SMPS full form means Switch Mode Power supply, the Power supply is an electrical device that delivers power to electronic devices and circuits. Most of the electronic devices require less power DC voltage as compared to the voltage supplied to the home or office. This led to the development of Switched Mode Power Supply (SMPS). This device converts from high voltage AC to Low voltage DC.

  SMPS provides power to various different system components such as the motherboard and the device drives. The key components of SMPS are power connectors and power supply fan. Power connector provides DC voltage to all the system components and power supply fan is used to keep the unit cool.

Types of SMPS

A number of SMPS available in the market depending on the size and the shape of the power supply. The different types of power supplies units are PC/XT, AT, Baby AT, LPX, ATX (NLX), SFX and WTX. Out of this PC/XT, AT and Baby-AT style are obsolete form factors whereas ATX and SFX styles are modern form factors and use ATX type power connectors. The power supply unit form factors must match the form factor of the system case and the motherboard.

Low profile Extended (LPX)

Low profile extended form factor is also known as slim Line form factor because its cases are known as slim Line cases. The main goal of this form factors is size reduction. Because of its low size and convenient rectangular shape, LPX can put in all sort of cases. The LPX form factor is found in low-profile cases (Desktop model as compare to a tower on the mini tower) with a riser card arrangement for expansion cards where expansion cards boards run parallel to the motherboard.

 Advanced Technology Extended (ATX)

The ATX system uses new low profile extended (NLX) power supply, hence the ATX form factors are known as ATX (NNLX) form factors. The ATX form factors do not have an outlet to connect the power cord of the monitor to the power supply unit. This power supply unit uses the ATX connector to connect the motherboard. In ATX power supplies, you will have 3.3VA and +5V stand By (SB) which are not there in AT power supplies.

Small Form Factor (SFX)

The SFX power supply unit has two fans to cool the system. This addition fan is placed on the top of the power supply. The internal fan of the SFX power supply is thermally speed-controlled. The output of the SFX power supply unit is 90W and is used on smaller systems that do not a lot of power. The SFX power supply unit uses the ATX connector to connect the motherboard. The one difference here is that the SFX power supply specification does not support the -5V compatibility voltage and therefore should not be used with motherboards that have ISA slots.

Output Voltages of SMPS

Different components of the PC require different voltages. The power supply has to be provided these voltages in varying amounts. Various voltages provided by the SMPS are
    Ø  -12 V(Blue)- Require by some of the serial ports. It is required by very serial ports and is provided to maintain compatibility with such hardware.
    Ø  -5 V (White) -required by some older floppy controller and ISA bus cards. It is provided for compatibility for older hardware
    Ø  0 V (Black) – required to complete a circuit with other voltages. It is ground voltage and is also known as earth.
    Ø  +3.3 V (orange) – Required by the processor, DIMMs, PCI/AGP cards. This Voltage was introduced in the ATX form factor and is available in the ATX/NLX, SFX and WTX form factors.
    Ø  +5 V (Red) – Required by the Disk drive logic, Voltage regulator, ISA cards, SIMM's and other miscellaneous components of the system. This is basic supply voltage for nearly all electronic components.
    Ø  +12 V (Yellow) – Used by motors in the system, form drive motors to cooling fans. It is also circulated to the system bus slots for usage by cards which may need it.

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