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I have 2 - 12VDC fans (the same kind that ventilate a tower pc). I want to vary the speed of the fans to change the cfm output. Is there a simple circuit I can use, or is it as simple as a var resistor, since the current draw is so low?
Most of those fans use hall-effect sensors embedded in the stator to sense the rotor position in order to switch the stator windings. They do not work at all if the voltage is reduced below a fixed threshold. I think you will have to obtain special fans which are designed for speed control.
Hook your existing fans to a variable bench power supply and see what happens.
Just drop the input voltage. I have yet to find a 12 volt computer fan that wouldn't spin at less than 3 volts. Just use a three pin adjustable voltage regulator (LM317 or similar) to change the fans input voltage. Three components and your good to go.
I have seen and tried the PWM method and very few fans I have like it.
Linear voltage regulation is better suited to small fan operations.
And yes its less efficient than PWM or SMPS but if you cant spare an extra watt you shouldn't be using this size or type of fans in the first place!