What is a PSU (Power supply unit)?

 The PSU(Power supply unit) is the device responsible for converting household alternating current (AC) into direct current (DC). It is also responsible for filtering and delivering the different tensions with which the computer components work. (Some circuits need 3.3 V; others 5V, and others 12.)

  The modern and most commonly used sources are of the ATX type, which replaced the AT model.