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