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From Sonic Retro

AC Adapter used for Genesis models 2 & 3, as well as the Sega Game Gear.

An AC Adapter is a simple device. It transfers current from an electrical socket (AC, or alternating current) into power usable by battery-powered devices (DC, or direct current). Many handheld devices (including most, if not all, handheld game consoles) have had some form of AC adapter released for them at one point or another.