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

F

FastEddie

Guest
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:


 
[quote author=DOK]
[quote author=RemusM]
If you own a 2000 USD rig is more than a stupidity to buy & use a cheap PSU for it.
[/quote]
You are rite but people are still using cheap psu's in hi end rigs, Go Figure :)
[/quote]

Unfortunately that's true.
And I told them: "Don't cry if your PSU will explode, catch fire, and burn down the building".
 
Its so sad most of the time, they make a ? 2000 machine but the PSU may only cost like ? 30 tops.
But they don't realise that a ? 30 PSU can kill their ? 1970 machine just like that.

In my opinion people that don't know about PSU's should be forbidden to build a computer in total.

I have said this a billion times before, a PC starts with a good powersupply, forget that and your system will have troubles and blowups all the time.
 
Well I tend to pick a PSU as the last choice. As you need to know you're power requirements before even the thought of what PSU to purchase. But the most important thing you need to chose when purchasing a PSU is the amperage on the +12v rail(s) [single rails preferred obviously]. There is also the factor of watts, it's just as important as amps. You could have 100A on the +12v rail, yet only have a total wattage rating of 500. So the PSU is only capable of delivering around 33A (assuming the other rails take 20% of the wattage requirement).

Getting the most amps with the most watts is the best way to go - but amps should always be thought of first.
 
I can say this, the first thing before anything else for my current rig is I bought my Corsair PSU +12V single rail.
 
Well, I caused quite some noise in here ... heh heh.

Did you see the TRIPPLE ... (no, no tripple rail, better ...) TRIPPLE FAN unit? I just love to make some NOISE!! :lol_anim:
 
On a serious note:

Qtec power supplies, have caused many fires around the World.

At the very least you risk your computer............. :yes:

Think once, think twice, is the lure of Qtec worth your life?

This is my opinion and not the opinion of the following:-

MSI

MSI Forum

MSI forum users





 
Del UK said:
On a serious note:

Qtec power supplies, have caused many fires around the World.

At the very least you risk your computer............. :yes:

Think once, think twice, is the lure of Qtec worth your life?

This is my opinion and not the opinion of the following:-

MSI

MSI Forum

MSI forum users

That is not entirely true. It IS the opinion of those running this forum.
Just go to Search, type "Q-Tec", select Hans as user to search for and see ...  :lol_anim:
 
Hans said:
That is not entirely true. It IS the opinion of those running this forum.
Just go to Search, type "Q-Tec", select Hans as user to search for and see ...  :lol_anim:

But, may also be opinion of MSI Forum User Hans.......

:oops:
 
funny, why i world  u want spend ur money and use somesing like Q-Tec?
would not even bother to look at anything like that.

my chose of PSUs:

Enermax Galaxy, or PC Power & Cooling (u cant beat this one : http://www.pcpower.com/power-supply/silencer-750-quad-crossfire.html) , or SliverStone, or OCZ (little bit expencive, but have 3 years warranty)

 
Choose PC Power & Cooling, Silverstone (Olympia or Decathlon series), Corsair (VX or TX series) or Tagan (2-Force II series)! Those are single rail all others are split rail!
Split rail is (what this thread is all about) a gamble....
 
flobelix said:
Choose PC Power & Cooling, Silverstone (Olympia or Decathlon series), Corsair (VX or TX series) or Tagan (2-Force II series)! Those are single rail all others are split rail!
Split rail is (what this thread is all about) a gamble....


its not realy gamble, its meter of reading inforamtion, provided by manafacturer, actualy reading between lines. and if information is purposly hided by manafacturer or seller, there no doubts its is crap PSU.
 
Seems you didn't have a read here so I'm wondering your posting. Even if the manufacturer tells you about the specs of a split rail, it's still a split rail.
OCZ for example says your psu would have 4 20A rails which means 75A "combined". But it's a split rail so how can amps on splitted rails be combined? If too much power is drawn from one rail that's it, no matter if there are 3 or 5 others left still offering full 20A. So it's a gamble because you don't know if there is maybe too much load on one rail although you would have enough power in total...

