PSU's---2 x 12v---The Dual Rail Myth

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FastEddie

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The Dual Rail Myth

In looking at the newest ATX standard, the ATX12VPower Supply Design Guide, ver 2.01, we find two major changes from the past in that 2 x 12v rails are now advocated, rather than a single 12v rail, in addition to a 24-pin main ATX power connector interface.  How does this dynamically impacts us AMD users?  First, let?s look at some of the changes made from the ATX 1.3 standard and see how they impact us, as power users.

The most important change is the incorporation of 2 x 12v rails, or ?Dual Rail,? technology, where the processor now has basically its very own power interface that is separated from all other power consuming devices on the motherboard.  A Quote from Section 1.2.1, entitled ?Increased +12VDC Output Capability? of the Guide:

?System components that use 12V are continuing to increase in power. In cases where
expected current requirements is greater than 18A a second 12 V rail should be made
available. ATX12V power supplies should be designed to accommodate these increased
+12 VDC currents.?
This may sound like a good thing, and I?m sure that one day it will be a major improvement over past power supply interfaces, however, as an AMD user, the major drawback is that this newest standard (being designed around, and in support of, Intel based technology), took no consideration of AMD/Sli.  ?Our? problem isn?t that AUX ATX 12v Rail-1 is isolated with anywhere from 15amps to 18amps of power for the processor alone, but that the remainder of the entire system must depend upon ATX 12v Rail-2, with it?s 15amps to 18amps, to power all the other devices on the motherboard.  This is a lopsided balance of power in regulating and controlling devices in the average computer system, and becomes more than simple balancing act when this ?design? becomes the limiting factor for system stability in our high powered enthusiast systems.

What?s added to ATX 12v Rail-2 that makes system stability an ?issue? is related to the plethora of new technologies that must all be powered by this single rail.  Aside from SATA/SATA II, standard PCI, in addition to all ?onboard? peripheral interface devices such as multiple gig-lan network connections, multiple (up to ten) USB 2.0 connections, IEEE 1394 interfaces, standard serial/parallel port interfaces, legacy IDE interfaces and audio functions, the ?Guide? takes into consideration a new power hungry interface, being PCI-E, that consumes anywhere from 40watts to 120 watts of power in a single card.  The cpu no longer has the distinction of being the highest power consumer in a computer system, when one considers the amount of power a 6800 Ultra needs to operate properly.  This is a vast amount of power to take into consideration when building a new computer system, and one in which is compounded two fold with the release of nForce 4 SLI, where two such power hungry graphic adapters can coexist in the same system.

Many of you have followed the Dual Rail ?vs- Single Rail argument as it?s been played out in the implementation of NF4 Sli, where I have maintained that dual rail psu?s, as they exist today, are not an efficient use of the power resources these power supplies provide.  Where AUX ATX 12v Rail-1 (for the processor) would run at approximately 35%-50% of it?s available output 60%-70% of the time,  ATX 12v Rail-2 would run at or near 100% of its available output 100% of the time.  Power Supplies are not designed to work under these conditions and it seems that the ?Guide? agrees and has taken this into consideration, where at Sec. 1.2.2. Minimum Efficiency, it points out:

