[Runtimemicro] has the answer, at least if you are writing PWM code for the Arduino. Their free application lets you set a few parameters, visually see the results, and then generates code for you.

If you've got an Arduino of any variety, you might have noticed some of the pins on the board have a tilde mark or PWM printed next to them. What is PWM?

Lastly on that connector is the spindle output and, just like the 36V fan output, the connections are: +ve directly connected to the 36V power input and -ve modulated to 0V from the cnc domain by ...