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A power supply unit (PSU) is a type of power converter that provides direct current (DC) voltage to internal computer components. Some of these units are designed specifically for use with 110 or 230 volts alternating current (AC), while others can be switched between the two or even accept any voltage within that range. The level of direct current voltage that they provide can also vary, typically between 3 and 12 volts DC. Desktop and laptop computers both use power supply units, though they are somewhat different in design. A desktop power supply unit usually has several bundles of wires designed to hook up directly to various internal components, while laptop "power bricks" are external units that typically utilize coaxial power connectors.
The most common type of power supply unit is designed specifically for use with the advanced technology extended (ATX) form factor. These ATX power supply units are similar to the earlier advanced technology (AT) units, aside from the way they connect to motherboards and power switches. An ATX power supply unit is a self-contained box that bolts inside a computer case and converts AC line voltage to DC. Several different DC voltages are usually provided due to the common needs of computer hardware. It is also possible for a power supply unit to provide some level of voltage regulation to protect against surges or spikes that could damage the internal components.
Desktop power supply units are typically rated based on the amount of power they can provide. This figure is usually given in watts, though there are other factors to consider. Some power supply units have multiple rails, which are different groups of wires that use separate current-limiting circuitry within the PSU. Each rail still uses the same power source, but the load can be split to prevent the chance of a failure in the wires. Power supply units that have multiple 12 volt rails are typically used in high-end systems that have significant power demands.
Laptops also use power supply units, though they are somewhat different in design. Most of these power supply units are external power converters that take AC line voltage and transform it into 12 volts DC. The DC voltage is then provided to the laptop via a coaxial power connection, which usually takes the form of a generic barrel connector. A variety of proprietary designs are also used by various manufacturers. Most laptops also have additional internal circuitry that is capable of converting the supplied 12 volts into the lower voltages required by various components.