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Description: The power supply is responsible for providing the electricity required by most of the components inside the PC box, and also some of its peripherals. The power supply converts standard utility 110V/220V AC power into several different DC voltages used by PC components. (Notebook machines don't have a conventional power supply, since they run off a battery or use an external AC/DC converter--I don't discuss those here; see the section on notebook components.) Since everything in the PC requires power, everything relies on the power supply to one extent or another. It is one of those "support" components that helps form the foundation of the PC.
Tip: For much additional
information on power issues, including more discussion of many of the technical details,
criteria and features mentioned below, see the
Reference Guide section on power.
Role and Subsystems: The power supply plays an important role in the reliability, cooling, efficiency and expandability of the PC. Poor-quality or under-powered supplies can lead to problems with virtually any other hardware item in the PC and can be very difficult to diagnose. They can also cause you trouble if you expand your hardware, or if future hardware requires more current than the supply can provide. The cooling fan in the power supply is primarily responsible for air circulation in most PCs. The power supply is not part of any of the major subsystems as I have defined them.
Related Components: The power supply must be matched in form factor with both the system case and the motherboard. It is often included in the system case, and too little attention paid to it--sometimes the case is purchased with the power supply as a "tag along" and nothing known about it aside from its total output. Needless to say, this is not a good idea. Power supplies can in fact be purchased separately from system cases if you don't want to buy them as a set (which usually means compromising either or both components).
Since some high-end CPUs now have rather dramatic power requirements, the selection of power supply may be influenced by the system processor as well, indirectly.
Key Compatibility Selection Criteria: There are several important criteria that will form the basis for selecting a power supply (and often, the case, since they are usually purchased together):
Performance and Capacity Selection Criteria: Power supply performance and capacity are about the amount of power the supply can provide, pretty much. There are several nuances to this however, beyond the total amount of power mentioned above:
Quality Selection Criteria: The quality of the power supply is very important, and unfortunately often not easy to determine. Here are a few attributes to help you consider it:
Important Features: There aren't many specific features to look for when selecting a power supply; you are going to focus primarily on form factor, output capacity and quality issues. One tip is that if you will be taking the computer to different parts of the world, it is best to get a power supply that will switch from 110 V to 220 V operation automatically--this obviously matters little to most people.
There are also redundant power supplies available, which are essentially two supplies in one. If one fails the other takes over. They are rather expensive and usually only of need for servers.
"Magic Numbers" To Watch For: The total output power of the supply, in watts, is about all that most people ever talk about. As I have tried to show you, this is important but only a rough guideline. Not all watts are the same.
Performance Impact: The power supply has no direct impact on overall system performance. Better supplies enable higher performance by supporting their power needs, but they don't make the system run faster.
Retail, OEM and Gray Market Issues: Many power supplies are sold as gray market. Since failures are rare beyond the first month this is not as much of an issue as it is with other components. Almost all supplies are sold "OEM" since few people need to buy power supplies retail.
Importance of Manufacturer: There are relatively few "big names" in the power supply industry, and lots of small and medium-sized companies making similar-looking units. Even if you buy a system from a major PC manufacturer, you may get a power supply whose manufacturer you have never heard of before. It's better to stay with a known brand, but most people get by acceptably with generics--as long as they aren't total junk. :^)
Typical Component Lifetime: Power supplies themselves can last for many years, but the cooling fan is a different matter--fans often clog after a matter of a year or two and cause overheating of the entire PC, so watch out for dirt accumulation near the fan. Obsolescence is not much of an issue as standards change slowly. Just make sure you don't get a unit that is under-powered--leave some room for expansion.
Driver Support Issues: Not applicable.
Warranty Issues: Failures beyond thirty days are fairly uncommon. Warranty length is a good indication of overall quality, but the odds of needing to use a long warranty are fairly low. Do watch out for very short warranties on cheap units.
Special Specification Considerations: Here are some additional tips to keep in mind when selecting a power supply: