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The Rails Explained
We have all, no doubt, been told that 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 controller 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. Until just recently, the +3.3V was used to exclusively power the CPU as well as some types of system memory, AGP video cards and other circuits.
+ 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 independant 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 enabling features such as wakeup from sleep mode, or wake on LAN technology to function. It also is what allows windows to turn your computer off automaticallly 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 mutha or 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, dual core based motherboard 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 is now two or more, each designated to power specific devices in order to ensure that nothing is underpowered.
Now as to why they are called rails, the best explanation that I can find is that the term comes from the wacky world of electronics and it refers to a long metal bar or strip that is used to proivde a particular voltage level. Perhaps someone with a deeper understanding of all things electrical could let me know whether or not that is true.
So now that you know the basics, let's move on shall we, and see how each of our contenders fared when put to the test.
ATX12v Specification Compliance
There are basically three main revisions of the ATX revision which can be found on power supplies sold today.
The 1.3 Spec introduced the additional 4pin CPU power connector into the mix. Originally this connector was needed only for P4 motherboard and many ATX 1.3 PSUs were labelelled at P4 compliant at the time. At this point in time anything 1.3 is legacy as it doesn't support PCI-E or the 24pin ATX power connector.
The 2.01 spec is basically a small revision beyond 2.0 which was the big new release which accompanied the jump from a 20 pin to 24 pin power connector. It states that the 4-pin CPU connector must be on it's own 12V rail and also updates minimum efficiency requirements. Additionally the recommended 12V current level was expanded as well as recommendation was made to add an additional 12V rail anytime requirements exceed 18A.
The jump to 2.01 is minimal and without any major additions, however it was the latest specification available between June 2004 and March 2005 so it is what you see many units branded with. If your unit is AT12V 2.0 though instead of 2.01 there isn't really anything you need to worry about.
2.20 is where we currently stand as of March 2005. It was released the same month as the short lived 2.1 revision which is why you won't see any units bearing that standard. The 2.20 again increased the efficiency requirements which now stand at a minimum of 65% or greater and a recommended level of 75% or greater. The +5VSB rail has also been increased to 2.5A
In short any power supply post 2.0 will do the job for you. The biggest advantage of purchasing a latter standar model would seem to be the increase in efficiency rating.
A full rundown on the ATX12V standard and its revisions can be found here.
EPS12V Specification Compliance
The EPS12V specification is one of those standards that most people have heard about, but very few know what it actually does. The EPS spec itself is defined in the Intel Server System Infrastructure (SSI) spec and is currently at v2.1.
The EPS12V spec dictates that Power Supply units must have a 24pin 12V connector as well as one additional 8-pin 12V connector. If a PSU is noted as EPS12V compliant you can be assured that it will have all the connections you need to power your new dual core based system, however it should also be noted that units do not necessarily have to meet EPS12V spec to include an 8-pin 12V connector.
On to the Show
Alright, school is out for the day. Now that we are all properly learned let's check out the list of contestants!
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