Power supply buyers guide

Voltage rails

The primary function of a PSU is of course changing 230V from your outlet into power the components in your PC can use. A PSU delivers five different voltages:12V, 5V, 3,3V, -5V and -12V. The last two negative voltages are no longer used in modern hardware, so the focus is on the first three mentioned.

We start with the most power hungry component in a PC, the processor. The processor is completely depended on the 12V circuit in a PSU and components on the motherboard itself convert the 12V from the PSU into different values for the processor. On average, a modern processor uses about 80W of power, quite a hefty load on a 12V circuit. Partly because of this modern ATX 2.0 PSU have two or more 12V rails. The second rail (12V2) is solely used to power the processor, which is why there is a separate 4 pins connector on modern PSU's and mainboards. The first rail (12V1) is used to power the other components that depend on a 12V current. The biggest advantage of 2 or more rails is that the processor will always receive a constant and stable supply of power, resulting in a more stable PC.

Some PSU manufacturers take it a step further and have introduced PSU's with three or even four 12V rails. Althought he official ATX standards does not include any specifications for the third or fourth 12V rail, nVidia has already implemented a SLI-ready certificate, which can only be issued to PSU's with a third circuit. This way the processor, graphics cards and other components are all fed through a different rail. PSU's with three or more 12V circuits do not always specify which circuit feeds what: the processor is exclusively fed by 12V2, but all other components are shared between 12V1, 12V3 and 12V4, one of which will exclusively feed the graphics cards. If you plan to build a Crossfire or SLI system, it is wise to contemplate the purchase of a PSU with three or more 12V rails. However, be careful as not all PSU's will have two 6-pin PCI-Express connections. PSU's with only one 12V circuit are basically only suitable for systems with a relatively simple processor and videocard.

A modern power supply features at least two 12V rails. For SLI and Crossfire we recommend to buy a PSU with three 12V rails. Models with no less than four 12V are also already available.