Finn Lassen wrote:

As discussed extensively on Matronics.com's Aero-Electric list, a

run-away  alternator can easily put out more than 100 volts! By run-away I
mean a failed voltage regulator feeding full 12 volts into the field winding
(B+ lead).

I don't think that the field lead amperage (from the voltage

regulator) is very high, so efficiency in the voltage regulator is not a
major factor.

Having said all that, I've still not modified my Mitsubishi (?)

alternator to include the crow-bar over-voltage protector. But if I had a
$10,000 avionics stack in my plane, you can be sure I would.

Finn

Somehow I doubt that 100 volt number.
Sounds like a good way to make a buck.

Must be a very rare failure mode or car radios and EFI/IGN  computers
would be dying all over the place. With 12 volts applied to the
field how long before the wires feeding it fused?

With a battery across the output from the alternator the connection
to the battery must also somehow disappear as the battery
would look like a giant short to 100 volts.

Further more 35 volt zeners on the input to most electronic equipment
would do the job. Perhaps there could be a 100 volt spike but a
capacitor could handle that and knock it down to way less than 35

volts.

Color me skeptical.
Paul Lamar

Ed Anderson wrote:

 
Well, I don't know about 100 volts, but when I decided to test my crow bar
device early on - I removed the battery load using the master switch (yes, I
was smart enough to have the avionics off), the voltage pegged my 18 V
voltmeter and blew all the voltage regulators in my engine instrument gauges
(I didn't know they had any and had to send them back to the factory).  The
factory asked me what I had been doing as the voltage regulators are suppose
to handle up to 35 volts.  The crow bar device I had did not react quickly
enough needless to say. Don't know if a failure voltage regulator would give
the same effect.

Ed Anderson

Well yes I can see how that could be done. But in a car it is
highly unlikely that the battery will be disconnected from the
charge wire while the field wire still has 12 volts on it. Do you follow me
on that? You obviously have separate wires that one of which (the charge wire)
you were able to disconnect. Most cars are wired such that the field
wire and the charge wire run together. Probably for this very reason.
Usually what happens is; if you disconnect the battery the
field wire is also disconnected hence no over voltage.

As long as the battery is hooked up to the alternator charge wire
the battery acts as a giant capacitor or a giant short in the event 
of an over voltage. 

As I said, this is an almost unheard of failure in a car.

Paul Lamar
 
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