Fan Smoother

This board impvoes the performance of 3D printer fans that are controlled by PWM power. It converts the PWM power to proportional DC voltage that is more compatible with the requirements of brushless DC fans. It's benefits:

1. Smoother fan operation.

2. Allows to use the fans also at lower PWM percentage range.

3. Allows to operate fans that don't well otherwise with PWM power.

The board has tree indepdenet channels for up to three fans and is intended for 12VDC or 24VDC fans that consume 300ma at most. To take advange of the smoother, sets the PWM frequency of your controller for at least 30Khz and higher if possible, the higher the PWM frequency, the better the filtering is.

This design is based on a circuit suggestion by @dc42 in the Duet forums but is not specific to Duet RepRap controller.