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



The energy stored in such a 200V pulse is actually quite difficult to
transmit to components beyond the charging system.  The energy is low -
the source is not tremendously inductive, and the peak current at low
RPM is.. low.  Plus, because the pulse is so narrow (fast rise, fast
fall), it has high frequency content.  Transmission of high frequency
signal is difficult within an impedance controlled environment (coax
cable).  Transmission of said energy through such lossy/dispersive
environment as battery cable is inefficient.  So, while the pulse may
pose a threat to internal regulator components (because of their
proximity to the pulse source), it's much less of a concern to remotely
mounted components.  Especially when the remotely mounted components
already must include protection from human body model ESD events
(several kV).

Cessna had some problems with early transistor avionics (Ge based, as I
recall) which were inadequately protected from ESD events.  I'd bet as
many radios were damaged by techs not taking adequate precautions as by
starting/alternator "spikes"..


Regards,

Matt-



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