Jeff wrote in :

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?

When he hooked it up backwards, the diode was forward biased straight
across the battery's terminals. Current went through the roof, welding the
junction to a short which, microseconds later, blew the fuse. Once welded
to a short, unless you can really melt the mounting, it stays a
short....across the coil, not in series with it. It's a short both ways at
that point.

This diode's function is to short the relay's coil when the coil's field
collapses, producing inductive kick which forward biases the diode for a
few milliseconds each time power is removed from the coil. Otherwise, the
inductive kick, which could be several hundred volts for a few microseconds
like the spark from an ignition coil, could destroy the control electronics
or switch contacts which arc each time it is de-energized. With the diode
in place, inductive kick produces .6v pulses, the forward bias voltage of
the diode as it sinks the kick's power current pulse.

Or is
there something else I'm missing? Should a small resistor have been
put in series?

No, as the diode is normally REVERSE biased by the applied power. I
doesn't conduct except for that little kick of the coil at de-energization.

Is the coil storing up too much energy? Why can't
everything just be digital?

It is digital...(c; 1 = relay on.....0 = relay off...but it's not
hexidecimal..making it harder for a software man to comprehend...hee hee.


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 ;-)


Any 1 amp, 50V piv or higher rectifier will work.....for this OR
circuit...(c;

Hook the anodes of the diodes, one to each power switch on each engine.
Hook the cathodes (banded end) together to the common power + lead to each
fuel guage. If either engine is turned on, its diode will conduct turning
on both fuel guages. (classic OR gate - DDL logic!)

engine 1----------------||---------\
\
\
engine 2----------------||------------\-----------fuel guage +

Circuit may be too simple for mainframe engineers...(c;