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:


 
Tiresmoke said:
Take a Tagan u22 for example. I use it as it has a trick. You can combine it's two rails into one with a switch on the back. It has a 30a total 12v output if you have it switched to single rail. Or if you select dual it is a 20/20 split. Tagan is a pretty good PSU maker. However you can see that it is only giving 30a total power output on the 12v rails no matter where the switch is set. So in split mode both rails will have to equal that number. That may be a 15/15 split or a 20/10 split or maybe something in between. I have seen folks have issues in split mode that goes away in single rail mode. So yu tell me what is going on then.

Tagan has a patent pending on this so we dont see this in other psus.  In my second rig, I have the tagan u22 set to a single rail setting.  I think at the time they made these dual rail was just coming out and Im sure they wanted to sell these psus in any flavor a consumer wanted.

But more to the point.  We have another major company coming out with 2 new huge single rail psus - Silverstone.  The first model is the 650w, 54a on the 12v rail:
http://www.silverstonetek.com/products-op650.htm

And the 1 megaton psu with 80a on a single 12v rail:
http://www.silverstonetek.com/products-op1000.htm

Now why would 2 major companies go back to single rail psus?  I was reading up on this and its cheaper to make than multirail psus plus they are finding that their is no safety issue with having so many amps on a single rail (like electrocution).
 
PCP&C also went back to Single Rail PSU's. Like I have said right along is that Split Rails are just marketing hype and they are taking your cash away for smoke and mirrors.
 
ChakkaSol, your second link is the same as the first. I think this is the link you wanted for 1000W, megaton as you put it, PSU: http://www.silverstonetek.com/products-op1000.htm
 
ive been having trouble with my stupid dual rail psu lately,what do you guys think of this http://www.ocztechnology.com/products/power_management/ocz_gamexstream_power_supply-nvidia_sli_ready_ the 600w it has 4*18a rail. im not able to buy any single rail psu, if i take the ocz it would be at cost price, so get a really good deal, btw ive it seen on a crossfire 1900xtx runnin fine
 
BG84

Here is a test report of the 700w version of that PSU.

http://www.jonnyguru.com/review_details.php?id=35

This is probably cheaper and does the job.

http://www.jonnyguru.com/review_details.php?id=30

 
Some of you might find this interesting.

PSU Recommendations for High End Gaming PC's

http://www.jonnyguru.com/forums/showthread.php?t=103
 
Link showing my end !!

Frankenputer

You must have read the post before I realised the link was not there & fixed it,3 minutes later.

Try again.

http://www.jonnyguru.com/forums/showthread.php?t=103
 
Guess what - Tiresmoke and Bas must have written the following for PC Power and Cooling...they have a myth link too:
http://www.pcpower.com/technology/myths/#8

PC Power states on their website:

8. ARE MULTIPLE 12-VOLT RAILS BETTER THAN A SINGLE 12-VOLT RAIL?
With all the hype about multiple 12-volt rails (ads claim that two rails is better than one, five is better than four, etc.), you?d think it was a better design. Unfortunately, it?s not!

Here are the facts: A large, single 12-volt rail (without a 240VA limit) can transfer 100% of the 12-volt output from the PSU to the computer, while a multi-rail 12-volt design has distribution losses of up to 30% of the power supply?s rating. Those losses occur because power literally gets ?trapped? on under-utilized rails. For example, if the 12-volt rail that powers the CPU is rated for 17 amps and the CPU only uses 7A, the remaining 10A is unusable, since it is isolated from the rest of the system.

Since the maximum current from any one 12-volt rail of a multiple-rail PSU is limited to 20 amps (240VA / 12 volts = 20 amps), PCs with high-performance components that draw over 20 amps from the same rail are subject to over-current shutdowns. With power requirements for multiple processors and graphics cards continuing to grow, the multiple-rail design, with its 240VA limit per rail, is basically obsolete.

PC Power and Cooling is once again leading the industry. All of our power supplies now feature a large, single 12-volt rail. The design is favored by major processor and graphics companies, complies with EPS12V specs (the 240VA limit is not a requirement) and is approved by all major safety agencies such as UL and TUV.
 
Yeah, but they only state it recently, we have been saying this right from the beginning :agrees:

In short, split-rails stink, single rail is the only good way to do it :lol_anim:
 
Very true Bas

The PC Power & Cooling Turbo-Cool 1kW-SR was first marketed as a 3 rail version.

Only recently has it reverted to single rail.
 
got today my ocz 600w gamextream, seems decent, i wanted a single rail, but i bought this one for the ridiculous price it was sold to me. i wanted a single rail one, but too much headache to find one. and like i showed the link before, the guys at dfi claim that its theyre favorite psu for high end systems (?), i hope theyre right.... ill be running some benchmarks and see the difference. i hope im lucky this time with a multiple rail psu..... :mad:
 
Just an update on another psu-brand. Have a look here:
PSU comparison table: http://www.qtec.info/products/comparison.htm?mpg=PSU
Can someone tell me what to compare it with?
And then this:
http://www.qtec.info/service/warranty/
What do they mean by "carelessness"? Do you think what I think? Can someone explain this?
 
Ow, I'm so dumb somethimes ... this is not dual rail, but dual fan ...  :lol_anim:
Oh well, who cares?  :mad:
 
Well, that's just the point: it seems that Q-Tec doesn't sell any psu's anymore and that, because of that, they stopped making them. This is probably the last one left in stock ...
 
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