William Chops Westfield writes:
>I'd guess low leakage, low esr, low inductance, high temp, and high tolerance
>as "special classes" that ought to be popular?  And where do tantalums fit
>in the picture, anyway.

        Electrolytic capacitors are definitely not precision devices.
I have been told that there is as much as a 50% tolerance in the
actual capacitive value from one sample to the next and, as several
others have stated, heat positively kills them.  More heat kills them
more quickly, but it's just a matter of time before they dry out and
stop being capacitors.  All the ones I have seen fail that didn't
short and become firecrackers did so by gradually loosing their
capacitance until they effectively took themselves out of the circuit.
In this case, a little heat from a heat gun, hair drier, or warm
soldering tip will bring them back a little until they cool back
down.  This helps find the bad ones much like Freeze Mist, only in
reverse.  You can also take a known good capacitor of suitable value
and rating and momentarily sub it across the suspect capacitor to see
if the trouble goes away.

        As for tantalums, they tend to be high in capacitance for
their size.  If they go bad or are connected in reverse, they tend to
announce this condition with a horrendous bang, lots of shrapnel, and
a bad odor.  All that's usually left afterwards are the two leads with
gobs of burnt material attached to the ends.  I had one go off in my
hand when I accidentally connected it to a reverse polarity terminal
on a board I was trying to trouble-shoot.  It burned a nice little
spot on my thumb and really got my attention.

        As for time constants, Most of the projects I have read about
or built that worked well  used silver mica or polistyreen capacitors.
I have built plenty of circuits with electrolytics as the C in a RC
constant, and they really don't work all that well over a long period
of time.

        After a while, they may start to reduce their capacitance as
they dry out or they  may develop leakage current which acts like a
resistor in parallel or they may just go "bang!"  I have joked before
that electrolytic capacitors should be socketed like tubes used to be
for all the grief they cause.

        Obviously, electrolytics serve a valuable function in
electronics, but they are the weak link.

Martin McCormick