A power supply receives electrical energy from a source (usually a wall outlet or a battery). It reformats this electricity for use by a computer, monitor, printer, or similar device. It also provides a secondary function of "cleaning" the incoming AC power by smoothing noise on the line, such as short-term voltage spikes or sags. Screening these line variations is important, so that the cleanest and most consistent power is provided to the equipment.
Power supplies can be internal or external. An internal power supply is attached to the inside of the unit. An example of this is the power supply in a personal computer. An external power supply sits outside of the unit, such as the "brick" power supply commonly used with notebook PCs. Using an external power supply unit allows the computer or peripheral device to be physically smaller, since there is no longer a reason to leave space for it inside of the cabinet. An example is the thick power adapter, such as those provided with a cell phone charger or external speakers. Power adapters are very high-efficiency power supplies. Although they may sometimes get warm, they remain cool without a fan.
Power supply design is tuned to the equipment that it supports. Server power supplies are unique to the manufacturer and are designed specifically for a particular model of server. Most data center servers are supported by dual power supply units. Desktop computer assemblers usually use an ATX sized power supply, so that it will fit into most computer chassis. ATX is an industry standard chassis size; it also standardizes the color code for output connectors.
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