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From: "Marc Wiese" <cardmarc@charter.net>
To: "ACRE NL" <rotaryeng@earthlink.net>,
        "Flyrotary" <flyrotary@lancaironline.net>
Subject: FWD: Alternator Voltage Spikes 
Date: Tue, 2 May 2006 16:17:28 -0500
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Subject: Alternator Voltage Spikes
 I have been noticing all the comments about voltage spikes during
starting/shutdown. One poster mentioned that a battery couldn't put out =
200
volts, so I thought I would toss my 2 cents in. I worked for several =
years
as an IC design manager at Texas Instruments in the automotive group =
where
we designed ICs for automotive regulators. I will assure you that =
charging
systems for cars (and airplanes) do indeed put out spikes well over 200
volts when the alternator is rotating very slowly. This voltage can =
easily
cause oxide failure, known as 'punch-through', which shorts VCC and =
Ground
inside the IC. Once that happens, heavy current flows through the IC =
causing
the smoke stored in the IC to escape. ICs don't work once you let the =
smoke
out! :-) To understand how a 12 volt battery/alternator can put out a =
high
voltage, you have to understand that in an inductor (i.e. the windings =
of
the alternator), the voltage across the inductor is equal to L*di/dt, or =
the
Inductance of the winding times the rate of change of the current =
through
the inductor. If you attempt to instantly stop or start the current =
through
an inductor, di/dt will become very large, and the voltage will increase =
to
whatever level necessary to collapse or create the magnetic field around =
the
inductor. When an alternator is turned very slowly, there are points =
where
the windings are open circuited (or routed through high resistance =
paths)
which causes voltages to rise to the level that breaks down the primary
protection - around 200 volts. In an automobile, there is a highly =
reliable
circuit that disconnects the radio bus from the charging bus during =
start
and shut-down to protect equipment from these spikes. This is what =
Cessna
attempted to do with some aircraft, but the circuit they used is =
somewhat
crude and potentially unreliable, so people disconnected its output that =
was
supposed to control the avionics power relay, and rewired this relay to =
be
controlled by a simple switch on the panel.
Marc Wiese