What About Them Rails
We have all no doubt been told when purchasing a power supply, that the number to look for is the amps on the 12V rail. What are each of the different rails for though, and why is the 12V rail typically the most important? Why the heck are they called rails? Let’s take at look at each and see.
-12V – This rail is pretty much obsolete now and is only kept on to provide backward compatibility with older hardware. Some older types of serial port circuits required both -12V and +12V voltages, but since almost no one except industrial users use serial ports anymore you as a typical home user can pretty much disregard this rail.
-5V – Again this is another obsolete rail, the -5V was used for old school floppy controllers and some ISA bus cards. Again, no need for the typical home user to worry about this rail.
0V – Though not listed on any manufacturer spec sheet, every power supply has a 0V ground line. The ground signal is used to complete circuits with other voltages and provide a plane of reference against which other voltages are measured.
+3.3V – Finally we are starting to get into something useful! The +3.3V rail was introduced with the ATX form factor in order to power second generation Pentium chips. Previously the CPU was powered by the +5V rail (along with the system memory and everything else on the motherboard), but a reduced voltage was needed in order to reduce power consumption as the chips got faster.
+5V – As mentioned above, the +5V used to run the motherboard, CPU and the majority of other system components on older pre ATX based systems. On newer systems, many of the components have migrated to either the +3.3V or +12V rails, but the motherboard and many of its onboard components still use the +5V rail so it is of importance to the typical home user.
+5V SB – The +5V Standby or “Soft Power” signal carries the same output level as the +5V rail but is independent and is always on, even when the computer is turned off. This rail allows for two things. First, it allows the motherboard to control the power supply when it is off by enabling features such as wake up from sleep mode, or wake on LAN technology to function. It also is what allows Windows to turn your computer off automatically on shutdown as opposed to previous AT supplies where you had to bend over and push the button. Every standard ATX power supply on the market will include this rail.
+ 12V – The +12V, also known as the mother of all rails, is now used to power the most demanding components in your system including the CPU, hard drives, cooling fans, and graphics cards. Historically the +12V rail was used only to power drives and cooling fans. With the introduction of the 4-pin CPU plug on P4 motherboards and then eventually AMD based motherboards, in order to supply newer power hungry CPUs, the +12V rail suddenly started to grow in importance. Today, multi core based motherboards require an 8-pin +12V connector to supply their power needs. High end GPU cards have also jumped on the +12V rail, which has required PSU makers to adapt. Where previously there was only a single +12V rail, there are now two or more, each designated to power specific devices in order to ensure that nothing is underpowered.