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Electrolytic caps?

Discussion in 'Electronic Components' started by Melissa, Feb 26, 2005.

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  1. Melissa

    Melissa Guest

    I used to work in electronics and have several large boxes of electrolytic
    caps in my lab. Does anyone still use them for anything? Is there even any
    electronics still being built in this country?
     
  2. What type / voltage/ capacitance are they?
     
  3. Yes and yes, but it depends on how old those caps are. Over five years
    old, and they're not 'fresh'; ten years and they're 'stale'.

     
  4. Joerg

    Joerg Guest

    Hello Watson,
    Nah. I have some 40+ year old radios where the electrolytic caps are
    just fine. A couple times I actually did a resistor charge and leakage
    test just to see. They were nearly as good as new.

    But sometimes when they had been in storage for decades they may have to
    be "formatted" slowly to get them used to the job of "being a capacitor"
    again.

    Regards, Joerg
     
  5. Jim Thompson

    Jim Thompson Guest

    Hi, Joerg. I do, literally, have some 40 year-old aluminum
    electrolytics in my part bins.

    What's the best way to "format" them?

    ...Jim Thompson
     
  6. I read in sci.electronics.design that Joerg <[email protected]
    It took a long time to discover that sodium chloride (common salt)
    contamination was responsible for short life of aluminium electrolytic
    capacitors, simply because traces of salt are almost everywhere. So if
    your old caps have, by chance or design, a low salt content, they may
    well still be good.
     
  7. Tim Wescott

    Tim Wescott Guest

    Its "forming" (technically "reforming" since it was already done once at
    the factory long long ago). You can do a web search on electrolytic
    forming.

    Basically you feed it rated voltage through a big current limiting
    resistor -- what you're doing is restoring the aluminum oxide layer on
    the foil. There are a number of techniques for verifying that the
    forming is complete (vs. the cap being bad) -- basically when you think
    you're done you test with more than the rated voltage to see if the
    thing leaks -- if so, you scrap it.

    I've never done it, but it's a common thing with tube-equipped boat anchors.
     
  8. It's actually "reform". Here's a web site that describes the process:

    http://www.vcomp.co.uk/tech_tips/reform_caps/reform_caps.htm


    Best regards,
    Spehro Pefhany
     
  9. I prefer to go for "High density". :)


    Actually, after you re-form the oxide layer with a limited current
    (<.1 ma) at the rated voltage you check the ESR to made sure the cap is
    good. Any signs of leaked electrolyte automatically makes it a bad
    part.
     
  10. Grass roots

    Grass roots Guest

    And that means what?
     
  11. Melissa

    Melissa Guest

    I don't recall, I'd have to go look. I have boxes of them all over the
    place in my lab, which still hasn't been unpacked since we moved.

    --
    Yours In Liberty, Melissa - Colorado, U.S.A.
    http://melissasliberty.blogspot.com/

    The last best hope for liberty, to give the world its first Bill of
    Rights: http://www.UPAlliance.org/billofrights.htm
     
  12. mc

    mc Guest

    The way I do it, which I thought was standard, was to gradually raise the
    voltage from zero over a period of minutes, monitoring for leakage all the
    time, and then leave it charged to the rated voltage for some time.

    You can measure quite an increase in capacitance from doing this, even if
    the capacitor didn't test out as defective to begin with.
     
  13. I read in sci.electronics.design that Michael A. Terrell
    Both residual leakage current and ESR are important, but for ESR you
    need some guidance on what is 'good' and 'bad'. It depends on a number
    of factors.
     
  14. It means that caps made during certain time periods and some
    companies are prone to failure from dried out seals and depending on the
    electrolyte used, the cpacitors may neeed to be re-formed.
     
  15. I try to find the OEM specs for the cap and check it with my Bob
    Parker ESR meter. Failing that, I compare the capacitor(s) to others on
    hand, or to the specs of similar caps from other OEMs.

    I have been running about a 50% failure rate for new old stock and
    salvaged electrolytics over the last year. I did the tests to entertain
    myself while I couldn't work, and had nothing to do and couldn't get on
    line for a year.

    I used to have a Sprague TO-6 capacitor analyzer to check capacitance
    and leakage. I'm thinking about designing a digital unit to replace it
    and maybe sell kits.
     
  16. BFoelsch

    BFoelsch Guest

    Personally, having played with electronics and salvaged parts since the
    early 1950s, I think the whole "reforming" thing is pretty much baloney.

    It's true, sometimes if you take an old electrolytic that tests poorly you
    can get it to improve by using the "reforming" process. Often, however, the
    resulting cap will have high ESR, and will want to leak for a while every
    time you apply voltage to it.

    I find that, in general, unless the leakage gets down to where it should be
    in not more than maybe 15 seconds, the reformed cap is going to "deform"
    spontaneously. There are some caps that will stop leaking over maybe 30
    minutes, but if you test the rest of their properties you will often find
    that you now have an electrolytic rectifier more than a capacitor.

    That is not to say that all old electrolytic capacitors are bad. I recently
    worked on an old Tek scope and the electrolytics with 1967 date codes were
    perfectly good. However, electrolytics used in HP equipment of the same
    vintage are almost always bad. I have some NOS capacitors that are fine, and
    I just tossed a bunch that weren't.

    I also worked on a 1992 fax machine recently. Every electrolytic cap was
    leaking, both physically and electrically.

    Test your old caps. Ramp them up to rated voltage while watching the
    leakage. Don't overlook ESR. If they are good, keep them and use them. If
    they are bad, toss them. I wouldn't put much faith in the reforming myth,
    though.
     
  17. Depends on a lot of factors including how long they have set without
    being used, how long they have been used, how high a temp they have been
    exposed to, etc. And of course how well they were made. The old timers
    seem to be better made back then, and withstand the test of time better.
    But the stuff being made today isn't as well made, IMHO. Smaller size,
    les ofecerything, etc, means they don't last like they used to.

    I just got thru replacing the main filter caps on two different HP PSes;
    both were dead and gone. In one case the PS was 40+ yrs old, so it's
    understandable. But 5 years is usually what they aim for. Hey, they're
    full of juice, don'tcha know.
    That's reformed, as opposed to the original forming. And even so, you
    may find that some of the juice has leaked, so you won't get the full
    rated capacitance, but just a fraction.
     
  18. It means that if the cap you are going to use is that old, you had
    better not assume that it has the rated capacitance at the rated
    voltage. So measure it and find out for certain.
     
  19. Melissa

    Melissa Guest

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