Overclock.net banner
1 - 20 of 160 Posts

·
Premium Member
Joined
·
16,521 Posts
Discussion Starter · #1 ·
First off, I'd like to give credit to Jonny Gerow (former star of JonnyGURU.com, now PSU project manager at BFG) for writing what I consider the definitive article on power supply "rails", one which educated me. If you want an accurate historical and mildly technical explanation of power supply rails, read his article here:
http://www.jonnyguru.com/forums/showthread.php?t=3990

Also, I'm going to not bother mentioning the +5V and +3.3V rails, because it just confuses the matter. In this article I will talk only about the +12V rail, and single/multiple versions of it.

So let's get down to it.

"Single rail power supplies are best!"
"No, multi rail units, stupid. You've got like five times as many rails to power your parts."
"No, single rail is best, are you an idiot? Look at how much more I can overclock!"
"You're just a moron, you don't know what a rail is!"
"You're just jealous because I've got a single, massive, powerful rail, if you know what I mean."

Children, children, calm down and shut your traps. You're both wrong! It doesn't matter, at least not in the way 99% of enthusiasts think.

What is a rail?



No, now stop being a smartass.

Seriously, most people have very strong opinions on what rail distribution is best, without understanding what a rail is. The common misconception is that a rail is a part in the power supply that provides power. This is utterly and completely false. You can have two power supplies that are 99% identical and one be single rail and one be multi.

No, rather a rail is a group of traces on the PSU's mainboard that are monitored by an OCP circuit.

"What?"

I said, it's a group of traces on the--

"Shut up until you can speak English."

Fine. A trace is a pathway of copper on a circuit board that carries electricity. You can see them as the faint copper-y lines running all over your motherboard. In this instance, I'm referring to the traces on the power supply that the wires in the cables are soldered to, specifically the ones carrying +12V power. With me so far?

"Yes. Speaking of tracing, I'm going to go get some stencils."

Whatever. Now, OCP is Over Current Protection. What OCP does is it monitors an output on a power supply. If the amount of current--

"What is--"

Read this: http://en.wikipedia.org/wiki/Electric_current
If the current going through a given output exceeds a certain amount, it will shut the power supply down.

What does this have to do with single or multiple rails?

"I was about to ask, yes."

Glad to know. The difference between single or multiple rails is this:

Multiple rail: each trace is monitored separately, so if, say, one trace goes over 25A the power supply will shut down.

Single rail: all traces are monitored all together, so if the total current going through the +12V outputs goes over, say, 60A, the power supply will shut down. Alternatively, no OCP may be present at all on the +12V rail.

Make sense? Nothing about single rail having extra power or being more stable, on either side. You could have two identical power supplies with the only difference being how the OCP chip is configured, and one could be single rail and the other multiple. Understand?

"No, I--"

Yes you do, if you don't go back and read through again.

So which is better? Well, neither has any affect on voltage stability or ripple suppression or efficiency or anything, really, at least not to a measurable degree. So all those arguments are bogus. A single rail PSU will not let you overclock any higher than a multi rail, or vice versa. However, there is a difference.

Let's imagine you have a short circuit on the +12V and the SCP (Short Circuit Protection) doesn't catch it. If you have a single rail power supply the draw on that one +12V wire and that one +12V trace will climb until either the combined OCP point is reached, or until something burns; the cable, your power supply, your motherboard. With large units, usually something will burn before the OCP point is reached.

In a multi-rail unit, the current would climb until it hit that rail's OCP point, at which point the PSU would shut down to protect itself and your computer. Ideally you'd have a rail for every wire, but that would be impractical, so you generally see 2-8, depending on the power rating of the PSU and the design philosophy.

So multi-rail is inherently safer, correct? Yes, but early on there were some problems. You see, when multi-rail units were first introduced, the specification at the time was poorly written, and power supply engineers made a mistake; they put all the cables that power heavy-draw components like your CPU, mobo, and graphics card(s) on one rail, and all the light-draw stuff like HDDs and fans on the other. This meant that you had one rail that might be pulling, say, 24A, and another pulling 5A.

"I see! They were unbalanced, so the man inside the PSU fell off his unicycle and--"

No. If your PSU's OCP trip point for that main rail was set to 24A, then your system could easily hit that trip point when under load, causing the PSU to shut down to protect itself. That could cause you to lose data, or leave your friends without backup while playing TF2.

"Wow that sucks."

Yes indeed. The power supply was just as powerful as the single rail units of the time, but would shut down under heavy loads because of the poor cable distribution among the OCP rails.

However, that is no longer an issue. The problem units only existed around 2006; since 2008 virtually all power supplies have had intelligent cable arrangements, with only a few gaffes here and there. On a modern multi-rail power supply there is virtually no chance of tripping a rail's OCP in normal use unless you're running way too much for the power supply to handle anyway.

So final verdict? Single rail or multi rail? Well, with low-wattage units it doesn't matter. OCP on a single rail is useful up to about 40A or thereabouts, which is where most 550W power supplies fall. So with 550W and under power supplies, it's a moot point. However, with high wattage units, >45A on the +12V (650W and higher) picking a multi-rail unit will provide you with an extra layer of protection. It isn't essential, and it has no impact on the power supply's performance. However, it does provide an extra layer of safety in case you get a short circuit. And I would consider it a must for >1000W power supplies; [H] recently tested the single rail Corsair AX1200, but they had an accidental short circuit, and since the PSU's OCP is set for over 100A, the short overloaded and destroyed most of their testing equipment. So there is a danger with single rail units over 1000W.

So multi-rail is mildly better, especially with high wattage units, but it won't have any impact on your performance or overclockability.

