>
>
> Sigh.  I though I knew what I was doing, and now I'm all confused!


Well, the 2003 diodes are better than nothing, most of the time.

I once was debugging a thermal printer that had problems with the occasional
"halt and catch fire"..  The design was in production, but was going through
a lot of printheads with no apparent cause.

After a bit of negotiation, we finally got the schematic.
The printhead elements were run from unregulated power, with an ADC to read
the voltage and correct burn time.  They were connected from unreg, through
the print elements, to a ULN2003.
Turns out they had connected the protection diode common to VCC.
This caused some really odd behaviour, since the current through the
printhead, and protection diodes was usually what was running the system,
NOT the output of the linear regulator.  The system would float around
5.24Vnominal, with VCC taking the expected hits whenever printhead
elements were
on.  I never could track things beyond that point to a specific cause, but
in all cases cutting the diode pin on the 2003 cured any problems they were
having, and we usually didn't see those units again.

I suspected that there was some problem in the regulation caused by the
output of the reg going higher than the regulation voltage, then back below.
Not a state that most linear regs are designed to operate in.
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