Cracking Capacitor skin

Discussion in 'General Electronics Chat' started by ioro, Oct 24, 2012.

  1. ioro

    Thread Starter New Member

    Oct 24, 2012
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    Hello,

    I'm using a DC-DC (boost) converter to bump 12V up to 24V (approx.85W) - it's well within it's operating spec (max. 150W) and I've just noticed after about two 8 hour sessions of continuous use that the plastic skin on one of the large capacitors has split.

    The X at the top of the cap shows no sign of bulge.

    Is a split plastic skin nothing to be concerned about and part of the everyday behaviour of any cap that sees temp fluctuations?

    It's fairly critical that this unit performs reliably over at least 7yrs and so if expansion of the cap suggests that there has been damage, however minor, then obviously this module is not for me! Perhaps even, the heat has caused the plastic skin to shrink and I need not worry about it at all!? Any thoughts would be hugely appreciated.

    The module sits at about 50 degrees C (measurement taken from the heat sinks). It's max temp rise is 45 degrees C and it's start temp is approx 22 degrees C.

    Thanks,

    Fred
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,766
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    The plastic most likely cracked due to heat issues.

    That is a good sign that the capacitor WILL NOT be reliable for 7 years.

    Any and all electrolytic capacitors in your circuit have an expected lifetime, measured in hours. Some special large caps have lifetimes of only 1000 to 5000 hours, while most common (1uf) types have lifetimes of 10,000 to 50,000 hours...

    Be VERY sure the caps you incorporate into your circuit are new ones and have not been sitting on a shelf, drying out and losing valuable hours of lifetime due to evaporation of the electrolyte. Be as sure as you can be, nobody can really tell except the vendor who sells them to you, and if they got them from a resaler then nobody will know for sure.
     
  3. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Maybe some photos of the setup would be helpful, and you can try using a caliper and compare the diamater of the suspect cap with a new one.

    How close are the caps to other heat generating parts?
     
  4. ioro

    Thread Starter New Member

    Oct 24, 2012
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    Thanks for such a quick reply.

    I will do some research on the cap to find the spec - lifetime, etc.

    This is the unit on ebay...!

    Out of interest, would you every rely on a module like this - it's fairly crude, but I don't have the experience to know if this is the kind of module that could just keep going and going?

    http://www.ebay.co.uk/itm/150W-DC-DC-Boost-Converter-10-32V-to-12-35V-Step-Up-Voltage-Charger-Regulator-/110773897686?pt=LH_DefaultDomain_0&hash=item19caa389d6

    Here's the photo of the module in use with the cracked skin. The caps aren't making contact with the heat sink but are very close, around .5mm

    [​IMG]

    Thanks again.

    Fred
     
  5. ioro

    Thread Starter New Member

    Oct 24, 2012
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  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Its from China.. enough said.
     
  7. timescope

    Member

    Dec 14, 2011
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    You can also check that the ripple current rating of the capacitors is adequate and the esr (equivalent series resistance) is low enough to prevent heating of the capacitors.

    Timescope
     
  8. ioro

    Thread Starter New Member

    Oct 24, 2012
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    The China part was definitely of concern!

    Will look into capacitor ratings. The 1000 hrs probably means this is a write off.

    Another option is a Traco power module which does much the same thing... but at a staggering £250 a piece.
     
  9. radiohead

    Active Member

    May 28, 2009
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    Choose a cap with a working voltage well above what will be applied to it.
     
  10. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    As radiohead said, take a capacitor with a higher voltage that applied.
    I always keep a factor of 1.5 * the working voltage and take the next higher voltage available.
    Also try to get the 105°C version of the capacitor.

    Bertus
     
  11. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    First I would ditch that board and make a new one, placing a cap right next to a heatsink is very unlikely to preform reliably for 7 years.
     
  12. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    China.. enough said again. :)
     
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