5v power on USB and 5v PIN

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Fox95

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This may be a dumb question....

if I have the teensy plugged into a usb cable and its plugged into my PC usb port for programming with arduino IDE and at the same time a seperate external 5v power supply is also supplying 5v to the Vin pin will that damage anything?
 
It probably won't damage anything -- the '5 V' will rise to the highest of the USB voltage and the external supply. In the worst case, the USB power supply will current limit or shutdown (in some MacBooks, you'll have to reboot to reset it). Most power supplies can handle their output being pulled above the setpoint. If you have a very high current power supply, it could end up back feeding the other loads on the USB (including ones inside the PC) and possibly overheating it.

I wouldn't recommend it, but brief (few seconds) overlaps should not be a problem. Note on Teensy there is a cuttable (& re-solderable) jumper to isolate these -- the 2 small rectangles at the top left of each schematic: https://www.pjrc.com/teensy/schematic.html . With that cut, you have to provide an external supply.
 
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