How does an AC DC transformer work?
I think what the question refers to are the small plug in power-supplies used to charge DC devices. The primary side of the transformer is the side that plugs into the wall. The secondary side of this transformer is still AC but reduced to the voltage that is required by the device that it plugs into. At the lower voltage side of the transformer it is connected into a half wave bridge or a full wave bridge rectifier. This rectifier changes the AC into DC. This rectifier is contained in the same housing as the step down transformer. From the output of the rectifier, a cord with a plug on its end is attached that connects into your device.