There, does that settle it?

".... HAHA TOLD YOU MULTI RAIL WAS BETTER."
"You can kiss my rail for all I care!"

*sigh*

This has been your friendly neighborhood Phaedrus, signing out.

Further Reading
 

·
Premium Member
Joined
·
65,162 Posts
What about the Enermax Galaxy 1000w or Corsair 1000HX? I heard they have "true" multiple rails.


Did you want to mention the one of the originators of the misconception and the reason for the "myth"?
 

·
Premium Member
Joined
·
16,521 Posts
Discussion Starter · #4 ·
Quote:

Originally Posted by DuckieHo View Post
What about the Enermax Galaxy 1000w or Corsair 1000HX? I heard they have "true" multiple rails.

I swear if you make me go into that I'll dock you 100 internet points...


It's still irrelevant though.
 

·
Data Recovery Engineer
Joined
·
20,229 Posts
LOL Very intersting way of explaining things! Good job!
 

·
Data Recovery Engineer
Joined
·
20,229 Posts
Quote:

Originally Posted by computeruler View Post
Very helpful. I've always wondered this.
Same
 

·
Registered
Joined
·
435 Posts
Very useful, was having this argument the other day with my buddy!
 

·
The Fabricat0r
Joined
·
5,934 Posts
Quote:

Originally Posted by DuckieHo View Post
What about the Enermax Galaxy 1000w or Corsair 1000HX? I heard they have "true" multiple rails.


Did you want to mention the one of the originators of the misconception and the reason for the "myth"?
One of? They where the origin
. But I'd like to keep my blood pressure down so let's not talk about that. They are dead to me anyway.

I find myself to be apathetic to the whole thing anymore. I can't be bothered to debate it even with people who know what they are talking about. It doesn't even factor into my own PSU buying decisions. In all of my years of computers I have never had a short or seen another computer in real life that shorted and melted/burnt something due to lack of OCP. I know it happens, but the frequency is so low I am much more concerned about the dangers of driving home from work than a freak accident with my computer.
 

·
Premium Member
Joined
·
16,521 Posts
Discussion Starter · #11 ·
Quote:

Originally Posted by shinji2k View Post
One of? They where the origin
. But I'd like to keep my blood pressure down so let's not talk about that. They are dead to me anyway.

I find myself to be apathetic to the whole thing anymore. I can't be bothered to debate it even with people who know what they are talking about. It doesn't even factor into my own PSU buying decisions. In all of my years of computers I have never had a short or seen another computer in real life that shorted and melted/burnt something due to lack of OCP. I know it happens, but the frequency is so low I am much more concerned about the dangers of driving home from work than a freak accident with my computer.
Yeah, it's no factor to me either, but I'm getting sick of debating it just like you are. So I decided the best way to settle it is an article. Multi-rail has a slight advantage because of OCP, but it doesn't really matter most of the time.
 

·
Premium Member
Joined
·
18,289 Posts
Nice read and easyer to understand then some others that try to explain it. 1 thing i would like psu makers to do is mark what rail goes to what so you can balance the load out.
 

·
Premium Member
Joined
·
16,521 Posts
Discussion Starter · #14 ·
Some already do.

In case you're wondering, your M12D is labeled as being multi rail when it's actually single. SeaSonic is weird like that.
 

·
Premium Member
Joined
·
2,857 Posts
Yes! Great read. This should certainly clear up any debate. Oh, and I vote to sticky and recommend everyone must read this before making a purchasing decision.
 

·
Registered
Joined
·
579 Posts
Great write-up.
One of the major reasons we see so many PSU's with single rails these days is because ATX2.2 lifted the 240va limit requirement and made it merely a recommendation. This is, I believe, a major contributor to why we are seeing rails that exceed 20A. PC P&C isn't such a bunch of rebels anymore.......not much of anything anymore actually.
 

·
Premium Member
Joined
·
18,289 Posts
Quote:

Originally Posted by Phaedrus2129 View Post
Some already do.

In case you're wondering, your M12D is labeled as being multi rail when it's actually single. SeaSonic is weird like that.
Yea i know but its still a nice psu i bought it cause i got it for 130 and jonnyguru gave it a good review. And i did notice its labled on inside but not outside i just need to remmber first 2 plugs go to rail 1 and 2nd to last one.

Quote:
This would be a good time to mention that the two 12V rails share a single source. They are split using overcurrent protection, just like 99% of the other multirail units out there.
 

·
Registered
Joined
·
887 Posts
At first thought it appears as if the different rails have different source, so load on one rail won't affect the other. However they share the same source, so all this multi rail thing is very misleading.
 

·
Registered
Joined
·
401 Posts
Quote:

Originally Posted by Kaninja View Post
PC P&C isn't such a bunch of rebels anymore
They weren't really in the first place - several industrial OEMs, most notably Zippy, never really adopted the 240VA requirement in the first place. They had some models that conformed to it, and some that didn't... both before and after the 240VA limit was imposed. It was just business as usual for them.

PC P&C actually tried on the 240VA limit for everything above the old 510W before realizing the SSA spec wasn't right for home computers and going to single 12V rather than make the rail distribution make sense.

Quote:

Originally Posted by lurkingdevil
At first thought it appears as if the different rails have different source, so load on one rail won't affect the other. However they share the same source, so all this multi rail thing is very misleading.
Only up until you realize it's the same sort of idea used by your household breaker box.
 

·
Premium Member
Joined
·
18,289 Posts
yea way i understand it is each rail has a failsafe and is only allowed say 40amps befor it triggers the failsafe compared to a single has to go way higher befor it trips it.
 
1 - 20 of 160 Posts
Top