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Do Capacitors Go Bad if Unused?

Discussion in 'TiVo Series3 HDTV DVRs' started by Mister B, Aug 20, 2012.

  1. Mister B

    Mister B New Member

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    I just ordered a new, never used S3 OLED. In reading up on this unit I now am concerned about this bad capacitor problem. Do the capacitors go bad just sitting in an unused Tivo, or do they only start to degrade once electrical current is applied?
     
  2. jakerock

    jakerock Hey ho howdy! TCF Club

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    I would think that they can go bad either way. Heat probably makes the problem worse though so a unit that hasn't been used (and thus the power supply hasn't gotten hot) is probably safer assuming that it hasn't been stored in a hot place for any length of time.

    Worst case the caps can be replaced for $20 or so.
     
  3. V7Goose

    V7Goose OTA ONLY and Loving It!

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    Actually, the worst thing for any electrolytic capacitor is to sit unused - and this has nothing at all to do with the plague of bad capacitors the industry has seen in the past few years.

    The bad caps from the plague go bad even with constant use, and they typically show up as bulging cans. Distorted cans are always a sure sign of a problem.

    But electrolytic capacitors that have gone bad from non-use will generally show no outward signs at all. So do not assume your power supply caps are good just because they do not show any bulging. If you run into strange problems that are not related to the HDD, it is always good to try swapping the power supply before you go too much further.
     
  4. steve614

    steve614 what ru lookin at?

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    :confused: What's the expiration date on electrolytic capacitors?

    Do electronics suppliers go through their inventory and discard "old" capacitors? :p

    Seriously, are you only considering capacitors that have already been put in use? How does non-use affect a capacitor?
     
  5. V7Goose

    V7Goose OTA ONLY and Loving It!

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    Electrolytic capacitors are different from other capacitors. They rely on an oxide layer that forms on the aluminum plates as an insulator to handle the high voltages - the oxide forms from current passing through the electrolyte. When an electrolytic capacitor sits for extended periods of time, that oxide layer slowly disappears, dramatically reducing the voltage capacity.

    I cannot recall any suggested shelf life numbers, but six years is unlikely to be a problem; 20 to 30 years IS a problem. I suspect 10 years sitting is enough to begin causing some problems with a few individual components. Just firing up a device every five years or so is probably enough to keep the capacitors formed, but I'm mostly guessing on that.

    "New" capacitors that are "old" can be reformed before being put into use, but if they have gone bad from sitting in an existing circuit, applying power can both damage the caps (even exploding them in rare cases) and other items in the circuit. A lot depends on just how much of the oxide is still there. If the caps are not too far gone, I have seen cases where the circuit had initial problems when first put back into use, but the capacitors actually reformed themselves before anything else was damaged. In these cases, multiple power cycles with very short 'on' times can be beneficial in trying to bring things back to normal.
     
  6. unitron

    unitron Active Member

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    semi-coastal NC
    ___________V7Goose speaks great wisdom.___________

    Back when electrolytic caps were made in the US with names like Sprague and Cornell-Dubilier, I think I remember them having manufacturing date codes, and just like supermarkets, you were supposed to rotate your stock physically, whether you did the accounting LIFO or FIFO.

    I know I remember my electronics teacher back in the mid '70s intentionally leaving "new" looking but faulty components in the parts bins to teach us the hard way to always test before installing, instead of assuming.

    And somewhere around here I've got an old Popular Electronics issue with a constuction project article about an oxide reformer for electrolytics that slowly ramped up the charging voltage to let you save them without risking blowing them up.
     
  7. Worf

    Worf Active Member

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    Electrolytic caps are one of the WORST possible caps around. They're only used because you can get them in large values with high voltage ratings economically (low voltage uses have supercaps which get you huge capacitances, but you're looking at 5V max voltage).

    They're basically metal plates separated by oil-soaked paper separators (hence the chemical nature of them). Over time, the paper dries out and capacitance changes - over 10 years it can drive 20%+. They're really only rated for around 3000 hours operation at rated voltage and temperature before they drift too far. That lifetime roughly doubles for every 10C decrease in ambient temperature (so they would last 16 times longer if an 80C cap runs at 40C, for example). And yes, during operation they heat up, and low-ESR caps (used in power supplies) heat up worse when they age as their ESR values increase.

