Power supply unit (computer)

The top cover has been removed (voiding the warranty) to show the internals of a computer Power supply Unit.

This article is about the common off-line switching power supplies used in desktop IBM PC compatible computers. There are many other kinds of computers with differing power supplies.

A computer power supply unit (Computer PSU) is the component that supplies power to a computer. More specifically, a power supply is typically designed to convert 100-120 V (North America and Japan) or 220-240 V (Europe, Asia and Australia) AC power from the mains to usable low-voltage DC power for the internal components of the computer. Some power supplies have a switch to change between 230 V and 115 V. Other models have automatic sensors that switch input voltage automatically, or are able to accept any voltage between those limits.

The most common computer power supplies are built to conform with the ATX form factor. The most recent specification of the ATX standard is version 2.2, released in 2004. This enables different power supplies to be interchangeable with different components inside the computer. ATX power supplies also are designed to turn on and off using a signal from the motherboard (PS-ON wire), and provide support for modern functions such as the standby mode available in many computers.

Wattage

Computer power supplies are rated based on their maximum output power. Typical power ranges are from 300 W to 500 W (lower than 300W for Small form factor systems), although units used by gamers and enthusiasts usually range from 500 W to 1000 W, with the highest end units going up to 2 kW for extreme performance computers with multiple processors and graphics cards (ATI CrossFire or NVIDIA SLI).

Appearance

External

Most computer power supplies have the appearance of a square metal box, and have a large bundle of wires emerging from one end. A label on one side of the box lists technical information about the power supply, including safety certifications maximum output wattage. Common certification marks for safety are the UL mark, GS mark, TÜV, NEMKO, SEMKO, DEMKO, FIMKO, CCC, CSA, VDE, GOST R and BSMI. Common certificate marks for EMI/RFI are the CE mark, FCC and C-tick. The CE mark is required for power supplies sold in Europe.

Dimensions of an ATX power supply are 150 mm width, 86 mm height, and 140 mm depth.

Connectors

Various connectors from a computer PSU.

Typically, power supplies have the following connectors:

• PC Main power connector (usually called P1): Is the connector that goes to the motherboard to provide it with power. The connector has 20 or 24 pins. One of the pins belongs to the PS-ON wire mentioned above (it is usually green). This connector is the largest of all the connectors. In older AT power supplies, this connector was split in two: P8 and P9. If you have a power supply with 24-pin connector, you can plug it into a motherboard with a 20-pin connector. In cases where the motherboard has a 24-pin connector, some power supplies come with two connectors (one with 20-pin and other with 4-pin) which can be used together to form the 24-pin connector.
• 4-pin Peripheral power connectors (usually called Molex for its manufacturer): These are the other, smaller connectors that go to the various disk drives of the computer. Most of them have four wires: two black, one red, and one yellow. Unlike the standard mains electrical wire color-coding, each black wire is a ground, the orange wire is +3.3 V, the red wire is +5 V, and the yellow wire is +12 V.
• 4-pin Floppy drive power connectors (usually called Mini-connector): This is one of the smallest connectors that supplies the floppy drive with power. In some cases, it can be used as an auxiliary connector for AGP video cards. Its cable configuration is similar to the Peripheral connector.
• Auxiliary power connectors: There are several types of auxiliary connectors designed to provide additional power if it is needed.
• Serial ATA power connectors: a 15-pin connector for components which use SATA power plugs. This connector supplies power at three different voltages: +3.3, +5, and +12 volts.
• Most modern computer power supplies include 6-pin connectors which are generally used for PCI Express graphics cards, but a newly introduced 8-pin connector should be seen on the latest model power supplies. Each PCI Express 6-pin connector can output a maximum of 75 W.
• A C14 IEC connector with an appropriate C13 cord is used to attach the power supply to the local power grid.

PSU always on

• Connect Power On (green) cable with the ground (black).

This is useful when testing PSU.

Internal

Inside the computer power supply is a complex arrangement of electrical components, including diodes, capacitors and transformers. Also, most computer power supplies have metal heat sinks and fans to dissipate the heat produced. The speed of the fan is often dependent on the temperature, or less often the power load. It may be dangerous to open a power supply even if it is not connected to an electrical outlet, as high voltages may still be present in charged capacitors. However, for most PSUs this can be fixed by unplugging the PSU and then pressing the power-on button, which will drain the capacitors. Still, care should be taken as some PSUs require a load on the output in order to discharge the capacitors fully. Even when the PC is turned off, a PSU will draw some power from the wall, most of it going to power the +5 VSB (standby voltage) rail.

