Dual 12v rails explained?

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db_

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I'm posting this question on the MSI forums because these forums over the past 3 years have been more helpful than most others I've encountered.  I seem to remember, way back when, there was a PSU forum here, but I could be mistaken.  So I'm shoving this here.

Can anyone explain the whole dual 12v rail thing to me?  I know it splits 12v power to different components that consume power on the 12v rail.  I just don't know how it goes about it.  I purchased a PSU yesterday with dual 12V in preparation for a new system I am building.  My current mobo doesn't support dual 12v so I'm just using one of the connectors.  My main question is - how do you know where the power for certain components is going?  Is it intelligently handled by the PSU?  Like if it sees a beefy video card, it'll put that card on one 12v rail, perhaps some case fans and whatnot, and route the CPU and other 12v-consuming components to the other rail?  Or does it just spread power out willy-nilly?  I'd like to think it spreads it intelligently, but I think that's being a bit optimistic.

So.  What's the deal with dual 12v?  Any solid links explaining how it all works? (I've googled to no avail)  Anyone willing to share their knowledge? :)  PSU's, ever since the 12v rail has been sucking up more power and had to be taken into consideration in building a PC, have been my sticky point.  I just don't get it.  Any help at all would be appreciated.  If this is an inappropriate place to post this, please direct me somewhere where you think I can be helped.  But like I said, I came here first cause everyone here has always been extremely helpful - and you're all responsible for a build I've been running for almost three years, rock solid, without a single problem.  :biggthumbsup:
 
D

Dancer

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Here's some reading on the 12v dual rail. It's more the total amps on the 12v rail, single rail is fine

DUAL 12V LINES: SPECS

Version 2.0 of Intel's ATX12V Power Supply Design Guide began recommending dual 12V lines for PSUs that can deliver more than 18A at 12V. Why? To abide by safety requirements of UL and EM 60950, which stipulates not more than 240VA on any wires or exposed traces. Intel's PSU Guide calls for a current limiter that keeps current to under 20A on each of the 12V rails: 12V x 20A = 240VA.

What is the safety reason for this 240VA maximum? It's the maximum recommended for an electronic device that a consumer will have reasonable likelihood of access. In plain terms, it might be to keep people from zapping themselves inside a PC, or more likely, accidentally creating a fire risk. This safety "rule" does not apply to any electronic or electrical devices where the chance of consumer exposure is low, such as a TV or CRT monitor, for example.

In PSUs that conform strictly to ATX12V v2.xx, it's important to know that even though there are two "independent" 12V lines, they still draw from the same main source. It's highly unlikely that there are two separate 120VAC:12VDC power conversion devices in a PSU; this would be much too costly and inefficient. There is only one 12VDC source, and each of the two lines draw from the same 12VDC source, but through its own "controlled gateway".

PSU makers' specs are misleading in that they rate the current capacity of each 12V rail independently. What really matters is the total 12V current: Generally, up to 20A is available on any one 12V line assuming the total 12V current capacity of the PSU is not exceed.

What the above means is that you don't need to worry about imbalances in power draw on the 12V lines ?as long as no single line is asked to deliver more than 20A. PSU makers seem to mark each line for max current on a purely arbitrary basis, probably more for marketing reasons than any other. A PSU rated for 32A max on the 12V lines can be labelled many different ways:

    * 12V1: 18A, 12V2: 14A
    * 12V1: 17A, 12V2: 15A
    * 12V1: 16A, 12V2: 16A
    * 12V1: 15A, 12V2: 17A
    * 12V1: 14A, 12V2: 18A

It could be marked 20A + 12A, but being a cautious bunch, the engineers will probably not specify more than 18A on any one line. This gives 2A headroom to allow some room for error for the current limiting circuit.

DUAL 12V LINES: REALITIES

Note that 12V2 is supposed to supply only the AUX12V (2x12V) 4-pin plug, which feeds only the CPU. With PSUs that adhere strictly to the ATX 12V v2.xx Guide, 12V1 then must supply 12V to all the other components that require it. This might lead to a problem with very high power gaming systems that utilize two high power video cards in SLI or Crossfire mode. Current high end VGA cards by themselves can draw >90VA each. Much of this comes from the 12V line via the 6-pin PCIe connector for the VGA card. If you add several hard drives and optical drives, the 240VA limit may be too low.

