PWM Fan Controller Design Based on 555 Timer
I designed three slightly different PWM fan controllers based on the 555 timer, used to control a large server fan – Delta FFB1212EHE, with a peak current of 3A and a normal load of 2A. The other features of these controllers include: boosting the fan to 100% for the first few seconds after power-on, and a “turbo” button to set the fan speed to 100% while pressed.
Version 1 (July 2010) of the controller did not work as expected during startup, staying at 0% instead of 100%. Another problem was that it would occasionally slow down to almost stopping, and I had to reset it to recover normal operation. I don’t know what caused this problem, but it seems to have no effect on later versions.
Version 2 (January 2011) fixed the startup boost problem by using a PNP output transistor. I changed the startup boost timer to reduce component count, but now the timer capacitor has a slow discharge time. Because of this, turning the controller off and back on too quickly would bypass the startup boost, as the capacitor voltage would still be above the 555 reset voltage threshold (~0.7V).
Version 3 (February 2011) is the same as Version 1, but with a PNP output transistor instead of …
