The splitting of the +12V rail

Discussion in 'Power Supplies' started by jonnyGURU, Mar 19, 2008.

  1. jonnyGURU

    jonnyGURU Moderator <BR> Power Supplies
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    What is "multiple +12V rails", really?

    In most cases, multiple +12V rails are actually just a single +12V source just split up into multiple +12V outputs each with a limited output capability.

    There are a few units that actually have two +12V sources, but these are typically very high output power supplies. And in most cases these multiple +12V outputs are split up again to form a total of four, five or six +12V rails for even better safety. To be clear: These REAL multiple +12V rail units are very rare and are all 1000W+ units (Enermax Galaxy, Topower/Tagan "Dual Engine", Thermaltake Tough Power 1000W & 1200W, for example.)

    In some cases, the two +12V rail outputs are actually combined to create one large +12V output (Ultra X3 1000W, PC Power & Cooling Turbo Cool 1000W, for example.)

    So why do they do they split up +12V rails??

    Short circuit protection only works if there's minimal to no resistance in the short (like two wires touching or a hot lead touching a ground like the chassis wall, etc.) If the short occurs on a PCB, in a motor, etc. the resistance in this circuit will typically NOT trip short circuit protection. What does happen is the short essentially creates a load. Without an OCP the load just increases and increases until the wire heats up and the insulation melts off and there's a molten pile of flaming plastic at the bottom of the chassis. This is why rails are split up and "capped off" in most power supplies; there is a safety concern.

    Is it true that some PSU's that claim to be multiple +12V rails don't have the +12V rail split at all?

    Yes, this is true. But it's the exception and not the norm. It's typically seen in Seasonic built units (like the Corsair HX and Antec True Power Trio.) It's actually cheaper to make a single +12V rail PSU because you forego all of the components used in splitting up and limiting each rail and this may be one reason some OEM's will not split the rails, but say they are split. Some system builders adhere very closely to ATX12V specification for liability reasons, so a company that wants to get that business but also save money and reduce R&D costs will often "fib" and say the PSU has it's +12V split when it does not.

    Why don't those PSU companies get in trouble? Because Intel actually lifted the split +12V rail requirement from spec, but they didn't actually "announce" it. They just changed the verbiage from "required" to "recommended" leaving system builders a bit confused as to what the specification really is.

    So does splitting the +12V rails provide "cleaner and more stable voltages" like I've been told in the past?

    It is true that marketing folks have told us that multiple +12V rails provides "cleaner and more stable voltages", but this is usually a falsehood. Quite frankly, the use this explaination because "offers stability and cleaner power" sounds much more palletable than "won't necessarily catch fire". Like I said before, typically there is only one +12V source and there is typically no additional filtering stage added when the rails are split off that makes the rails any more stable or cleaner than if they weren't split at all.

    Why do some people FUD that single is better?

    Because there are a few examples of companies that have produced power supplies with four +12V rails, something that in theory should provide MORE than ample power to a high end gaming rig, and screwed up. These PSU companies followed EPS12V specifications, which is for servers, not "gamers". they put ALL of the PCIe connectors on one of the +12V rails instead of a separate +12V rail. The +12V rail was easily overloaded and caused the PSU to shut down. Instead of correcting the problem, they just did away with the splitting of +12V rails altogether. Multiple +12V rail "enthusiast" PSU's today have a +12V rail just for PCIe connectors or may even split four or six PCIe connectors up across two different +12V rails. The rails themselves are capable of far more power output than any PCIe graphics card would ever need. In fact, Nvidia SLI certification these days REQUIRE that the PCIe connectors be on their own +12V rail to avoid any problems from running high end graphics cards on split +12V rail PSU's.

    There's less components and less engineering to make a PSU that DOES NOT have the +12V rail split up, so it's cheaper to manufacturer (about $1.50 less on the BOM, $2 to $3 at retail) and typically this cost savings is NOT handed down to the consumer, so it actually behooves marketing to convince you that you only need single +12V rails.

    But some people claim they can overclock better, etc. with a single +12V rail PSU

    B.S. It's a placebo effect. The reality is that their previous PSU was defective or just wasn't as good as their current unit. If the old PSU was a cheap-o unit with four +12V rails and the new one is a PCP&C with one +12V rail, the new one isn't overclocking better because it's a single +12V rail unit. It's overclocking better because the old PSU was crap. It's only coincidental if the old PSU had multiple +12V rails and the current one has just one.

