[ The PC Guide | The PC Buyer's Guide | Designing and Specifying PC Systems and Components | Detailed Considerations and Tips for Specifying Particular Components ]

Power Supplies

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):

• Form Factor: The power supply must be of a form factor that matches those of the system case and the motherboard. When it comes to the case, the most important issue is physical matching in terms of size; when it comes to the motherboard, the issue is the connections between the power supply and the motherboard. There are several different power supply form factors, some of which are used with more than one kind of case or motherboard; ATX is the most popular form factor. To help avoid some of the confusion associated with form factor matching, refer to this table. Note that on new systems the power supply and case form factor are usually selected to match that of the motherboard; on an upgrade it is often the other way around.
• Total Capacity: You need a power supply that is of approximately the right size for the application you need. While the specific output at each voltage level is very important (see below for more), you will be able to make a "rough cut" elimination of certain models based on their approximate capacity. As a rule of thumb, a supply that is 250 W or more is best for larger systems. For less demanding systems with fewer peripherals and less performance, 200 W or less may be sufficient, though the extra power of a 250 W supply isn't bad to have.
• Component Requirements: Some high-end systems have additional specific power demands that you must consider. A server will require additional power to handle its multiple processors and storage devices. Also, some newer system processor platforms have been tested with particular units and you may be best off going with a specific model recommended by the manufacturer. For example, AMD has specific compatibility lists for systems using its Athlon CPU.

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:

• Power At Specific Voltages: The total output rating of the supply (e.g., "300 W") is an aggregate of the output provided at each of several voltage levels. Some peripherals use mostly one specific voltage while others use a different one, so a single number isn't sufficient for determining capacity. You need to know how much current the supply can produce for (at least) the three key voltages: +3.3 V, +5 V, and +12 V. See here for more detail.
• Peak Power: Some peripherals, such as hard disk drives, draw more power when they are starting up than when running "steady state". To deal with this temporary additional demand, power supplies have a rating for how much they can put out for a brief time at startup. Some supplies are better than others at providing this peak capacity.

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:

• Certifications: One good indication of at least basic quality and adherence to safety standards is to look for certification markings from at least one of the big certification organizations, such as UL (United States), CSA (Canada), NEMKO (Norway) , TUV or VDE (Germany). The more certifications the better, but any one of these tells you at least something about the fundamental quality of the unit.
• Warranty Length and Policies: What is the length of warranty on the supply? Better ones have warranties of three or even five years. What is the warranty service policy?
• MTBF: This stands for mean time between failures. It is a theoretical estimate of the number of hours, on average, this model operates between failures when considering a large number of supplies running continuously. Larger numbers mean theoretically higher quality. Note that since MTBF is both an average and an estimate, you should not assume that this means the particular model you buy will last that long. Use this for guidance only.
• Electrical Characteristics: There are several specifications that indicate the actual quality of the power produced by the supply. The most important ones are load regulation, line regulation, ripple, transient response and holdup time. For all but holdup time, lower figures are better. See here for more.
• Noise: Users are becoming increasingly concerned with how much noise the system produces, and the cooling fan within the power supply is a major source of this noise. Better units generally are quieter, though there is a tradeoff here: more power requires more cooling and may therefore require a bigger and noisier fan. There are units specially designed to produce less noise than regular supplies.

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:

• If the supply you purchase has a manual voltage selection switch, be sure you put it to the correct voltage for your location or Bad Things May Happen. :^)
• Check all the specifications of the supplies you are considering, looking for small differences that may reflect better design on the part of one model.
• If you are going to put a bunch of hard drives in the system, pay special attention to the +12 V rating, which is what the drive's spindle motor uses.
• Beware of proprietary supplies that look just like standard ones.
• Watch out for some of the incredibly bad power supplies that are bundled with cases, especially very cheap ones. If the case and power supply cost $30 total, you are almost certainly getting an el-cheapo supply (and the case probably isn't great either).

Next: Motherboards

Home  -  Search  -  Topics  -  Up