Some models even include heat pipes.

AT vs. ATX

A typical installation of an ATX form factor computer power supply.

There are two basic differences between old AT and newer ATX power supplies:

• The PC main connectors (see above description of connectors).
• The soft switch. On older AT power supplies, the Power-on switch wire from the front of the computer is connected directly to the power supply. On newer ATX power supplies, the switch goes to the motherboard, allowing it to control the turning off of the system via a message from the operating system.

Laptops

Most portable computers have power supplies that provide 15 to 100 watts. In portable computers (such as laptops) there is usually an external power brick which converts AC power to one DC voltage (most commonly 19 V), and further DC-DC conversion occurs within the laptop to supply the various DC voltages required by the other components of the portable computer.

Energy efficiency

Computer power supplies are generally about 70–75% efficient; to produce 75 W of DC output they require 100 W of AC input and dissipate the remaining 25 W in heat. Higher-quality power supplies can be over 80% efficient; higher energy efficiency uses less power directly, and requires less power to cool as well. As of 2007, 93%-efficient power supplies are available. Resonant-transition or quasi-resonant switching regulators could achieve over 90% energy efficiency, and also reduce radio frequency interference.

It's important to match the capacity of a power supply to the power needs of the computer. The energy efficiency of power supplies drops significantly at low loads. Efficiency generally peaks at about 50-75% load. The curve varies from model to model (for examples of what this looks like see the test reports of efficient models found on the 80 PLUS website [1]). One rule of thumb is that a power supply that's over twice the required size will be significantly less efficient, and waste a lot of electricity.

Small facts to consider

Redundant power supply.

• Life span is usually measured in mean time between failures (MTBF). Higher MTBF ratings are preferable for longer device life and reliability. Quality construction consisting of industrial grade electrical components and/or a higher speed fan can help to contribute to a higher MTBF rating by keeping critical components cool, thus preventing the unit from overheating. Overheating is a major cause of PSU failure. MTBF value of 100,000 hours is not uncommon.

• Power supplies may have passive or active power factor correction (PFC). Passive PFC is a simple way of increasing the power factor by switching in and out banks of capacitors. Active PFC is more complex and can achieve higher PF, up to 99%.

• In computer power supplies that have more than one 12V power rail, it is preferable for stability reasons to spread the power load over the 12V rails evenly to help avoid overloading one of the rails on the power supply.
  • Multiple 12V power supply rails are separately current limited as a safety feature; they are not generated separately. Despite widespread belief to the contrary, this separation has no effect on mutual interference between supply rails.
  • The ATX12V 2.x and EPS12V power supply standards defer to the IEC 60950 standard, which requires that no more than 240 volt-amps be present between any two accessible points. Thus, each wire must be current-limited to no more than 20 A; typical supplies guarantee 18 A without triggering the current limit. Power supplies capable of delivering more than 18 A at 12 V connect wires in groups to two or more current sensors which will shut down the supply if excess current flows. Unlike a fuse or circuit breaker, these limits reset as soon as the overload is removed.
  • Because of the above standards, almost all high-power supplies claim to implement separate rails, however this claim is often false; many omit the necessary current-limit circuitry (one example), both for cost reasons and because it is an irritation to customers.[2] (The lack is sometimes advertised as a feature under names like "rail fusion" or "current sharing".)

• When the computer is powered down but the power supply is still on, it can be started remotely via Wake-on-LAN and Wake-on-Ring or locally via Keyboard Power ON (KBPO) if the motherboard supports it.

• Most computer power supplies are of type switched-mode power supply (SMPS).

• Most computer power supplies have short circuit protection, overpower (overload) protection, overvoltage protection, undervoltage protection, overcurrent protection, and over temperature protection.

• Some power supplies come with sleeved cables, which is aesthetically nicer, makes wiring easier and cleaner and have less effect on airflow.

• There is a popular misconception that a greater power capacity (watt output capacity) is always better. If all else is equal, this is true, but since supplies are self-certified, a manufacturer's claims may be double or more what is actually provided. Although a too-large power supply will have an extra margin of safety as far as not over-loading, the more excess capacity there is, the less efficient it will be, and the more electricity it will waste. Additionally, computer power supplies generally do not function properly if they are too lightly loaded. Under no-load conditions they may shut down or malfunction.

• Power supplies do not always live up to what they are marketed. Noise can be measured from different distances and at different room temperatures.

• Power supplies can feature magnetic amplifiers.

• Power supplies can feature double-forward converter circuit design.

