Stupid question: What are rails????

[Greasetattoo]

Hello,
I think I allready know what they are, but what are rails on a power supply????
Thanks,
Grease

 

[Greasetattoo]

Figured it out, just as I though....

This is one of the many issues that you see in online forums. People talk about that their “rails” are low and that they need a new power supply. First off, the “rails” that they are referring to are the 3.3V, 5V, and 12V lines that come off the power supply. The 3.3V and 5V generally power the digital devices like the motherboard, PCI slots, AGP slot, etc. The 12V is usually used for things that are motorized like the hard drives, CD-ROM, fans, etc. These lines are rated to run at a certain specification. It is usually somewhere around plus or minus 5%. Technically, if these lines stay within this margin, the system should run fine with no instabilities but if they do run out of specification a number of problems can arise. It is rare that somebody posts a problem with their power supply going over 5% above what the line is supposed to be. Usually power supplies go under the minimum voltage when used over a long period of time. When the rails are low this usually leads to blue screens and various other instabilities like random reboots. When these lines are overvolted problems arise again like drive failure, blue screens, damaged drives and damaged motherboards. Therefore, it is easy to see why it is important to have a good power supply.

 

[sailor]

A "rail" is a single output of the PSU. Things connected to the same "rail" are electrically connected to the same output. Note that this does not mean necessarily that different rails are different voltages as you can have different rails with the same voltage. For instance, you can have two different rails both at 12V. This can be done for different reasons but mainly it can be that you want a clean 12V for one thing while another 12V for other things and you don't want them interfering with each other. Just think a rail as a single strip at the output where you can connect different things and they are all in parallel. I hope that helps.

 

[Ice Czar]

Greasetattoo
while it true most comments you see address low rails, and that is a sign of an overloaded supply,
the part about high rial outside of the 5% spec needs a bit of clarification
supplies dynamically alter the voltage as the draw from the components changes
this is the load regulation \ transient response the spec given for a supply is by the rail and generally when documented as transient response will read that for a 20% load change the rail will return to within 5% of the nominal voltage for that rail within less than 1ms.

failure to bring back down a high voltage during one of these changeups is called an overshoot, and considering how quickly this all occurs its often missed by monitoring software the SMBus (System Management Bus) and obviously the user

while low rails can lead to instability, overshoots kill things
refer to the ESD & Electromigration Rant
either immediately or more slowly by wearing on them

sailor excellent post
Id add that isolation of the rails is another factor
in most supplies the +3.3V and +5V have a shared capacity, but not in all supplies,
however in the latest ATX12V spec (v2.0) the main reason a second rail was adopted was for isolation
following the lead taken by the EPS12V spec, where there are 2 flavors a "common plane"
which has 2 CPUs on one rail, mobo on another and peripherals on a third
the alternative being a "split plane" where there are four +12V rails and each CPU gets its own
capacity between the rails in all the above is not shared and each is individually regulated