A PC Power Supply Unit (PSU) can accept the voltage of 110-120 VAC (Volts of Alternating Current), or 220-240 VAC. PSU's operation can be adjusted to the supplied voltage either with the use of a manual selector switch on the back of the device or automatically by the device. PSUs equipped with manual selector switch are referred to as fixed-input devices, PSUs offering automatic voltage adjustment are known as auto-switching PSUs.