A single rail offers all available +12v power on one rail so all +12v power is drawn from that rail and you can be sure there will be no problems as long as you have enough amps.
 
 
flobelix said:
Seems you didn't have a read here so I'm wondering your posting. Even if the manufacturer tells you about the specs of a split rail, it's still a split rail.
OCZ for example says your psu would have 4 20A rails which means 75A "combined". But it's a split rail so how can amps on splitted rails be combined? If too much power is drawn from one rail that's it, no matter if there are 3 or 5 others left still offering full 20A. So it's a gamble because you don't know if there is maybe too much load on one rail although you would have enough power in total...

A single rail offers all available +12v power on one rail so all +12v power is drawn from that rail and you can be sure there will be no problems as long as you have enough amps.
? ?

actualy ON BOX state, Max Output Current for all 4 rails 66A, and i manual sed, its can be provided only for 60sec, so there is enouth information.? to have idea how much u can have on each rail, its simply get 80% out of Max. combined output, and divide on amount of rails.? that what aprox. u can get from each rail. to calculate aprox. Max. combined Currant output not difficult too, from max Power output PSU (for example my ) 1010W? deduct 25W (output from -12V&+5Vsb) 1010-25 = 985W, then deduct 175W (output from +3.3V&+5V) 985 - 175 = 810W, the divide by 12V 810 / 12? = 67.5A , bum for rough calculation not to bad, as long OCZ state 66A. then 80% out of 67.5 x 0.8 = 54A, split on 4 54 / 4 = 13.5 A that what u can expect form each rail of my PSU anytime at any moment. u not need to be Rocked scientist to calculate this. another thing, if u dont bother to learn, but in this case nothing i can do.
Its is true for unexpirenced user better use single rail PSU, at least they can see what is what, compere to dual or quad rail, but they , unexpirenced, anyway will go for cheap PSU with big Watts on box, they never consider PSU vital part of the computer, untill they RIG/system run into the trubble(overheating, low performance, or even damanging/killing parts of RIG/system), then they start cry and blaim manafacturer for bad quality and ect. ect. ect.

actualy, from ur post i can see u dont know how to calculate "combined" currant output.

about overlading one rail, if u building ur own RIG, for sure u know which hardware u use and u know how much roughly its requare Watt/Amps. and for sure u know to which rail u connect each hardware. example: again take for example my PSU and my RIG .on PSU state, +12V1 - CPU rail, +12V2 - pirmary Video adapter, +12V3 - peripherals, +12V4 - secendary Video adapter. rail N1 , my CPU , at stock speed, requare 125W. even if i push it hard , i would not be able overload this rail. rail N2 pirmary video adapter, according ATI on average its need 100w, OK my is O/Ced, well 120W, nop, not overloded. rail N3? peripherals, gosh u realy need try hard to overload this one,unelss of couse u manage connect cofe making mashin to ur RIG, or Microwave. Rail N4 practicly same with N2, exept its take nominal wattage, u cant Overcloak secendary video card in CrossFire, no meter how hard u try. he we go, load spread nicely over all 4 rails, non of them overloaded. to overload one rail and leave all other underloaded, u have to, have no ideas what u do whatsoever. in this case, there no point even try build ur own RIG, just pup up to nearest computer shop and ask specialist do it for u. its will save ur time, cash and headaches.

P.S. sorry for misstakes and bad spelling, english never was my strong side.
 
Nice calculation....

1. Basically this is what OCZ states for GameXStream 1010W:

http://www.ocztechnology.com/products/power_management/ocz_gamexstream_power_supply_1010w

can't find 66A max output in there... BTW: 66A max output would also be ridiculous for a 1010w psu...

2. Obviously you didn't understand my point: If rails are really split they can't be combined as a rail from which is drawn too much power can't be compensated by another. That's the idea of splitting rails to dedicate a rail e.g. to the vgas.
The combined value you impressivly calculated is a theoretical value of total +12v power available on all rails. This on the other hand does not mean that a rail could compensate the lack of power on another rail.
With a psu as powerful as yours this won't have much effect because the rails are strong enough but for a psu from a lower price level this could be a crucial point.
 
flobelix said:
Nice calculation....

1. Basically this is what OCZ states for GameXStream 1010W:

http://www.ocztechnology.com/products/power_management/ocz_gamexstream_power_supply_1010w

can't find 66A max output in there... BTW: 66A max output would also be ridiculous for a 1010w psu...

