Hi. I have a strange problem.
I'm using my arduino to drive motor through a L298N motor driver board. The motor driver is powered from a 0-14V power supply I built at school many many years ago, set to 12V.
In order to make it work, I had to connect the GND-port on the driver to GND on the arduino.
When powering the arduino through a 9V battery pack, it works fine. However, if the arduino is connected to USB (for troubleshooting over the serial monitor), it seems to drain the external power supply.
If I turn on the power supply, it quickly builds up to 12V, but as soon as I connect the arduino to USB, it drops to ~2V.
My arduino board is a funduino.
Connections:
Digital 2/3 - optical sensor
Digital 5/6 - IN1/IN2 on the L298N board
Digital 11 - ENA on L298N
+5V/GND1 - optical sensor
GND2 - GND on L298N
I don't understand how this drains the external power supply, as the only thing connected between power supply and arduino is GND - GND.