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>> ----- Original Message ----- From: "Al Gietzen" <ALVentures@cox.net>
All of the
failures I've seen (2) occurred in the first few seconds of testing
(1) or because the electronic assembler installed the capacitor
backwards (reverse polarity) (1).
This may be the most important point, Ed. My experience was on a test fixture at the end of the assembly line. It was the first time the caps had seen a biasing voltage. Think of it as dramatic crib death. It was stated at the time that the insulating oxide formed on the tantalums when the voltage was applied. Don't know if it was true, but the solution is to bring the voltage up rather slowly to give the chemistry a chance to work.
No doubt, Ernest, properly designed/tested etc, Tant Capacitors are viable, however, given I've never had a board chrisp before and now I have had three go into failure mode since the first of the year, something is happening. In any case, I've reworked the PC board design to use ceramics - now, if the failure mode continues then clearly, the cause will be something other than the TanT capacitor usage. Since I am dealing with Pulsed inputs from the injection circuit that may have some influence even though I have no Tants in those circuits.
In looking at my circuit, it appears that the TanT capacitor I have on the +5V lead feeding the PIC Chip is a likely culprit for shorting and causing the current to run-a-way. I am going to check that out later today.
Ed
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