Power supply

A Typical ATX Power Supply

In home and office computing, a power supply is the device responsible for converting the alternating-current from a wall socket into the low-voltage direct current required by devices like personal computers. For the purpose of this article, the term power supply is used exclusively to reference units that comply to the ATX standard.

What Every System Builder Should Know

Amperage, Voltage, and Wattage

For the purposes of this article, it is enough for the reader to understand that Wattage = Voltage * Amperage and that the watt is the unit representing electrical energy. If you are interested in learning more, which you should be, follow the Wikipedia links.

Rail Voltages

Any modern computer power supply will be constructed to the ATX12V v2.2 standard. Such a unit will offer a +3.3v rail, a +5v rail, at least one +12v rail, a -12v rail, and a +5VSB rail. In most cases, only the amperage of the +12v rail or rails will be of any concern to system builders, since these rails supply energy to modern video cards and CPUs. Hard drives are typically powered by the +5v rail.

The +12v Rail or Rails

Powerful +12v rails are especially important for system builders who plan to utilize high-end video cards, since such cards tend to have immense power requirements under load. An ideal unit should be capable of supplying nearly all of its advertised wattage to its +12v rail or rails. The maximum wattage on a given rail is equal to the voltage multiplied by the amperage. Remember that the amperage rating of a given rail represents the amount of current that it is rated to handle, but not necessarily the amount of current that the unit is capable of supplying to it.

Example A: One 650 watt unit might offer a +12v rail rated for 53 amperes, indicating that 636 watts should be available on that rail. Another unit promising the same wattage might offer +12v rail rated to 46 amperes, indicating that 552 watts or less is available on that rail.

Example B: A certain 850 watt unit might offer four +12v rails, each rated for 18 amperes. This indicates that it can supply 216 watts to any one of its +12v rails. The total wattage available on the +12v rails might be 768 watts. This indicates that only 64 amperes of combined current will be directed through the +12v rails in total, despite the fact that they could handle more. Another PSU rated for 850 watts might have a single +12v rail rated for 70 amperes, indicating that it is capable of supplying 840 watts on its +12v rail.

How Many +12v Rails?

The short answer is that the number of +12v rails does not matter for a typical system builder, so long as 80%-90% of the wattage is available across the +12v rails. Ignoramuses (mostly gamers) often claim that the monorail design is objectively superior or that it simplifies load balancing. The first claim has always been blatantly false, while the second primarily resulted from attempts to use inadequate power supplies with high-end video cards. Multi-rail units designed for use with high-end video cards naturally use rail layouts designed to accommodate them properly. If anything, the multi-rail design is probably superior for safety reasons explained by the third-party link at the end of this section.

Multiple +12v rails tend to be derived by splitting a single +12v source up into multiple outputs with each output receiving a limited capacity. Units supplying 1000 watts or more frequently do have multiple +12v sources, but the individual sources still tend to be split up as previously described. In some cases, two separate +12v sources may even be combined into a single high-energy rail. Traditionally, the ATX12v standard required that the +12v rails be split up like this for safety reasons, but this standard was eventually relaxed. As with household wiring, the goal is to prevent a single wire from becoming dangerously hot by limiting it to the amount of current that it is capable of carrying safely.

It is highly recommended that you read Single vs. Multiple +12V rails: The splitting of the +12V rail if you are interested in truly understanding power supply +12v rail configurations.

Efficiency Ratings

Power supplies are not perfectly efficient, meaning that the amount of power being draw "from the wall" will be greater than that available to components. Since power consumption tests are taken "at the wall" and are not always corrected for inefficiency, your actual power needs may be significantly lower than a give reviewers tests might indicate.

Most power supply manufacturers participate in a voluntary certification program called 80 PLUS™. To be 80 PLUS™ certified, a unit must operate at a minimum of 80% efficiency under loads of 20%, 50%, and 100%. Efficiency in this case refers to the percentage of power actually delivered to components. An 80 PLUS™ certified unit with a rating of 600 watts for example would draw 720 watts from the wall at 100% load. The more efficient your unit, the lower your electric bill. Since they often include superior components, highly efficient units tend to have longer lifespans and higher retail prices.

There are several levels of 80 PLUS™ certification, as you can see in the following chart.

80 Plus test type	115V internal non-redundant				230V internal redundant
Fraction of rated load	10%	20%	50%	100%	10%	20%	50%	100%
80 Plus		80%	80%	80%
80 Plus Bronze		82%	85%	82%		81%	85%	81%
80 Plus Silver		85%	88%	85%		85%	89%	85%
80 Plus Gold		87%	90%	87%		88%	92%	88%
80 Plus Platinum		90%	92%	89%		90%	94%	91%
80 Plus Titanium					90%	94%	96%	91%

Selecting a Power Supply

Power Supplies are Fucking Dangerous

Considering its purpose, it should go without saying that the power supply is one of the more dangerous components found in a personal computer. A defective, overloaded, or poorly constructed unit has the potential to destroy the rest of your system if it fails. It could even explode or cause a fire.