    The only "problem" the occurs with multiple +12V rails is that when a +12V rail is overloaded (for example: more than 20A is being demanded from a rail set to only deliver up to 20A), the PSU shuts down. Since there are no "limits" on single +12V rail PSU's, you can not overload the rails and cause them to shut down..... unless you're using a "too-small" PSU in the first place. Single +12V rails do not have better voltage regulation, do not have better ripple filtering, etc. unless the PSU is better to begin with.

    So there are no disadvantages to using a PSU with multiple +12V rails?

    No! I wouldn't say that at all. To illustrate potential problems, I'll use these two examples:

    Example 1:

    An FSP Epsilon 700W has ample power for any SLI rig out there, right? But the unit only comes with two PCIe connectors. The two PCIe connectors on the unit are each on their own +12V rail. Each of these rails provides up to 18A which is almost three times more than what a 6-pin PCIe power connector is designed to deliver! What if I want to run a pair of GTX cards? It would have been ideal if they could put two PCIe connectors on each of those rails instead of just one, but instead those with GTX SLI are forced to use Molex to PCIe adapters. Here comes the problem: When you use the Molex to PCIe adapters, you have now added the load from graphics cards onto the rail that's also supplying power to all of your hard drives, optical drives, fans, CCFL's, water pump.. you name it. Suddenly, during a game, the PC shuts down completely.

    Solution: To my knowledge, there aren't one-to-two PCIe adapters. Ideally, you'd want to open that PSU up and solder down another pair of PCIe connectors to the rails the existing PCIe connectors are on, but alas... that is not practical. So even if your PSU has MORE than ample power for your next graphics cards upgrade, if it doesn't come with all of the appropriate connectors, it's time to buy another power supply.

    Example 2:

    Thermal Electric Coolers take a lot of power and are typically powered by Molex power connectors. I, for one, prefer to run TEC's on their own power supply. But that's not always an option. If you had a power supply with split +12V rails and powered your TEC's with Molexes, you would be putting your TEC's on the same +12V rail as the hard drives, optical drives, fans, CCFL's, water pump.. you name it, just as you did with the Molex to PCIe adapters. The power supply could, essentially, shut down on you in the middle of using it. A power supply with a single, non-split, +12V rail would not have any kind of limit as to how much power is delivered to any particular group of connectors, so one could essentially run several TEC's off of Molex power connectors and not experience any problems if one had a single +12V rail PSU.

    Typical multiple +12V rail configurations:

    • 2 x 12V rails
      • Original ATX12V specification's division of +12V rails.
      • One rail to the CPU, one rail to everything else.
      • VERY old school since it's very likely that "everything else" may include a graphics card that requires a PCIe connector.
      • Typically only seen on PSU's < 600W.
    • 3 x 12V rails
      • A "modified" ATX12V specification that takes into consideration PCIe power connectors.
      • One rail to the CPU, one rail to everything else but the PCIe connectors and a third rail just for PCIe connectors.
      • Works perfectly for SLI, but not good for PC's requiring four PCIe connectors.
    • 4 x 12V rails (EPS12V style)
      • Originally implemented in EPS12V specification
      • Because typical application meant deployment in dual processor machine, two +12V rails went to CPU cores via the 8-pin CPU power connector.
      • "Everything else" is typically split up between two other +12V rails. Sometimes 24-pin's two +12V would share with SATA and Molex would go on fourth rail.
      • Not really good for high end SLI because a graphics card always has to share with something.
      • Currently Nvidia will NOT SLI certify PSU's using this layout because they now require PCIe connectors to get their own rail.
      • In the non-server, enthusiast/gaming market we don't see this anymore. The "mistake" of implementing this layout was only done initially by two or three PSU companies in PSU's between 600W and 850W and only for about a year's time.
    • 4 x 12V rails (Most common arrangement for "enthusiast" PC)
      • A "modified" ATX12V, very much like 3 x 12V rails except the two, four or even six PCIe power connectors are split up across the additional two +12V rails.
      • If the PSU supports 8-pin PCIe or has three PCIe power connectors on each of the +12V rails, it's not uncommon for their +12V rail to support a good deal more than just 20A.
      • This is most common in 700W to 1000W power supplies, although for 800W and up power supplies it's not unusual to see +12V ratings greater than 20A per rail.
    • 5 x 12V rails
      • This is very much what one could call an EPS12V/ATX12V hybrid.
      • Dual processors still each get their own rail, but so do the PCIe power connectors.
      • This can typically be found in 850W to 1000W power supplies.
    • 6 x 12V rails
      • This is the mack daddy because it satisfies EPS12V specifications AND four or six PCIe power connectors without having to exceed 20A on any +12V rail
      • Two +12V rails are dedicated to CPU cores just like an EPS12V power supply.
      • 24-pin's +12V, SATA, Molex, etc. are split up across two more +12V rails.
      • PCIe power connectors are split up across the last two +12V rails.
      • This is typically only seen in 1000W and up power supplies.

