In reply to:

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Hmm, I can't understand what you were trying to do with the 7805 in that circuit...theoretically your fan could only get 5V max..

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You can use a fixed voltage regulator as an adjustable one by varying the voltage at the reference input just like you do with an adjustable one. It probably doesn't function as well as one which is has been designed to be adjustable but it works well enough for this application.

The Transistor was supposed to remain off until it's Gate voltage had reached a certain value (650mV I think) and then turn the fan on. This would prevent the situation where the fan isn't quite getting enough voltage to start it properly and it sits there oscillating back and forth.

Using the remaining Op-Amps in parallel as a current driver sounds like a good idea and is something I hadn't thought of. I'll look into that. Thanks for the tip .