Hello,
I want to power my Arduino Mega using a 9V battery (barrel jack).
No I bought one of these (including wires on the right side) 
But what is the + and -? I mean there is a connection inside the connector and on the outside (tip). I don't want to fry my board.....
Another thing. I already have another battery pack (separate motor control). The battery pack and the Mega have a common ground. But should the 9V battery also have a common ground?
/me
On the arduino external power jack the inside contact is + and the outside is -.
thanks
Another thing. I already have another battery pack (separate motor control). The battery pack and the Mega have a common ground. But should the 9V battery also have a common ground?
The battery pack and the Mega have a common ground. But should the 9V battery also have a common ground?
The external power connector on the Arduino board will already have made the common ground connection to the ground pins on the Arduino.
I think you find that battery life will be very poor using standard 9vdc battery to power the Arduino board, they have very poor MAH ratings and are quite expensive for the amount of energy they contain.
Lefty
they have very poor MAH ratings and are quite expensive for the amount of energy they contain.
Agreed. I was looking at a Duracell 9V battery datasheet the other day, and it looks like it could only supply about 500 mAH of power at a usable voltage for the Arduino.
And just one costs $3.50 at my local grocery store (which, admittedly, is about the worst place to buy batteries ;D).
Re the barrel jack, centre pin is usually the +, but always test your plug before connecting for the first time.
Also, you can see if it's center positive/negative by a label on the back of all (that I've seen) AC -> DC converters ("wall-warts").