Computer power supply

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The top cover has been removed to show the internals of a computer PSU.

A computer power supply typically is designed to convert 110 V or 230 V AC power from the mains to usable low-voltage DC power for the internal components of the computer. 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 of many computers.

Computer power supplies are rated for certain wattages based on their maximum output power. Typical rated wattages range from 200 W to 500 W, although units used by gamers and enthusiasts usually range from 500 W to 800 W, with the highest end units going up to 1 kW (although 1.1kW and higher power supplies are available for extreme performance computers with multiple processors and graphics cards (eg. ATI CrossFire or NVIDIA SLI)

Various connectors from a computer PSU.

Most computer power supplies have a large bundle of wires emerging from one end. One connector attached to the opposite end of some wires goes to the motherboard to provide power. The PS-ON wire is located in this connector, and is usually green. The connector for the motherboard is the largest of all the connectors. There are also other, smaller connectors, most of which have four wires: two black, one red, and one yellow. Unlike the standard electrical wire color-coding, each black wire is a ground, the red wire is +5 V, and the yellow wire is +12 V.

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 depending on the temperature, or less of often the power load. It may be dangerous to open a power supply even if while not connected to an electrical outlet as high voltages may still be present in charged capacitors. These may remain even while the unit is switched off, as there can be electric charge stored in the capacitors. However, for most PSU's 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 PSU's 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.

In desktop computers, the power supply is a box inside the computer; it is an important part of the computer because it provides electrical power in a form that is suitable for every other component inside or attached to the computer in order for it to work. In portable computers (such as laptops) there is usually an external power brick which converts AC power to one DC voltage (most commonly 19v), and further DC-DC conversion occurs within the laptop to supply the various DC voltages required by the other components of the portable computer.

The original ATX main connector is a 20-pin connector. Most newer boards have a 24-pin connector; however many will function perfectly with only the original 20 pins connected. Similarly, most PSUs split the connector into 20+4, so that it will be compatible with both past and future motherboards.

1 Small facts to consider
2 Modular power supply
3 See also
4 External links

[edit] Small facts to consider

Most power supplies have an On/Off switch on the back.

All modern power supplies should be RoHS-compliant.

Common certification marks are the CE mark, UL mark, GS mark, TÜV, NEMKO, SEMKO, DEMKO, FIMKO, CCC, CSA FCC, etc. The CE mark is required for power supplies sold in Europe.

There usually is a sticker on the PSU with a list of certifications, the specification, and a warning not to open the enclosure.

Some power supplies have a passive PFC, some have active PFC.

High-end computer power supplies usually have about 80% energy efficiency, the higher energy efficiency the better. A power supply with a higher energy efficiency will decrease the amount of energy lost to heat. As of 2006, there are power supplies with 85% efficiency.

Some servers come with two or more power supplies for redundancy.

Life span may be measured in MTBF and should be at least 100,000 hours. Higher MTBF ratings are preferable for longer device life.

Some power supplies have a switch to change between 230v and 115v. Some high-end models have automatic sensors that switch input voltage automatically.

It is common to connect the power supply to a power strip, along with the computer display, computer speaker and other peripherals. The power outlet should preferably be grounded. Some people connect the power supply through a surge protector or UPS.

In computer power supplies that have more than one 12v power rail, it is preferable to spread the power load over the 12v rails evenly to help avoid overloading one of the rails on the power supply.

Computer power supplies come with several different connectors depending on the need.
- All computer power supplies come with a 20 or 24 pin main power connector which plugs into the motherboard.
- All computer power supplies have 4-pin Molex connectors which plug into peripherals such as hard drives and CD-ROM drives.
- Most modern computer power supplies have a 4-pin auxiliary connectors which can plug into the motherboard to provide additional power. This connector is commonly called the "P4" connector because it was originally used to provide more power to the Intel Pentium 4 based motherboards (and was later adopted by AMD).
- Most modern computer power supplies include a SATA power cable for components which use SATA power plugs.
- Most modern computer power supplies include one or two 6-pin connector which are generally used for PCI Express graphics cards. Each PCI Express connector can output a maximum of 75watts.

When the computer is powered down but the 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).

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

[edit] Modular power supply

A modular power supply is a specialized power supply unit. Unlike regular PC power supplies, it has cables that can be plugged into the unit as the user wishes. This helps eliminate unwanted clutter inside the computer. Most companies' modular PSU cables are not compatible with one another.