Re: Frying your avionics


GTH wrote:
I challenge you to
do this test on your plane at cruise rpm with lots of avionics load and
prove me wrong!


That's what overvoltage protection is made for : in the airplane I
built, within a few milliseconds, the OV relay opens and disconnects the
alternator from the ship's circuit.


Regards,
Gilles Thesee
Grenoble, France
http://contrails.free.fr
Hi, Gilles!
Do you know all the time-constants of the over-voltage protection
loop? You say your OV has a relay which opens; a relay can take
20-60msec to drop out, especially if you have a diode across the coil
for inductive transients. What is the L/R time-constant of your
alternator's field when fully charged? Do you know? Most of the OV
circuits I'm familiar with use a solid-state device, such as an SCR,
which is triggered on in a transient that then shorts out the field
supply source, causing its circuit breaker to open. What is the
time-constant of the SCR's trigger circuit to reduce nuisance triggers
from short transients; do you know? Do you also know how long it takes
for the field circuit breaker to open? I wrote this posting to make you
people aware of a possible failure mechanism in your plane's charging
system that could have disastrous results with your avionics. If you
wish to dismiss this warning, that is your prerogative. But please, if
this situation ever happens to you, pull the field breaker before
closing the alternator through! Paul

.

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