Replacing Your Computer's Power Supply: When and How


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If you’re in doubt about your computer’s power supply, you may consider replacing it. There are two primary times when people consider taking this step.

The most common is if a major upgrade is being contemplated, particularly installing a newer, faster CPU. Let’s say your computer has a mid-range processor, graphics card, CD burner, lots of RAM, and is depending on a 350 watt power supply. That step up to a faster CPU could be more than the power supply can handle. 450 watts would be in order, particularly if any future changes, like moving up to a high end graphics card, are a possibility.

Remember, today’s fastest processors can take 100 watts all by themselves. Graphics cards are equally power hungry, with top end units gobbling up another 100 watts. You want more power available than your computer will use.

I’ve had people ask me “Won’t too much power burn something up?" The answer is no, it doesn’t work that way. If your computer requires only 370 watts from a 450 watt supply, then that is all it will use. The extra 80 watts will simply be there if you need it.

The other time people change their power supply is if it starts acting up. Often this comes with no warning. Either they push the button, and nothing happens, or worse, it dies right in the middle of some important task. To complicate matters, problems other than the power supply can have similar symptoms, things like wiring gone bad.

One failure, that can be catastrophic, is for the power supply’s fan to quit. If the computer tended to run hot before, loss of that fan could push it over the edge. The CPU could be damaged.

If your fan stops, immediately save your data, and turn off the computer. If you’ve caught it soon enough, your machine will be saved. All you’ll have to worry about is changing the power supply.

Never try to change the fan itself, or open up a power supply for any reason. Just replace the whole unit. There’s a capacitor in there that can knock you into next week. They’re nasty, viscious things that will hold a charge until something, or some idiot, discharges it. Don’t be an idiot.

The first thing a beginner thinks when replacing a power supply is “How will I ever get all these wires hooked back up?" Just remember that all those four wire plugs are interchangable. You can swap the plug that goes to the hard drive with the one that goes to the CD/DVD, and it will work just as well. There’s no danger of turning the plugs backwards, because they are keyed to fit only one way. The power plug to the motherboard is unlike any other, a big rectangular block with many wires leading to it, and can be mistaken for nothing else. (Older computers have two power plugs to the motherboard, marked P8 and P9. Just note the location of each on the board. )

Once the wires are unhooked from the drives and motherboard, take out the four screws that hold the power supply to the back of the case, and ease it out. Put the new supply in, tighten up the screws, and hook up the wires.

Michael Quarles is the author of Building a PC for Beginners. His website is


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