COMPUTER POWER SUPPLY
Figure 6. The top cover has been removed to show the internals of a computer Power supply Unit.
A computer power supply unit (Computer PSU) is typically designed to convert 110 V (USA) or 230 V (Europe) 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. 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.
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)
Figure 7. 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 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.
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.
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; though only a few will function perfectly with only the original 20 pins connected. Similarly, most PSUs have the connector split into two parts, with 20-pin and 4-pin respectively, which can be used together to form the 24-pin connector. This is for the purpose that it will be compatible with both past and future motherboards that either use 20-pin connector or 24-pin connector.
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