POWER SUPPLY
A computer power supply unit (Computer PSU), or Modular Power Supply Unit (MPS) 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 230V and 115V. 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.Contents [hide]
1 Wattage
2 Appearance
2.1 External
2.2 Connectors
2.3 Internal
3 AT vs. ATX
4 Laptops
5 Energy efficiency
6 Small facts to consider
7 Wiring diagrams
8 False Advertising
9 Modular power supply
10 Troubleshooting
10.1 Capacitors
10.2 Noisy fan
11 See also
12 External links
12.1 Computer Power Supply Calculators
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Wattage
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 2 kW for extreme performance computers with multiple processors and graphics cards (ATI CrossFire or NVIDIA SLI).
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Appearance
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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 maximum wattage.
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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.
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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 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, most of it going to power the 5Vsb (standby) rail.
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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.
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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 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.
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Energy efficiency
Computer power supplies are generally about 70–75% efficient[1]; to produce 75W of DC output they require 100W of AC input and dissipate the remaining 25W 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 [1]. Resonant-transition or quasi-resonant switching regulators could achieve over 90% energy efficiency, and also reduce radio frequency interference.[citation needed]
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 [2]). One rule of thumb is that a power supply which is over twice the required size will be much less efficient, and waste electricity.
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