If you're updating your PC's power supply or building a new PC from scratch, there are so many PC power supplies these days that it's easy to get confused and settle for whatever a vendor is offering. But, with a little time and research, you can improve your odds of obtaining the best PSU (power supply unit) and installing it correctly.

Basically, a power supply supplies power, right? But it actually does more than that. A proper power supply needs to fit and work well with your computer hardware. Your new power supply should:

1. Fit your PC case.
2. Fit your PC motherboard and CPU combination.
3. Provide ideal airflow to keep the computer hardware and power supply cool.
4. Be manufactured to high standards of fit and finish, and for reliability and safety.
5. Provide stable electrical power and include some level of protection for your computer hardware, and generate low EMI (electromagnetic interference).


Loading On The Power

When power is provided to your computer hardware, the power supply converts AC to DC, and then the computer hardware "draws" electricity much like the flow of water. This electrical flow is usually expressed in watts, which is volts multiplied by amps. Normally the flow of electricity to your computer hardware will be small, and many off-the-shelf power supplies handle this adequately. But what happens when your computer hardware needs much more power, such as when it is processing digital video? At times like these, the computer hardware will demand more continuous power, power needs may peak, and this is when your power supply will either flow or falter. If the power supply falters, your computer may crash, hang, or reboot.

Ideally your power supply should provide more power than your computer hardware will ever need, so you must determine your needs. Do you have a stock PC to which you have added a lot of extra (or more powerful) components? Then you'll need more power flowing reliably to those components. If your power supply has been consistently driven too hard it can eventually become unstable, damaging expensive computer components.

The best way to tell if you need more power is to add up the power consumption of all components, but this can get complex. For example, AMD suggests ". . . calculating the power supply minimum output capacity as the power required by the processor plus 80% of the total wattage for all the other components in a desktop/tower system." Here is what that means: Take the maximum power consumption of the CPU in watts, which can be obtained from Intel's or AMD's datasheets for your CPU.

Total the maximum power consumption of your computer hardware and multiply by 0.8.

Add the results to get your power consumption needs.

Sounds easy, right? It is, for a seasoned electrical engineer. For practical purposes, just abide by this simple rule: Buy the most power you can afford. If the power supply you are running doesn't fit your needs, it may shut off when components draw too much power. This risks the loss of computer components (possibly even a fire) and most importantly, your data. (By the way: When was the last time you backed up your critical data?)


Select Your Power Supply

In today's market, there are many ATX power supplies available. ATX power supplies are specific to ATX cases, which are currently the most common on the market, so that is what we will be discussing here. (NOTE: ATX 12V V1.3 compatible power supplies support the latest P4 and Athlon 64 platforms.)

Power supplies come in various power configurations designed to fit your needs. Some power supplies come with one fan, some with two. Some come with the Serial ATA interface, which is used to power external devices. Some come with a variable speed fan (either manual or automatic). Some come with over-current and over-temperature monitors, which means that they'll shut off at predetermined points in order to preserve your computer hardware. Some even come with cool lights to impress that geek girlfriend (or boyfriend) of yours.

Overwhelmed yet? Don't worry. The typical after-market power supply configuration is 300 to 350 watts with one fan. Look for the following:

Power. Purchase a power supply with a minimum of 350 watts; 400 watts is better for the average high-end computer. If you have a tall PC case (generally called a tower), keep in mind that internal cables are also very likely long, and long cables require more power. Remember to get the most wattage you can reasonably afford. (NOTE: Be sure to read the specifications for the power supply unit. It may be sold as a P350 but may only supply 325 watts of power.)

Warranty. If a power supply provides a good warranty, it usually means that it's built better than some other units. Also, look for the acronym MTBF, which means "mean time between failure." The higher the MTBF, the better.

Fan. AMD recommends that you get a power supply with a lower inlet fan, not an upper one. This is because the company feels that air flows better when the fan is on the lower part of the power supply. Some of the better power supplies include a passive heatsink on the back, which provides extra cooling capacity and means that the fan may not turn on until enough heat builds up in the unit. Some power supplies may have no fan at all, only heatsinks. (NOTE: It is important that your power supply fan not work against the airflow of another fan that may be installed in your PC case. If you have multiple fans installed, they should work together.)

Quiet. If the system's noise level is important to you, get a power supply that creates at most 45dB (decibels) of fan noise (the sound of a quiet library is about 35dB). Any louder, and the unit may prove annoying when the fan is operating at high speed. (NOTE: The more power a unit provides, normally the more noise the fan will make.)

