The amount of power required by a particular computer system will depend on the power requirements of the motherboard, processor and RAM, and on the number of add-on cards and peripheral devices drawing power from the PSU.
The highest power consumption is found in commercial network servers or high-performance personal computers featuring multiple processors, a number of disk drives, and multiple graphics cards. Systems used for gaming have much higher power requirements (typically 450 to 800 watts), mainly because they employ high-end graphics adapters which consume large amounts of power. Small form factor computer systems tend to have low power supply requirements in the order of 300 watts or less. The maximum rated power output of a PSU can range from around 250 watts up to as high as 2 kilowatts, depending on the type of system they are intended for.
The illustration below shows a typical ATX PSU.ĪTX PSUs are designed to work specifically with the ATX family of motherboards and fit into an ATX system case, and can be turned on or off (or placed into standby mode) using signals generated by the motherboard. The cables are often grouped and colour-coded according to the type of device they will be connected to.Īlthough in the past several form factors have been used for the power supply unit, some of them quite heavy and bulky, most desktop personal computers now use power supplies that conform to the standard ATX format, the most recent version of which is 2.3.1 released in 2008. Inside the case, a bundle of cables emerges from the front of the PSU. In some cases, there may be a voltage selector switch to allow the user to select a voltage according to their geographical location (the United States, for example, has a domestic power supply operating at a nominal 120 volts). The same aperture also allows air to flow into the PSU's cooling fan. It is mounted inside the system case using four screws in a standard location such that the on/off switch and power cord socket mounted on the back of the PSU are accessible via an aperture in the rear of the case. The PSU usually takes the form of a metal box 150mm wide x 86mm high x (typically) 140mm deep. The computer's power supply unit (PSU) converts the domestic alternating current (ac) mains supply voltage (220-240 volts in Europe) into various regulated, low voltage direct current (dc) outputs required by the components that make up the computer system.