"RST Engineering" wrote in message
...

"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


I totally missread the initial question when this thread first started, and
somehow I suspect that I may not have been the only reader who did so.

The battery contactor, usually operated by the master switch, should
normally be a trivial case with regard to voltage spikes the circuit is
protected by the battery. It does no harm to protect the circuit and, if
you want to be really anally retentive, you could measure the current drawn
by the coil of the contactor and size the diode for a little more current
than that. Once the current is flowing in an inductor, such as a contactor
coil, it will try to continue flowing at the same rate when you first turn
it off--which causes a voltage spike several times the normal operating
voltage. The protection diode simply allows the current to flow around in a
circle--through the diode and around back through the coil for a few
milliseconds until the resistance of the wiring in the coil uses up the
energy. Thus, presuming that the diode is located at the coil, the voltage
spike
should be limited to the forward voltage drop of the diode--usually about
six tenths of a volt.

With regard to possible damage to avionics, the contactors which have their
coils powered from the master buss (which presumably is activated by the
battery contactor) are the greater issue and all should be protected by
diodes--although capacitors may also work. Technically this is airframe
wiring and therefore in the mechanic's job description--so that is who to
see in the case of a certified aircraft.

Otherwise, nearly every EAA chapter has members who are or were mechanics as
well as members who are or were avionics technicians--and a couple of cups
of coffee with the three of you together, and maybee a couple of additional
members, will quickly solve a lot of problems. Remember that is a lot of
the original and continuing reason for chapters!

BTW the 1N400X series of diodes are 1 amp, and I believe that the next
common size is/was around 3 amps.

Regards,
Peter