Single rail or multi rail?

[ddarko]

It seems like a lot of the newest PSUs are single rail designs. A couple years ago, it seemed like multi rail designs were all the rage and single rails were supposed to be for "cheap" or bargain PSU. Now, the pendulum seems to be swinging back to single rail. I know a lot of this is marketing spin and take it with a grain of salt - PC Power makes a big deal about how single rails are inherently more efficient than multi rail - but what are the pros and cons for single rail vs. multi rail? Personally, this is also in the context of searching for a new PSU and narrowing it down two Corsair models, the 620 watt unit, which is multi rail and modular, or the 650 watt, which is single rail and non-modular.

 

[Billb2]

No matter how many "rails" a psu has, it can only put out it's rated power.

The amp ratings of the individual rail are only the current carying capacity of that rail, not how how many amps it can actually deliver.
If the sum of all the rails adds up to more than the total capacity of the PSU then if you load all the rails to their rated load, well, the blue smoke will come out.

I't kind of a non-issue. The marketing hype has everyone confused.

 

[MarcVenice]

You could stop worrying about it. Why ? Because the 620hx from Corsair is also a single rail. Yes, I know it sais it's a multiple 12v rail design, but it actually isn't. Single 12v rails aren't inherently better, but you can't really go wrong with them.

http://jonnyguru.com/forums/forumdisplay.php?f=3 Check the Powersupply FAQ and the Powersupply Basics threads. Enjoy your reading

 

[Modelworks]

I prefer single rail.
Just because I know that any connection I make will have access to the full output of the supply. With split rail, you might hit the limit for one rail.

Only reason they were split anyway is because of safety.
Ground a 12V 50A wire and you get some serious heat

 

[jonnyGURU]

Originally posted by: Modelworks
With split rail, you might hit the limit for one rail.

True. But the limit on the rail should be considerably more than what the connectors on that rail should ever need. For example: putting two PCIe connectors on it's own 20A rail. Or putting a CPU on it's own 20A rail.

 

[TigerMonsoonDragon]

doesn't a dual 12 rail psu combine into one when one is only used? i'm confused about that

 

[robertk2012]

Not exactly. It really depends on the power supply and what they call rails. There may or may not be OCP which would come into effect onces the amps on a single rail hit a certain point.

As for power being trapped. Thats just marketing garbage.

 

[jonnyGURU]

Originally posted by: kyubi
doesn't a dual 12 rail psu combine into one when one is only used? i'm confused about that

No.

Here's how it works if a PSU has multiple +12V rails:

You have ONE +12V rail. This rail is split up into multiple separate rails. Each rail is "capped off" with a limiter called an OCP (Over Current Protection).

Power isn't borrowed from other rails because that would mean you would defeat the OCP rendering completely useless. the OCP is there to PREVENT excess current from being delivered to the connector. Not to allow it to get MORE POWER from another rail.

There's no "trapped power" because the Amp rating you see on each rail is not a measure of capability. It's a measurement of limit policed by the OCP. That's why the sum of all of the rails is often far greater than the actual +12V capability of the PSU. For example: A 500W PSU with 360W capability on the +12V rail. This rail is then split into two, each with a 20A limit on it. 12V @ 20A = 240W. 240W X 2 = 480W. 480W > 360W. The 120W overlap is what actually debunks the "trapped power" theory.

Some lower power units, like 430W, 480W, 500W, etc. may have 17A or 18A rails in tandem. This could essentially "trap" power since so much is going to the CPU and only 17A or 18A to everything else. But these PSU's are simply inadequate for high end graphics cards, SLI, Crossfire, etc. in the first place that discussion is really moot.

 

[robertk2012]

Originally posted by: jonnyGURU

Originally posted by: kyubi
doesn't a dual 12 rail psu combine into one when one is only used? i'm confused about that

No.

Here's how it works if a PSU has multiple +12V rails:

You have ONE +12V rail. This rail is split up into multiple separate rails. Each rail is "capped off" with a limiter called an OCP (Over Current Protection).

Power isn't borrowed from other rails because that would mean you would defeat the OCP rendering completely useless. the OCP is there to PREVENT excess current from being delivered to the connector. Not to allow it to get MORE POWER from another rail.

There's no "trapped power" because the Amp rating you see on each rail is not a measure of capability. It's a measurement of limit policed by the OCP. That's why the sum of all of the rails is often far greater than the actual +12V capability of the PSU. For example: A 500W PSU with 360W capability on the +12V rail. This rail is then split into two, each with a 20A limit on it. 12V @ 20A = 240W. 240W X 2 = 480W. 480W > 360W. The 120W overlap is what actually debunks the "trapped power" theory.

Some lower power units, like 430W, 480W, 500W, etc. may have 17A or 18A rails in tandem. This could essentially "trap" power since so much is going to the CPU and only 17A or 18A to everything else. But these PSU's are simply inadequate for high end graphics cards, SLI, Crossfire, etc. in the first place that discussion is really moot.

You did a much better job explaining that than I did. You did forget to mention that some PSUs like antec say they have two, three or four rails but really only have one.

 

[jonnyGURU]

Yeah... but what does that really tell anyone?

Yeah... our marketing department was trying to convince you to add up the separate rails and assume that the +12V is much greater than it actually is?

In the grand scheme of things, the only thing it really means is if something shorts out your PC is going up in flames. No biggie.

 

[robertk2012]

Yeah I always like a good fireworks display ;-)

 

[Yellowbeard]

Originally posted by: jonnyGURU In the grand scheme of things, the only thing it really means is if something shorts out your PC is going up in flames. No biggie.

