BeQuiet! StraightPower 700w PSU

PSU - Power Supply Units 108 Page 3 of 8 Published by

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Rails

Power Supply Rails

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Recently we took a look at a couple of Zippy PSUs (here) and if you've read that review you'll notice they both had one large 12v rail. Since Intel introduced dual 12v rails in their guidelines, people seem to have forgotten all about single rail PSUs and the craze is all about the PSU with the most 12v rails apparently. I see people recommending that others buy a dual rail PSU because it's dual rail. Apparently a PSU isn't worth buying if it has a single 12v rail. It's like they forget single 12v rails ever existed. I guess that's marketing for you.

Dual 12v rails were first recommended in V2.0 of Intel's Power Supply Design Guide. They were recommended for any PSU that can deliver more than 18A at 12 volt because of safety requirements. Intel's guide limits the number of amps to under 20A on each 12v rail (12v x 20A = 240VA) as 240VA is the maximum figure recommended for any electronic device that a consumer may have access to. In other words, it's supposed help prevent people injuring themselves. Note the word "supposed". As far as I know it's not been proven.

On a dual rail PSU, the 12v2 rail will supply power to the 4-pin 12v connector on your motherboard which feeds the CPU. The 12v1 rail is supposed to power everything else that requires 12v such as hard drives, optical drives and video cards. Not all manufacturers will stick to this arrangement, but that's how it's supposed to be. (There's the word "supposed" again. ;-))

But actually, most dual rail PSUs only have one 12v rail. Even though it says there are two "independent" 12V lines they actually still draw from the same main source. It would be too expensive and inefficient to have two 120VAC:12VDC power conversion devices in a PSU. They also only have one Over Current Protection circuit for the sole 12v rail.

Now imagine you want to run an SLI or Crossfire setup and your PSU did actually have two independent 12v rails of 20A a piece and kept to the power arrangement as specified above. A 7900GTX or X1950XTX consumes roughly 9A each. Two of those is 18A. Now remember, one of your 12v rails is powering your CPU, the other has to do everything else and the most manufacturers can place on this rail is 20 amps, according to the guidelines. We already have SLI/Crossfire running off this rail, and that's 18A. We have two amps left to power everything else. You need hard drives, fans and optical drives. There more than likely isn't going to be enough power to go around. Depending on where the limiter is the PSU would simply shut down once it reaches that point. The load on 12v1 is very heavy but the load on 12v2 is very light. The power would need to be distributed better.

However the above doesn't happen. I know plenty of people running heavily overclocked SLI/Crossfire configs on dual rail PSUs. This is proof that dual rails are quite pointless as the dual rails are usually in fact a single rail. They are also supposed to improve safety towards the consumer yet that has never been proved and now Intel has removed the 240VA limit from their guidelines completely.

The BeQuiet PSU today has four 12v rails and this is how those rails are divided up:

Copyright 2006 Guru3D.com

The rails above are 18A each, and you have four of them. The load is shared out and the PCIe connectors are on seperate 12v rails. Yes, they are sharing with other connectors, but they won't pull as much power as another video card so essentially as long as the two PCIe connectors and thus your SLI/Crossfire setup isn't running off the same 12v rail, you should be trouble free and have plenty of room for overclocking too.

To be honest I'm not too sure what a quad rail PSU offers over a single 12v rail PSU. With a single rail you can place whatever you like where you like and not have to worry about balancing the load since everything is powered from the same source. On a PSU with 4x independant 12v rails you need to think about that a little more. Split rails does mean though that if there's a short on one rail the seperation provides protection and prevents harming of other components on a seperate rail.

Anyway, it goes without saying that you need to buy a quality PSU whatever config it has and you can find a list of quality PSUs here at XS. It also says which to avoid. You can't go wrong.

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