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In light of some recent events, I'd like to post this
following means of justifying renewal of your avionics:
To those of you who have a circuit
breaker between your alternator's output and your main buss, here's a wonderful
way to fry your present old avionics so you can replace them with new ones. This
is a real help in justifying new avionics to your significant other! If your
alternator field-circuit-breaker is connected to your main buss, then if your
alternator breaker opens, either through an overload or manually, your main buss
voltage will drop. The regulator, sensing this, will increase the alternator's
field current. 'Course, there is no load on the alternator, so its voltage will
rise. Eventually, in a few milliseconds, the regulator will be pumping maximum
current through the field. Since we drive our alternators at 7000-9000 rpm, the
alternator will put out well in excess of 100 volts. Closing through the
alternator-breaker will put this high voltage on your buss. Your over-voltage
protector, if present, will crowbar and short the field supply, opening its
breaker, but by the time the field collapses, the damage will have been
done. Here's two things you can do to prevent this and keep your old avionics.
Any time your alternator breaker opens, pull your field breaker before restoring
the alternator's breaker. If you don't want to depend on your memory in a time
of stress, rewire your field circuit breaker to the alternator side of the
alternator breaker. That way the alternator's field will serve as the alternator
load and the regulator will be sensing the alternator output, thus keeping it in
regulation. If you don't think this scenario could actually take place, I
challenge you to do this test on your plane at cruise rpm with lots of avionics
load and prove me wrong! Maybe you shouldn't try this while in heavy
IFR! By the way; putting one of those load-dump transient MOV (metal oxide
varistor) protectors on the output of the alternator will not protect
against this. Their power and current ratings are very high, but only for
very short transients. One or several seconds do not a short transient
make! Pumping hundreds or thousands of Watts through them for more than a few
milliseconds will turn them into slag!
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