The alternator field winding is driven by a PWM (Pulse Width Modulated)
signal that goes from zero to 12V. The more output current needed from the
alternator, the longer the ON time and the less OFF time of the PWM signal,
affecting the average voltage to (and current in) the field winding.
12V x 10% ON time = 1.2V
12V x 50% ON time = 6.0V
12V x 90% ON time = 10.8V
Thus, the duty-cycle for 5V would be approximately (5 / 12) x 100 = 41.67%.
The current in the field winding is directly related to the average voltage
to it. However, since Power = Volts x Amperes, the power consumed in the
field isn't a straight line.
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