A Barrel Jack Adapter is cable that power to on-board PCBs and can adapt from many different style connectors. For example, a USB port to a 5.5x2.1mm barrel connector, center positive or a 9V battery connector to a 5.5x2.1mm, center-positive barrel jack. A standard PCB-mountable barrel jack for DC power adapters with 2.1 mm center holes. This jack is compatible with our wall-adapter power supplies: