Frying your avionics

[ELIPPSE]

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!

[GTH]

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

[cavelamb]

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...

[GTH]

>>
>> 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

[Dale Scroggins]

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.

[GTH]

>
> 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

[ELIPPSE]

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

[GTH]

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

[Dale Scroggins]

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.

[Dale Scroggins]

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?

[GTH]

> 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

[ELIPPSE]

> 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!

[[email protected]]

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.

[David E Koehler]

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!
>