09-08-2012, 12:56 PM
A power supply unit (PSU)
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converts mains AC to low-voltage regulated DC power for the internal components of the computer. Modern personal computers universally use a switched-mode power supply. Some power supplies have a manual selector for input voltage, while others automatically adapt to the supply voltage. Most modern desktop personal 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. The desktop computer power supply changes alternating current from a wall socket to low-voltage direct current to operate the processor and peripheral devices. Several direct-current voltages are required, and they must be regulated with some accuracy to provide stable operation of the computer. A power supply rail or voltage rail refers to a single voltage provided by a power supply unit (PSU). Although the term is generally used in electronic engineering, many people, especially computer enthusiasts, encounter it in the context of personal computer power supplies.
ATX standard
Voltage converter for 80486DX4 processors (5 V to 3.3 V). Note the heat sink on the linear regulator, required to dissipate the wasted power.When Intel developed the ATX standard power supply connector (published in 1995), microchips operating on 3.3 V were becoming more popular, beginning with the Intel 80486DX4 microprocessor in 1994, and the ATX standard supplies three positive rails: +3.3 V, +5 V, and +12 V. Earlier computers which wished to operate on 3.3 V typically used a simple but inefficient linear regulator to generate it from the +5 V rail.
The ATX connector provides multiple wires and power connections for the 3.3 V supply, because it is most sensitive to voltage drop in the supply connections.Another ATX addition was the +5sb rail for providing a small amount of standby power, even when the computer was nominally "off".There are two basic differences between AT and ATX power supplies: The connectors that provide power to the motherboard, and the soft switch. On ATX power supplies, the front-panel power switch provides only a control signal to the power supply and does not switch the mains AC voltage. This low-voltage control allows other hardware or software to turn the system on and off.
AT standard
Original IBM power supplies for the PC,XT and AT included a line-voltage power switch that extended through the side of the computer case. In a common variant found in tower cases, the line-voltage switch was connected to the power supply with a short cable, allowing it to be mounted apart from the power supply.An early microcomputer power supply was either fully on or off, controlled by the mechanical line-voltage switch, and energy saving low-power idle modes were not a design consideration of early computer power supplies. These power supplies were generally not capable of power saving modes such as standby or "soft off", scheduled turn-on, or "last state" power controls, as these concepts didn't exist yet.
ATX12V standard
As transistors become smaller on chips, it becomes preferable to operate them on lower supply voltages, and the lowest supply voltage is often desired by the densest chip, the central processing unit. In order to supply large amounts of low-voltage power to the Pentium and subsequent microprocessors, a special power supply, the voltage regulator module began to be included on motherboards. Newer processors require up to 100 amperes at 2 volts or less, which is impractical to deliver from off-board power supplies.
Initially, this was supplied by the main +5 V supply, but as power demands increased, the high currents required to supply sufficient power became problematic. To reduce the power losses in the 5 V supply, with the introduction of the Pentium 4 microprocessor, Intel changed the processor power supply to operate on +12 V, and added the separate P4 connector to the new ATX12V 1.0 standard to supply that power.
Because of this change, it is important to consider the +12 V supply capacity, rather than the overall power capacity, when using an older ATX power supply with a more recent computer. As all of the rails come from one transformer and primary-side switching components, there is an overall maximum power limit. Power requirements for a modern desktop personal computer may range from 300 watts to more than 1000 watts for a file server or a computer with multiple processors. The power rating of a PC power is rated by the manufacturer. Simple, general purpose computers rarely require more than 300–350 watts maximum.
It is possible to overload one voltage from a power supply well below the total rating of the power supply. For example, most PSUs create their 3.3 V output by regulating down their 5 V rail. As such, 3.3 V and 5 V typically have a combined limit as well. A 3.3 V rail may have a 10 A rating by itself (33 W), and the 5 V rail may have a 20 A rating (100 W) by itself, but the two together may only be able to output 110 W. In this case, loading the 3.3 V rail to maximum (33 W), would leave the 5 V rail only be able to output 77 W. The most important factor for suitability for certain graphics cards is the PSUs total 12V output. 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. However a system will have other loads on the 12 volt supply.