On my PC, a delay timer fires a relay two secs after start up. Before the relay is fired, it sends 12v to my fans. When the relay engages it changes the fans supply line to one that is resistance adjusted by a pot as a speed control. This gives good starts and a low final speed. Just another way to do it...
Sounds crude but its worked that way for quite some time now. The trick is to select the right size pot for a good range of adjustment. Trial and error is fine for that since they are just a dollar each at surplus stores.
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