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If there is any one component that is extremely vital to the operation of a computer, it is the power supply. The power supply should be the highest priority when choosing components; if working with a set budget, invest in the power supply and make compromise elsewhere. The power supply is perhaps the most significant module in every computer system as it supplies stable, consistent power to every other component in the system,
Power supplies must not produce and dissipate too much heat or noise. Bad power supplies are almost always overrated in their peak wattage number. A thumb rule is that one should purchase as much good quality power supply as one can afford to ward off sudden breakdowns, data corruption and possibly physical component damage. If you are building a brand new system, go for anything over 400 watts. Select the power supply that fits into the cabinet you have chosen. Also make sure that there is enough clearance so that it does on interfere with other occupants of the cabinet. Choose a Power supply compatible with the motherboard and the other components you are planning to fit into the case and also one that offers additional plugs in case of upgrades of the computer later on.
Almost all computer power supplies have three voltage rails (3.3V rail, the 5V rail, and the 12V rail).
Wiring coming off an industry standard circuit board will be:
ORANGE +3.3 V
YELLOW +12 V
BLUE -12 V
RED +5 V
WHITE -5 V (May not be present on recently manufactured supplies)
BLACK GND
GREEN POWER-ON (Active high -- must be shorted to ground to force power up)
GRAY POWER-OK
PURPLE +5 V STANDBY
BROWN +3.3 V REMOTE SENSING
(please check with the specification of the manufacturer which may vary)
Ensure that the sockets fit into the components well. Go for one with 20-pin/24-pin ATX Socket (as required) and enough 12V Molex Sockets (4-Pin Molex Connector/Serial ATA Connector/PCI-Express Connector) to power different peripherals inside the computer box. Now-a-days modular power supplies are available where the cable can be plugged and unplugged to many sockets provided in the power supply. In this case one has to use only as much cables as needed. This prevents the inside of the cabinet from cluttering. It is wise to use sleeved cables. Go for the power supply with a cooling fan and lesser noise. Never trust a vender for his words. Go for a more superior, trusted and reliable one, even if it costs more. It will give you years of comfort.
I must repeat again that the power supply should be the highest priority when choosing components; if working with a set budget, invest in the power supply and compromise elsewhere.