The Apple II, also introduced in 1977, was noted for its switched-mode power supply, which was lighter and smaller than an equivalent linear power supply would have been, and which had no cooling fan. This standby voltage may be generated by a small linear power supply inside the unit or a switching power supply, sharing some components with the main unit to save cost and energy.įirst-generation microcomputer and home computer power supply units used a heavy step-down transformer and a linear power supply, as used, in for example, the Commodore PET introduced in 1977. Standby power allows a computer to be started remotely via wake-on-LAN and Wake-on-ring or locally via Keyboard Power ON (KBPO) if the motherboard supports it. Some PSUs also supply a standby voltage, so that most of the computer system can be powered off after preparing for hibernation or shutdown, and powered back on by an event. A power supply rail or voltage rail refers to a single voltage provided by a PSU. Several direct-current voltages are required, and they must be regulated with some accuracy to provide stable operation of the computer. The desktop computer power supply converts the alternating current (AC) from a wall socket of mains electricity to a low-voltage direct current (DC) to operate the motherboard, processor and peripheral devices. Internals of a PSU with passive PFC (left) and active PFC (right)