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The purpose of diodes across
inductive coils like a starter relay is fairly simple. When power is
abruptly removed from an inductive coil, the collapsing magnetic field generates
a brief, but intense current "spike" with polarity opposite that of the
previously applied power (voltage). If you have a diode reverse-wired
across the terminals of the coil, it will not conduct during normal operation,
but will conduct the reverse current of the inductive spike. This
effectively shorts the spike out across the coils and largely prevents it being
propagated into your electrical system.
Almost any silicon diode will
perform this function. Common types like the 1N4002, 1N4003, or 1N4004 are
fine, and can be bought for maybe a nickel apiece. The negative end, which
is usually indicated by a solid band around the diode body, goes to the positive
pole of the relay; the other end goes to the ground terminal. Current
ratings are probably not very important, since they're for continuous current,
and inductive spikes last for milliseconds or less. Reverse (breakdown)
voltage is another diode specification you can mostly ignore because at least in
silicon diodes, you're not likely to find anything with breakdown voltage much
less than 100V, which is more than plenty.
Jim Cameron, N143ES
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