Citat:
Although the ATX power supply specifications are mostly vertically compatible in both ways (both electrically and physically), there are potential issues with mixing old motherboards/systems with new PSUs, and vice versa. The main issues to consider are the following:
* The power distribution biases across 3.3 V, 5 V, and 12 V rails are very different between older and newer ATX PSU designs, as well as between older and newer PC system designs.
* Older PSUs may not have connectors which are required for newer PC systems to properly operate.
* Newer systems generally require larger power supplies than older systems.
This is a practical guidance what to mix and what not to mix:
* Older systems (until Pentium 4 and Athlon XP platforms) were designed to draw most power from 5 V and 3.3 V rails.
* Because of the DC-DC converters on the motherboard that convert 12 V to the low voltages required by the Intel Pentium 4 and AMD Athlon XP (and subsequent) processors, such systems draw most of their power from the 12 V rail.
* Original ATX PSUs have power distribution designed for pre-P4/XP PCs. They lack the supplemental 4-pin 12-volt CPU power connector, so they most likely cannot be used with P4/XP or newer motherboards. Adapters do exist but power drain on the 12 V rail must be checked very carefully. There is a chance it can work without connecting the 4-pin 12 V connector, but caution is advised.[15]
* ATX12V 1.x PSUs have power distribution designed for P4/XP PCs, but they are also greatly suitable for older PCs, since they give plenty of power (relative to old PCs' needs) both on 12 V and on 5 V/3.3 V. It is not recommended to use ATX12V 1.x PSUs on ATX12V 2.x motherboards because those systems require much more power on 12 V, and much less on 3.3 V/5 V than ATX12V 1.x PSUs provide.
* ATX12V 2.x PSUs have power distribution designed for late P4/XP PCs and for Athlon 64 and Core Duo PCs. They can be used with earlier P4/XP PCs, but the power distribution will be significantly suboptimal, so a more powerful ATX12V 2.0 PSU should be used to compensate for that discrepancy. ATX12V 2.x PSUs can also be used with pre-P4/XP systems, but the power distribution will be greatly suboptimal (12 V rails will be mostly unused, while the 3.3 V/5 V rails will be overloaded), so this is not recommended.
* Systems that use an ISA bus require ATX/ATX12V 1.2 or earlier because the ISA bus requires a −5 V power rail unless the board provides a DC to DC converter that supplies −5 V. ATX/ATX12V 1.3 and later prohibit the −5 V rail so power supplies built to these versions are usually unsuitable for ISA systems.