    Truly horrible, horrible caps. But they're the only ones available. The "Bad capacitor plague" was when a bad copied formula made it to China, and the oil in the paper degraded significantly and outgassed, causing them to bulge and split (they have pressure reliefs just in case). Of course, that completely destroys the capacitor - it no longer has the capacitance rated, the ESR, and may completely short out, destroying other parts in the process.

    10 years if you derate them well in service. Unused, they degrade rapidly - a few weeks. However, that degradation is reversible - you put them through a reformation process that basically places charge on the caps and they slowly regain their abilities over several days/weeks. But unused caps will be significantly off spec until reformed.
     
  8. Mister B

    Mister B New Member

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    El Paso...
    Thanks for all of the pertinent information. The bottom line is that I realize there is some risk in buying equipment that is "new, still in the box" which may be up to 6 years old. The purchase is through the Amazon Marketplace so if it is DOA, their returns are no hassle.
    If the S3 OLED unit or my current "Tivo HD" fails later, this forum has information on home repair, probably beyond my skills, or the advertisers on the top of this screen appear to be very reliable professional options.
     
  9. unitron

    unitron Active Member

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    There are so many "new in box" TiVos out there, at least if you believe all the Craigslist ads, that TiVo would have more money than Google if all of them had been subbed.

    Instead of paying a premium for an unused one, I like the idea of one that's already proved it's not going to let the magic smoke out the first time you plug it in.

    But then, it's not like I'm going to leave the original hard drive in there very long.:)
     
  10. dlfl

    dlfl Cranky old novice

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    So.... The degradation period when unused is either "a few weeks" (per Worf) or 6 years (per V7Goose). :D I hope it isn't a few weeks -- I strongly doubt the stock of caps or the power supplies and other equipment containing them is being rotated every few weeks!
     
  11. lillevig

    lillevig Hot in West Texas

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    If it's a few weeks then I'm buying stock in the capacitor manufacturers. Talk about planned obsolescence. :rolleyes:
     
  12. Worf

    Worf Active Member

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    Well, it works both ways actually.

    The degradation period of a few weeks is valid, but they will reform themselves with use. They'll be temporarily out of spec in the mean time, but once reformed, will be good as new. It's just if they're not used, they degrade and will need reformation after a few weeks idle (you don't usually have to do anything special as the circuits are built with those tolerances in mind so reformation happens transparently)

    The few years is their basic lifespan. It's actually around 3000 hours or so, at rated maximums. Derate them and they work better - roughly double for every 10C below max temperature they're operated at. Prime spec caps are rated up to 105C, so if they operate at room temperature most of their lives (25C, say), that's extending their lives by 256 times. Normal spec parts are 80C, and TiVo's love to be around 40C, so it's still around 16 times or so. Or roughly 6 years before they go out of spec (hit 20% lower - they lose capacitance as they age).

    Will the circuit they're in still work? Probably, because electrolytics are horrible caps, and have tolerances of around 20% or more. If they're filter caps, the downstream circuit gets a bit more ripple (and probably still works fine until you get to the point where it's so bad it hangs/reboot/does odd things). Hence why you don't usually need to do anything special to reform an idle cap. (Plus, they're usually quite overspecified so stuff works even if they badly degrade.)

    But after 5 years or so, it's probably better to toss the caps away and get new ones.

    And if you want to invest in stock - invest in Li-Ion battery producers. Those things lose capacity the moment they leave the factory, and usually die of old age before people hit their cycle limit. (Avoid "New Old Stock" Li_Ion batteries at all costs - if they're as old as your existing battery, they ain't gonna have much more life than yours).
     
  13. theraven146

    theraven146 New Member

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    Jul 25, 2012
    If an electrolytic capacitor is simply unused for an extended period of time, the dielectric will degrade; the longer it is not used, the worse the dielectric becomes. The capacitance is reduced, and the leakage rate increases. If the leakage rate becomes excessive, there will be enough power dissipation in the package to cause the electrolyte to boil, rupturing the package forcefully.
     

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