?Minimum measured efficiency is required to be 70% at full and typical (~50%) load and
60% at light (~20%) load. New recommended guidance has been added to provide
direction for future requirements.?
What this tells us is that, indeed, ATX 12v Rail-2 shouldn?t be required to run at more than 70% MAX of it?s available output on a constant ?full and typical? basis.  And it means that Dual Rail psu?s are not the best choice for powering nForce 4 Sli as these power supplies weren?t designed to work in such a lopsided environment.  It is much more efficient to use a high output Single Rail psu with at lease 26a on that 12v rail, where 28a for the Sli dual 6800 Ultra?s is a much more stable environment to run these top of the line cards.  And for those that say, ?well these boards were designed around the ATX 2.01 standard, so they should work just fine,? my answer is that they weren?t ?designed? around NF4 Sli by nature of the limitations the ?Guide? itself places on that second 12v rail.  Another response regards accumulation of the two 12v rails, where 18a & 18a = 36a and plenty to power any motherboard.  My response is this is a total misconception as these power supplies have no ability what-so-ever to accumulate their output current from the two totally separate rails on the fly.  Sure, there are one or two psu?s that do have the ability to combine their output, via a switch, but that ?switch? turns them into essentially a more powerful single rail unit, with less amperage than the total of both 12v rails. Until a psu is designed for Sli, don?t be drawn into the marketing hype about ?Sli Ready? and ?Sli Capable? as their use goes contrary to the very guide that implements them.  You would be much better served with a single rail power supply that can ?efficiently? power that Sli gamers box.  :cool:


 

thespin

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Thanks, fastEddie.  But are you saying that the 12V pins on the 24pin power connector share the same transformer as 12V Rail#1 ?  I have the Silverstone ST60F (600W) with FOUR 12V rails and I could use rails2-4 for peripherals leaving rail#1 (18A) unused and reserved for the motherboard if that's the case (I am not using SLI or a high-end video card) ....
 

Supershanks

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Hi thespin,


This as i understand it is the rail arrangement for the S/stone Strider 600/ Zeues650 & Akasa 600.  12v3  serves the pci-e & mobo. 12v4 the molex (excluding sata).

The loading shown is from my own rig when researching a new psu. I didn't opt for this model as i was concerned about the 12v4 rail being overloaded. 8A max load doesn't give much to play with.

Silverstone are supposedly revamping the 12v rails see Best PSU ?

luck
 
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skippy9146

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I thought it best to avoid the whole "Dual Rail Debacle" & use a Power Stream with a single 33A 12v rail.

Call me crazy, but I never have bought into the whole dual rail thing.
 

Supershanks

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Hi dudes
I thought it best to avoid the whole "Dual Rail Debacle" & use a Power Stream with a single 33A 12v rail.
Hehe ;) :lol_anim:
see My new PSU

Remember FastEddie  & syar sharing issues on dual rails , all that's happened since is more complexity & confusion.

This is a decent analysis.Dual Rails ? The true story

luck :smile:
 
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skippy9146

Guest
Supershanks said:
Hi dudesHehe ;) :lol_anim:
see My new PSU
38W... That'll work! :biggthumbsup:

Seems like those guys have gotten that Big Fat Single Rail religion.... I like this quote -

"Powerful single +12V rail (38A) for special performance requirements."
 

Supershanks

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LOL and we all like to be special. HeHeHe
- there's no answer to that :lol_anim: :D

was really pleased with ? as well cost ?80($140), had been budgeting a bit more going by the 600w units but in the end i got headache & kept it simple. ;D
 
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skippy9146

Guest
I like more power & all that... but there's just something wrong about a full Kilowatt PSU in a "regular" PC. :shocking:

Seems to be getting a bit out of hand.
 
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Tiresmoke

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LOL But they are certified... Ya Certified to remove cash from your wallot. HeHeHe

All kidding aside as I told another person here on the forum. If after all the years I have been involved in building and servicing rigs proffessionally, I have a rough time sorting it out then how can the average joe computerist figure out what is right or wrong with multirail PSU's. Why bother. Stay with what you know works and works in a proven way.

Makes good old fashioned horse sense to me.
 

Supershanks

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well u can see where it's driven from The Grand Clash for Watts: Power Consumption of Modern Graphics Cards with X1900XT pulling 120.6 watts about 9.5 12v Amps( ex memory) each , but I'm not a big gfx user so won't be bothered.
I have a rough time sorting it out then how can the average joe computerist figure out what is right or wrong with multirail PSU's
is exactly the point & i really think this is where pps can get caught out.
 

Sealance

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I read on the first of the post about the duel rail myth. Well you are right by a sort. But as an engineer I can tell you that what you are not taking into this is the loading range and loading amp feed on the rails.

