Power supply unit (computer)

Power supply unit (computer)
Power supply unit with top cover removed

A power supply unit (PSU) supplies direct current (DC) power to the other components in a computer. It converts general-purpose alternating current (AC) electric power from the mains (110 V to 120 V at 60 Hz [115 V nominal] in North America, parts of South America, Japan, and Taiwan; 220 V to 240 V at 50 Hz [230 V nominal] in most of the rest of the world) to low-voltage (for a desktop computer: 12 V, 5 V, 5VSB, 3V3, −5 V, and −12 V) DC power for the internal components of the computer. Some power supplies have a switch to select either 230 V or 115 V. Other models are able to accept any voltage and frequency between those limits and some models only operate from one of the two mains supply standards.

Most modern desktop computer power supplies conform to the ATX form factor. ATX power supplies are turned on and off by a signal from the motherboard. They also provide a signal to the motherboard to indicate when the DC power lines are correct so that the computer is able to boot up. While an ATX power supply is connected to the mains supply it provides a 5 V stand-by (5VSB) line so that the standby functions on the computer and certain peripherals are powered. The most recent ATX PSU standard is version 2.31 of mid-2008.


Power rating and efficiency

Computer power supplies are rated based on their maximum output power. Typical power ranges are from 500 W to lower than 300 W for small form factor systems intended as ordinary home computers, the use of which is limited to web-surfing and burning and playing DVDs. Power supplies used by gamers and enthusiasts mostly range from 450 W to 1400 W. Typical gaming PCs feature power supplies in the range of 350–800 W, with higher-end PCs demanding 800–1400 W supplies. The highest-end units are up to 2 kW strong and are intended mainly for servers and, to a lesser degree, extreme performance computers with multiple processors, several hard disks and multiple graphics cards. The power rating of a PC power supply is not officially certified and is self-claimed by each manufacturer. A common way to reach the power figure for PC PSUs is by adding the power available on each rail, which will not give a true power figure. Therefore it is possible to overload a PSU on one rail without having to use the maximum rated power.

This may mean that if:

  • PSU A has a peak rating of 550 watts at 25°C, with 25 amps (300 W) on the 12 volt line, and
  • PSU B has a continuous rating of 450 watts at 40°C, with 33 amps (400 W) on the 12 volt line,

and if those ratings are accurate, then PSU B would have to be considered a vastly superior unit, despite its lower overall power rating. 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. Simple, general purpose computers rarely require more than 300–350 watts maximum.[1] Higher end computers such as servers and gaming machines with multiple high power GPUs are among the few exceptions.


Most computer power supplies are a square metal box, and have a large bundle of wires emerging from one end. Opposite the wire bundle is the back face of the power supply, with an air vent and an IEC 60320 C14 connector to supply AC power. There may optionally be a power switch and/or a voltage selector switch. A label on one side of the box lists technical information about the power supply, including safety certifications maximum output power. 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 and India.

A RoHS or 80 PLUS can also sometimes be seen.

Dimensions of an ATX power supply are 150 mm width, 86 mm height, and typically 140 mm depth, although the depth can vary from brand to brand.


Various connectors from a computer PSU.

Typically, power supplies have the following connectors (all are Molex (USA) Inc Mini-Fit Jr, unless otherwise indicated):

  • PC Main power connector (usually called P1): This 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 (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. A power supply with a 24-pin connector can be used on 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.
  • ATX12V 4-pin power connector (also called the P4 power connector). A second connector that goes to the motherboard (in addition to the main 24-pin connector) to supply dedicated power for the processor. For high-end motherboards and processors, more power is required, therefore EPS12V has an 8 pin connector.
  • 4-pin Peripheral power connectors: 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 red wire is +5 V, and the yellow wire is +12 V. In some cases these are also used to provide additional power to PCI cards such as FireWire 800 cards.
  • 4-pin Molex (Japan) Ltd power connectors (usually called Mini-connector or "mini-Molex"): 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.
  • 6-pin 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.
  • 6+2 pin For the purpose of backwards compatibility, some connectors designed for use with high end PCI Express graphics cards feature this kind of pin configuration. It allows either a 6-pin card or an 8-pin card to be connected by using two separate connection modules wired into the same sheath: one with 6 pins and another with 2 pins.
  • A IEC 60320 C14 connector with an appropriate C13 cord is used to attach the power supply to the local power grid.

