- an Arduino UNO
- a 12V PC fan with 1.68W rating (140ma).
- 2n3904 transistor
- 12V 500ma power supply to connect to Vin.
What I would like to do is to simply use a digital output pin on the Arduino to turn the fan on and off (no need to PWM). What resistor should I use at the base of the transistor and in series with the fan if I want to switch the fan on and off using this transistor?
Without the transistor I see that a 100-ohm resistor seems to power the fan nicely, but in conjunction with the transistor and 100-ohm it doesn't run. My guess is that is might be due to lack of proper current management on my part of the base of the transistor and that I'll need to take into consideration the current gain.