The current ATX12V v2.2 spec was created before dual VGA card gaming configurations for Intel boards were announced. SLI, being an nVidia feature on nForce 4 chips for AMD CPU motherboards that came many months earlier, may have been ignored by Intel's PSU design guide team.

Not all PSUs with 6-pin PCIe connectors follow ATX12V v2.xx to the letter. In fact, they can't, as the guide does not cover the 6-pin 12V PCIe outputs. This connector and its current delivery capacity was specified by nVidia, the originator of the SLI concept. nVidia maintains a list of power supplies that they have certified as being suitable for SLI systems. The question is, Where should this 12V come from? More to the point, which line DOES it come from?

I interviewed a number of engineers from several power supply manufacturers to pose this very question. The answers were surprising. All of the engineers I spoke with wished to remain anonymous. This is a summary of what they told me:

    * Some PSU makers are using 12V2 to supply more than just the 2x12V or 4x12V connectors. It is often used to power the 6-pin 12V PCIe outputs.
    * Many PSUs marked as having dual 12V lines actually have only a single 12V line ? they do not feature two 240VA current limiters specified by ATX12V v2.xx; they have only one Over Current Protection (OCP - current limiter) for the single 12V line.
    * The 240VA current limit is considered a high cost, useless annoyance by most PSU makers. If multiple 12V lines are used, because the vast majority of components now use mostly 12V, the 18~20A limit for any line means that the precise power distribution to the various 12V output connectors can become critically important in some cases.
    * The engineers point to the many high power pre-V2.xx ATX12V PSUs that had as much as 30A on a single 12V line. As a product class, those have not proven to be any more dangerous in any way than other ATX12V PSUs. Even if exceeding 240VA in a single wire run was dangerous, this is extremely unlikely to occur in a PC because 12V is distributed to many different components on many different wire runs.

What's really interesting is that Intel has tacitly waived the 240VA limit requirement in its PSU validation program for the better part of a year. Intel maintains a web page listing all the ATX12V they have tested that "meet MINIMUM electrical, mechanical fit and functional compatibility" with Intel desktop boards and processors. For the 32 ATX12V v2.2 PSUs tested in 2005 that are on this list, 17 models are identified as having at least one output line that exceeds 240VA. And yet, these 17 models are on Intel's approved list.

According to the engineers I spoke with, the majority of these 17 models have just one 12V line. They also point out that there are another 20 or so ATX12V v2.0 PSU models on the Intel list, and none of them were tested for the 240VA current limit conformance. My sources say that if these models had been tested, more than half would not conform to the 240VA current limit because they have only one 12V line.

In the last couple of months, my PS engineering sources report, Intel has verbally informed them that the 240VA limit has been removed. A single 12V line is now "officially" approved, never mind what ATX12V v2.2 specifies.

What does all this mean? Essentially, the only potential benefit of dual 12V lines is improved safety, and this is questioned by the engineers I spoke with. There are many downsides to dual 12V lines, including higher cost and the extra headache of ensuring adequate 12V current for all the components in complex, high power systems. For the consumer who is trying to make a choice among the myriad of PSUs available on the retail market today, the most practical approach regarding dual 12V lines and power capacity is to consider only the combined 12V current capacity. (It will certainly be interesting to see how Intel handles this issue in the next version of ATX12V.)

 

Surv1v0r

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Dancer said:
nVidia maintains a list of power supplies that they have certified as being suitable for SLI systems.
Look here : http://www.slizone.com/object/slizone2_build.html
 

ex_forum_user_3

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The Dual-line spec is 100% rubbish.....
Also the 240VA max output per line, what balony.....

Intel wrote those specs, clearly somebody wrote them without technical skills in total.
Have you read the spec's?
I did and they make me sick.
As all lines come from the same input, yep your wall-socket.....and then spiltting them into multiple lines, for what purpose?
Being more stable, rubbish.
Being more safe, rubbish

What is safety? 12V 100A is that more unsafe then 12V 20A? No way....you can easily burn down a house with 12V 1A as with 100A.....
Will touching 12V kill you? No way, the number of amps flowing thru a body is way to low, even if you are wet.

