I've started out very simple to get going, since I do need a solder fume extraction fan right now, but also to test out some of the things in my comfort zone, putting a bit of throw-away software together to get things rolling. My current hack uses a basic Arduino skeleton, poking directly into the avr registers to get the target pwm frequency of 25kHz. It mostly works but falls down at very low/high duty cycles, since in the current pwm mode I need to update the threshold value in the interrupt handler at each counter overflow. But that's OK since I'm just testing things out. I'm currently running the fan directly off of 12V from a lab supply and the arduino from USB.

I also had some fun with a serial based UI where you can control the pwm speed with the arrow keys and get a nice volume meter style gauge to show you the current duty cycle setting.

Right now I've started experimenting with openscad to make a simple, printable design to replace the cardboard and tape prototype.