CompTIA A+® Certification All-In-One For Dummies®, 2nd Edition

6.2. Identifying the Purpose of Power Supply Units

NOTE

Remember the tasks and specifications listed in this section.

A computer's power supply is responsible for several tasks. The following list contains typical tasks and specifications related to power supplies:

• Conversion of voltage from the building-supplied AC voltage to various DC voltages used by components inside the computer.

• DC voltage regulation to provide a very small tolerance of error to the devices receiving the power. Typically, a +/−5% voltage differential is allowed through the specification for 12VDC, 5VDC, and 3.3VDC connections.

• Over-current protection specifies that the power supply should handle some level of abnormally high current to prevent fuses in the power supply from blowing unnecessarily.

• Input-under voltage specifies that if the voltage being supplied by the building drops below the normal operating level, it should not cause damage to the power supply itself. It will, however, likely cause your computer to turn off. For a 120V power supply, this low voltage level is usually 100V; so, at voltages below that level, the power supply will simply turn off.

• Energy Star is a specification that defines how much power the power supply should provide to the computer components during reduced power states, such as when the computer is put to sleep or on standby.

• The PS_ON# feature allows the motherboard to control the power supply though a soft power switch. Power supplies for ATX motherboards do not use a physical ...