Determining Your Power Needs

Ideally, a power supply should be operated at around 80% or less of its maximum load to ensure a long lifespan and to accommodate for the possibility of capacitor ageing. If you plan to over-clock, the increased energy requirements of the over-clocked components must be considered. A well-constructed unit should serve you for about a decade if you treat well, so future needs should be considered while making your selection. To get a reasonably accurate estimate of what your build's power requirements might be without engaging in a preposterous amount of research, use this calculator or a similar one.

Warning: Most Nvidia SLI and AMD Crossfire configurations tend to have greater power requirements than comparably performing single-card configurations. Any unit intended for one of these multiple-card configurations will be advertised as being "SLI Certified" or "Crossfire Ready".

Cable Length & Connectors

Cable should be checked against the measurements of the case being used in the build, although this is only likely to be a concern with very large full towers. More importantly, the connectors offered by the unit should be checked against the motherboard, video card, and number of SATA drives being used in the build.

Modular Vs. Non-Modular

Modular units offer detachable cables, allowing you to use only those required. Some units are only partially modular, while others are fully modular. Cable management is simplified to such an extent that in either case the difference should be considered largely academic. Modular cables do introduce some extra electrical resistance, but not enough to represent a serious concern. Non-Modular units are often more affordable and may be longer-lasting.

Branding & OEMs

An original equipment manufacturer or OEM is firm actually responsible for the construction of a product. Firms like Channel Well, Delta, HEC, and Seasonic are examples of PSU OEMs. While OEMs do generally market their own lines, many also perform contract manufacturing for or market their products through firms like Antec, Corsair, and XFX. While build quality often varies wildly from one manufacturer to next, purchasing based upon brand loyalty is still not a good idea.

A few brands popular on /g/ have been mentioned, but there are of course far too many to cover in this article. For a detailed explanation of power supply brands, see Tom's Who's Who In Power Supplies: Brands, Labels, And OEMs.

Qualified Reviews

Once you have identified a unit that seems to meet your needs, it is important to vet it against qualified reviews. Unfortunately, this can be a problem in a world of paid reviewers and ignoramuses. JonnyGURU is quite knowledgeable and tends to write detailed reviews involving opinion, disassembly, and stress-testing. Primarily the last two.

Troubleshooting

Cleaning

Like most PC components, the PSU requires regular and thorough dusting with a pressurized air duster. If you allow dust and pet hair to build up, the unit will fail more quickly than most components. Keep in mind, however, that as with all moving components, forced movement must be avoided. Try holding down the fans with a set of tweezers or something similar while blasting them with air to avoid excessive wear on the bearings.

The Fan

Occasionally, a PSU's fan will fail long before the unit does. Should this occur, you can save the unit if you cut power at once. If the unit is still under warranty, the manufacturer should repair the unit for you without voiding your warranty. If the unit is no longer under warranty and you have the skill, attempt the following:

• Remove the unit from the PC and put it in a safe place. Do not disassemble it for at least 24 hours to give the capacitors ample time to drain.
• Find a nearly identical fan. Do your research. The new fan must be compatible with the fan controller's voltage at idle for example.
• Open up the PSU and check to see whether or not the fan's wiring is soldered to the PCB. Most of the time, it will be.
• If it is, unscrew the old fan and snip the fan's wires as close to the fan as possible. Take care not to damage the solder.
• Braid the wire, exposing slightly more than one centimetre of metal.
• Do the same for the replacement fan after deciding how long you would like the wire to be.
• Splice the wires together properly. Crush the ends flat with pliers, put them together, then twist.
• Using high-quality electrical tape or liquid sealant rated for high temperatures, cover the spliced wires.
• Test the PSU.

Warning: Repairing a PSU is always somewhat dangerous and certainly not a task for amateurs. A shock from its capacitors could result in hospitalization or death. Take proper precautions and wear electrician's gloves with leather over-gloves while working. If you do not already own proper safety equipment, it is exceedingly unwise to proceed.

External Links

• Wikipedia's Explanation of the Watt (SI Measurement).
• Wikipedia's Explanation of the Volt (SI Measurement).
• Wikipedia's Explanation of the Ampere (SI Measurement).
• Single vs. Multiple +12V rails: The splitting of the +12V rail
• This is one of the few regularly updated tools for calculating power requirements.
• Tom's Who's Who In Power Supplies: Brands, Labels, And OEMs
• Anandtech offers various benchmarks, including idle and peak power draw, for a wide range of components.
• JonnyGURU is one of the few genuinely reliable PSU review sites.
• Realhardtechx has an interesting PSU review database. It makes finding reviews for specific units a bit easier than a basic search engine would.