[Michael Kleinigger] posted a lengthy discussion on Pulse-Width Modulation that goes beyond the traditional beginner tutorial. He starts a bit of background info on PWM and a tip about using a camera to judge frequency and duty cycle of LEDs. From there it’s down the rabbit hole with some testing of power-loss versus frequency.

When you change from frequencies of 50 Hz to 1 MHz how does the parasitic power loss from switching affect the overall efficiency of the circuit? It turns out there’s a rather large amount of loss at the highest level, around 1.5 mW. The greatest balance of low power loss and elimination of flicker seems to be right in the 300-500 Hz range.

Pictured above is a Parasitic Wind Turbine built as an experiment to see if any useable energy could be captured from a standard home air conditioning unit. Although not listed on the website, the 6 phase 90 V 4.0 the motor used here is available from Skycraft or auction websites like eBay. Using the rotor of a standard box fan and placing it at a 45 degree angle yielded the best RPM.

Lastly, these applications of parasitic power remind us of an art project by Michael Rakowitz from 1998. Rakowitz created temporary homeless shelters that were inflated by a building exhaust vents. The air flow from these vents provided both structure and heating for the shelters. They also served to draw attention to the homeless housing situation and encourage cities to think of new ways to address the problem.