Yes they feed off the same transformer but the load is not just on one rail and it does not have to suffer from amp pull from another line  in front of it. Or a output line that was already loaded to full amp draw before the second line started to load up.
  On the robotic systems that I build for lines in factorys we have lessend the lost of units by going to two and three rail systems.

The making of two rails does make it better over all and I can show you where load balancing of the two rails does help while running the computer. That is if you have a big enough output from your PS to start with.
 
  So do not make it sound like buying a two rail system is a waist of money as it is not. 
Will a singel rail system work just fine , yep it will. Is a two rail system  better , yep it is.
  Now weather you want to spend the extra cash for the two system is up to the person.

  But the main object here is that you have a large enough power Supply for your given need in the first place. So when you add up the watts that you will need on you system then try to come as close to doubbling that amount as you can aford. There are other factors that come into play on the amout that you will need and use besides the total amount watts/amps and the number of rails in a given system power supply.
 
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Tiresmoke

Guest
I could buy that if and only if we all built our rigs the same. HJowever some folks may have 7 HDD's and Opticals to deal with and maybe very little used from a MB otherwise. Others make have multi Vid Cards and Multicore CPU's or even multiple CPU's. Others may have a huge amount of RAM or other devices. Which rail needs power and how will it affect the overall power balance acrossed two or more rails? If you are talking a purpose built machine that is configured and then mass produced with little or no change from model to model or even could be custom engineered that is a whole different situation and you would certainly have more control over how power is distrivuted.

Yes done right multi rail units could have an advantage but how many computer builders have what it take to determine which is right for their needs. You aren't talking engineers for the most part.
 
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Tiresmoke

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I am not here to decry a standard I personally see as bogus. I am trying to make folks use their heads and investigate this further. I am not an engineer. I am a tech. Most folks know that here. I give advice like many other users here do based on experience and what we can clearly see by helping others with their issues. Most folks get allong just fine with multi rails PSU's and some simply don't. My advice is simple. If you are unsure and don't understand this then either have someone prove the new standard works for your build or stay with something you know will work. If you are already using one but suspect PSU issues then try a single rail and see if the problems are solved by it. Ask a freind who may have one if you can borrow it. Take it to a shop that can test it. Whatever but at least know that there is potential there that a multirail is not OK for your application and be ready to deal with that.
 
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ChakkaSol

Guest
Minimum PSU Requirements from the evga website:

1.  7900gtx single - 450w psu with 22a on the 12v rail
2.  7900gtx sli - 550w psu with 34a on the 12v rail

3.  7900gt single - 400w psu with 20a on the 12v rail
4.  7900gt sli - 500w psu with 28a on the 12v rail

5.  7600gt single - 350w psu with 18a on the 12v rail
6.  7600gt sli - 450w psu with 24a on the 12v rail

Plus, the new nvidia cards require less power than the previous 7800 and 6800 series video cards.  Also if you decide on an ATI card(s), they require more power than these 7900 series video cards too.  There you have it in black and white directly from a video card manufacturer.
http://www.evga.com/products/moreinfo.asp?pn=512-P2-N570-AX
 
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Tiresmoke

Guest
Personally I feel the trend will be to go back to good quality single rails in the end. This Dual or more Rail idea seems to me to be a failed marketing concept. But hey I have been wrong in my past predictions on occassion as well. Just not very often. LOL

Maybe just an idea that had this hit two or three years earlier it would have worked at the time and by now more conventions would have been worked out to make it work with the rigs we have today. LOL like an idea whose time has already past.
 

thespin

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Supershanks said:
8A max load doesn't give much to play with.
NO JOKE, amigo! Thanks for this information!  It may explain a lot about the problems I am having.  Your links also talked about the problems caused by unbalance loads:
http://www.extremetech.com/article2/0,1558,1932947,00.asp

I am going to try to RMA my ST60F for Silverstone's redesigned version.  Thanks again!
 
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