The idea for this came from http://www.cpemma.co.uk/sdiodes.html NOW http://www.pcsilencioso.com/cpemma/sdiodes.html and I wanted to explain it a little more and build a few of them. Basically a diode type 1N4001 drops around 0.75 volts with a typical fan. When you link them together then the voltage can be varied.
This is taken from the site
Using a chain of diodes mounted on a rotary switch, a series of voltage steps can be produced giving adequate control for fans.
This control method has several advantages;
- Cost is less than that of a half-decent rheostat, and much less than the price of a good one.
- number of fans wired in parallel (or to a fanbus) can be controlled from the unit without danger of overload. Cheap rotary switches will handle 1amp at low voltage, rheostats to handle such currents are big and expensive.
- The full 12V can be used if required. Basic semi-conductor voltage regulators lose about 2V and would need a by-passing switch to run the fans at full speed.
- The heat produced is spread over a number of diodes. Each one will only generate about a watt at the maximum 1amp load, and less than 0.1W per diode with a typical single fan.
- Unlike resistors, voltage drop is about the same at all fan currents, so fan start-up at low voltages is more assured and a wide range of fans can be used on the same unit.
- Last but not least, adding display lights to form a voltage bargraph or show switch position is easy.
I did one of the switches as a speed up video.