Tuesday, June 24, 2008



It turns out that creating a variable voltage at more than a few hundred milliamps efficiently from a power supply is not that easy. Pulse Width Modulation (PWM) is a common technique for using simple on-off switching to produce the effect of a truly variable voltage. Motors, lights and a large variety of other devices can be controlled in this way. House light dimmers work the same way.

What happens is that instead of a device either solidly on or off, it is switched on and off many times a second. In the diagram below, voltage is plotted against time. The waveform in gray shows the rapid switching output of the PWM circuit, and the effective voltage is shaded in as an overlay.

PWM Concept Diagram

On the left portion of the diagram, the power is only on for a small amount of time, so the effective voltage the device under control experiences is very small. In the mid portion, the power is on for half of the total time, so the controlled device experiences approximately half the full voltage. Finally in the right portion of the diagram, the power is on for all the time, each cycle, in fact it never goes off at all, and so the voltage the device experiences is the full value of the supply.

The portion of each cycle for which the power is switched on is called the duty cycle, and is often expressed as a percentage ranging from 0% to 100%.

The effect this has on lights is like a dimmer - the light glows dimly and brightly depending on the duty cycle. If this technique is applied to a normal DC brushed motor, the motor speeds up and slows down.

No comments: