Madshrimps posted a guide about building a 300W Fan Controller from an ATX PSU

What do you if you have more than 20x high speed fans in your PC and
want to control them through an easy front-end? Build your own fan controller of course! In this tutorial we show you how to convert that old 300W ATX PSU into the most powerful rheobus you have seen.

Building a 300W Fan Controller from an ATX PSU