Hi Jim,
Schematically, the cathode is the bar and the anode is the arrow part.

With the diode connected with its cathode to the positive voltage and
its anode connected to ground, it is reverse biased and looks like an
open circuit during normal operation. When the contactor relay (master
solenoid) is turned off, the collapsing magnetic field will generate a
field of opposite polarity (the voltage spike we are talking about) such
that the side of the coil that is connected to the +12V bus will now be
at a large NEGATIVE voltage, putting the cathode of the diode more
negative than its anode and it will now conduct. The voltage across a
conducting diode is about 0.7 Volts, so the spike will be limited to
about a -0.7V along the +12V bus in the aircraft. Without the diode,
hundreds of negative volts could be placed on the bus (for a short
period of time, in the milliseconds), but solid state devices don't get
along well with these high voltages, ESPECIALLY since it is opposite
polarity of what they are designed to operate at.

Scott


Morgans wrote:

"Scott" wrote in message
.. .

Nope, not really a concerning factor. Any of the "garden variety"
diodes in the 1N400X series should be fine. Just be sure to put the
cathode to the +12 or +24 volt side of the contactor coil and the anode
to the ground side (assuming a negative ground system).


I am not very good with the electron understanding, but I would be grateful
to understand this whole discussion.

Is the cathode normally the downstream side of the diode? What does the
installation like this, do, to help with the current spike?