A power supply unit (PSU) converts mains AC to low-voltage regulated DC power for the internal components of a desktop computer. Modern personal computers universally use switched-mode power supplies. Some power supplies have a manual switch for selecting input voltage, while others automatically adapt to the main voltage
Most modern desktop personal computer power supplies conform to the ATX specification, which includes form factor and voltage tolerances. While an ATX power supply is connected to the mains supply, it always provides a 5-volt standby (5VSB) power so that the standby functions on the computer and certain peripherals are powered. 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 voltages are in spec, so that the computer is able to safely power up and boot. The most recent ATX PSU standard is version 3.0 as of mid
The overall power draw on a PSU is limited by the fact that all of the supply rails come through one transformer and any of its primary side circuitry, like switching components. Total power requirements for a personal computer may range from 250 W to more than 1000 W for a high-performance computer with multiple graphics cards. Personal computers without especially high performing CPUs or graphics cards usually require 300 to 500 W.[14]
Power supplies are designed around 40% greater than the calculated system power consumption. This protects against system performance degradation, and against power supply overloading. Power supplies label their total power output, and label how this is determined by the electric current limits for each of the voltages supplied. Some power supplies have no-overload protection.
The system power consumption is a sum of the power ratings for all of the components of the computer system that draw on the power supply. Some graphics cards (especially multiple cards) and large groups of hard drives can place very heavy demands on the 12 V lines of the PSU, and for these loads, the PSU's 12 V rating is crucial. The total 12 V rating on the power supply must be higher than the current required by such devices so that the PSU can fully serve the system when its other 12 V system components are taken into account. The manufacturers of these computer system components, especially graphics cards, tend to over-rate their power requirements, to minimize support issues due to too low of a power supply.
ATX motherboard power connector (usually called P1): This is the connector that goes to the 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, i.e. 20+4-pin form) which can be used together to form the 24-pin connector.
motherboard
12V only power connector (labelled P1, though it is not compatible with the ATX 20 or 24 pin connector): This is a 10 or 16-pin supplying the motherboard with three or six 12 V lines with common returns, a 'supply OK' signal, a 'PSU ON' signal and a 12 or 11 V auxiliary supply. One pin is left unused.[34]
Molex connector
12V only System monitoring (P10): This is a 171822-8 AMP or equivalent connector carrying a supply to the PSU fan and sense returns.
[34]
ATX12V 4-pin power connector (also called the P4 power connector). A second connector that goes to the motherboard (in addition to the 24-pin ATX motherboard connector) to supply dedicated power for the processor.
4+4-pin For the purpose of backwards compatibility, some connectors designed for high-end motherboards and processors, more power is required, therefore EPS12V has an 8-pin connector.
4-pin peripheral power connector4-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 US 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, mini-Molex, or ): This is one of the smallest connectors that supplies a 3.5-inch floppy drive with power. In some cases, it can be used as an auxiliary connector for Accelerated Graphics Port (AGP) video cards. Its cable configuration is similar to the Peripheral connector.
Berg connector
Auxiliary power connectors: There are several types of auxiliary connectors, usually in 6-pin form, designed to provide additional power if it is needed.
power connectors: a 15-pin connector for components that use SATA power plugs. This connector supplies power at three different voltages: +3.3, +5, and +12 V, in three pins per wire, one designed to precharge capacitive loads on for hot-plugging designed backplanes.
Serial ATA
6-pin Most modern computer power supplies include six-pin connectors that are generally used for graphics cards, but a newly introduced eight-pin connector should be seen on the latest model power supplies. Each PCI Express 6-pin connector can output a maximum of 75 W.
PCI Express
6+2-pin For the purpose of backwards compatibility, some connectors designed for use with high end graphics cards feature this kind of pin configuration. It allows either a six-pin card or an eight-pin card to be connected by using two separate connection modules wired into the same sheath: one with six pins and another with two pins. Each PCI Express 8-pin connector can output a maximum of 150 W.
PCI Express
12-pin for graphics cards, each PCI Express 12-pin connector can output a maximum of 648 W (12V, 9A), 2 150 W 8-pin can be combined via an adapter cable to form one 648 W 12-pin.
PCI Express
for PCI Express graphics cards, each PCI Express 16-pin connector can output a maximum of 662 W (12V, 9.2A), 12 power pins, 4 contact pins. Introduced on ATX 3.0.
