Hey, so I followed this thread (https://forum.arduino.cc/index.php?topic=653252.0) because I have the same fan (5V, 0.2A) and I want to power it from an Arduino.
I want to change the transistor into something more common (like BC337) and want to calculate the required base resistor.
I found the DC current gain is 10 (Ic/Ib at VCEsat=0.7V is 10)
The base current is: Ib=0.2A/10=0.02A
To calculate the base resistor Rb, the supply voltage 5V is subtracted by 10% of 5V as tolerance and 0.7V as the drop over Base to Emitter.
So Rb = (5V-0.1*5V-0.7V)/0.02A = 190 Ohm
Is this calculation correct?