2. Obviously you didn't understand my point: If rails are really split they can't be combined as a rail from which is drawn too much power can't be compensated by another. That's the idea of splitting rails to dedicate a rail e.g. to the vgas.
The combined value you impressivly calculated is a theoretical value of total +12v power available on all rails. This on the other hand does not mean that a rail could compensate the lack of power on another rail.
With a psu as powerful as yours this won't have much effect because the rails are strong enough but for a psu from a lower price level this could be a crucial point.

ab135c49c10d74e790197965d0da56c26667b5c.JPG


that what u can see on BOX. anyway, actualy rails can compensate, if one or 2 rails underpowered? u can get some, not much, Amps to other rails. say in sutuacion if u use for rail N1 89W CPU and have not many hardrives and optical drives on N3, rails N2 and N4 can exide to 16A. That numbers 20A on each rail? its test values when all other rails set to 1A. 75A value come from when +3.3&+5 set to 1 A and +12 rails pushed to the limit.
 
75A value come from when +3.3&+5 set to 1 A and +12 rails pushed to the limit.

That's peak not max! Max output is just maximum +12v. If OCZ just states peak values on their website and doesn't point that out then we got Q-Tec style.

The 75A on the sticker provided by me is for OCZ1010GXSSLI while your's is OCZ1010GXS. Maybe it's a different version for your local market. As the stickers are besides the combined +12v value absolutely identical it would be a bit odd if two calculation models are used for the same product.

 
To put it real simple:

Splitrail = Gamble

Singlerail = Exactly knowing what it can/will do

Fond of gambling? By all means do get a splitrail :lol_anim:

Can a splitrail do 66A on a single line? They state not, they say it can do only 20 or such, then it should shutdown.
Does it? Is it a splitrail at all? Many brands say they are splitrail but they are not.
But you don't know unless you open the PSU itself (void warranty) and follow all lines.

As said, if you like gambling, go splitrail :lol_anim:
 
Bas said:
To put it real simple:

Splitrail = Gamble

Singlerail = Exactly knowing what it can/will do

Fond of gambling? By all means do get a splitrail :lol_anim:

Can a splitrail do 66A on a single line? They state not, they say it can do only 20 or such, then it should shutdown.
Does it? Is it a splitrail at all? Many brands say they are splitrail but they are not.
But you don't know unless you open the PSU itself (void warranty) and follow all lines.

As said, if you like gambling, go splitrail :lol_anim:

The Corsair HX620W was a great example of this. It said 3 x +12v rails rated at 18A on the box/website. Upon opening the PSU, I found that +12v led to a single point on the PSU. Therefore it wasn't split rail, but single.

One-go said:
actualy ON BOX state, Max Output Current for all 4 rails 66A, and i manual sed, its can be provided only for 60sec, so there is enouth information.  to have idea how much u can have on each rail, its simply get 80% out of Max. combined output, and divide on amount of rails.  that what aprox. u can get from each rail. to calculate aprox. Max. combined Currant output not difficult too, from max Power output PSU (for example my ) 1010W  deduct 25W (output from -12V&+5Vsb) 1010-25 = 985W, then deduct 175W (output from +3.3V&+5V) 985 - 175 = 810W, the divide by 12V 810 / 12  = 67.5A , bum for rough calculation not to bad, as long OCZ state 66A. then 80% out of 67.5 x 0.8 = 54A, split on 4 54 / 4 = 13.5 A that what u can expect form each rail of my PSU anytime at any moment. u not need to be Rocked scientist to calculate this. another thing, if u dont bother to learn, but in this case nothing i can do.
Its is true for unexpirenced user better use single rail PSU, at least they can see what is what, compere to dual or quad rail, but they , unexpirenced, anyway will go for cheap PSU with big Watts on box, they never consider PSU vital part of the computer, untill they RIG/system run into the trubble(overheating, low performance, or even damanging/killing parts of RIG/system), then they start cry and blaim manafacturer for bad quality and ect. ect. ect.

actualy, from ur post i can see u dont know how to calculate "combined" currant output.

