What's a dual rail?

[Smartazz]

I heard people talking about how this is important, what is a dual rail exactly?

 

[imported_Imp]

They're usually talking about the 12 volt rail when they say dual rail. Anyways, there's a big debate on the usefulness of two 12v rails. They initally made it to follow some new standard for safety whivh gives the CPU it's own 12v line and the other parts (video, HDD, etc.) on another. They draw from the same 12v source or something so it really doesn't make much of a difference. To find your total Amps on the 12v line, you just add up both rails. So in the end, it really doesn't matter, but from what I hear, it's better to just have one strong 12v rail than two.

 

[dguy6789]

Dual rail is supposed to be more "stable" but I have not seen any evidence to support this claim.

 

[Navid]

For safety, there used to be a maximum requirement on the current flowing through a single rail.

As the need for current from power supplies grew, the manufacturers addressed the need by providing two 12V rails. Then, they could provide more total current without violating the requirement since the current on each of those rails was less than the specified limit.

The two rails are completely isolated. So, if each of them can provide 15A and you need 16A from one of them, you will be out of luck even if the other rail is only loaded by 1A!

I have heard that the requirement has been removed or relaxed. But, I am not sure about that.

 

[JimPhelpsMI]

Hi, RAIL comes from the use of heavy copper bars across the back of the rack when individual boards were socketed into the rack to form a computer or controller etc. The copper bars were called RAILS. Dual rail should be individual sources within the PSU suppling two sets of +12 Volt leads from the PSU. To reduce signal mixing within the leads suppling the different devices drawing from the PSU. Simplified!
Hope this helps a bit, Jim

 

[pkme2]

Here another explanation.
http://www.techenclave.com/forums/dual-12v-rail-psus-why-they-4622.html

 

[Ken90630]

Originally posted by: Navid
The two rails are completely isolated. So, if each of them can provide 15A and you need 16A from one of them, you will be out of luck even if the other rail is only loaded by 1A!

Hey, Navid,

This is something I've been wondering about for awhile now. Don't take this the wrong way, but how sure are you of this? I think you're right, but I've been looking around & trying to find a definitive answer re whether or not the 12V rails can 'share' their power and I haven't been able to find a credible source to say one way or the other. And an article I read while back kinda implied, vaguely, that some dual-rail PSUs do share. So I dunno. Mind if I ask where you got your info?

Incidentally, I happened to be on the phone with a Seasonic 'tech guy' recently and I asked him about it. He, too, said that the two 12V rails are isolated & don't share, but he kinda hesitated when I asked & didn't sound too sure of himself. So I'm less than 100% confident about the answer.

If anyone has any links or other defintive, credible info on this, feel free to chime in.

EDIT: Just saw pkme2's post. Interesting info, and it seems to corroborate that the rails aren't shared. However, then I saw this. Scroll down the page to post by "Crazy Eddie" and read what he has to say. He's apparently of the opinion that PSU manufacturers can produce dual-rail PSUs that share their 12V rails if they choose to. So does the ATX 12V 2.x standard not require that the rails be isolated?

Aaaack ... the waters get muddier. :laugh:

 

[imported_Seer]

http://www.silentpcreview.com/article28-page3.html

Scroll down to "Dual 12V Lines: Specs." According to them (they asked industry engineers), it's a laregly useless feature since both rails still draw from the same power 12 converter. It's just supposed to minimize the current flowing through any one wire in case you get shocked.

It's very interesting, and I suggest you read it, as he covers a lot more than what I just summarized.

 

[Ken90630]

Originally posted by: Seer
http://www.silentpcreview.com/article28-page3.html

Scroll down to "Dual 12V Lines: Specs." According to them (they asked industry engineers), it's a laregly useless feature since both rails still draw from the same power 12 converter. It's just supposed to minimize the current flowing through any one wire in case you get shocked.

It's very interesting, and I suggest you read it, as he covers a lot more than what I just summarized.

Thanks for the link. This is the kind of credible info I've been looking for. :thumbsup:

Apparently the ATX12V 2.x standard is more of a 'preference' or 'suggestion' than an enforced requirement. As such, it looks like it's up to each mfgr to decide how they wanna handle the dual rails in their particular PSUs and what numbers they wanna use to rate their outputs on the 12V rails. Kinda leaves the door open to number manipulation and marketing 'hype,' but I guess that's become par for the course in the computer industry, eh?

After all is said & done, I don't know that it matters a heckuva lot if the PSU has dual rails or not. As long as high quality is otherwise there, in terms of wiring, heatsinks, fan(s), the fan controller, capacitors, efficiency, active PFC, warranty, etc., I don't think I would necessarily choose a dual-rail PSU over a single-rail one at this time. Although having a truly dedicated rail for just the CPU could, in theory anyway, contribute to overall stability (particularly in an overclocked rig with a power-hungry video card). That's what I gleen from all this, anyway.

I like the looks of the Seasonic S12 series and may end up getting one soon, but it'll be more for the quietness, the aluminum electrolytic capacitors, efficiency, sleeved cables and overall quality (& warranty) than the fact that it's a dual-rail unit.

 

[Navid]

Originally posted by: Ken90630

Originally posted by: Navid
The two rails are completely isolated. So, if each of them can provide 15A and you need 16A from one of them, you will be out of luck even if the other rail is only loaded by 1A!

Mind if I ask where you got your info?

I have never used an Amp meter and a variable high-power resistor to test this. By the way, that could be quite dangerous. I hope no one gets any ideas!

If the two rails can share current, they are not two rails anymore! If one rail can deliver more than 20A, the safety requirement is not satisfied because if there was a short, it could deliver too much power.

Whether there are separate rectifying diodes and coils, or only one set but two current limiters, the two rails will be isolated and the current on each rail will be limited to less than the total, which is the objective of the regulation.

Are some manufacturers not telling the truth about their products and effectively have just one rail when the product description suggests that it has two? That is quite possible.

 

[Ken90630]

Originally posted by: Navid

Are some manufacturers not telling the truth about their products and effectively have just one rail when the product description suggests that it has two? That is quite possible.[/quote]
Yeah, that's the impression I got from one of the articles or posts linked to earlier (I forget which one it was). I think OCZ was mentioned as one company that was 'sharing' 12V among its "two" 12V rails, but don't hold me to that (I was really busy this morning and had to skim thru the articles/posts).

Anywho, I still think you're right in that the ATX12v 2.x spec calls for the rails to be dedicated/isolated and that they not 'share.' But as I mentioned earlier, I also get the impression that the spec is merely the preferred/recommended way of doing things (preferred by Intel, who came up with the spec), but it's not a legal requirement. In other words, there's no PSU Police Force that's gonna knock some mfgr's door down if they're not complying. :laugh: That kinda annoys me 'cuz how are we users gonna know who's telling the truth about their "dual 12V" PSUs and who's not? Guess the only way right now is to look for credible reviews and hope they mention this.

That SPCR article linked to by Seer looks like a great read, BTW. I just read the first page this morning, but I intend to read the rest of it as soon as I get time.

 

[JimPhelpsMI]

Hi, If the PSU plate shows two +12 listings then the supplies should only share the Source, but be supplied from two different switching regulators. Again - to prevent signal mixing in the leads. I X R still = E and all leads have a small amount of resistance. Jim

 

[Skott]

Do we know of any dual rail PSUs that cannot supply the needed amperage in a system? Most people look at voltage alone for their needs when buying a PSU but commonly not much is said about how many amps a particular PSU supplies or how much a system uses at idle and peak usage.

I guess what I'm asking is how many amps should I be looking for a psu to supply wether it be a single rail or a dual rail? Anyone know that?