All important computer hardware need specific amount of el. energy. This means that computer needs constant, stabile and reliable el. energy source with exact defined voltage and power values. With the development of PC computers, power supply unit also has made bigger or minor design changes. First models PC-XT used XT power supply units with 150-200W. Starting with AT286 computers used AT PSU with 200-250W, until first Pentium 2 computers. Today it is mostrly used ATX (for Pentium 2 and Pentium 3 computers) and ATX-P4 with 300-350W. Voltages that request computer hardware components are mostly standardized:



Comparing to standard ATX PSU ATX-P4 has extra 4-pin connector with +12/6+6A
In addition to standard PSU for PCs are used unstandard versions, mostly for server configurations. Those PSU allow so called tandemic work, meaning working two or more PSU at the same time so reliability of power supply could increase and leave redundancy which is of high importance for network servers. Those PSU are usualy allowed replacement with power on (hot plug). Mostly are montated on industrial boxes, rack.