Variable Resistor

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leesichello

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Hey guys. Im currently running two 12 Volt DC fans in parrallel (one is .25A and the other is .44A). They are hooked up to a AC DC transformer with 12 Volt DC output rated for 1.5A. I'd like to put a variable resistor in the circuit to control the speed of the fans but I'm not sure what kind to use or if it is okay to run the fans at less than 12 Volts. I plan on the circuit looking like the thumbnail below. Where can i find the resistor i'll need and is this okay?
 

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It's a circuit that controls the pulse width.

If you connect a series variable resistor, you'll vary the voltage over the fans, but they will lose "torque".

So it's better to switch them on and off very fast, so that they will always be feed with 12V.
To vary the speed, you'll control the ON time (duty cycle), the shorter the ON time is, lesser the speed is.
This way, you only vary the average power, but the instantaneous power will be always the same.
 
If you connect two variable resistors to a NE555 chip and like the output to a transistor you should be able to create a PWM unit.
 
Here's a much simpler circuit, instead of the MOSFET, use a normal NPN transistor with a 180R base resistor.
 
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