16-pin 12VHPWR connector
An connector with an appropriate C13 cord is used to attach the power supply to the local power grid.
IEC 60320 C14
An SFX form factor PSU
A TFX form factor PSU
A FlexATX form factor PSU
A formerly used AT PSU, mechanically of the same size as the ATX PSUs
PS3 power supply, shorter than ATX, only, 300 W maximum (not to be confused with the PlayStation 3)[39]
An adapter allowing an SFX PSU to substitute an ATX or PS3 PSU
The Small Form Factor with a 12 V connector (SFX12V) configuration has been optimized for small form factor (SFF) system layouts such as microATX. The low profile of the power supply fits easily into these systems.
The Thin Form Factor with a 12 V connector (TFX12V) configuration has been optimized for small and low profile Mini ITX and Mini DTX system layouts. The long narrow profile of the power supply fits easily into low profile systems. The cooling 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 components.[37]
Most portable computers have power supplies that provide 25 to 200 W. 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.
External power supply could send data about itself (power, current and voltage ratings) to the computer. For example, genuine Dell power source uses 1-Wire protocol to send data by third wire to the laptop. The laptop then refuses a non-matching adapter.[38]
Some computers use a single-voltage 12 V power supply. All other voltages are generated by voltage regulator modules on the motherboard.[27]
Life span is usually specified in mean time between failures (MTBF), where higher MTBF ratings indicate longer device life and better reliability. Using higher quality electrical components at less than their maximum ratings or providing better cooling can contribute to a higher MTBF rating because lower stress and lower operating temperatures decrease component failure rates.[40]
An estimated MTBF value of 100,000 hours (roughly, 140 months) at 25 °C and under full load is fairly common.[41] Such a rating expects that, under the described conditions, 77% of the PSUs will be operating failure-free over three years (36 months); equivalently, 23% of the units are expected to fail within three years of operation. For the same example, only 37% of the units (fewer than a half) are expected to last 100,000 hours without failing.[a] The formula for calculating predicted reliability, R(t), is
where t is the time of operation in the same time units as the MTBF specification, e is the mathematical constant approximately equal to 2.71828, and tMTBF is the MTBF value as specified by a manufacturer.[42][43]
Power supplies for servers, industrial control equipment, or other places where reliability is important may be hot swappable, and may incorporate N+1 redundancy and uninterruptible power supply; if N power supplies are required to meet the load requirement, one extra is installed to provide redundancy and allow for a faulty power supply to be replaced without downtimes.[44]
Ripple tests are performed with an external load and monitoring equipment[48]
A 'power supply tester' is a tool used to test the functionality of a computer's power supply. Testers can confirm the presence of the correct voltages at each power supply connector. Testing under load is recommended for the most accurate readings.[47]
Most of power supply fans are not connected to the speed sensor on the motherboard and so cannot be monitored, but some high-end PSU can provide digital control and monitoring, and this requires connection to the fan-speed sensor or USB port on the motherboard.
(DC power supply for notebook computers)
IEC 62700
List of computer power supply manufacturers
Power management
Quiet PC
Switched-mode power supply applications
PS-ON Signal
ATX12V Power Supply Design Guide, v2.01
ATX12V Power Supply Design Guide, v2.2
(Power Supply Design Guide for Desktop Platform Form Factors, v1.1)
ATX12V Power Supply Design Guide, v2.3
(Power Supply Design Guide for Desktop Platform Form Factors, v1.2)
ATX12V Power Supply Design Guide, v2.31
(Power Supply Design Guide for Desktop Platform Form Factors, v1.31)
ATX12V Power Supply Design Guide, v2.4
How PC Power Supplies Work
Website with Information & Research on Active Mode Power Supply Efficiency
PC Repair and Maintenance: In-depth Look at Power Supply
![PS3 power supply, shorter than ATX, only, 300 W maximum (not to be confused with the PlayStation 3)[39]](http://upload.wikimedia.org/wikipedia/commons/thumb/1/16/PS3_PC-PSU_IMG_2431.JPG/155px-PS3_PC-PSU_IMG_2431.JPG)
![Ripple tests are performed with an external load and monitoring equipment[48]](http://upload.wikimedia.org/wikipedia/commons/thumb/3/36/PC_PSU_Noise_Test_Setup.svg/220px-PC_PSU_Noise_Test_Setup.svg.png)