John Larkin wrote:

On Fri, 27 Jul 2007 21:50:33 GMT, Joerg
wrote:


John Larkin wrote:


On Fri, 27 Jul 2007 16:03:35 -0400, "Tom Del Rosso"
wrote:



"John Larkin" wrote in
message


It's like with Saabs: great cars to drive, between fires.

That's just a Saab story.

I've heard of tantalums exploding if reversed, but otherwise there must be
some parameter that's being pushed. Voltage not derated enough?


High currents (ie, high dV/dT) causes tiny little sintered projections
to heat up. Tantalum metal is fuel and the MnO2 electrode is the
oxidizer. Once triggered, it becomes a chemical reaction that is
impressively exothermic, even if a lot of electrical energy is not
available.

The fixes are to severely under-rate the parts on voltage (1/2 to 1/3
rated voltage are suggested) and soft-start supplies to avoid high
peak currents.

Interestingly, most tant suppliers say to *never* use their parts
above 80% of "rated voltage."


I find that a rather dishonest specsmanship. "Here is our product and
here are the specs but please never use it up to those specs". Why don't
they just say 100uF/28V instead of 100uF/35V? All the more reason for me
not to use any tants.



In use, solid tantalums seem to have a declining failure rate versus
time, whereas aluminums, both electrolytic and polymer, eventually
fail... one from losing water, the other from gaining water!


Electrolytics often get too bad of a rap. I've got dozens and dozens
here that are about my age or older. Like the Hammond organ from 1961
where they put the cap right next to (!) the big rectifier tube. First
cap, still fine. Same for the ones in the Sachsenwerk radio from the
early 50's and the Astor BPJ radio here in the office which is also from
the 50's. Then there is the Rohde&Schwarz SMF generator which often ran
all day long. No fan, totally shielded, gets quite toasty in there. Has
the original electrolytics from around 1955, only the steel tube for the
oscillator is developing arthritis. But after over 50 years of dutiful
service that's ok.


It depends on the crimped seals, which is a matter of luck. My other
problem with alums is that I want my gear to work to at least -20C,
perferably -40, and elec esr's skyrocket below 0C, so my switchers
will go crazy or at least make huge amounts of ripple.


Yes, that's a problem, one that is often overlooked in automotive. For
hi-rel stuff with temperature extremes I try to limit myself to
ceramics. It's amazing that you can get 10uF for pennies nowadays.

--
Regards, Joerg

http://www.analogconsultants.com