On Tue, May 02, 2006 at 10:53:12AM +0100, Michael Rigby-Jones wrote:
> >Tantalums are YEARS reliable, while electrolytics might be classed as 
> >MONTHS if the temp is elevated.
> >
> 
> Tantalums have some specific problems that electrolytics don't however.  They are VERY voltage sensitive, they have very little over-voltage capacity so in many applications (specificaly supply rail bypassing) they have a higher voltage rating than might be specified for an electrolytic, with the corresponding size/cost penalty.  Manufacturers design notes usualy suggest to apply no more than ~30% of the devices rated voltage in a low impedance circuit.  When tant's do fail, it's very often short circuit and given enough current capacity flames and smoke will often be generated.

Good rule to follow for sure. I've heard electrolytic caps need a
certain *minimum* voltage or the plates degrade. Is that true for
tantalum too?

My electronics class has blown up a whole lotta electrolytic and
tantalum caps belive me... By the end of the last semester, we had quite
literally blown up every cap in the room, even taken ones out of junked
equipment. Electrolytic make nice booms, but it's the tantalum ones that
flame and release nasty, nasty smoke.

In any case in my designs I always buy caps with 3x more voltage rating
than I plan to put on them, unless it's not something I'm selling and
I'm feeling broke. :)

-- 
pete@petertodd.ca http://www.petertodd.ca
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