    Ok... What's the bottom line?

    The bottom line is, for 99% of the folks out there single vs. multiple +12V rails is a NON ISSUE. It's something that has been hyped up by marketing folks on BOTH SIDES of the fence. Too often we see mis-prioritized requests for PSU advice: Asking "what single +12V rail PSU should I get" when the person isn't even running SLI! Unless you're running a plethora of Peltiers in your machine, it should be a non-issue assuming that the PSU has all of the connectors your machine requires and there are no need for "splitters" (see Example 1 in the previous bullet point).

    The criteria for buying a PSU should be:

    • Does the PSU provide enough power for my machine?
    • Does the PSU have all of the connectors I require (6-pin for high end PCIe, two 6-pin, four 6-pin or even the newer 8-pin PCIe connector)?
    • If using SLI or Crossfire, is the unit SLI or Crossfire certified (doesn't matter if a PSU is certified for one or the other as long as it has the correct connectors. If it passed certification for EITHER that means it's been real world tested with dual graphics cards in a worst case scenario).

    Figure out if there are any variables that may affect the actual output capability of the PSU:

    • What temperature is the PSU rated at? Room (25° to 30°C) or actual operating temperature (40°C to 50°C)
    • If room temperature, what's the derating curve? As a PSU runs hotter, it's capability to put out power is diminished. If no de-rate can be found, assume that a PSU rated at room temperature may only be able to put out around 75% of it's rated capability once installed in a PC.

    After that, narrow selection down with finer details that may be more important to others than it may be to you....

    • Does the unit have power factor correction?
    • Is the unit efficient?
    • Is the unit quiet?
    • Is the unit modular?
    • Am I paying extra for bling?
    • Do I want bling?

    Hope this helps eliminate some of the questions that come up over and over again. I'll be editing this throughout the day and hope to be putting up more FAQ material later! :D
     
  2. nerp

    nerp Diamond Member

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    SPOT ON. I learned a bit by reading this and I'm glad to know others will too. This should be bolted for a while at the top of the forum, imho.
     
  3. magreen

    magreen Golden Member

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    Amazing stuff, Jonny. Congrats. Things are really shaping up around here. We should see more meaningful and less repetitious discussions here on the forum with this kind of authoritative info stickied to the top.
    :thumbsup:
     
  4. Mr Fox

    Mr Fox Senior member

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    This is why you are the PSU Mod !!

    Great resource/reference !!

    The last mod did a good job answering questions, he just never placed Reference type work at the top of the Forum.




     
  5. flexy

    flexy Diamond Member

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    A++ reference. JonnyGuru is the PSU master and i am glad we have people like him, spending time writing FAQs and giving help. Awesome !!
     
  6. arameth

    arameth Member

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    Thanks again to everyone for all the great info...so new PSU get's added to the list. Going to read through all the stickies there...

     
  7. Martimus

    Martimus Diamond Member

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    Very nice post, JohnnyGURU! I am glad that you are here, because you really helped me understand a lot of things about the PSU that I didn't understand before. (Not just in this post, but throughout all of your replies over the last year or so.) You have a way of explaining these things in a way that is easy to understand, where most posts I have seen on PSU's lacked detail, and made you wonder where the conlusions came from (including my own conclusion on my Antec failure.)
     
