Originally Posted By: mlord
Power the empeg from Vin, and measure with a voltmeter the voltage at one of the +5V pins on the Arduino.

3.9V on my setup. Not quite enough for reliable operation of the Arduino, which wants 4.75V to 5.25V. Interestingly, this was enough for the Arduino's onboard 3.3V PSU to output a full 3.3V.

Quote:
Then, plug in a laptop over USB (with Vin still connected), and measure it again.

Here, the measured voltage went up to 4.8V. Then, I connected an inline current measuring device on the Vin line. Current draw was about 0.06A, until I connected the "laptop" USB in parallel, at which point current draw doubled to 0.12A. Which means it is very likely back-feeding some current to the laptop (not good).

This was expected, as one can measure voltage as "present" at the USB connector with Vin connected and nothing else. So it definitely is going to try and harm any PC that gets plugged in.

Note: I did not actually risk a laptop, but rather used a USB power dongle instead.

Arduino Mega reference design schematic is here:
https://www.arduino.cc/en/uploads/Main/arduino-mega2560-schematic.pdf
The boards we have may or may not match this schematic. smile


Edited by mlord (10/01/2018 15:17)