Originally Posted By: mlord
The Arduino Mega schematic has some circuitry around the Vin/USB parts that looks like it is intended to cut out the USB power when powered from Vin. So it _might_ actually be safe as-is.


Yeah, I think it is fine. I'm powering the Arduino with +9V via the black power jack (aka. "Vin" plus a diode), and I've added some mechanical relays onto the +5V output to act as a significant load (about 0.22amps total at 5V). All with the Betz board powered from the 3.3V output of the Arduino.

Then.. connected +5V to the USB connector through my voltage/current measuring dongle to look for current flowing there. It reads 0.01amps, which is effectively ZERO here.

So, yeah, with sufficient voltage on Vin, the USB connection appears to be safe. Which kinda makes sense I suppose, given the intended customer base for these things. smile

EDIT: Note that if you connect to the +5V pin on the Arduino rather than to Vin, it will probably power just fine at 5V. But this also bypasses the USB safety circuit, so I don't advise it for this case.





Edited by mlord (11/01/2018 03:03)