AT vs. ATX

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

There are two basic differences between AT and ATX power supplies: The connectors that provide power to the motherboard, and 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 power switch on the front of the computer goes to the motherboard over a connector labeled something like; PS ON, Power SW, SW Power, etc. This allows other hardware and/or software to turn the system on and off.

The motherboard controls the power supply through pin #14 of the 20 pin connector or #16 of the 24 pin connector on the motherboard. This pin carries 5V when the power supply is in standby. It can be grounded to turn the power supply on without having to turn on the rest of the components. This is useful for testing or to use the computer ATX power supply for other purposes.

AT stands for Advanced Technology when ATX means Advanced Technology eXtended.

Other Form Factors

The Thin Form Factor with 12 Volt connector (TFX12V) configuration has been optimized for small and low profile microATX and FlexATX system layouts. The long narrow profile of the power supply (shown in Figure 1) fits easily into low profile systems. The fan placement can be used to efficiently exhaust air from the processor and core area of the motherboard, making possible smaller, more efficient systems using common industry ingredients.[2]


Most portable computers have power supplies that provide 25 to 200 watts. In portable computers (such as laptops) there is usually an external power supply (sometimes referred to as a "power brick" due to its similarity, in size, shape and weight, to a real 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.


Some web servers use a single-voltage 12 volt power supply. All other voltages are generated by voltage regulator modules on the motherboard.[3]

Energy efficiency

Computer power supplies are generally about 70–75% efficient.[4] That means in order for a 75% efficient power supply to produce 75 W of DC output it would 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 efficient PSU's waste less energy in heat, and requires less airflow to cool, and as a result will be quieter. Google's server power supplies are more than 90% efficient.[3] HP's server power supplies have reached 94% efficiency.[5] Standard PSUs sold for server workstations have around 90% efficiency, as of 2010.

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 (examples of how this curve looks can be seen on test reports of energy efficient models found on the 80 PLUS website). As a rule of thumb for standard power supplies it is usually appropriate to buy a supply such that the calculated typical consumption of one's computer is about 60% of the rated capacity of the supply provided that the calculated maximum consumption of the computer does not exceed the rated capacity of the supply. Note that advice on overall power supply ratings often given by the manufacturer of single component, typically graphics cards, should be treated with great skepticism. These manufacturers want to minimize support issues due to under rating of the power supply specifications and advise customers to use a more powerful power supply to avoid these issues.

Various initiatives are underway to improve the efficiency of computer power supplies. Climate savers computing initiative promotes energy saving and reduction of greenhouse gas emissions by encouraging development and use of more efficient power supplies. 80 PLUS certifies power supplies that meet certain efficiency criteria, and encourages their use via financial incentives. On top of that the businesses end up using less electricity to cool the PSU and the computer's themselves and thus save an initially large sum(i.e. incentive + saved electricity = higher profit).