It's OK, I'll just add some fire dampening material to my case which should also help with noise.

 

[Modelworks]

Originally posted by: Yellowbeard

Originally posted by: jonnyGURU In the grand scheme of things, the only thing it really means is if something shorts out your PC is going up in flames. No biggie.

It's OK, I'll just add some fire dampening material to my case which should also help with noise.

I prefer halon sprays made from old water coolers

 

[DerwenArtos12]

Originally posted by: Modelworks

Originally posted by: Yellowbeard

Originally posted by: jonnyGURU In the grand scheme of things, the only thing it really means is if something shorts out your PC is going up in flames. No biggie.

It's OK, I'll just add some fire dampening material to my case which should also help with noise.

I prefer halon sprays made from old water coolers

oh god, lets just hope aigomorla doesn't read that!

 

[flexy]

Originally posted by: ddarko
It seems like a lot of the newest PSUs are single rail designs. A couple years ago, it seemed like multi rail designs were all the rage and single rails were supposed to be for "cheap" or bargain PSU. Now, the pendulum seems to be swinging back to single rail. I know a lot of this is marketing spin and take it with a grain of salt - PC Power makes a big deal about how single rails are inherently more efficient than multi rail - but what are the pros and cons for single rail vs. multi rail? Personally, this is also in the context of searching for a new PSU and narrowing it down two Corsair models, the 620 watt unit, which is multi rail and modular, or the 650 watt, which is single rail and non-modular.

Hello, i just posted an extensive thread on this subject.
http://forums.anandtech.com/me...=2166273&enterthread=y
The point is that even corsair, ocz, whatever currently "multi rail" in reality are only "virtual" pseudo multiple rails. The 12V rails are run togeter in one point in the PSU, effectively making it a single-rail CPU.

They're only called "multi rail" because each wire might have a limiter before it internally goes to the common 12V point for all "rails".

FAR from "independent, separate rails". This is only a markting "stunt" making the current drives conform to ATX specs - since each "wire" (so to speak) has each own Amps limiter, each "rail" cant draw over a certain amount of amps.

To make this easier understandable, typical shot of a circuit of a so called "multi rail' PSU

http://www.hardwaresecrets.com/fullimage.php?image=5095

You see that the 4 "independent" rails are all connected to the one point with only a resistor or bridge/jumper in between.

In other words:

99% of all current multi rail PSUs do actually NOT have ANY advantage, a disadvantage even because of the artificial limitation of draw on each of the pseudo rails. Nothing can be further from the truth than statements like "4 independent, seperate 12V rails" - suggesting that there are 4 power sources and the draw of each rail wouldnt affect the remaining --> thus being more "stable" than a single rail PSU. Common Myth.

I actually wonder which PSU actually has "real" separate rails..i would need to investigate further into this. According to the site above all of the current consumer grade high-end PSUs use that "virtual rail" design. Will digg more.

 

[jonnyGURU]

Flexy, I appreciate your enthusiasm for your new found knowledge but all of this was explained and covered in this thread four days ago.

Furthermore, your interpretation of the information also has some falsehoods in it. Not your source, but your interpretation: "99% of all current multi rail PSUs do actually NOT have ANY advantage." As previously explained, the splitting of the +12V does have a safety advantage and that is the true reason for it's implementation. Anyone marketing it as something else is lying, I will agree with you there. But to say that the rails are split for no reason outside of that marketing is completely false.

 

[flexy]

jonny, whow..i just came back from your awesome site. respect!

You are right...maybe i went too far saying "for marketing reasons only"..and its obvious that there must be a limit per rail for safety reasons. My input was indeed based on my "new found knowledge" - in regards to what's actually up with "separate rails" and (my) thinking this would increase stabilty, thinking "separate rails" is truly equal to independent power distribution. Now i read this is actually rarely the case, and (obviously) only a few PSUs starting from 1000W and up might have actually two real separate rails. Apologize i did not read the whole thread.

What is your thought as respected reviewer if companies clearly label with slogans like "independent rails"...or would you just "blame" it on the average Joe who reads something into it what in reality is not the case? (Eg the common myth about increased stabilty once it says "multiple rails" on the package)

G.

add:
Jonny, it's pure timing coincedence that i posted my rant in regards to my findings about the same time when you explained it here in detail. I didn't see your posting, so yes, my thread was kind of redundant

 

[jonnyGURU]

Check out your other thread. I posted a VERY long reply breaking down EVERYTHING you would want to know about splitting +12V rails.

Do I agree with the marketing? No. Marketing sucks. Marketing lies. If you make a good product that you can honestly stand behind and know deep down that you're giving the customer what they need AND want, you shouldn't have to lie.

That said, alot of false marketing is done out of pure ignorance. There's only a handful of OEM's, but five times as many power supply companies out there. If one OEM says that splitting +12V rails makes the voltages more stable, everyone that they make PSU's for is likely to buy into that. I must admit that early on I bought into it too. Until I started poking around and realized "hey... how can there be any improved stability without additional filtering components??" DOH! Of course, I have a tech background that consists of a lot of PC grunt work (installing servers, doing upgrades, building clones, etc.) including working in an RMA depatment or two and I have seen my share of burnt PCB's and PSU wires, so I can fully appreciate the REAL REASONS why there's OCP's on the rails.

Of course, any implication that the product you're selling is potentially "unsafe" is a marketing no no.