Mark deHaas writes,
>To block back EMF on any coil such as a fuel pump winding use any good
>sized regular diode from Radio Shack etc. Make the diode good for
>a few hundred mils in forward current flow. Then solder the diode in
>at either the fuel pump or at the switch. Just remember to connect the
>cathode to the positive side and the anode to the negative side.
>Otherwise the diode will burn out as soon as the power is restored.
>So now any EMF pulse will break down the reverse voltage of the diode
>and short it out thus saving the contacts on your switch.
The diode must be placed across the Fuel Pump coil with the cathode
twoards the "positive" connection. What happens is as the contacts open
the laws of electromagnetism require that the inductor current (fuel
pump coil = inductor) continues to flow in the direction it was flowing
just previous to the contacts opening. This causes the "negative" end
of the inductor to swing more positive (current flows out of a positive
node of a source if you use conventional current descriptors) than the
"positive" end. Since without a diode or a condensor there is very
little to restrict the magnitude of this "induced" positive voltage
swing (back EMF), this can result in an arc across the contacts. The
diode clamps this induced voltage across the inductor to about a volt.
A condensor slows down the rate of induced voltage rise (dV/dT=I/C) and
reduces the peak voltage (Vc=intg(I*dT)).
As far as I know most rectifier diodes will become permenantly damaged
if you exceed their reverse break down voltage, they usually become
short circuits. You might be able to place a zener diode across the
contacts. Zener diodes are designed to work in reverse break down but
you would need to make sure that the selected unit could handle the
energy/power involved (E=.5*L*I^2, P=E*T).
Your best bet would be a rectifier diode across the coil, use one with
at least a 1 amp 100V rating.
Frank Marrone at: marrone@optilink.com
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