Why have multiple rails on PSU's?

[TLH]

This Silverstone Olympia OP650 I'm looking at getting has one big 54amp 12v rail.
I can see the point of 2 rails,one for cpu and one for the rest.That makes sense,putting dedicated power to the main component of a system,but what is the big deal about having 3 or even 4 rails,surely the components have to shove the same initial power down the tubes to start with so what's the point in splitting it up?

[freaksavior]

this should answer it,

http://forums.techpowerup.com/showth...=multiple+rail

it's pretty long, 60 replies, explaining the whole thing

[niko084]

Yes to sum it up so you don't read all the replies, I was pretty big into studying psu's at that time and learned a lot! Very very few psu's are truly independent multi rail, in fact its close to non. The big push on them was for safety with the early P4s that were overly power hungry. Although in the ultra high end where they have multiple transformers there are just god sends, they are huge, heavy, and VERY expensive, also pretty rare by all means. Really the multi rails psu for the average or even 99% of the high end gaming systems will go back to single rail.

Both work fine, but single rail is definitely easier for the average person to figure out ifs its going to work or not. If you are end up with a multi rail and it wont quite hold, and you "KNOW" what you are doing, you can take it apart and solder all the rails together and make it one big single rail in turn.

[TLH]

Well I'm not sure I'd want to take apart a brand new psu to do that anyway,even if I was that electrically minded.I'm doing the research now so that I get what I need straight off the bat.I'm not about to shell out close on £100 for something that won't do the job.

The reason I bought it up was I read something about the atx spec somewhere that single rails shouldn't really be more than 20amps.Now that sort of went against what I thought already as the Tagan TG480 u01 I already have has 28amps on the 12v rail.

Ah,here it is http://www.jonnyguru.com/forums/showthread.php?t=1036

Specifically this paragraph.

So why do they split up 12V rails?

With the demand on +12V becoming greater and greater, Intel decided it would be "safer" to split the duty of supplying +12V across two rails. It's "safer" because inexpensive transistors capable of supplying more amperage (say more than 34A) at any kind of decent efficiency (70% or better) are subject to blowing up. I guess that's not very “safe.” ;-)

To split the duty up between two (or more) +12V rails, one can use cooler running, cheaper transistors to supply the power. Furthermore, this isolates devices on one rail from another, so EMI introduced by lighting inverters and drive motors can be isolated from sensitive components like the CPU and video card.

Some people have questioned the principle of multiple 12V rails.

And for good reason! But I don’t think multiple 12V rails in general should be shunned. But it’s best to know what rails go where when considering using a multi-12V rail power supply with a high end system.

ATX specifications only say that the CPU (the 2x2 4-pin connector) is put on a separate rail from the ATX connector (the 20 or 24-pin) and the drive (also used for fans, lights, etc.) power connectors. They also specify that no one rail should have more than 20A available on it (that’s their “safe” limit, so to speak.)

So if you breeze through reading that, you would say “Ok. The CPU gets it’s power from the 12V2 and everything else gets it’s power from the 12V1.” But then you realize there’s a problem with that. 20A for just a CPU, even a dual core or even a dual CPU, is overkill. And 20A may be enough for some drives, lights, fans, etc. But what about PCI express video cards that regulate their voltage from the 12V rail via an auxiliary 6-pin connector? High-end video cards can easily tax 7A or more EACH off of the 12V rail. 20A leaves zero overhead.

Unfortunately, some power supplies adhere to the “quick read” version of the ATX standard and put everything but the CPU on one rail. This is where everyone seems to be running into problems. Fortunately, some other power supply companies have gotten creative with rail distribution. I’ve seen power supplies with the PCI express connectors on 12V2 and even some with one PCI-e connector on each of the two 12V rails. THESE are the kind of dual rail power supplies you need to look for.

Some power supplies have more than two rails. The Antec NeoHE, for example, has three. Two modular connectors are labeled for 12V3 use. These are the two ports one should plug their PCI-e connectors into. Other power supplies have four 12V rails. These typically adhere to a standard other than ATX called “SSI” but PCI-e is taken into consideration by keeping the PCI-e off of the same rail as all of the drives. Even if a PCI-e is plugged in using a typical drive Molex, that rail is still separate from the ATX connector, and the 2x2 4-pin connector.

To be honest I nodded off while reading that other thread last night.lol.It was gone 2am.So I'll have another read after some grub and a cuppa.

The other psu I was considering before I spotted that OP650 was a Silverstone Strider ST75F 750W.I like this because of the modular aspect and it still comes in under a ton.It has plenty of juice for the 12v's to use (max of 648w or 54w) which is the same as the op650 as far as I can see,just split up instead.

Now both have a real good Jonny Guru review but the ST75F gets a little shaky at high loads while the ripple on the OP650 is virtually non existent,20mv compared to the ST75F's 75mv.Even 75mv isn't close to being bad though,both psu's have good quality components I presume (I'm not up on good caps),the only thing I have to seperate the two is the modularity and rails questions.Jonny seems to have proved that the Strider has seperate rails so which psu is better?

[largon]

Seasonic M12 500W / 600W / 700W are advertised as quad-rail but they really have one big rail because there's no over current protection associated with Intel ATX12V spec.
48A for 12V (576W), and rock solid voltages.

Check the review at jonnyguru if you haven't already.

[niko084]

I don't know much about the OP650, but that Silverstone will be enough power to suffice just about any system out there.

Largon- the reason they are quad rail and more like 1 single rail is because they combine them at the end where they output to the cables. Weather it was done on accident or purpose, thats the reason. Multi rail or single rail you can't combine them without physically connecting the wires.

As for taking apart a new psu, I understand your thoughts there, personally it doesn't bother me, but I can relate to as much.