Power supply unit (computer) A power supply unit (PSU) converts
mains AC to low-voltage regulated DC power for the internal
components of a computer. Modern personal computers universally
use a switched-mode power supply. Some power supplies have a
manual selector for input voltage, while others automatically
adapt to the supply voltage. Most modern desktop personal
computer power supplies conform to the ATX specification, which
includes form factor and voltage tolerances. While an ATX power
supply is connected to the mains supply, it always provides a 5
V standby (5VSB) voltage so that the standby functions on the
computer and certain peripherals are powered. ATX power supplies
are turned on and off by a signal from the motherboard. They
also provide a signal to the motherboard to indicate when the DC
voltages are in spec, so that the computer is able to safely
power up and boot. The most recent ATX PSU standard is version
2.31 of mid-2008. The desktop computer power supply changes
alternating current from a