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Tantalum caps.

Discussion in 'Electronic Design' started by Henry Kolesnik, Jan 21, 2004.

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  1. Over the last few years I've acquired quite a few consumer electronincs pcbs
    including TVs, VCRs, stereos, etc, so when I discovered that I needed a
    tantalum to repair some test equipment I was going to salvage a tantalum. I
    couldn't find one anywhere, so I assume they're too expensive or too
    unrelaible for high end consumer electronics. A couple of the boards were
    from my personal stuff purchased new. One example is a MGA Mitsubishi rear
    projection TV that operated flawlessly for nearly 20 years of daily use.
    Most of my test equipment comes from hamfests and is surplus after becoming
    obsolete and non-operative in less than 20 years. That leads me to wonder
    what the real story is behind tantalum capacitors. What do the experts have
    to say?
    tnx
    hank wd5jfr
     
  2. They can have very good characteristics (small size, low esr, high
    parallel resistance and good capacitance stability) but have some
    strange failure modes if they are misapplied. Digikey sells a great
    variety of them. I can seldom justify their cost in production
    designs, but use them quite often in one offs.
     
  3. ddwyer

    ddwyer Guest

    a failure mechanism associated with the source resistance and how close
    the operating voltage is to the maximum specified.
    Modern aluminum can have very low esr and an adequate alternative to
    tantalum.
     
  4. So in a production design, what would you use to get the equivalent
    performance? An aluminum electrolytic in parallel with a ceramic?


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  5. John Larkin

    John Larkin Guest

    We often use surface-mount tantalums on high-density, high-cost
    boards. They are very reliable (don't dry out like aluminums) if used
    carefully, but high peak currents can ignite them, so they are
    generally a bad idea for bypassing power rails.

    Polymer aluminums (don't dry out) or polymer tantalums (don't explode)
    seem like a good idea, but I haven't tried them yet.

    I think multilayer ceramics are pushing 100 uF these days.

    John
     
  6. A production design usually pays for the engineering necessary to
    reduce the need for premium quality components. Your solution is
    often a cheaper alternative to a premium quality tantalum.
     
  7. I have a Racal 9301A where a tantalum must have caught on fire because all
    that was left was 2 leads, some crisp blackish ash and a little hardened
    crust on the pcb where it burned. There's probaby 10 other tants on the
    board and one or more are shorted but still intact and I'm trying to find
    the bads one/ones with least effort without a schematic. The other unit is
    a Wavetek 188-S1257 where a tantalum had a dead short but was intact. I
    repalced it with an electrolytic. The cap is on a 15 volt rail where I
    think it shorted and took out the regulator. Ireplace the regulator with
    what I assumed was a good one out of a new box but it was bad and it put 23
    volts on the rail that had a 20 volt rating but no more failed. Sometime I
    have good luck.
    73
    hank wd5jfr
     
  8. Over the last few years I've acquired quite a few consumer electronincs pcbs
    Consumer electronics are costed down to the lowest possible level. If
    they can use something cheaper, they WILL use something cheaper. A TV
    set or VCR has had people go over the design hundreds of times with
    BOMs and catalogs, checking to see if they can shave a penny here or a
    penny there.

    Computer equipment is a good source for tantalums - motherboards, hard
    drive PCBAs, etc. Of course, it will be surface-mount :)
     
  9. Tim Wescott

    Tim Wescott Guest

    I wouldn't assume that just because your test equipment comes to you broken
    is a result of tantalum caps -- perhaps your sample is skewed by buying at
    hamfests instead of burgling active technology companies? Maybe if you only
    acquired your home entertainment equipment from dumpsters you'd conclude
    that aluminum electrolytics are bad?

    I recently escaped from a company that does aero (but not space) systems.
    They get mounted on aircraft and are expected to survive being shipped in an
    unpressurized cargo hold at 50000 feet. At that altitude a wet aluminum
    electrolytic will dry out, but a tantalum will be fine. There are even
    wet-slug tantalums for high-altitude applications that will not dry out at
    these altitudes.

    The problems with tantalum are their fragility (we've had exploding caps on
    our boards, with one manufacturer's part being fine and another being
    horrid), cost, and the relative scarcity of tantalum. Does anyone remember
    the Great Tantalum Shortage of a couple of years ago? One of the big
    tantalum supplying regions is central Africa, and a combination of wars
    reducing supply and increased demand led to some supply problems for a
    while -- I remember that at least one of the manufacturers even came out
    with a Niobium cap as a substitute.
     
