Headset Noise

Hi Travis,

First, Glad that you seem to have fixed your problem. Your alternator
must have had a bum diode, and my prediction was wrong.

Second, I want to point something out to you. Disconnecting the battery
while the alternator is spun-up (and maintaining field exitation) is
EXTREMELY DANGEROUS. Doing this could cause the bus voltage to spike
beyond 100+V, and destroy every piece of avionics in your panel. Rather
than trying to explain why here, do a Goggle search on the phrase
"ALTERNATOR LOAD DUMP".

Every aircraft I have seen is wired such that turning off the battery
master simultaneously breaks the path to the alternator field, thereby
shutting down the alternator. Some aircraft accomplish this with a
mechanical interlock on the switch (Cessna split master/ALT switch), or
by the way the ALT switch is wired. The fact that your Lake is NOT
wired this way seems to be a major screw up.

Disconnecting the battery in order to diagnose an alternator is NOT an
accepted diagnostic tool.

Third, to show the expected bus voltage under different conditions, I
did four different Spice simulations and put the results he
http://tinyurl.com/lkzt4

Normal.jpg = 14V with Battery, loaded to 25A, showing typical
resistances. (50mv of ripple)

NoBattery.jpg = Same as above, but with no battery to act as a filter.
(2V of ripple)

BadDiode.jpg = same as Normal, except one diode is open. (400mv of
ripple)

BadDiodeNoBattery.jpg = same as above, but no battery to act as a
filter. (15V! of ripple)

Look at all four, and you can see the difference in the waveshape and
amount of ripple.

I also posted four more files based on the four simulations above,
which show the Spectra (FFT) of each case. Note that when a diode is
missing, the fundamental frequency is related to the rate of rotation,
while with six intact diodes, the fundamental frequency of the ripple
is six times the rotation rate.

MikeM