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General Questions about Tantalum and Electrolytic Capacitors

Discussion in 'Electronic Design' started by Bernhard Krämer, May 3, 2005.

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  1. Hello,

    While working on my circuit, I found that I have some basic questions about
    simple capacitors:

    1)
    Is there any minimum operating voltage for Tantalum and Electrolytic
    capacitors?
    I think that the formation of an electrolytic layer in such a capacitor
    could perhaps require some minimum operating voltage.

    2)
    Until which frequencies does an electrolytic capacitor work well? When
    should I consider tantalum capacitors?
     
  2. Mark

    Mark Guest

    1 generally no

    2 the main diff between al and tan is the ESR especially at low
    temperatures. ESR = Effective series resistance

    A second diff is leakage (shunt resistance) , tant are lower leakage
    (higher shunt resistance) generally.

    Mark
     
  3. John Larkin

    John Larkin Guest

    Third diff is that al elecs gradually lose capacitance (over years or
    decades) whereas tants like to explode violently if enough current is
    available.

    John
     
  4. Joerg

    Joerg Guest

    Hello John,
    Depends on who makes them. I have elecs in old radios that are around 50
    years old and when last measured still had their 10uF or whatever and
    little leakage. Some are from companies that stopped making caps when I
    was in diapers. I usually check these electrolytics when I repair or
    restore a vintage radio.

    But then again there were certain PC motherboard caps that had a life
    span of a few weeks, I heard.

    Tant explosions: I remember when they stuffed a proto board wrong, all
    the tants reversed. Then it was connected to a 5V/100A supply. Some said
    it was like popcorn, others likened it to a machine gun. We had to
    replace some acoustic ceiling tiles later.

    Regards, Joerg
     
  5. Guest


    Over long periods the working voltage of trolytics drops if that
    working voltage isnt applied. Unlikely to get you into trouble, but can
    if the thing is stored 20 years unused then needed.

    The good side is they they will reform, but whether theyll reform in
    use depends onteh circuit, and whether the circuit will function
    meantime is another question.

    But by far the most important difference is that tants are prettier.


    NT
     
  6. Ross Herbert

    Ross Herbert Guest

    Hemetically sealed (wet) tantalum's make a bigger bang than solid
    tantalum's.
     
  7. Robert Baer

    Robert Baer Guest

    However, one can make an aluminum electrolytic explode like a
    firecracker (lots of paper seen afterwards) if excessive voltage is applied.
     
  8. Robert Baer

    Robert Baer Guest

    A trick used to reform the aluminum capacitors on Tektronix tube
    scopes was to slowly raise the input line voltage with a variac.
    The idea is to have a capacitor voltage increased slowly, to minimize
    internal leakage currents and possible damage.
     
  9. Guest

    That works nicely with transistorised kit.

    With the old valve stuff that needs reforming, the rectifier valve
    starts to conduct over a very small v range, so you have to wind the
    variac up very slowly indeed around that area to get that method to
    work.

    I've reformed 1930s 'lytic caps that way, though not with 100% success,
    but earlier kit tends to use paper caps housed in wax, and I've not
    found any way to deal with those. Can they be oven cooked at just above
    100C safely? If so it might be poss to dry them out that way.


    NT
     
  10. John Larkin

    John Larkin Guest

    Sure, but you have to furnish enough energy to make the bang, and you
    generally have to exceed the cap's specs. An MnO2 tantalum provides
    its own explosive chemical energy, and they detonate when operated
    well within their ratings. The new polymer tants are better.

    John
     
  11. Joerg

    Joerg Guest

    Hello John,
    But electrolytics can explode "nicer". Not by excess voltage but when I
    exceeded the current rating for too long one of them (a really big one)
    sent its aluminum can skywards. Straight up like a Saturn V except that
    the ceiling was in the way. It also made a nice roaring sound. The paper
    then flocked down like snow in a fairy tale.

    That day I learned that there is indeed such a thing as a maximum
    current rating for caps.

    Regards, Joerg
     
  12. Joerg

    Joerg Guest

    Hello Bigcat,
    Depends on who made the cap. I have some that are fine after 40-50 years
    on the shelf. True new-old-stock.

    The death sentence for an old cap in, say, a radio or a TV is when
    replacing an old selenium rectifier with silicon diodes. Unless there is
    a power resistor in series it can blow the cap right out of its moorings.

    Sometimes, to preserve the antique look, I have scraped out a cap and
    mounted a modern electrlytic in the can. That way it still looks like
    the old can. The most pretty can was a Ducati cap. I guess that this
    motorcycle company must have made caps for a while after WW2 until the
    cycle biz picked up again. Maybe to tide them over the economic slump.

    Regards, Joerg
     
  13. Ben Bradley

    Ben Bradley Guest

    Firstly, a little terminology: Tantalums are electrolytics too,
    just made with something more expensive (or it's a more expensive
    process), what are commonly called electrolytics are technically
    "aluminum electronlyics."
    So tantalum is short for tantalum electrolytic, and electrolytic is
    short for aluminum electronlytic. This shorthand also gets by having
    to used the alternative international spelling/pronunciation of
    aluminum/aluminium.

    I'm don't know about tantalums, but I've heard suggestions that
    electrolytics be operated at at least half their max rated operating
    voltage, else they're more likely to lose capacitance over the long
    term (several years). It's also suggested that anything that has
    electrolytics in it be powered up for at least a few minutes about
    every six months or so, so the electolytics get voltage and don't
    deform.
    Have you looked at capacitor manufacturers' websites? Surely they
    would have something on this.
    I'm sure it's related - the initial forming of the layer in
    manufacturing is done by applying a voltage.
    You have to get info on the ESR and inductance of whatever
    particular value of capacitor (and for the manufacturer and type) you
    use, to decide if it's acceptable at the frequency you use. Many of
    the better "suitable for switching power supply use" electrolytics
    work fine into the hundreds of kHz. The Digikey catalog has some short
    blurbs about each type of capacitor and what applications it might be
    best for, but again, the maker's websites ought to have the best info.
     
  14. John - KD5YI

    John - KD5YI Guest


    Lots of really great information...

    http://www.cornell-dubilier.com/framepdf.htm

    Enjoy.

    John
     
  15. Matt Flyer

    Matt Flyer Guest


    A couple of differences that I am aware of:
    1 - Tantalums tend to have a higher capacitance for their size.
    2 - Aluminum Electrolytics performance tends to degrade as they age.
    This may be an issue if you are using them for de-coupling.
    3 - Tantalum supposedly works better at high frequencies (you will need
    to compare datasheets and look the ESR and ESL)
     
  16. John  Larkin

    John Larkin Guest

    4. Tantalums are more expensive.

    5. Tantalums explode.

    John
     
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