Will it be more stable? No, as it's just a matter of regulators and caps, they are all over the board and plugin cards, no problem there.

Will it lead to problems in high-end systems, yes it does, you don't know what rails take what current.

Ergo, it's one utterly stupid specification....Intel blew that one real big!

As for the SLI certification, that's another stupid one, it only tells you the CPU has SLI Videocard connectors, other then that, nothing at all. The certification is just a marketing trick, nothing more.

I can write better specs for PSU's then Intel can, it's not that hard to do.....a bit of common sense is all that it takes.
As it's just a matter of giving enough power, single line, as the board and cards just take what they need.
The only protections that a PSU need to have is:

Input fuse, to prevent overload from the wall-socket.
Output shortage protection, that's simple enough, short the wires and it must turn off, simple as that.
Over/under voltage protection, +/- 10% voltage and turn off, also very simple.
Overheat protection, just put a sensor in the box, when it goes over say 50C, plain turn off, again, very simple.
Overload protection, just measure the max amps + 10% and shut off.

Nothing complicated about it or is it?
Anyone can write better specs then the stupid ATX 2.x spec's from Intel :lol_anim:

Do you need multiple lines for that, no way, it makes no sense, except as a marketing instrument....
 
R

RogerP

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On the specifications being stupid I will agree with you 100%.  I remember the first one, was a good I idea, because it mainly covered the physical standards for an ATX power supply and standardized  connectors, but even then I had the impression it was a manufacturing specification, not a industry standard, Version 2.x and later proved that.

I looked for a single rail PS for my new upgraded system, because I new the 40A total of the Dual 20A Rail supply, I was using would not take the spin-up surge of the hard drives and fans that were in the system.

Could not, find Single-Rail Power Supply that would either.  Tried 54A unit but could not get the system to boot reliably, just not enough +12V power.  The +12V power to run it is only about 30-35A, still do not have full system and I am running two dual-rail supplies at 20A per-rail, that is 80A total.  Had to take one of the PS apart and rewire it.  It kept tripping that dam spec V2.2 overload and shutting down on startup.  The one +12V rail on that PS was not being used in the Dual-Supply mode.  It is the rail that supplies the motherboard.  Therefore, I took it apart and removed all unused motherboard wiring.  Then moved the PCI-E Graphics Cards cable connectors to the unused motherboard +12V rail and its running find now.  Could have just removed the little circuit board with made it a 40A single rail, may still do that.  If you know anybody looking for a good 600W single rail for $79.99.  Need to have a technician rewire it if your do not have the skills.

Question, I am running just one ATi X1900XT and that is drawing lot of more power then the old X1800XL.  ATi states it needs 19A, but I have not been able to find time to do it yet to make a test jig to measure it has anybody?

How are people firing up systems with two of those X1900XT's in Cross-Fire mode on +12V rails that are limited too 20-22A, I have test several and they trip about 10-15% over there maximum rating.  Those cards must not be drawing 10A or more when running 3D Graphics Games.

Could not, find any single Power Supplies that would function in a Dual-Power-Supply mode.  Almost as hard, to find multi-rail-power-supply that would function in a Dual-Power-Supply mode.  When I did find one, it was the same PS I have been using it all my systems.  Therefore, know it?s a good one, because I have abused them a look running different types of tests.

Well, I have been babbling again.

Roger
  When submitting a problem, include a complete list of your system components; include part numbers, all Power Supply Voltages, and their output ratings.  It is almost impossible to estimate what your problem is without knowing something about it. 
 

ex_forum_user_3

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MSI Europe has measured it ;)

I have attached details of that some posts up.

Anyway, your have been looking in the wrong directions, I think....

