Walter Harley wrote:
"Roy Lewallen" wrote in message
...

analyzed the circuit carefully, and discovered that at an extreme
setting of one control, the tantalum cap could have a very small reverse
voltage applied. I modified the circuit to eliminate the possibility of
any reverse voltage of any level, and the capacitors quit failing.


How much voltage? We talking tens, hundreds, or thousands of mV?


As I recall, it was a couple of tenths of a volt. The capacitor was
probably a 12 or 25 volt unit.

A quick scan of the web shows that some manufacturers claim their
tantalum capacitors will withstand something like 10% of rated voltage
(not to exceed 1 volt) at 25 degrees C, and 3 - 5% of rated voltage (not
to exceed 0.5 volt) at 85 degrees C, with a time limit on application of
reverse voltage. Because of my experience, though, I'd consider it to be
bad design practice to allow any reverse voltage at all until I saw some
reliability figures for capacitors used under those conditions. The
problem is that it doesn't cause immediate failure, or even assured
failure -- it just increases the probability of failure. Some
applications can tolerate the increased failure rate, and some can't. At
Tek, a great deal of importance was placed on reliability.

Roy Lewallen, W7EL