I have found with electrolytics is they go short
    circuit blowing the fuse.
     
  12. /dev/phaeton

    /dev/phaeton Guest

    No Microsoft here.

    -phaeton
     
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