Larry W4CSC wrote:
"Roger Long" wrote in
:


Did I blow the diode by hooking up with reverse
polarity?



Yep. The diode only conducts when the field in the coil of the relay
collapses, when you turn the power to it off. The diode is reverse biased
(+ power to its banded cathode end, - power to its anode) when the relay is
energized. You had it connected up backward, zapping the diode to a short.

Oops....

+12-------coil----------12
| |
--|--
band noband


I'm confused. Perhaps this is why I left hardware engineering for
software. I thought the purpose of a diode was to pass current one
way, and block it the other. Did this fail because the passed current
was too high, or because the blocked voltage was too high? Or is
there something else I'm missing? Should a small resistor have been
put in series? Is the coil storing up too much energy? Why can't
everything just be digital?

While I'm on the subject, since I have twin engines, some of my
instruments, such as the fuel gauge are fed by the key switch from one
engine. Is there a way to hook up 2 diodes to allow either feed to
work/ Radio Shack part numbers please ;-)