I noticed sparks coming out of the jack-connector as soon as I plug it in (9V) while running the Arduino with USB. As I use the Arduino mega 2560 for an university-project I want to be sure if this can harm the board.
Do I have to be concerned?
Can the Arduino be powered in parallel? Do I need to power the jack before plugging the Arduino to USB?
I am sourcing the jack with 9V/1A. May I want to reduce the current?
If not, is there an easy work-around to automatically select between these two sources?
First off, I think you should be concerned. With my experience, this can harm the board. However, I use an Arduino Uno, which can only take 5v power. I once tried a 9v power supply and almost killed my ATmega. I think you should be concerned, but I have no suggestions otherwise.
I think there's a misunderstanding. I am talking about the rounded 2.1mm jack-input soldered on the board. According to the arduino reference (like so: http://arduino.cc/en/Main/ArduinoBoardUno#.UxsLlGd6el4) it's recommended to use the jack-inputs between 7V and 12V. So I use 9V.
I wonder if it's possible to have these 9V plugged in and connect it to an USB or vice versa. The board is working nicely when I do. I can just see sparks coming out of the plug when the plug touches the jack the first time during USB-mode. And this is something I want to avoid.