Comments below:
"Jeff" wrote in message
...
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?

Simple answer, explained elsewhere, repeated here. Roger hooked his new
sump pump system to a test battery to test it, but unfortunately connected
the battery backwards (+ where - should have been). The diodes (two pumps,
two relays, two diodes) normally would have blocked the 12 v. from the
battery and passed only the reverse current from the collapsing relay coil
field when the circuit opened. Instead, because the polarity of the circuit
was reversed, the diode passed the full current available from the battery,
essentially, the diode was a dead short accross the battery, so the diode(s)
burned up and shorted itself (themselves) permanently.

Rodger won't do that again I'll bet.

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 ;-)

Yes, two diodes, arranged in a Y, each diode feeding power from each key
switch coming together, then feeding power to the instruments you want to
continue to operate with only one engine on. The diodes need to be rated at
a high enough wattage (volts x amps = watts) to pass enough current to
operate the instruments. There will be a slight voltage drop across the
diodes so you may experience lower than expected readings on the
instruments.
--
Ken Heaton & Anne Tobin
Cape Breton Island, Canada
kenheaton AT ess wye dee DOT eastlink DOT ca