The AC adapter, AC/DC adapter or AC/DC converter is a type of external power supply, often enclosed in a case similar to an AC plug. Other names include plug pack, plug-in adapter, adapter block, domestic mains adapter, line power adapter, or power adapter. AC adapters are used with electrical devices that require power but do not contain internal components to derive the required voltage and power from mains power. The internal circuitry of an external power supply is very similar to the design that would be used for a built-in or internal supply.
External power supplies are used both with equipment with no other source of power, and with battery-powered equipment, where the supply, when plugged in, can sometimes charge the battery in addition to powering the equipment.
Use of an external power supply allows portability of battery-powered equipment without the added bulk of internal power components, and makes it unnecessary to produce equipment for use with a specified power source.