posted on Feb, 21 2009 @ 07:39 PM
These circuits are mostly used to control DC motors. Their benefit is that you can maintain high current and voltage to the motor while controlling
its speed so that the motor maintains adequate torque at low rpm's compared to just lowering the voltage to the motor, which would also reduce the
current to the motor, and the motor's torque would also drop.
A good example of this can be found in electric RC airplanes and RC cars.
Technically a DC motor isnt actually DC at all. The armature windings and their contacts are arranged in such a way so that when the brushes make
contact with the armature winding contacts on the shaft, they create a swtiched DC, or a false AC, thus making the armature windings change polarity
and push against the permananet magnet, thus creating the rotation of the armature.
This switching effect can be seen by connecting a DC motor to an oscilliscope and turning the armature by hand and watch the pulsing effect from zero
line to the positive swing and negative swing.
Early PWM circuits used simple 555 timer chips with a varying resistance on the timer input, and the output connected to a high current drain
transistor or FET that connected to the motor. Today's PWM circuits are more complex and can adjust both the pulse width time domain as well as how
much pulse current they output.
[edit on 21-2-2009 by RFBurns]