My buddy Dave has been pestering me to SUBSCRIBE to this newsletter
for
over a year now.

My name is Steve Wilke, I've been involved on and off in aviation
since I
was a kid.

I spent over 20 years in Aerospace & Defense, both active duty in the
Marines and as a civilian working on the radar and missile systems
of such
aircraft as the F-4J/S Phantom II, the F14A/F14D Tomcat, the F-15
Eagle,
F/A-18
Hornet, the YF-22 prototype, the AMRAAM Missile, and many other very
fun
toys.

I decided to subscribe because I am concerned about people using
commercial-grade parts in an aviation environment.At the very least,
they
should
be using automotive-spec parts, but most preferably mil-spec parts.

I have a feeling that many of your newsletter readers/posters are
not aware
of what happens during "load dumps", such as turning off your landing
lights.You can get pretty high voltage "spike" transients on the
electrical
system (60v) when you suddenly turn off a heavy load. This will wipe
out
circuits that don't have protection against such
transients,particularly
those like the 4000 series CMOS.

I really don't care so much for the "newsletter" type of things; it
would
be preferable to have everything on a forum - it is so much easier to
maintaincontinuity that way.

I have been a member of "AllAboutCircuits.com" as "SgtWookie" for
several
years - and I have managed to accumulate more posts than any other
member,
by along shot.

Anyway, rather long - I guess "SUBSCRIBE" would've been adequate.

Regards,
Steve in Orlando.



It sounds like Steve has some useful and practical experience on real
hardware..

None the less, at the risk of sounding argumentative (though I'm not
trying to be), I have a little different perspective.I work as an
engineer at an IC manufacturer.From my perspective, I know that
_many_ commercial, automotive, and military grade IC's often don't
have any internal differences, and in fact are built on the same
production line and taken from the same lots.

Sometimes the different product markings imply a slightly different
test environment/spec.For instance, automotive grade IC's are
spec'ed to operate over a wider temperature range than commercial
parts.Early in a product life product destined for automotive
marking will be tested over the wider range to verify performance.
However, once defects are analyzed, in order to lower manufacturing
costs, all of the devices are tested in the same environmental
conditions (usually the more relaxed, commercial conditions).Often
mil-spec and automotive spec parts go through the production line
along side the commercial parts.

I agree with Steve's assessment that many people don't understand load
dumps.I question his use of landing lights as an example..Load
dump is caused by the storage of electrical energy due the inductive
reactance of a circuit, and how much current is going through the
circuit.In the case of an incandescent light bulb, the inductance is
very low, so the stored energy is very low, even though the operating
current may be somewhat high.

The only thing that's sort of strange about incandescent lights is
that they have relatively high inrush current when starting from a
cold filament.The rated current through the filament is limited by
its resistance while at it's normal operating temperature.When the
filament is cold, the resistance is _much_ lower, so when you first
flip on the switch, the current will be higher for a short time (in
the milliseconds if I recall).Much before the you get useful
brightness from the lamp, the inrush current has dropped off.

This inrush current doesn't have any bearing on turn-off transients.
The filament takes a long time to cool (many seconds) to a temperature
where its resistance is again low.The load dump from a simple
landing light will be exceedingly low.

Again, load dump happen with highly inductive sources and inductive
loads.


Alternators can sometimes put their regulators at risk if they are
running at full output and they are then disconnected from the system
load (disconnect relay).I agree that 60V is quite possible
(probable).Normal (factory) regulators are usually reasonably well
protected from this.The el-cheapo internal regulator from
Checker-Schucks-Kragen might not be (but probably is).

In actuality, an alternator load dump isn't of the purely inductive
kind.Wound-field alternators (the kind in your car) are regulated by
adjusting the current going through the field windings.Regulation of
the field current has damping which delays response - improving
stability.The delay causes the output voltage to lag changes in
load.When the alternator is disconnected from the load, not only do
you get an inductive kick, it last a bit longer because the regulator
needs time to respond to the changed load and reduce the field current
appropriately.



A landing gear motor may cause a purely inductive load dump.
Switching off a large motor with a small switch can damage the switch
contacts.That's why such loads are often controlled by relays.



Finally, a very small scale load dump always occurs when opening the
switch which controls a relay's coil.The current through the coil
includes some stored magnetic energy.Opening the switch attempts to
halt the current through the coil, and the stored energy causes the
voltage across the coil to rise for a short time - until the energy is
depleted.

A diode can be connected across the coil.With the diode, the voltage
is limited to no higher than the forward bias voltage above the bus
voltage - eliminating the problem.

The scale of the load dump on the coil is always determined by the
current through the coil, and is completely unrelated to how much
current the relay is controlling (through the contacts).

The voltage caused by the dump is caused by the very small stored
energy in the coil.It lasts a very short amount of time (decaying
pulse).

Normal system wiring doesn't conduct spikes (decaying pulses) very
efficiently.The spike will be flattened by bus capacitance.This is
why usually only the controlling switch is put at risk.


I hope some of this helps.If someone disagrees, I'd be happy to
discuss, and even be shown wrong..;)


Regards,

Matt-

Always helpful Matt, I am grateful.

Steve: Semper Fi Marine!
Thanks for your service...

Ed Carver


Hi I am a member since a year and mostly listen in to the
conversations and gathering data for a future  Wankel driven plane of
my own.

I was a little taken aback by the recent discussion on dump loads from
Steve . Although important to consider I didn't think this was a
difficult  problem to solve . I am a researcher in the medical field
and not an electronics engineer But have had some experience building
chemiluminescence analyzers from scratch,some of my experiences could
have bearing on this problem:
As we wanted a very good signal to noise ratio we tested a variety
small switched PSUs  that was needed to drive different parts of the
analyzer; We found on oscilloscope rather ugly peaks on turnoff as
well as noise during operation that got worse with load. The same was
the case with a high voltage section used  to generate ozone (very
similar to an ignition coil).
 we completely eliminated the problem by replacing as much as
possible with gear that could run directly of a separate 12 V battery.
What we couldn't replace was separated from sensitive parts and
shielding of all sensors& cabling from inductive noise was done, well
as heavy use of Zener diodes.
We constructed a  separate 220 v system for some power hungry parts
such as vacuum  pumps that could otherwise gotten us into trouble on
the 12 v side.

Something similar procedures must surely already be used  for
experimental aircrafts?
you don't mix the power supply for avionics and control systems with
that of pumps and motors ?

What would be the appropriate protection measures for cpu and mother
board as well as sensors of the proposed ignition system ?

I  would be more concerned with the problem of redundancy of the
engine control systems.

--
Robert Lindwall

We have at least ten people flying over ten years with
no reported problems with dump loads. The systems are all configured
with a common 12 volt buss. No different than a car.

Most of those people have Tracy's dual electronic EFI system which has
come in handy more than once.

The one I am working on will also be dual.

Paul Lamar


I agree 110% with the idea of a forum format. It would help to
maintain specific issues in a more ordinate form and easier to follow.

I am sorry, I am no Internet geek to give some help, but it has been
my personal experience in other forums. You can always and easily
backtrack to the issue you want to review or follow.

My 2¢ and please, don't shoot the messenger :),

Alfonso