> For Ta caps, allow a generous over-voltage margin (50%) as
> they are wont to fail if pushed anywhere near their voltage rating.
> For example use a 10V on a 5V supply, 25V on a 12V supply etc

Let me tighten that specification (although I will be flamed for it :-) ).

Tantalum capacitors MUST have a rated voltage which is higher than the
highest voltage or transient voltage which will EVER appear across them.
When exposed to voltages not very much higher than their rated voltage they
will fail magnificently.

Failure is ultimately a very hard metallic short circuit but along the way
you can hope for bad smell, noise (shrieking, moaning etc), smoke, flame and
explosion. In the "best" case I have had all of these in the order listed.

For applications that DEMAND the Tantalum's capabilities consider using a
SOLID aluminium capacitor instead. They have most of the capabilities of
Tantalum and are of similar physical size for equivalent spec but do not
self destruct in the same manner and may be cheaper. Note this is SOLID
Aluminium - not wet film electrolytic. Philips, at least, make them.

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 ;-)


regards


                Russell McMahon

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