Some units do not have an internal circuitry system that makes them functional, so an external power supply, or adapter, is necessary to provide power. Many external power supplies come enclosed in what looks like an oversized AC/DC adapter. Some of the other names for them include plug pack, adapter block, plug-in adapter, domestic mains adapter, power adapter, power brick and wall-cube.

An external power supply unit can be used with electronic components that have a battery. When a battery powers an electronic item on a regular basis, you can use the adapter to help connect with the unit and provide the necessary power. A power supply unit works well with equipment that has no power source. When this is the case, the unit can be connected from the equipment into an electrical wall socket.

Laptop computers often are accompanied by an AC adapter that can be used to power their battery. Laptops usually operate with a battery power pack, but when the battery has expired you can use an external power supply like an AC adapter to power up the laptop. The external power supply will charge up the battery while the computer is in use.

Many companies that make electronics like laptops, cell phones, iPads and iPods, and other battery-powered electronic equipment usually include an AC adapter with their products. The batteries on these units usually expire after a certain length of time and need to be recharged, but an external power supply unit allows the user to keep using the electronic device while charging up the battery. Thus, the external power supply unit has become a valuable piece of equipment for users who depend heavily on modern technology.

External power supplies were first created for use by the military, but it was soon discovered that an AC adapter would be beneficial to use with all the electronic devices we have today. Some of the adapters are small with just a small plug-in, while others can be large with a larger plug-in. The size of the adapters depends on the electronic device for which they are intended.

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