I have a simple 12V 4-pin computer fan which I can control using PWM. The fan speed does not go below 1000rpm so I am using a TIP120 to stop the fan when I need it to go below 1000rpm. I have also tried the switching off with a MOSFET and both the circuits work. I have provided both the circuits below. The issue is that by accident I unpowered the arduino supply (I was supplying it using USB) and I saw that the arduino was still powered on!!!! I guess the supply from the fan is somehow leaking to power the Arduino. Is this normal? If not, what might be happening? Thank you in advance for your help.