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 larger or 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 (about 11 years continuous operation) 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 putting a coil in series with the primary filter 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,[6] both for cost reasons and because it is an irritation to customers.[1] (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.
  • Early PSUs used a conventional (heavy) step-down transformer, but most modern computer power supplies are a type of switched-mode power supply (SMPS) with a ferrite-cored high frequency transformer.
  • Computer power supplies may 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 detrimental effect on airflow.
  • Since supplies are self-certified, a manufacturer's claimed output may be double or more what is actually provided.[7][8] Although a too-large power supply will have an extra margin of safety as far as not over-loading, a larger unit is often less efficient at lower loads (under 20% of its total capability) and therefore will waste more electricity than a more appropriately sized unit. Additionally, computer power supplies generally do not function properly if they are too lightly loaded. (less than about 15% of the total load.) Under no-load conditions they may shut down or malfunction.For this reason the no-load protection was introduced in some power supplies.
  • The most important factor for judging a PSUs suitability for certain graphics cards is the PSUs total 12V output, as it is that voltage on which modern graphics cards operate. If the total 12V output stated on the PSU is higher than the suggested minimum of the card, then that PSU can fully supply the card. It is however recommended that a PSU should not just cover the graphics cards' demands, as there are other components in the PC that depend on the 12 V output.
  • Power supplies can feature magnetic amplifiers or 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 ATX12V 2.x power supply connector
(20-pin omits the last four: 11, 12, 23 and 24)
Color Signal Pin Pin Signal Color
Orange +3.3 V 1 13 +3.3 V Orange
+3.3 V sense Brown
Orange +3.3 V 2 14 −12 V Blue
Black Ground 3 15 Ground Black
Red +5 V 4 16 Power on Green
Black Ground 5 17 Ground Black
Red +5 V 6 18 Ground Black
Black Ground 7 19 Ground Black
Grey Power good 8 20 Reserved N/C
Purple +5 V standby 9 21 +5 V Red
Yellow +12 V 10 22 +5 V Red
Yellow +12 V 11 23 +5 V Red
Orange +3.3 V 12 24 Ground Black
  • Pins 8, and 16 (shaded) are control signals, not power:
    • Power on is pulled up to +5 V by the PSU, and must be driven low to turn on the PSU.
    • Power good is low when other outputs have not yet reached, or are about to leave, correct voltages.
  • Pin 13 supplies +3.3 V power and also has a second thinner wire for remote sensing.[9]
  • Pin 20 (formerly −5 V, white wire) is absent in current power supplies; it was optional in ATX and ATX12V ver. 1.2, and deleted as of ver. 1.3.
  • The right-hand pins are numbered 11–20 in the 20-pin version.

Modular power supplies

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

While modular cabling can help reduce case clutter, they have often been criticized for creating electrical resistance. Some third party websites that do power supply testing have confirmed that the quality of the connector, the age of the connector, the number of times it was inserted/removed, and various other variables such as dust can all raise resistance. However, this is somewhat inconsequential as the amount of this resistance in a good connector is small compared to the resistance generated by the length of the wire itself.[10]

See also


  1. ^ a b "Power Supply Fundamentals (page 3), silentpcreview.com". http://www.silentpcreview.com/article28-page3.html. Retrieved 2008-04-20. 
  2. ^ "TFX12V Thin Form Factor with 12-Volt Connector Power Supply Design Guide, formfactors.org". 
  3. ^ a b "Google plans to go carbon neutral by 2008" by Bridget Botelho 2007
  4. ^ Strong Showing: High-Performance Power Supply Units | Tom's Hardware
  5. ^ http://www.80plus.org/documents/80PLUS_PLC-DSC.pdf
  6. ^ "OCZ GameXstream 700 W Power Supply, Hardware Secrets". http://www.hardwaresecrets.com/article/397/5. Retrieved 2008-04-20. 
  7. ^ Oklahoma Wolf (September 14, 2007), The Bargain Basement Power Supply Roundup, jonnyGURU.com, http://www.jonnyguru.com/modules.php?name=NDReviews&op=Review_Cat&recatnum=13, retrieved 2008-01-31 
  8. ^ Rutter, Daniel (2008-09-27). "Lemon-fresh power supplies". dansdata.com. http://www.dansdata.com/gz086.htm. Retrieved 2008-09-28. "The lemon-market in PC power supplies has now officially become bad enough that no-name generic "500W" PSUs may actually barely even be able to deliver 250 watts. A realistic constant rating for these units is more like 200 watts. So the capacity inflation factor's hit 2.5, and it's still rising." 
  9. ^ "ATX Specification Version 2.1". http://www.formfactors.org/developer/specs/atx2_1.pdf. 
  10. ^ Gerow, Jon (2006-08-10). "Modular Power Supplies: The Reality of the Resistance". motherboards.org. http://www.motherboards.org/articles/guides/1488_1.html. Retrieved 2008-03-30. 

External links

Computer power supply calculators

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