> Since the failure in shorted mode is so rare, my detective instincts lead
me to suspect something else -

Someone else noted the need for reverse voltage protection diodes. These are
covered in the data sheet. The need arises when the input voltage falls much
more rapidly than the output and the output cap discharges "backwards" into
the regulator. This can occur if the input is shorted suddenly OR if the
output load is small relative to the output cap provided while the input can
discharge rapidly when turned off - either because there is little or n
input cap or because some other load rapidly discharges the input cap.

In these cases and others the LM293x family are specifically designed to
survive. Their forte is automotive situations replete with alternators, car
batteries, inductive loads and numerous somewhat silly people wielding
jumper leads and reverse connected batteries. I strongly recommend this
regulator family. (Standard disclaimer, no shares in NatSemi or other
financial etc interest yada yada).

You said 7805 - hopefully NOT a 78L05 - the TO92 miniature version. These do
die for reasons that I have never bothered to determine - not using them is
easier. The TO220 version costs less usually and the TO220 dissipates more
power when unheatsunk than the TO92 pkg.


        RM

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