How SMPS Works

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power-supplyPower supply unit is an electrical device that provides power to electronic devices and circuits. Most of the electronic devices require less power voltage as compared to the voltage supplied to home or office. This led to the development of Switching Mode Power Supply (SMPS) technology. This device is responsible for the power conversion from high voltage to low voltage. The Power supply unit supplies power to different system components such as the motherboard and the device drives. You must protect the system from extreme temperature changes like overheating using cooling devices. You can use a voltage stabilizer to protect the system from power fluctuations. You can protect the system from failure due to power cuts by installing an UPS.

Switch Mode Power Supply:

The SMPS unit is mostly found in the top corner of the system case. It accepts the Alternating Current (AC) from the power socket in the wall. It then converts the AC to DC for the system. The power supplies are available in the range of 250W to 400W. The current supplied by power supply unit to the different components of the system are 3.3V, 5V and 12 V. The microprocessors and   chipsets in the system use 3.3V and 5V of the electric current, while the disk drives such as hard disk drives and other removable disk drives use 12V of electric current.

Functions of SMPS:

The Power supply junction supplies AC current to the power supply unit (SMPS). However, the various components of the system require DC. The SMPS unit converts AC to DC for its use. This power is then distributed among the different parts of the computer. The power supply unit of the system generates high-frequency signals during the electric current conversion from AC to DC. This frequency may harm the components of the system. To prevent damage to the system the SMPS unit is packed in a metal box.

Colour specifications of an ATX Power connector:

Pin Signal Colour 1 Colour 2 Pin Signal Colour 1 Colour 2
1 3.3 V Orange Violet 11 3.3 V Orange Violet
2 3.3 V Orange Violet 12 -12 V Blue Blue
3 GND Black Black 13 GND Black Black
4 5 V Red Red 14 PS_ON Green Grey
5 GND Black Black 15 GND Black Black
6 5 V Red Red 16 GND Black Black
7 GND Black Black 17 GND Black Black
8 PW_OK Grey Orange 18 -5 V White White
9 5V_SB Violet Brown 19 5 V Red Red
10 12 V Yellow Yellow 20 5 V Red Red


Factors to be considered before selecting a SMPS:

Before selecting an SMPS, you need to consider certain factors such as size, wattage and different connectors.

Size: SMPS is available in various sizes. Before selecting a power supply, you need to select power supply of appropriate size which fits in your system case.

Connectors: Check for the available number of power supply connectors that can provide supply to all the devices. Also check if the power supply connector can be attached to power supply slot.


Servers and desktop computers work in power rating. Power rating is measured in terms of Watts. Rating for SMPS varies between 250W-400W. Maximum power consumption occurs at start up time of SMPS. Computers have rating between 250W-300W and servers have rating between 300W – 400W. It is always better to have excess wattage on a power supply.

To calculate Wattage requirements manually, consider the below table:

PC Item Watts
Accelerated Graphics Port (AGP) card 20 to 30 W
Peripheral Component Interconnect (PCI) card 5W
Small Computer System Interface (SCSI) PCI card 20 to 25 W
Floppy Disk Drive 5W
Network Interface Card 4W
50X CD ROM drive 10 to 25 W
RAM 10W per 128 MB
1000 RPM Hard disk 15 to 20 W
Motherboard without CPU or RAM 20 to 30 W
Dual core processor 1.7 GHZ 50 W


Testing an SMPS:

  1. Check the AC line for functionality.
  2. Check the power cord and Line voltage selector switch if it has been set to 230 VAC.
  3. Remove all daughter cards, FDD, HDD and supply connector and check the PG signal(+5VDC grey)
  4. Short pin no.13 and 14 to see that fan is rotating and then check the output voltage by DMM.
  5. If the SMPS does not work when connected to motherboard then check whether there is a problem with SMPS.
  6. SMPS may make hissing sound because of loose transformer vibrating at a sub harmonic frequency or dried up electrolytic capacitors. To troubleshoot this problem.
  1. Ensure that there is no short circuit or loose connection.
  2. Ensure that the power supply is not overloaded.