  10. OK1SIP

    OK1SIP Guest

    Hi all,
    tantalum caps seem to be too expensive for consumer-grade equipment.
    They contain pricey material - silver and, of course, tantalum, so
    making them cheaper is impossible. AFAIK they are widely used in
    military-grade equipment, where the price is not an issue. Their main
    advantages are a longer life (they do not dry out nor leak) and a
    bigger temperature range (frost resistance).
    About using cheap parts in consumer electronics: At least 80 percent
    of failures of certain types of TV sets were caused by dried-out
    aluminum caps. The good practice when repairing these sets was: first
    check all electrolyte caps by adding a good one in parralel. It was
    successful very often.

    BR from Ivan
     
  11. Jeroen

    Jeroen Guest

    Yes, but alas, only with zero volts across them. Capacitance
    drops precipitously with DC bias. For a cap with Y5V dielectric,
    at half the rated DC voltage, there's only 10% of the initial
    capacitance left. Most manufacturers don't tell you.
     
  12. Fred

    Fred Guest

    I didn't think it was quite as bad as that. Also very temperature
    dependent. These type of ceramics are also pyroelectric as well as being
    piezoelectric!
     
  13. Bill Turner

    Bill Turner Guest

    _________________________________________________________

    One odd characteristic of tanalums is they will work with reversed
    polarity for a while before failing. I have seen them inadvertently
    installed backwards and get through test ok and out in the field before
    failing.

    Just something to be aware of. Aluminums normally fail immediately
    under the same circumstances.
     
  14. Bill Turner

    Bill Turner Guest

    _________________________________________________________

    Well, maybe. Any manufacturer who has been nailed with thousands of
    dollars in warranty costs caused by saving a penny might disagree with
    your statement. I've seen it happen.
     
  15. Caused by bad engineering (or purchasing), I would say.

    Best regards,
    Spehro Pefhany
     
  16. John Larkin

    John Larkin Guest


    Which opens up the possibility of using them as parametric amplifiers
    or modulators. I have a paper somewhere that uses the nonlinearity of
    ceramic caps to make a nonlinear transmission line - a shock line -
    that sharpens the rising edge speed of kilovolt pulses.

    John
     
  17. John Larkin

    John Larkin Guest

    Wet-slug tants are expensive (do they still have silver cases?) but
    don't blow up like the dry ones. The dry slugs coat the sintered
    tantalum (fuel) with MnO2 (oxidizer).

    This is really erratic. One spool of tants will be bombs, another
    can't be made to fail by deliberate abuse.

    John
     
  18. The ONLY problems I've ever had with tantalums are where:

    (1) The part was defective from the manufacturer.

    (2) The voltage rating was consistently exceeded.

    (3) The thing was installed backwards (reverse polarity).

    I have no less than five Tektronix O-scopes here, all vintage
    late-70's to mid-80's. This means not one of them is less than 20 years
    old. They all use lots of tantalums, and they all work great, but then
    again Tek was (in those days) proud of what they put out, and was most
    definitely engineer-driven (which means at least a 20% 'fudge factor'
    built into everything they made).

    Tantalum caps are very stable and durable, but they are much more
    costly than aluminum types. In consumer electronics, the manufacturers
    will try to shave every penny they can off the cost of the design, often
    contrary to good common (engineering) sense.

    Such considerations are (usually) not so critical when it comes to
    non-consumer stuff.

    Keep the peace(es).

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  19. Tim Wescott

    Tim Wescott Guest

    Boy are they ever. I have some surplus ones, but they have cases that are
    more of a silver-gray than that nice yellowish-white look you get from
    silver-plated connectors. These are special parts, but if you want a
    generally high-performance cap in a (relatively) small package they're hard
    to beat.

    I worked for a while on a project to make a power-wire networking device.
    During testing I accidentally dragged a scope ground across a circuit that
    was referenced to the 115V power line, thereby exceeding the tantalum cap's
    voltage rating -- er -- "slightly". Little pieces of flaming capacitors
    bounced around the lab. After that all of my digital logic (3.3V and lower)
    coworkers _never_ messed with my bench.
    Interesting. I'll have to remember that. Thanks. In any case when
    designing with _any_ electrolytic capacitor it's best to use specify a cap
    for 20-50% higher voltage than what you think it's ever going to see,
    particularly because many voltage regulators overshoot on power up and the
    output cap sees more voltage than you think.
     
  20. Ken Finney

    Ken Finney Guest

    < snip >

    Silver cased wet slug tantalums DO explode, most contracts that
    allow the use of wet slugs require the use of tantalum cased parts.
     
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