Battery Contactor Diode?

On Sun, 03 Sep 2006 13:40:12 GMT, "Skrud"
wrote:

What is the correct type of diode (p/n) to put across a garden-variety
battery contactor to prevent the inductive spike?


Why wouldn't you use a capacitor?

Joaquin

Joaquin Murrieta wrote:
On Sun, 03 Sep 2006 13:40:12 GMT, "Skrud"
wrote:

What is the correct type of diode (p/n) to put across a garden-variety
battery contactor to prevent the inductive spike?


Why wouldn't you use a capacitor?

Joaquin

A capacitor has limited capacity, and as Jim Weir pointed out,
we're talking about a lot of current here, even though it's a brief
pulse. The diode shorts the current during the inductive spike
generated when the master switch ( or starter switch) is opened,
keeping that spike from entering the rest of the aircraft's electical
system. A capacitor would store some of it, but there'd be enough left
over to cause trouble. A capacitor thus charged also keeps the
contactor closed for some time after shutoff as it discharges through
the coil.
I have measured a 600 volt spike across the master contactor
coil. Gives a person a really good jolt if he's touching the terminals
at the time. Makes one respectful of the system, and makes him more
aware of the need to shut off avionics before shutting off the master.
We can't really rely on that diode always being in good shape.
A Zener diode won't work here any better than a switching diode.
We need a diode that allows forward current flow for a very short
period, while preventing reverse flow during normal operation. A Zener
is designed to break down at a specific reverse voltage, and the only
reverse voltage this application offers is the usual 14 volts or so
during aircraft operation.

Dan