Active Cooling 5v brushless fan with Motor Speed Controller soft PWM

[MarkHaysHarris777]

Pardon my ignorance regarding electronics, but why do you need a transistor? Is it because the fan needs 5V but the GPIO is only 3.3? Also why is a resistor required?

Please Read through this Link:

hi Chris, the transistor is needed as a current switch. The GPIO being used as a pwm pin is 3v3 on|off with a duty cycle to control the fan motor speed.  The fan brushless motor requires 5v period.  So, we are using a transistor (controlled by a 3v3 pwm signal) to switch on|off a 5v 400ma current with a duty cycle which slows the motor speed.

The 3v3 pwm signal on the base of the transistor switches on|off the motor current which has an EMF of 5v, and is being limited (collector emitter) somewhere between 200ma and 300ma. The current limiting resistor (ballast resistor) keeps the 3v3 pwm signal current (at the base) quite low just a few milli amps. Failing to put the base-emitter limiting resistor in the circuit has the same bad consequence for the transistor that trying to drive an LED without a ballast resistor has--- blown transistor.  Consequently the GPIO current is very low.  The motor current is relatively high at 5v,  and the GPIO signal voltage is 3v3 (its limit) with a relatively small current.  

The transistor-resistor combination is the simplest of the "driver" circuits (or switches) for controlling a relatively high load with a relatively low signal source.

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Please Read through this Link:

The explanation in the link is pretty clear ; with diagrams, for the theory behind transistor switching.