Power supply info for the novice

Posted by: TCPMeta

Power supply info for the novice - 01/28/10 06:03 AM

Thought I would share some priceless info on power supplies for the novice modder.

Typical power supplies are known as a ATX platform power supply.

Power Supplies range in different watts but most people do not know about the amps. For a example you're building a custom PC. Yea you can go with the biggest and best power supply money can by but you have to stop and think for a minute "What about the amps?". When you read up on a motherboard check out the detailed specs such as amps needed. They usually list the amp ratting for the fastest CPU it can support and SLi video. Same thing goes with say a old sawtooth mac that you decide to toss in a ATX power supply into it. Always go with something equal or greater then that was in use. Yes you can use a lower amp ratting but it will cut the life span in half or even have power issues if you decided to load up the PCI BUSes or slap in a RAID.

Ideally you can get away with 300 watts in a typical home system and around 400 watts for a power house gaming PC. Once again a example a sawtooth you can load up everything imaginable into it and still have a couple watts to spare from that 200 watt power supply. I myself always go for 500 to 600 watts so if I ever upgrade I don't have to turn around way down the road and buy a new power supply.

Color code
90% of power supplies use the same color code for the wires.

Orange = +3.3 Volts DC
Red = +5 Volts DC
Yellow = +5 volts DC
Black = Ground

Blue = -12 Volts DC
Purple = +5 Volts DC = Standby lead (always a hot lead when the power supply is plugged into a electrical outlet.)
White = -5 Volts DC
Green = Power On (motherboard sends signal to turn on the power supply. Modders like to ground this out so it is constantly on.)
Gray = Power supply OK lead (should be asserted high by the power supply to indicate that the +5VDC and +3.3VDC outputs are above the undervoltage thresholds.)

Brown = 3.3 Volts DC = Sense lead (A remote 3.3 V sense line can be added to the optional connector to allow for accurate control of the 3.3VDC line directly at motherboard loads. Because of potential voltage drops across the connector and
traces leading to the motherboard components, it may be advantageous to implement a 3.3 V sense line that remotely monitors the 3.3VDC power level at the load on the motherboard. The implementation of this signal should be such that if an NC condition is detected on this line, the default 3.3 V sense line on the main connector would be used for sensing the 3.3VDC voltage level.)

Misc info
A typical 20 pin ATX power supply can be used with a 24pin motherboard. The extra four pins add less resistance to the rail insuring that the said device receives the proper voltage.

You can also use a 24pin power supply on a 20pin motherboard. It would look a bit awkward but it works. To keep things safe I would wrap the last for pins with electrical tape.

A lot of people do not know this but a SATA drive's power connector does not require the Orange 3.3 Volt lead. Some older drives do require it but most newer drives do not need it. This also explains why if you buy a power adapter there isn't a 3.3 Volt Orange wire.

If you want to learn even more about power supplies check out this link
Posted by: ctan

Re: Power supply info for the novice - 02/06/10 01:58 AM

Thanks TCPMeta,

The electrical aspect of getting under the hood has always been the most daunting for me. Always afraid I'll smoke another CPU or worse!
Posted by: johnodd44

Re: Power supply info for the novice - 01/27/11 10:02 AM

except your forgetting something.

this mod if done on a g4 tower will cause the tower to fail boot posts