Those people with Crossfire X1900XT's are not using multiple rails, they are using single rails....
And the PSU that is known to cope with it without any problems is the OCZ PowerStream 520W.
Those dualies fail miserably, and the only one that does do it is the PCP&C 1KW....all the rest...nope!
Forget the Watt's please, those are stupid figures, I only use that to inform you of the model PSU, not the expected power or such.
For me Wattage has become the new type of modelnumber :lol_anim:

I don't know what PSU you tested as you forgot to mention a brand, but trust me the OCZ Powerstream 520W will do it, with a nice 33A @ 12V....
So many people use it in SLI or Crossfire systems, and so far it never failed.

I did a math-calculation once, and came to approx 29A for a normal Crossfire-system with 2 X1900XT's, that leaves 5 amps for the rest, more then enough.
The Inquierer tested my Single-Rail theory, and hey it worked just fine....where all Multiple's they tried failed, no matter the Watt's or amps they had listed.  ;)
Funny was that either nVidia or ATI came to that conclusion, only after I suggested it ;)
 

Surv1v0r

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Bas

Again, many thanks for all your hard work on this.  Does this mean that MSI will remove all dual-rail PSUs from their approved list ?  If not, isn't their list a bit misleading ?
 

ex_forum_user_3

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This is MSI Europe doing the tests.....
Not Taiwan!

I don't think anybody has done this kind of extensive testing yet, as this testing is already going for like 2 months.
The person I know at MSI Europe, is doing these tests in his own spare time, as we wanted to know what goes where on amps.
As you notice, we don't care about voltages nor split-rail-PSU's.....you might think the Coolermaster used is a tripple-rail, but it's not, we checked that out before testing started, it's a single-rail :lol_anim:

BTW, MSI Taiwan is aware of these tests and figures and they get this same file as well, just a bit more often then I post in here.
 

Surv1v0r

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I would like to nominate Bas for an all expenses paid (by MSI) trip to Taiwan.

. . . how far from Taiwan is Thailand ?  :lol_anim:
 

syar2003

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All dual rail are dual rails .
I think you're all missing the point here .

It's still a single AC-+12v transformer witch is routed through seperate (dual,triple etc) current limiter circuits (rails) .

Quad SLI was demontrated at cebit by nvidia and used tagan 900/1100 , and theese PSU's have dual +12v transformers and four or more rails.
 
T

Tiresmoke

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However the PSU that the charts are being made with are supposed to be Dual Rail. However when they opened the unit they found that the three rails were hardwired together. As such not all multi rail PSU's are what they claim to be.

You pointed out multiple transformers and that is the corect way to do so. You completely isolate both rails by truely having seperate circuts. That is a much better solution however you will find it is in fact quite rare and costly.
 

ex_forum_user_3

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syar2003 said:
All dual rail are dual rails .
I think you're all missing the point here .

It's still a single AC-+12v transformer witch is routed through seperate (dual,triple etc) current limiter circuits (rails) .

Quad SLI was demontrated at cebit by nvidia and used tagan 900/1100 , and theese PSU's have dual +12v transformers and four or more rails.
Syar, the picture of the inside of that PSU is somewhere in this topic :lol_anim:
 

syar2003

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My point is , a single ac-dc +12v transformer equipped PSU can ony supply xx amount of +12 total current nomatter how they have wired the +12v outputs(rails) . The problem with some PSU comes from the current limiter/trigg circuits design.

 

ex_forum_user_3

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Syar, all very true....
Most "multiple rails" have seperated limiters.....and not realy splitted rails....
However, the CoolerMaster we use for the tests is supposted to have 3 rails.....so you would expect at least limiters per line.
So we opened the box and found this....with all the best arguments in the world we can't call this a tripple-rail or tripple-limited....
Have a look, it's a plain old single rail.
 

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syar2003

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Agree with that .
However it's hard to say from the pic how this one really is . But i belive you have been able to trace the +12v circuit design of it , and don't doubt your conclutions .
I can only see the output solderings on the wires running from the PCB labeled point named +12v2.
 

ex_forum_user_3

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Well it could be that the PSU in question could be delivered with true limiters in the USA or somewhere else.
But a fact is, the MSI Europe PSU supplied by Coolermaster has no split-limiters and is shorted between the "seperated" lines.
Maybe they put real resistors in place for other parts of the world, that we don't know.
And so far Coolermaster didn't respond to this.
 
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