I have a few DC power jacks of various sizes and shapes with 3 pins, 2 of them are connected when not plugged in, 1 is isolated for positive. When a plug is plugged into the jack usually the 2 that were connected disconnect, one goes to negative, the other - which I will call the third pin - isn't connected to anything. This is true for all power jacks and none of them show voltage in the third pin except for one.
That's strange, where is this voltage coming from to the third pin if the pin is disconnected? Is this something to worry about and should I get a new adapter?
This is a picture of the adapter that shows voltage, I only have one piece:
5.5mm x 2.5mm DC Power Jack