"Morgans" wrote in message
news

"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).

Scott, at and above the 1N4004, the chip inside the case is physically more
robust and will take a spike of current a little better than, say, a 1N4001.





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?


Jim, as you know, a diode conducts current in one direction and blocks it
from conducting in the other direction. A positive voltage on the anode
forward biases the diode and the voltage will appear as a positive voltage
(less some small conduction drop of a volt or so) at the cathode. A
negative voltage on the anode will reverse bias the diode and it will not
appear as a negative voltage at the cathode.

Likewise, a negative voltage at the cathode will appear as a negative
voltage at the anode.

When a coil has a collapsing field, it produces a spike of energy as the
field attempts to keep the current through the coil constant. THis large
negative spike can and will cause some of the aircraft electronics to fail
catastrophically. One way of making sure that negative spike doesn't kill
the avionics is to shunt it to ground through a forward biased diode. That
way the most that can sneak through is a volt or so as opposed to several
hundred volt spikes without the diode. How do you forward bias a diode with
a negative voltage? You connect the CATHODE to the "hot" +12 volt terminal
of the coil and the ANODE to ground. THe negative spike is effectively
clipped at a volt or thereabouts.

Jim