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very new and dont know enough about capacitors

Discussion in 'General Electronics Discussion' started by morngrym, Apr 17, 2007.

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  1. morngrym

    morngrym

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    Apr 17, 2007
    Hello,
    i am trying to fix a motherboard i have and the capacitors all kinda blew up
    (still work but stuff came out of them but its gives memory errors) but i found a few others from older motherboards but not enough. i was wonder if a 6.3v 1500 uf will be ok to use for a 6.3v 1000 uf. because i dont know if it more of a voltage thing or the uf is a factor considering as well

    thanks
    Lance
     
  2. Wabajig

    Wabajig

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    Apr 14, 2012
    Measure the voltage across the cap now, if its a constant 5Volts for example, then yes most likely fine. If the voltage can only be read on AC, then use exact value. Hope this helps.
     
  3. Raven Luni

    Raven Luni

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    Oct 15, 2011
    Interesting. The exact same thing happened to one of my motherboards once. Started with the memory, then I tried changing it and noticed the capacitors looked like they had cheese growing on them (they had all popped). I'm guessing its quite a few years old then?

    Getting to the bottom of the problem is most important I'd say. Something had to have made them pop - maybe a problem with the power supply or failure of other components on the board. A group of components like that dont all fail at once.
     
  4. KJ6EAD

    KJ6EAD

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    Aug 13, 2011
  5. timothy48342

    timothy48342

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    Nov 28, 2011
    A group of components don't spontaneously fail all at the same time without a reason. (It would be statistical crazyness!) So, likely something has made them all fail together.

    But, I remember reading about an issue with electrolytic caps that due to bad quality control and age they slowly fail and start to bulge and leak over time, but still function well enough that it goes unnoticed for a while. (Their ESR goes up, but their capacitance reamains mostly unchaged.) Then when a circuit finally fails it is discovered there are multiple bad caps.

    It should be noted that in the case of what I'm describing, they all fail for the same reason, quality+age, but will be in different stages of failure, so would all be in slightly different in appearence. Some exploded, some bulging, some apparently fine. Does that match up with what you are seeing on your motherboard? If so, the fix is just to replace all the failed caps plus any that are suspect.

    If that doesn't match up... If it seems more like a sudden spontaneous failure, then back to what Raven Luni was saying, Find the reason.

    --tim

    Ah, and I just noticed that KJ6EAD posted a link to what I think I was talking about. Going to go check that out now.
     
  6. timothy48342

    timothy48342

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    Nov 28, 2011
    Yeah, Capacitor Plague, that was it.

    To morngrvm,
    The capacitance number, like 1500uf, describes a characterister of how it functions regularly. It should be matched. There are some cases where you could use a greater value, but you would have to know how it is being used to make a judgement of whether it is ok to use a larger value. If you can't get an exact match for the uf(microfarad), then error on the high side slightly for large electrolytic caps. 1000uf in place is 1500uf is erroring on the wrong side and by way to much. I think 1000uf is no good. 1500uf to 1600uf should be fine and if it can be determined that this is just smoothing out a ripple current, then 1500uf to 2000uf or possibly even more is ok. (Anyone disagree with that???? I'd like a little confirmation to be sure.)

    As far as the voltage rating, like 6.3v, that just means that it can hadle up to 6.3v. Replace with something that can handle the same or more. More is totally fine. Never less.

    --tim
     
  7. Wabajig

    Wabajig

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    Apr 14, 2012
    If he was changing alot of caps with 50% or greater in value, then there would be concern for current draw on initial turn acting like short possibly blow a fuse or crowbar the switchmode. I've found that most these electrolytic caps on a motherboard leak from either a bad power supply or the prolonged heat from the CPU heatsink boiling out the electrolyte. Just a tidbit.
     
  8. KJ6EAD

    KJ6EAD

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    Aug 13, 2011
    They can cascade fail in short order once the first one does and the others are left to cope with the ripple current. These capacitors are connected in parallel to share the load in a penny pinching circuit design.
     
    Last edited: May 23, 2012
  9. Raven Luni

    Raven Luni

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    Oct 15, 2011
    Capacitor plague - wow thats the first I've heard of that.
     
  10. Tywaser

    Tywaser

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    May 17, 2012
    I had an old computer that worked for 8 years. I was from 2002 and it had cap plague but I still used it for 6 months after I noticed. It was stored for a month and then it was time to recover the book my mom wrote. By the time I was ready to turn it off for the last time it was buzzing like an abused mot. maaaaap maap maaaaaaaaaap. And it had a 5 amp fuse but it still drew enough to immediately heat up the 13/14 amp cord so much that the rubber was sticky. It was obviously fooling the 15 amp breaker because I have done experiments with this exact wire and it took 19 amp to melt the rubber in 4 minutes. Approximate values. I dont remember exactly. maybe all of my appartment's plugs are 20 amp cicuits. My brand new 1200 watt microwave draws 18.2 amps at 121 volts.
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,419
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    Jan 21, 2010
    I'm not sure how much there is in the stories about capacitor plagues and bad electrolyte recipes being stolen.

    I would hazard a guess that capacitor failures became more common as boards started moving to lower and lower CPU voltages and having onboard switchmode regulators. Those capacitors were (and are) exposed to huge ripple currents and this really does accelerate failure.

    The main things to be careful about when replacing motherboard capacitors are:

    1) temperature ratings
    2) ESR
    3) Physical size

    Of all of these, the third can be the most tricky.
     
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