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
maintain  continuity 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 a  long 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-