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Just as a clarification - the "spike suppression diodes" do not protect
the electrical system from high voltage spikes; they protect the switched
device, which in this case is the solenoid coil of the relay being turned off,
and the switch itself. The coil really doesn't care, and the switch is
designed to tolerate the resulting arc. The normal wiring (I'm no good at
putting graphics in an email so I'll do it "verbally.") Goes from the battery
and all your expensive electronics to the switch, to the relay coil, and to
ground(return). The high voltage spike exists only between the switch and
relay coil, no where else.
Gary is correct. When current to the relay
coil is interrupted the inductance of the coil will cause a voltage spike at the
coil. How big depends on the inductance of the coil and the amount of
current that was flowing when the switch was opened. Whether or not the
switch will tolerate this depends on the switch. Personally I think a
diode across the coil is a good idea, just be sure it's installed the right
way. I don't see the need for anything fancier than a diode, something
like a 1N540x (where x is 4, 5, 6, 7, or 8). If you want to protect larger
parts of your electrical system against voltage transients you will need
something fancier than a diode.
Tom Gourley
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