How to limit current in order to power small car radiator fan with laptop charger?
Is there a reason you're not controlling it via PWM? The main argument against is the complexity, but would give you the effective reduction of voltage you're after. Though there are potential issues with using PWM on an input to (what I assume is) a DC brush-less motor, you may want to end up smoothing it out with capacitance, but then it becomes a DC-DC converter...

Alternatively, as your current plan is to change the voltage across the motor, it isn't insane to considering using a DC-DC converter with a potentiometer on the voltage setting pin.

Third option would be to put a known fixed resistor in series with the fan, and a lower rated pot across the pins of the motor.



So R1 is a high wattage (100W), low resistance device, R2 is a high resistance POT (you should really put a high value resistor to go in series with the POT to make sure you can't set it too low). But his is a pretty wasteful way of controlling the fan, burning energy in the resistors rather that switching off as you would with a switched DC-DC or PWM option.