  8. WoodButcher

    WoodButcher Platinum Member

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    Thanks for your efforts in putting your knowledge in print. The forum format makes this a great reference. thanks again:thumbsup:
     
  9. flexy

    flexy Diamond Member

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    jonny is one of the view doing "real life" PSU testing with appropriate equipment as compared to many other sites/reviewers who do a halfa$$ job or use only a DMM which is totally insufficient in measuring quality and performance of PSUs.
     
  10. Ratman6161

    Ratman6161 Senior member

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    Great analysis - thank you! That really explains some things I've been concerned about.

    Some specific questions though:

    I just ordered a Q6600 and Abit IP35-e. The rest of my components (including power supply) will be migrated from my existing system. My PS is an Ultra X-Finity 600 Watt. What can I say, it was cheap at the time! Its configuration looks like what you described as "old school", i.e.

    "2 x 12V rails
    * Original ATX12V specification's division of +12V rails.
    * One rail to the CPU, one rail to everything else.
    * VERY old school since it's very likely that "everything else" may include a graphics card that requires a PCIe connector.
    * Typically only seen on PSU's < 600W. "

    According to the manufacturer, 12v1 is 18 amp and 12v2 is 20 amp

    The 12v cpu power connector has two 4 pin plugs that can supposedly be used together in an 8 pin socket. So, I'm assuming that those cpu power connectors are on their own rail? But the documentation doesn't say if its the 18 or the 20 amp going to the cpu.

    It also has two 6 pin PciE power connectors, but with only two rails, I'm assuming that those are actually sharing with the molex connectors, right?

    Anyway, my video card is a 7900GS, so not as power hungry as many. I have no desire for SLI and the motherboard I ordered doesn't support it anyway. Other than the CPU and video card, the system will have 8GB DDR2 800 ram, 1 SATA2 hard drive, 1 IDE hard drive, and a DVD burner, and thats it. As to over clocking, I'm shooting for 3.0 Ghz since at that speed I should be just overclocking the CPU and running everything else in spec.

    So I think I should be fine with what I have as far as power supply goes. Can anyone give me a warm fuzzy that I'm not going to destroy my new cpu with insufficient power?
     
  11. jonnyGURU

    jonnyGURU Moderator <BR> Power Supplies
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    If I had to guess, I would say the CPU is on the 18A rail since the PSU has to have enough power to do low end SLI.

    Right.

    You should be fine. There's enough power on that +12V rail for a pair of Nv 6x cards or ATI 1950's, so one 7900GS is nothing.
     
  12. Ratman6161

    Ratman6161 Senior member

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    Thanks. I went ahead and did the install and everything is AOK!
     
  13. Big Lar

    Big Lar Diamond Member

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    Great info on All the new threads, Thanks Jonny!

    Larry
     
  14. imported_wired247

    imported_wired247 Golden Member

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    Thanks, you done got copy pasted on my350z.com forums, credit given of course
     
  15. jonnyGURU

    jonnyGURU Moderator <BR> Power Supplies
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    For those who question the validity of the "it's for safety" argument, please have a look at these photos:

    http://www.jongerow.com/BFGPOWER/images/DSC02898.jpg

    http://www.jongerow.com/BFGPOWER/images/DSC02899.jpg

    http://www.jongerow.com/BFGPOWER/images/DSC02900.jpg

    I actually see this quite often. Usually with the floppy connector when attached backwards to a Creative sound card or a floppy drive, but this time it was a four pin Molex and it was NOT attached backwards. What it was plugged into shorted and caused this.

    The PSU works fine. In fact, if I isolate the exposed wires so they don't short, I can actually hook it up to the load tester and run it.

    The short caused the +12V wire to heat up and melt it's insulation off. The wire never shorted so short circuit protection never shut the PSU off. If the PSU had not shut off once the resistance of the short created a > 20A load, the heat would've melted the other wires' insulation, the sleeve, etc.
     
