> Russell McMahon wrote:
>
> > If you want to produce a product which MIGHT smell, shriek, explode and
> > catch fire and/or short out various parts of its circuitry at
indeterminate
> > future dates then Tantalum caps are definitely the way to go ! Because
this
> > isn't what happens in all cases and because, if you design well and are
very
> > careful and lucky you may not have any problems, you are now liable to
have
> > people telling you what fine caps Tantalums are ;-)
>
> I've seen resistors smoke and explode a lot
> too, maybe we should all stop using resistors. ;o)
> I don't know what bad experiences you had with
> the tantalums but I haven't seen any trouble with
> them. But then I never use a 16v cap on a 16v
> power supply rail.

Resistors fail when you exceed there specifications substantially or if they
fail to meet specification.
Tantalum caps fail when you exceed their specs by a very small amount.
Also resistors usually miss out on the shrieking, smell much less evilly
impressive and the explosion from a tantalum puts the average resistor to
shame. Try it - you;; like it. Get a lab supply and a lowish voltage
tantalum cap. Apply increasing voltage above rated value and see what
happens. Great fun - as long as it isn't in a circuit that is meant to be
reliable and useful :-).

A PDF from Tantalum Capacitor maker NEC titled "Notes on the correct use of
Tantalum Capacitors" makes very interesting reading.
See -
    www.ic.nec.co.jp/compo/cap/english/products/c_tape/EC0332EJ1V0UM00.pdf

I find the first comment in their preface interesting in view of the rest of
the paper - something lost in the translation perhaps.

It provides a formula and nomogram for failure rates which would make many
designers blanch when they realised its significance.
They provide interesting guidelines
(following are as verbatim as possible but with some editing to reduce size)

eg

Operating voltage relative to rated voltage

- General applications - 70% max
- Power line or low impedance circuit 30% (50% max)
- Derate voltage by 1% per degree C above 85C
(many Al caps are only rated to 85 C and have very short lives at that
temperature so this is not a major problem).

Note that this means that, whereas an Al cap should be operated NEAR it's
rated voltage, a Tantalum should be operated at about 1/3 of its rated
voltage maximum or 50% if voltage transients are extremely well controlled.


Also

"Reliability is increased by inserting 3 ohms pere volt series resistance
into switching circuits, charge-discharge circuits etc".

"If the capacitor is in a low impedance circuit applied voltage should be
less than 1/2 to 1/3 of rated voltage.

"Do not connect two (or more) tantalum caps in series!!!"

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