On the back side of newer DCUs (Digital Control Units) are two different sockets to accept a DC power supply:
1) The smaller socket is a 3.5mm OD socket, center positive. This socket was designed to fit the standard 5-volt LabQuest Power Supply, order code LQ-PS, $11. Another option to use with this socket would be a LEGO 10-volt, center-positive power supply (used with EV3 Robotic System).
2) The larger socket is a 5.5mm OD socket, center negative. This socket will fit the 6-volt LabPro (Vernier AC Adapter, order code IPS, $12)or 9-volt ULI power supply.
You can also connect a non-Vernier power supply or a battery power supply (one lantern battery or four to eight 1.5V cells connected in series) to either socket, as long as you follow the polarity guides for the sockets and do not exceed 12V DC.
Without a power supply connected to the DCU, it will draw its power from the interface to which it is connected. While the interface may be able to provide enough current to power a small electronic component like an LED, it will not be able to power larger electronics, like a motor. Be sure to always connect a power supply to your DCU before using it!