about overlading one rail, if u building ur own RIG, for sure u know which hardware u use and u know how much roughly its requare Watt/Amps. and for sure u know to which rail u connect each hardware. example: again take for example my PSU and my RIG .on PSU state, +12V1 - CPU rail, +12V2 - pirmary Video adapter, +12V3 - peripherals, +12V4 - secendary Video adapter. rail N1 , my CPU , at stock speed, requare 125W. even if i push it hard , i would not be able overload this rail. rail N2 pirmary video adapter, according ATI on average its need 100w, OK my is O/Ced, well 120W, nop, not overloded. rail N3  peripherals, gosh u realy need try hard to overload this one,unelss of couse u manage connect cofe making mashin to ur RIG, or Microwave. Rail N4 practicly same with N2, exept its take nominal wattage, u cant Overcloak secendary video card in CrossFire, no meter how hard u try. he we go, load spread nicely over all 4 rails, non of them overloaded. to overload one rail and leave all other underloaded, u have to, have no ideas what u do whatsoever. in this case, there no point even try build ur own RIG, just pup up to nearest computer shop and ask specialist do it for u. its will save ur time, cash and headaches.

P.S. sorry for misstakes and bad spelling, english never was my strong side.

You can't calculate the combined output of a split rail PSU. The rails are split, not combined. If you could combine them, they'd call them combined railed PSUs. They are called split rails, because they are split.

75A is (like Flobelix said) a joke for a 1010W PSU... My Corsair TX750W can do 60A @ +12v. At peak I would assume it's about 70A and because it's single rail; I know it will do the 60A stated on the box.
 
AaronYuri said:
My Corsair TX750W can do 60A @ +12v.

realy, can do 60A. let me see, what is formula for WATTS, ahhh W = A x V. well if ur PSU be able do 60 A , its mean its would draw 720W. max output on ur PSU 750W, r u planning supply +3.3V, +5V, -12V and +5Vsb with only 30W? good luck with that.

split rails , in reality not completely split, they attached to same power source. all true, if u do not use one rail, u cant transfer all Amps to other, but this allow other rails perform better. all 4 rails together can draw more power, than PSU can produce its self. technicly each rail have capacity of 240W, but in reality they cant draw so much, its would exide Max.output of PSU. 1010 - 25 ( to-12V and +5Vsb) , then u have to allocate some power to +3.3V and +5V (not need go for Max. values 100W would be enouth), 985W -100 = 885W left, its alot less than all 4 rails can draw 240 x 4 = 960W , i dont know why im argue with u, go back to school and refresh Physics course. to make it little bit simpler, in overal, if u drop out one rail, u lose Amps: lets go with max Currant, 66A for 4 rails, 60A will for 3 rails(when u use only 3 rails), 40A for 2 rails(when u use only 2 rails) , 20A for single(when u use 1 rail , technicly inposible to achive.)

its easy and cheaper produce multi-rail PSU than single rail, also, if u dont know single rail PSU more efficent than multi-rail PSU, so there no surprise 850W single rail PSU would achive same numbers (Amps) as 1kW multi-rail PSU


P.S.
"The 75A on the sticker provided by me is for OCZ1010GXSSLI while your's is OCZ1010GXS. Maybe it's a different version for your local market. As the stickers are besides the combined +12v value absolutely identical it would be a bit odd if two calculation models are used for the same product."

look at sticker again, by 1010W? u will see small print, what its say?

"That's peak not max! Max output is just maximum +12v. If OCZ just states peak values on their website and doesn't point that out then we got Q-Tec style."

all companies do that, corsair too, i would hardly believe they achive 60A on 750W units with proparly loaded +3.3V&+5V
to stop this argue, why do u simply go and ask Corsair? how much load was on +3.3V&+5V , when they achive 60A.( would not surprise if they say 9W , which is 1A for each)

and last one, good or not, but by some reason i cant see Corsair TX750W certified to run RIG with 2x HD 2900XT/XTX or 2x HD 3870X2, when my "s***y" unit does, and yes its was tested , i had 2 x HD2900XT my self , and sold it on ebay. if ur PSU (Corsair one) realy can provide 60A, that would be more than enouth to supply them. (ATI recomend 52A continuos for CrossFire with HD2900/HD3870X2)
good luck in future

 
Back
Top