I searched the forums without luck. Or my search was poor. At one time I think I understood why: the question is if powering the Photon at VIN with a 5 v wall wart can I simultaneously monitor Serial using micro USB from a laptop? Interactions between power supplies I’m assuming my wall wart has enough capacity to keep voltage within required range during Photon operations. It would cost me days of effort if I fried something while monitoring it. It took days to solder this together. Thanks!
You won’t damage your computer or the Photon, but you could damage the 5V supply on VIN, but probably won’t.
The reason is that there is a Schottky diode between USB power and VIN, so VIN won’t supply power to USB, which could damage your computer. Also it allows USB to power 4.9-ish volt devices from VIN.
However, since the diode is unidirectional, it does mean that if you supply power on USB, there will be 4.9-ish volts on VIN, which you are also connecting your external power supply to. A second diode could protect against that case to be safe.
Also on Gen 2 only (Photon, P1, Electron, E Series) you can use a data and GND USB cable. That doesn’t work on Gen 3 (nRF52840) devices, which require VUSB to power up the USB port.
Knowing about the diode in the Photon made it clear. I should add a diode to the wall wart wiring. Thank you.