DC barrel socket with 3 contacts question

Using Arduino General Electronics
1

So, here's a simple one for you guys... can someone explain to me what the 3 contacts are for in a 3-contact DC barrel socket? I know one is hot (+) and one is for ground (-)... just not sure how the other comes into play.

Thanks!

2

It’s a switched contact. The two live, other than the earth, form a connection.

3

There is a third switched ground pin. It is normally closed with nothing plugged into the jack, that allows one to have an internal battery powering the circuit but upon inserting the external power plug the batteries ground gets disconnected from the circuit ground.

Here is a decent datasheet page of a typical barrel power connector from SparkFun:

Lefty

4

Thanks for the explanations!!!