Wiring diagrams

AT power connector (Used on older AT style mainboards)

Color	Pin	Signal
	P8.1	Power Good
	P8.2	+5 V
	P8.3	+12 V
	P8.4	−12 V
	P8.5	Ground
	P8.6	Ground
	P9.1	Ground
	P9.2	Ground
	P9.3	−5 V
	P9.4	+5 V
	P9.5	+5 V
	P9.6	+5 V

24-pin ATX power supply connector
(20-pin omits the last 4: 11, 12, 23 and 24)

Color	Signal	Pin	Pin	Signal	Color
	+3.3 V	1	13	+3.3 V sense
	+3.3 V	2	14	−12 V
	Ground	3	15	Ground
	+5 V	4	16	Power on
	Ground	5	17	Ground
	+5 V	6	18	Ground
	Ground	7	19	Ground
	Power good	8	20	−5 V (optional)
	+5 V standby	9	21	+5 V
	+12 V	10	22	+5 V
	+12 V	11	23	+5 V
	+3.3 V	12	24	Ground

False advertising

The DIY boom has led to power supply manufacturers marketing their products directly to end-users, often with grossly inflated specifications. Some of the main tricks employed are...

• Advertising the peak power, rather than the continuous power.
• Determining the power at unrealistically low temperatures (below 40°C).
• Advertising total power as a measure of capacity, when modern systems are almost totally reliant on the number of amps on the 12 volt line(s).

So if...

• PSU A has a peak rating of 500 watts at 25°C, with 25 amps (300 W) on the 12 volt line, and
• PSU B has a continuous rating of 500 watts at 40°C, with 33 amps (400 W) on the 12 volt line,

and those ratings are accurate, then PSU B would have to be considered a vastly superior unit. PSU A may only be capable of delivering a fraction of its rated power under real world conditions.

This tendency has led in turn to greatly overspecified power supply recommendations, and a shortage of high-quality power supplies with reasonable capacities. Very few computers require more than 300–350 watts maximum, and more like 200 watts typically, but "everyone knows" that a high-performance computer requires at least a 500 W supply.

Modular power supplies

A modular power supply is a relatively new approach to cabling, allowing users to omit unused cables. Whereas a conventional design has a numerous cables permanently connected to the power supply, a modular power supply provides connectors at the power supply end, allowing unused cables be detached from the power supply, producing less clutter and a neater appearance. It also make it possible to supply a wider variety of cables, providing different lengths or Serial ATA instead of Molex connectors.

The disadvantage is an extra connector at the power supply, increasing cost and electrical resistance, which can affect the performance of the power supply, and potentially decreasing reliability.

While it is hoped that eliminating the excess cables will improve the flow of cooling air inside the computer case, the modular connectors tend to block airflow inside the power supply itself. The emphasis on appearance in modular power supply marketing tends to underscore this point.

Troubleshooting

A power supply is tasked with providing electricity to every component in a computer, so a faulty or under performing unit will often cause a wide range of symptoms, including, but not limited to:

• Failure to power on.
• General instability.
• Rebooting, either randomly or when the computer is stressed.
• Hardware failure, due to sudden power spikes/dips or long term electronic noise.

Capacitors

Bad or leaky capacitors. See capacitor plague. Solid capacitors are good.

Noisy fan

Most desktop computer power supplies are equipped with an exhaust fan, which helps to keep internal components cool and operate more efficiently. Abnormal fan noise is generally caused by a lack of internal lubrication or failing motor. While it's relatively easy and inexpensive to lubricate/replace a fan, opening a power supply can be very dangerous and usually voids the warranty, so it's best left to a professional.

See also

• Green computing
• Power management
• Power supply
• List of manufacturers

External links

• "80 PLUS" Certified Energy Efficient Power Supplies Program
• Website with Information & Research on Active Mode Power Supply Efficiency
• How to Buy an Energy-Efficient Power Supply
• Computer Power Supply - Schematics, Reviews, Repair Guides
• PC Repair and Maintenance: In-depth Look at Power Supply
• X-bit labs - ATX Power Supply Units Roundup. Part V
• ATX12V Power Supply Design Guide v2.2
• Power Supply Design Guide for Desktop Platform Form Factors Revision 1.1
• How to Discover Your Power Supply Real Manufacturer
• Dell non-standard Power Supplies

Computer Power Supply Calculators

• eXtreme Power Supply Calculator (frequently updated)
• Schrock Innovations' Power Supply Requirement Calculator (outdated)
• Computer Power Supply Calculator (outdated)
• JOURNEYSYSTEMS' online power supply calculator (outdated)