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