Frying 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 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!

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

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


Well, yeahbut...

Ellipse's method avoids the situation in the first place...

That's what overvoltage protection is made for : ....


Well, yeahbut...

Ellipse's method avoids the situation in the first place...




The Ellipse's method is just for the load dump case, with a perfectly
working voltage regulator.
Now, what if it is the regulator that goes berserk ?

An overvoltage protection is intended to catch ANY OV condition,
regardless of it's cause.

Regards,
Gilles Thesee
Grenoble, France
http://contrails.free.fr

cavelamb wrote:
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



Well, yeahbut...

Ellipse's method avoids the situation in the first place...


Having a storage battery in the circuit avoids the situation too. The
battery, barring bad connections, will soak up the initial surge. The
overvoltage protection device is unlikely to trip in Ellipse's scenario,
because it won't need to.

Having a storage battery in the circuit avoids the situation too. The
battery, barring bad connections, will soak up the initial surge.


Yes, the initial surge. And then...?


The
overvoltage protection device is unlikely to trip in Ellipse's scenario,
because it won't need to.




?? Can't figure out that one. OV is OV, and the avionics pay no mind as
to the original cause : zap all the same.

It is a good thing to try to fool proof an architecture. But OV
protection is there to protect against even things that should not happen.
You'd be surprised at the number of things that happen in aviation, and
yet should not happen : empty tanks, wheels up landings, popped
breakers, fried radios, engine failures, accidents...

You may wish to have a peek at www.aeroelectric.com. The load dump
scenario has been covered extensively.



Regards,
Gilles Thesee
Grenoble, France
http://contrails.free.fr

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

Hi Paul,

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!

Read again, I didn't dismiss your load dump issue and proposed solution.
I was trying to convey that your suggestion is valid for "normal" cases,
and that an overvoltage protection should be included in the ship's
architecture to take care of *any* OV condition, including voltage
regulator runaway.

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

GTH wrote:


Having a storage battery in the circuit avoids the situation too. The
battery, barring bad connections, will soak up the initial surge.



Yes, the initial surge. And then...?


The

overvoltage protection device is unlikely to trip in Ellipse's
scenario, because it won't need to.





?? Can't figure out that one. OV is OV, and the avionics pay no mind as
to the original cause : zap all the same.

It is a good thing to try to fool proof an architecture. But OV
protection is there to protect against even things that should not happen.
You'd be surprised at the number of things that happen in aviation, and
yet should not happen : empty tanks, wheels up landings, popped
breakers, fried radios, engine failures, accidents...

You may wish to have a peek at www.aeroelectric.com. The load dump
scenario has been covered extensively.



Regards,
Gilles Thesee
Grenoble, France
http://contrails.free.fr

Upon your advice, I peeked at the link. At
http://www.aeroelectric.com/articles/spike.pdf
Mr. Nuckolls makes exactly the same point as my previous post. Page 2,
last paragraph.

Mr. Nuckolls and I seem to have quite a bit in common. I, like him,
have been poking around aircraft avionics and electrical systems for
over thirty years. Currently I'm doing my best to assure the integrity
and reliability of avionics and electrical systems in a well-known
French-designed line of business jets.

I agree with you that overvoltage protection is necessary in an
electrical system using and alternator as a power source. I agree with
Mr. Nuckolls that most load-dump fears are unfounded, and that a system
using a well-designed regulator, effective overvoltage protection, a
well-maintained battery, and good wiring design and installation will
protect today's avionic systems quite well. Adding additional
protection devices or following Ellipse's recommendation carry their own
subtle hazards.

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.

Over the last thirty years, I've seen many avionics systems damaged by
lightning. I've seen a few damaged by reversed polarity, from improper
battery or alternator installation. I've never seen significant damage
from load-dump surges.

ELIPPSE wrote:
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


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?