> > Tantalums tend to explode
>> pretty much right at their rated maximum.

> Well before that, even, if you do it right (it's advisable to de-rate 2:1 or
> even 3:1 for some applications with current surges).

Solid (not wet electrolytic) aluminum caps have about the same
advantages as Tantalum but without Tantalums charming flaw.

Tantalum caps are made by sintering a mass of Tantalum particles
together, attaching a wire and then creating an oxide layer on the
outside of the sintered mass. An outside electrode is then added over
the oxide and ther very large surface area of the particles and the
very thin oxide layer provide superb capacitance density. The voltage
rating is related to oxide thickness. typically its very very very
very thin by any normal standards. Being thin, if it's punctured the
hole width to depth ratio is large for even a "snmall" puncture.

What typically happens with Tantalums is that the voltage level
exceeds the "punch through" voltage of the insulating oxide layer and
a small non healing conduction path forms. If the circuit is low
energy enough the capacitor may be walking wounded but still walking.
However, if there is substantial energy available the break through
becomes increasingly severe and a very low resistance is offered to
the available energy to have fun with.

The important point is that the excess voltage can either come from
general over voltage or from a small spike or from ripple. The
tolerances are very fine and the failure is catastrophic. So, the 2:1
or 3:1 recommended overrating is to keep spikes etc below the rated
voltage. If the spike comes from eg inductive ringing and if the
capacitance is small enough relative to the capacitance or if the wind
is blowing south east and it's a bank holiday (let alone dead fish
being waved) then the spike may 'reach up" from very low levels.

Once it starts there is no going back.

Solutions

- Never use tantalum caps
- Ensure supply never exceeds cap voltage rating
- Never use tantalum caps
- Ensure spikes do not exceed etc
- Never use tantalum caps
- Use only on low energy circuits
- Never use tantalum caps
- Use solid Aluminum caps which are as good but don't have the problems
- Never use tantalum caps
- Never use tantalum caps

Also - Never use tantalum caps
But, the world does, and they are extremely good at what they are good at.
In low energy circuits they are superb
If you get paid at good rates for callouts and for distance and for
weekends then by all means use tantalum caps.
Otherwise ...


           Russell
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