Frying your avionics

My point in my first post was that the battery would absorb the initial
transient in Ellipse's scenario. The system voltage would never reach
the level required to trip the overvoltage protection. If the battery
or its wiring were in poor condition, then the overvoltage protection
device could trip. Today's radios would not be threatened in either event.

Dale,

I agree with you.
My point was just that the battery *could* be unavailable to soak up the
load dump consequences, rendering the OV protection indispensable
whatever the case.

Best regards,
Gilles,
http://contrails.free.fr

Paul, have you ever tried to reset a 50, 70, or 100-amp circuit breaker
in flight? In the few cases I've seen when such breakers have tripped,
the crews were unable to reset them. We reset them on the ground, using
a soft mallet.

I remember one airframe manufacture putting the alternator output
circuit breaker in the floor beneath the pilot's feet. That allowed the
pilot to stamp his foot on it to reset it.

How much experience do you have with aircraft electrical systems, Paul?

I have a switchable 50A CB on my gear hydraulic pump; 'hasn't needed
a mallet yet to turn it off or on. As for experience, I've designed
solid-state voltage regulators with series OV protection, buck-boost
regulators for permanent-magnet alternators to keep the charge going
when the alternator voltage output is less than the battery voltage, CD
ignition systems, electronic fuel injection systems, augmenter exhaust,
propellers, rocket engine igniters, O-200 external engine starter, VHF
and UHF antennas. Lots of previous experience designing computer mods,
a total radar simulator, L, P,and X-band receivers and transmitters,
X-band antennas, antenna simulator equations, phase-lock loops, motor
controllers, ATLAS autopilot simulator equations, radio and light
refraction equations for ATLAS guidance, radar-tracking tropospheric
noise model. So I have a little electronics knowledge and experience.
I have a friend who took off with a low battery that required him to
get a hand-prop. 'Seems that on the short 10 mile return to his home
airport, his alternator breaker popped and wouldn't reset. During his
short taxi at low rpm, the alternator didn't put out sufficient current
to pop the breaker, but during takeoff and climb it did, well in excess
of the breaker rating because of the discharged state of the battery.
Again I'll say it; if you don't think what I've written can happen,
then just ignore it! BTW, have you ever put an O'scope across a
battery's terminals to see what it does with narrow, high-voltage
transients? I have!

ELIPPSE wrote:
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 shut off the field supply, 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!

This is known as a load dump. Disconnecting your battery from the
alternator can do the same thing. Transients can go as high as 150VDC.
Modern avionics systems are supposed to be designed to withstand this
scenario (per DO-160). If older stuff gets fried, I suspect it must
pre-date the DO-160 standard.

Greetings,
I'm in the process of building, and appreciate your message. Thanks, I'll
wire them together before the breaker.
david

"ELIPPSE" wrote in message
oups.com...
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 shut off the field supply, 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!