> we had a proto board that halfway through building developed a short, part
> had to be out that day of course. After 3 people looked at it for half an
> hour we were still no closer to finding it. Wound up pulling the valuablle
> chips and connecting a 12V SLA battery across 5V and gnd, found the fault,
a
> tantalum cap and failed

Electronics lessons to learn the hard way.

103.    ***NEVER*** use tantalum capacitors in other than low energy high
impedance circuits if the voltage may exceed the capacitors rating for any
period of time. Here the word "any" assumes the normal meaning. eg a 1 uS
pulse is included in the period "any of the time".

A power supply rail is never a low energy high impedance circuit.

104.    *** NEVER *** listen to people trying to assure you that rule 103 is
bunkum.

105    Tantalum capacitors belong to the common class of electronic
components which contain crisis detectors. These allow them to detect when
the worst possible time to fail will be and to act accordingly.

__________________

Tantalum capacitors are great fun. I have seen one, in sequence:

- smell terrible
- emit smoke
- emit a jet of flame
- shriek loudly
- explode
- short the power supply rail.

Solid aluminium capacitors (NOT common Al electrolytics) are much more
boring. Apart from having about the same capacitance for size as Tantalum
caps, they lack all the interesting features.

YMMV but only in terms of what combination and sequence of events will
happen to you.



        RM

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