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What happens when a computer power supply is turned on PDF Print E-mail
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1. When the computer's Power Supply is plugged into power socket, there is a 5 volt Standby power present for the power supply at all times.
This 5 Volt Standby power is what activates or turns on the Power Supply.When the "power on" button is pressed.
The "power on" switch located on the computer chassi, is a Momentary Contact Switch


2. Power Supply activates, sends power to the motherboard.
The LED light on the motherboard, indicates the motherboard is receiving power. Note this has nothing to do with the CMOS battery located on the motherboard


3.The first chip to receive power is the BIOS chip. This Chip is a slang terms for I.C.(Integrated Circuit)
Power to the BIOS chip initializes the BIOS program, which is burned into the chip.
BIOS program looks to see what devices are installed, does a Ram Memory count, turns the Processor on, and hands the computer over to the OS (Operating System, Windows XP and Windows 7 are examples of OS)


Useful Info: All the LED lights use less than 1 Watt of power. These include the Power On LED, the LED on the motherboard, and the LED light on the Power Supply etc.

  • Each fan uses 2 to 3 Watts.
  • A typical Processor uses 51 to 125 Watts all are DC volts
  • The SMPS (Switched-Mode Power Supply) in your computer produces three main Voltages.

> 3.3 Volts wires with Orange insulation
> 5 Volts wires with Red insulation
> 12 Volts wires with Yellow insulation


Click here to see power wire from the SMPS