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Are Multiple 12-volt Rails Better Than A Single 12-volt Rail?

Discussion in 'General Hardware' started by freaksavior, Feb 14, 2007.

  1. freaksavior

    freaksavior To infinity ... and beyond!

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    I've been asking myself that question ever since i decided it was time to get a new psu. so


    "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"

    Read all the psu facts @ http://www.pcpower.com/technology/myths/
     
    Crunching for Team TPU
  2. Chewy

    Chewy New Member

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    Good post, man. The 1950xtx asks for 30amps on the 12v rail for a fully loaded system... its hard to find that on a multiple rail 12v psu... but I guess thats the max it would ever use.
     
  3. niko084

    niko084

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    They ask for 30amps combined rail. Thats the key there that most people miss. The 1950xtx will NOT draw 30amps. The problem with multiple rail systems is very true though. Although its not an issue if you have dedicated high output pci-e rails.

    For instance a 20 amp rail that only powers 2 pci-e plugs. That is more than enough power for 2 even 8800s or x1950s.

    The problem with most psu's is that they share the same rail for the pci-e with other things like hard drives or your processor. The other issue comes down to 3 rails at 20 amps, although max output is only 42amps, because what they draw off is bottle necked.

    But yes multi rail psu's that are not high end, don't waste your time if you have a hot machine. There is a plus to multi rail setups, and thats voltage stability, but then again only in a good psu.
     
  4. mullered07

    mullered07 New Member

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    so could i x-fire 2 x1950pro' on my ocz powerstream 450w with 30a on my single :( 12v rail ?

    without saying "this amount of amps/volts" is " reccomended" but in real life terms could i do it ?
     
  5. niko084

    niko084

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    30 amps on your 12volt leaves you with 360watts for 12 volt...

    That being said its really close... You could probably I hear that OCZ makes good psu's..
    But I would be looking for something with a little more punch.
     
  6. mullered07

    mullered07 New Member

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    well my 2nd pci-e slot is only agei (pci-e x4 and b4 anyone says, it DOES support x-fire on the x1950pro) would that make a diff on the draw from the psu being as its only 4x as opposed to 16x, i do plan on doing x-fire ( and yes i know guys there will be perf loss on 4x but there will still be perf gain vs 1 single x1950pro and it will do me fine until my next build/ mobo/dx10 grfx upgrade as i plan on keeping my ram and getting another gig and also my e4300 as i can get 3ghz with my next board upgrade)
     
  7. niko084

    niko084

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    Ya it would keep that card a little slow, but I don't know if that would effect the power requirement... Honestly I don't think I would try it... It's not worth the chance at blowing stuff up.
     
  8. freaksavior

    freaksavior To infinity ... and beyond!

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    thanks, there are so many psu question i thought it could be usefull! ;)
     
    Crunching for Team TPU
  9. mullered07

    mullered07 New Member

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    goddam it niko, dont tell me i need another x1950pro AND a new psu lol :banghead: i figured so much anyway but i do like to see what i can get away with :p who doesnt?

    touchwood ive never blown one component before out of 6-7 builds (not including builds for other people) ;) (although have scratched some mem modules on my old ti4200 with a screwdriver and fooked it, you know the thing im talking bout, multicoloured lines all over the screen :banghead: )
     
  10. niko084

    niko084

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    Lol... Ya I haven't personally blown anything up on "accident" either. But I'm pretty careful about it.
     
  11. newtekie1

    newtekie1 Semi-Retired Folder

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    Everyone always points to that article, but they never seem to realize that it is posted on the website of a company that builds power supplies.

    The fact of the matter is that single rails are easier and cheaper for a power supply maker to produce, which explains why they want to say it is the better design.

    However, the fact of the matter is that the groups passing the standards for power supplies say that multiple rails are better and they usually say something like the limit on a rail should be around 25-30A. However, they don't ever make that a requirement, just a recommendation.

    I disagree with the majority of what that site says, IMO it creates more bad myths than it actually addresses.
     
    Crunching for Team TPU 50 Million points folded for TPU
  12. Chewy

    Chewy New Member

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    My psu makes me wonder... it originally had 4 12v rails, but they decided to combine the 4 rails into one (fuse the 4 rails together) and combined they make 36 amps.. I wouldent want to have the 36 amps going through just one pci-e conector, from what I`ve read the wire could burn-out.. but is my psu an exception to the 25-30amp rule? since it has 4 rails fused together to make 1? :p
     
  13. niko084

    niko084

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    I don't think they are realy combined. If they are giving you a combined total number in the specs its because you can have 3 rails at 20amps per and have a combined total of 50amps. Which obviously isn't 60... The key being if you are pulling 10 on one you can pull up to 20 on the other two. Because they all pull from the same source. Thats where most cheaper brands fall off they say something like 600 watts, then they fail to tell you only 340 of them are 12 volt, and at 70c you get 200 watts.

    This is where the real spec sheets come into handy and being able to fully understand them. I couldn't before today, then after about ohh 50 pages of misc reading *and multiple headaches* I got it pretty well figured out.
     
  14. niko084

    niko084

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    A good point here.... Multiple rails are used in about every high end power supply. They offer much better stability and they are more efficient. *When built properly*

    Don't trust every piece of mumbo jumbo you read.

