I'm powering my Arduino clone with an LM317 circuit that regulates Vin voltage to 10V (measured with a multimeter). While the Arduino is powered by USB only, everything works. When it's powered using Vin, I get odd behavior. I then tried plugging in the USB to see what was happening via the serial monitor and my laptop shut down. I read the Arduino is supposed to handle both USB and Vin simultaneously without shorting out, but now I'm scared to try it again. Is it safe to try again? Any idea what would cause my laptop to shut down, or what I could be doing wrong? Thanks.
It should be fine if you don't have Vin less than 6V6. There is a circut that compares half of Vin against 3V3 "reference" and if it's less, then it enables "ideal diode" switch from Vusb (but it also allows backfeeding) Another way how to bakcfeed usb from the Arduinos 5V is short 3V3 to 5V and having 9V on Vin (eg. less than 10V).
However backfeeding USB port shouldn't be causing restarts. Maybe there are a power supply issues and you've got a big voltage potential difference before connecting it into the USB? You can try to measure it