Power supply rail
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A power supply rail or voltage rail refers to a single voltage provided by a power supply unit (PSU).
[edit] Original IBM PC standard
In the original IBM PC PSU design, there are two voltage rails: +5V volts and +12V. These PSUs can also supply -5V and -12V, but with limited amounts of power.
- +5V: For most of the standard silicon microchips at the time, this was the voltage required to power them. Of the maximum 63.5 watts these PSUs could deliver, most of it was on this +5V rail.
- +12V: This rail was used primarily to operate fans and floppy disk drives. However, as faster chips and additional devices (such as hard disks and optical drives) came about, more power was delivered on the 12V rail. However, since most of the power is consumed by chips, the 5V rail still delivered most of the power.
[edit] ATX standard
On a modern ATX computer PSU, there are usually three primary voltage rails: one +3.3V, one +5V and as many as four +12V rails. Each of these supplies power to the various components of the computer system.
- +3.3V: This rail has its own circuitry in the PSU which generates the voltage. It has many wires and power connectors to distribute the 3.3 volts to components which require 3.3V. As microchip technology improved by squeezing a larger number of small transistors onto the chips, the voltage requirements for these transistors drops. The ATX standard added this 3.3V rail to power newer chips.
- +5V: Has its own separate set of circuitry, wires, and connectors.
- +12V: In modern PSUs, the most important voltage rail is the +12V rail which supplies power to components that draw the most amount of current. These are usually the cpu, hard drives or other storage peripherals, cooling devices, and video cards. Current fluctuations will usually cause malfunction, hence the importance of a good +12V rail. As such, modern ATX12V PSUs may have as many as five separate +12V rails. Like the 3.3V and 5V rails, each 12V rail has its own set of wires and connectors.
Note: Most PSU's create their 3.3V output by regulating down their 5V rail. As such, 3.3V and 5V typically have a shared rating. For example, a 3.3V rail may have a 10 amp rating by itself (33 watts), and the 5V rail may have a 20 amp rating (100 watts) by itself, but the two together may be able to output 110 watts. In this case, to load the 3.3V rail to maximum (33 watts), the 5V rail will only be able to output 77 watts.
As all of the rails come from one transformer, there is also an aggregate maximum power for the 3.3V, 5V, and 12V while simultaneously operating.
