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-

Easily fixed.
Just connect a VDR (voltage dependant resistor) across the 12volt feed to
any electronics. These devices are nomally open circuit. When the voltage
across them exceeds their rating they are short circuit.

Ian. RV7 Renesis, bolting the wings on.


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