Electrical and heat protection. This is a must. Excessive power draw may cause heat to build up in your power supply. It's not likely to cause a fire, but it is possible. If the power supply fails by going up in smoke, it will very likely ruin a computer component, and possibly the entire computer. For extra insurance, we like to use a battery backup, which prebuffers and stabilizes the electric supply, but that's a subject for another article. (NOTE: Most electrical devices built for or coming into the United States carry the UL [Underwriters Laboratories] rating, but it is best to confirm these certifications. So look for this and other industry standard electrical designations.)

If you have a tower PC case, you can normally purchase any industry standard ATX power supply and it will fit. If you have a horizontal PC case, it may mean that the power supply is not necessarily a standard fit. If you are unsure, ask your sales representative. He may want to see your computer.


Install Your Power Supply

After all the talk of selecting the right power supply, the mechanics of installing it turn out to be fairly straightforward. When you open the box, you will find specifications and directions for installing your power supply. It pays to read these to ensure that the power supply model you purchased truly fits your needs and to familiarize yourself with the unit. We'll provide general instructions, but keep in mind that your particular power supply's instructions may differ somewhat.


Removing An Old Power Supply

If you are upgrading, you will want to detach all external components and then remove the old power supply.

For safety's sake, unplug your PC from the AC outlet before you begin the process of removing your old power supply. While power supplies are built with safety in mind, they can deliver a potentially life-threatening shock. The best way to avoid this is to first unplug the three-prong electrical connector from the wall and then unplug the electrical connector from the power supply.

To remove the old power supply, first open your PC case. Many older style ATX PC cases come with screws that require a screwdriver (a power screwdriver works best), but some of the newer ones have tabs or screws with knurls or knobs so that you can remove them by hand. Some PC case covers remove by taking off the entire shell, but on most newer ones you only have to remove the left side of the PC case cover (assuming a vertical PC case and that you are facing the front of the computer), to expose the motherboard, CPU and memory, and power supply. Be careful of sharp edges.

Now that your computer case is open, the old power supply is exposed. Again, ensure that the main AC electrical cord is unplugged from the unit. Then remove the male ATX power connector by unplugging it from the motherboard. (See the pictures describing both the male and female connectors.) To remove the male connector, use a knife to loosen the attachment clip and then gently rock the connector from side to side, until it comes out of the female socket.

Now remove any other power cables connected to your computer. Remove the DVD/CD-ROM power cable and hard drive power cables, as well as any other power supply attachments made to the old power supply. (NOTE: Cables connected from your motherboard to other peripherals in your computer, such as the ribbon cables, do NOT need to be removed.)

Finally, remove the power supply from the PC case by removing the four screws on the back. The unit will usually come out the side, but some cases are set up so that it will come out of the back of the case. Once the old unit is out, you can simply reverse this process to install, by slipping the new unit in its place, attaching it, and reattaching the connectors. (NOTE: The PSU will come out of your computer encased in a steel or aluminum box. Do not take the power supply itself apart. The possibility of getting a shock from an unplugged power supply is remote, but it does exist, especially if you remove the unit's protective covering.)


Install The New Power Supply

If you have a new PC case in need of a power supply, you may have probably skipped to this section. To see how to open the PC case, see the instructions above on removing an old power supply.

Insert the power supply unit either from the side (which is typical) or from the back of the PC case. Then temporarily secure it with a single screw (you'll see why in a minute). (NOTE: Check to see if your power supply comes with a switch setting the input voltage. Before you plug in the AC, be sure the switch is set to the U.S. standard of 120 volts or you risk damaging the unit.)

Now attach the DVD/CD-ROM power connector, hard drive power connectors, and any other power supply attachments needed. Attach the motherboard ATX power connector last.

Excess heat is a common cause of computer component failure. Make sure that the cables inside the PC case are arranged so that air can flow freely into and through the case.

Once you are sure the power connectors are secure, you can test the system by attaching the external AC connector to the power supply, reconnecting external components (mouse, keyboard, sound, video, etc.), and turning the unit on. Once all the hardware on the computer is powered, and the system starts correctly, you are in business. Shut down the computer. Use the remaining screws to ensure that the power supply is firmly attached to the PC case, reattach the PC case cover, and connect all external peripherals.


Finishing Up

We're hoping that you don't have any leftover screws and such, because in theory, you're finished. Once you have purchased the power supply, installing it should only take about 20 minutes. You can now sit back, relax, and watch a movie on your PC's DVD player. Your computer hardware is getting all the good power it needs to do its work.