PWM variable control of lights and motors
Pulse Width Modulation (PWM) as it is used in electronics is the greatest thing since sliced bread.
What it allows us to do is regulate the flow of energy with out wasting so much of it. In the early days if you had a 12 volt battery and you wanted to drive a 6 volt lamp you could use a transistor to regulate the voltage down to 6 volts but half of the energy was wasted as heat in the transistor. PWM uses the adveatage of time. A 6 volt light buld or a motor or many other devices can actually run at 12 volts just not for a long time. So we take advantage of that by pulsing. For instance, having the lamp on for 1/2 of a second and then off for 1/2 of a second would be like delivering 6 volts for one second. Of course you could call that blinking but if you speed that up about 100 times and do it at a frequency of 100 Hertz it’s so fast you can’t see the blink. Whats more the filiment of the light buld doesn’t have time to heat up to the 12 volt level so it dosen’t know the difference eather. It thinks its running at 6 volts. Caution - results may vary depending on the type of lamp your using but negative effects can be reduced by adjusting frequency and duty cycle (on and off time).
What about the transistor? Yes there is still a transistor involved but idealy it is working just like a switch. When it’s off there is no current flow and no power lost. And when it’s on all the current goes through it to the lamp and none is lost in the transistor (switch).
Here are some cleaver links I found that might help.
http://www.cpemma.co.uk/pwm.html