    I came by another write up about how bad modular cabling is... Uh huh, they compared a mid-level Corsair to about 6 other mid-level psu's and found that the Corsair did indeed have .1 lower volts on no load, but on full load had a higher voltage still. They compared 6 of them and they were all decent brands in the same price line. Third party people.
     
  15. niko084

    niko084

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    Your power supply has 4- 18amp rails. But a combined max of 38 amps. That gives you a raw 456 watts of 12volt, which isn't too bad for most x-fire/sli setups. The deal is you can only use a total of 38 amps across all rails, each one independently capable of 18amps.
     
  16. Chewy

    Chewy New Member

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    I`m not sure :p but originally the m12 series was seposed to have 4 rails (mine was 18 amps per rail), but in some review it said after getting a go ahead from a QC or safety place seasonic decided it was best to combine the rails to one and soddered the 4 rails together in the psu to make one. I seen a picture where it showed the 4 combined inside.. this is prob not the 1st psu that has had this done... I figure since the 4 rails were combined they can handle 36 amps.. but one pci-e conector would burn if it tried to soley deliver 36 amps.

    Its advertised as a quad rail psu but it really just has one :p since they soddered them together.
    Meh no biggy I`ve been tired lately so I have theses stupid questions lol
     
  17. niko084

    niko084

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    Ya but they wont take the *38* amps. I looked up your psu to get the info.
    I guess its slightly possible they decided to join them, but then they are guilty of false advertisement.. I would be taking it apart to find out personally. Either way it goes you are pretty good and solid for most setups.
     
  18. freaksavior

    freaksavior To infinity ... and beyond!

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    Crunching for Team TPU
  19. Namslas90 New Member

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    It's better to have a multi Rail PSU; Providing the PCI-E Rails support the minimum per rail that each card needs (actualy needs, not "recomended").

    @ NIKO084; Hey looks like you've been doing some research, good deal. You've about got this PSU thing figured out!!!
    So, Niko, what PSU are you going to get?
     
  20. niko084

    niko084

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    Silverstone ST60F 48amps on 12 Volt :cool:

    It's works right into what I planned on spending-
    LIAN LI PC-61 USB
    ASUS P5B
    ATI x1950pro 256mb
    Silverstone ST60F
    Intel e6300
    Corsair XMS2 DDR675 2 gigs 1024x2

    I think after this system for a bit, if I decide I want something more again, I'll end up getting another case I was looking at also a Lian Li and might just go dual power supplies if I feel the need.

    But this psu should more than power this system. Infact by all means it should power 2 x1950s, x1950xtx's would be pushin it pretty good but, most I think I'll need anytime soon is 2 1950pros, if I even use those... I really play a lot of older games, and quite a bit of cs:source.. Pretty rare I find a new game I like, and non of them really push graphics too much, besides GR:AW. Which I hear runs pretty decent on a single 1950 anyways.

    I can't say for most but the 7900 series cards at max draw 5 amps from the pci-e power plug and 5 amps from the board. I was lucky enough to find that info. So 10 amps total actual draw from a video card. Which means a few good rails and 50amps or so of good output at 12volts should hold a set in sli without too much trouble. That puts you around the $200-$250 range for a decent psu.

    But like I found its all about the power supply having good total power, and the pci-e rails being dedicated to the pci-e plugs. And most cheaper ones mix them with other power like IDE... Which is why 2 rail psu's are a horrible idea 1 or 3+.
     
  21. niko084

    niko084

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    Heh forgot to mention there, the Neo HE 500 is actually a good psu for a single 1900 series including the x1950xt, as long as it was the revsion after A3.. You very well could have had a revision before that which I think shared the pci-e rail with the processor or something else... Which with a hot amd chip and over clocked, would have been pretty bad.. Very well could have been your issue.
     
  22. Namslas90 New Member

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    No mine was SLI certified with 2 PCI-E rails each w 18 amps per rail,(3 total 12v rails), still wouldn't power my card. I got the same symptoms most others describe when there isn't enough power.
     
    Last edited: Feb 15, 2007
  23. tkpenalty New Member

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    lol SLI Certified does not mean Crossfire nor ATI Certified.
     
  24. niko084

    niko084

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    No what I stated was power supplies that are SLI and Crossfire Certified have dedicated rails to the pci-e auxiliary power plugs.

    I should have said to better clarify though SLI and/or Crossifre.

    Basically if you buy a psu that is SLI and/or Crossfire certified it will have one of its 12 volt rails dedicated to the pci-e auxiliary power plugs. Thats the most important part to using a multiple rail psu with hot video cards.

    I personally very strongly suggest 1 rail or 3+, as dual rails do not provide more than 40 amps, and must share the rails to a great extent, so they can't dedicate 1 rail to pci-e power.

    Anyways I'm going to go to bed far too much reading has been done today and its almost 4am here. I have read close to 900 pages on power supplies and the current draws of video cards, processors, hard drives, ram, fans, optical drives, name it.. My mind is roasted.
     
  25. Wile E

    Wile E Power User

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    And SLI, Crossfire, or ATI certified doesn't mean squat anyway.
     

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