Automotive Circuit Protection - Part 2

[garwillis@msn.com (Garfield Willis)]

OK, now that we've discussed Load Dump some, how bout all those other
varieties of "Automotive Transients" the SAE & ISO orgs would have us
worry about? Here's a short basic (abbreviated) list; trust me you don't
wanna see the whole litany, unless you suffer from insomnia.

Name				transient V			frequency/cause
---------				----------------			-------------------

Load Dump			pos/~70V			each load disconnect
ALT Field Decay		neg/~100V			shutdown of elec. sys.
Ind. Load Decay		pos/neg/-+50V		varied
Reverse Bat			neg/14V			screw-up
Double Bat			pos/28V			jmp-start/boosters

We've touched on L-D in Part 1, so next up is ALT Field Decay. This is
referring to what can happen on the *field* winding terminal of the ALT
(not it's output), when the ALT spins down after shutdown (has
mechanical and magnetic energy it tries to pump back into the field
winding, which IS still connected to the bus, unlike it's output).

Inductive Load Decay, applies to any/ALL inductive loads (including the
ALT field winding) that suddenly switch off when you shut down: relays,
motors, IGN coil & INJ windings etc are all included. The polarity and
magnitude depend on how the load is dangling from the bus AND if it has
it's own freewheeling diode in place.  It's kinda a catch-all for worst
case turning off inductive loads, such as suppose you had all your power
windows going up, the power antenna going down, the cigarette lighter
plugged in and glowing (yeah, not much of an inductive load, but humor
me, OK? :), the A/C on (clutch) and the cabin fan motor on full tilt,
the engine cooling fans going full tilt, blah blah you get the idea, and
while all that's happening, you turn the key OFF. Battery and ALT output
are now both disconnected from entire +BATT bus (no nice big filter
there anymore), and all the inductive loads are collapsing.

Reverse battery is fairly self-explanatory, and fairly embarrassing. :)

Double battery doesn't refer to some bizarre phenom where if you jump
two 12V batteries in parallel, suddenly the voltage doubles cosmically.
Heh. It's related to jump-starting simply because alot of the BATT
booster carts have two 12V in series for big boost during cold weather
jump-starting.

OK, that's a VERY cursory look at the issue, and the SAE/ISO specs
actually detail not only the range of voltage more elaborately, but how
much *energy* is behind/contained in each kind of transient, so you can
estimate how much energy a transorb for example must be able to absorb
to squelch the threat. Handling all these conditions and meeting the
specs IS of course possible. :)  You can just assume any modern
vehicle's electrical system has the needed protection designed in (gawd
I hope so), but when WE add circuitry to the bus, THIS is the kind of
threat consideration ya oughta give to anything you attach, assuming you
want it to live a longNhealthy life.

Now, finally, to put some meat on these bones, I'd like to suggest an
exercise both easy and revealing. With the "categories of threat" listed
above, amble on over to the National Semi site, and compare the specs
for two likely denizens of the +BATT bus, both are +5V regulators.
	LM2931 vrs. LM7805
Pay attention to the bullet items listed for the newer LM2931, and see
if you don't recognize many of them from the threat list above. You
won't find them in the LM7805's data sheet, however. It wasn't intended
to go into automotive use, altho it shows up there in alarming numbers.

And even tho devices like this have been designed with automotive in
mind, if you also pay close attention to the NUMBERS in the spec, you'll
see that in some cases the protection may still be barely adequate.
That's why with mission critical systems especially, you want to USE
these devices because they are hardened against these kinds of threat,
but you ALSO probly might want to think about still having a first line
of defense with transorbs in front of them. If that seems too paranoid
for your application, at least stick with these more robust parts that
explicitely detail having normal automotive protection built in.

Not sure what should go into a Part 3; let's see what the cat drags
home.

Gar


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