I found a very funny and informative series of videos on YouTube that go over some important electronics principles.
Here is the PWM video: http://www.youtube.com/watch?v=Lf7JJAAZxEU
Here is a playlist of videos by the same guy where he goes over some important electronic hardware like diodes, resistors and transistors. http://www.youtube.com/watch?v=-mHLvtGjum4&feature=list_related&playnext=1&list=SP970BF3F6D77B12E8
Here is another playlist by the same guy: http://www.youtube.com/watch?v=aOJuCYgmPPE&list=PLDE23FAC8A681FA46&feature=plpp_play_all
I really enjoined these videos and I think you guys will too.
What is described in that video is but one electronic principal referred to as pulse width modulation.
A better term for what is described in that video is variable duty cycle. The amount of power supplied in that case is achieved by varying the duty cycle (the ratio of on to off time) of the power supply. As noted in the video if you supply a 12v signal with a duty cycle of 50% you will see the same result as providing a constant 6v signal. Provide a 12v supply at a 25% duty cycle and the result would be the same as providing a constant 3v signal. The actual pulse width is basically irrelevant as long as the “off” time is less than the reaction time of the device. IE 1ms (millisecond) on and 1ms off will give the same result as 2ms on and 2ms off as both are “on” 50% of the time and “off” 50% of the time.
In FIRST terminology what is used to provide the control signal to the motor controllers not operating in CAN mode is also commonly called PWM but it is actually VPW or Variable Pulse Width. In this application the width of the pulse is what carries the data. Because the frequency of the pulses is constant this also results in a varying duty cycle, but it is the length of the pulse that carries the data. This method is what is also used in the Automotive industry by some mfgs to carry the On-Board Diagnostic info to a “scan tool”.
Another definition of PWM is also used in the automotive industry. In this case it is used to drive a high power consumption device such as low resistance fuel injectors with a low current driver (transistor) and prevent overheating of both the driver (transistor) and the actuator (injector). For example you may want an injector open time of 8ms to give the desired amount of fuel. Because of the reaction time of the injector you can keep it open for that entire 8ms duration by providing an initial pulse of say 1ms to get the injector fully open and then switch to a duty cycle of as low as 20% (.2ms on and .8ms off) to keep the injector fully open for the rest of the pulse. This is also sometimes referred to as Peak and Hold or PNH in the automotive industry.