SMPS IS Switch Mode Power Supply. The primary function of Smps is to convert the alternating current (AC) power available in homes into direct current (DC) required for a computer system. In desktop computers, you will find a metal box in the corner of the CPU case supplies power to various components in the CPU box. The power supply converts 115-230 volt AC into DC that is required for computer components to work.
AC stands for alternating current. It is an electric current whose magnitude and direction vary cyclically.
DC stands for direct current. It is considered as the constant flow of electrons in a single direction from low to high potential. Computers use DC power. Because of the advantage of alternating current over direct current in transforming and transmission.



The SMPS or power supply of a computer comes in different industry standard. The commonly used SMPS are AT and ATX. There are also other SMPS standard like LPX, SFX... etc. But we will describe about ATX SMPS.

In AT SMPS system we have to manually Shutdown the computer. After giving Shutdown command “It is now safe to turn off your system” message appears then press the power button to shut down. In ATX SMPS shutdown is automatic, that is just give the Shutdown command.
The main power connectors in a SMPS are as follows.

TO CHECK WETHER SMPS IS WORKING OR NOT

Take a small piece of wire and connect one end in GREEN wire port and other in the BLACK. Switch ON the supply if the fan in the smps is working the smps is good. If not open the cover and check whether the Capacitors inside the Smps is bulged on the top. If bulged replace the capacitor with the same value of the earlier.Check the voltages with a multimeter.

A Good ATX SMPS wires gives the following voltage as the output. And work for different purpose

WIRE         VOLTAGE      FUNCTION
       
Black                 0                  Ground
Blue                 -12                 Serial Data
Brown/Violet    +3.3               Sensor
Gray                  +5                  Power good supply
Green                +5                  Power supply on/off
Orange              +3.3               D Ram Refreshing
Purple               +5                  Stored by voltage
White                -5                   Refreshing Ram
Yellow              +12                Motor Rotation
Red                  +5                   Logic circuit

In case of a defect or problem with power supply, it is not recommended that an inexperienced user open a power supply for repairing. The power supply has dangerously high voltage. Even after unplugging, power supplies can retain dangerous voltage and must be discharged (like a monitor) before servicing it. These internal repairs are not recommended unless the technician is specially trained to repair such components.