Hi,
NOOB here, be warned 
I just build my first thingie by mixing different projects together. It is an Arduino Nano based fan control with output of fan sensor values to a 16x2 LCD.
The fan must run on 12v, therefore I use a BEC to power the arduino with 5v. Everything is working fine but I can't upload code to the arduino if the 12v battery is not connected. So something is "stealing" power if I only connect USB.
Two questions here:
- is there a danger of drawing to much power from USB and damaging my computer?
- what can I do to use the arduino only with USB power?
I documented the wiring in the attachment. I would be glad to get some tips to improve that.
Greetz,
darktim
FanControl.ino (6.92 KB)