  16. NinjaJedi

    NinjaJedi Senior member

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    Great guide. thanks for the time you spent. I will be upgrading my gpu soon. most likely to a 800gts (g92) I have an antec neoHE 550. it has 2 12v rails rated at 18 amps. (each) If I understand everything correctly if I'm using just 1 rail I'm getting 36 amps? I ask this cause I'm a bit confused on what exactly the (g92) card will need for the 12v rail. I have seen anywhere from 22-28 amps. I have checked the slizone site at certified crossfire PSUs for the (g92) cards and they have listed PSUs that are only 18 amps on the 12v rail. To get to the point of this. I am just concerned if the 18 amps will be enough for that gpu? Since I see PSUs with the same amps on 12v rails listed on the site I think it should be. Just thought I would ask someone who knows a bit more about PSUs than I. My system specs are listed below. Thanks a bunch for any help you can give.

    ga-965p-s3
    e6600 @ 3.2
    2gb pc-6400
    raptorX
    x-fi extreme gamer
    1 dvd burner
    antec neoHE 550
    x1800xt
     
  17. jonnyGURU

    jonnyGURU Moderator <BR> Power Supplies
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    Ok.. This FAQ is a good place to settle this....

    When a graphics card company tells you you need "22-28 amps" for a particular graphics card, they're telling you that you're going to need this much power FOR THE ENTIRE PC. NO graphics card NEEDS 22A. That's 264W!

    "Mechanically" you can't even deliver over 150W to the card (75W through the slot and 75W through the 6-pin). Never mind 264W! ;) In reality, you're probably only going to need 6 or 7A of power to the card. Everything else is gravy.

    So don't sweat it... your PSU is way more than enough for your proposed graphics card upgrade.
     
  18. NinjaJedi

    NinjaJedi Senior member

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    thanks a bunch Jonny
     
  19. VulcanX

    VulcanX Member

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    I recently went on a hunt to search for a good overcloking/info gathering forum ( central location ) to find it all and just learn, but threads like yours just help everyone so much and thanks a lot, its really helped me understand it now * impressed *
    Hope to read more posts like this one, really what everyone thinks about but never really get to it
     
  20. lightstar

    lightstar Senior member

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    great post- thank you! never understood the different rails- much clearer now ;-)
     
  21. StanTheMan

    StanTheMan Senior member

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    there r other kind of PSU, such as Thermaltake Toughpower 1000 w and beyond. This unit sports 2 PSU in 1. One with 2 rails +12V and +3.3V and the other with 2 rails +12V and +5V. Cool =)
     
  22. jonnyGURU

    jonnyGURU Moderator <BR> Power Supplies
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    Second paragraph:

     
  23. pmv

    pmv Golden Member

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    Great faq. This is such a useful site. I only worry about pc hardware every few years when I get an itch for a new machine, and every time I do I find all the issues have changed and I have to educate myself all over again. The whole issue of multiple rails was a new one on me.
     
  24. mindless1

    mindless1 Platinum Member

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    Your post was interesting but not quite accurate. I'm apathetic enough about it that I won't argue as someone either has their PSU work ok or not and get a refund (unless a crap design that fails later) so for practial purposes it won't matter, but since we're on the discussion...

    How you define split rails, subjectively, is not how all do. How anyone defines it determines whether they can say this one unit does and this other unit doesn't have them.

    In fact, many designs people say don't have split rails employ parallel RC circuits. In fact, an RC circuit is known to combat noise and provide isolation on power rails. Not in all scenarios, the worse the problem the more it would tend to help. What does a RC circuit look like? This is where many get confused. A resistor won't jump up and shake your hand, nor does it necessarily have color bands or a value stamp on it. All a resistor has to be is a metal less conductive than copper. Many see what they are calling "jumpers" and don't realize this is a resistor. Open any cheap multimeter and you will see similar wire used for current measurement purposes.

    Let's say it does have the RC, but the JonnyGuru interpretation or definition is more involved. I'm not saying the more involved definition is wrong per se, only that it is subjective and arguably not any more correct than some other definitions. You wrote there is only one 12V source, but before the RC filter there could be individual inductors, a further isolation of the two rails. Even before this, it is possible there could be individual rectifiers for even more isolation and yet, it still wouldn't satisfy the narrow interpretation of "only one 12V source".

    I'm not saying either/or is a better 12V design, just that discussion of whether there are multiple rails is really not very significant, only whether a particular unit meets it's own labeled specs per 12V rail up to the total 12V capacity, and those in the context of a reasonably close current ratio per other rails as rated - not necessarily whether it can perform well in an average system even, because it is up to the system integrator to pick a PSU appropriate for the system, not just look at whether it is a good brand, ample total wattage, and worked in a potentially dissimilar system load test. It is good that not all PSU have the same current per rail ratio, so long as people recognize it when selecting one. Perhaps manufacturers should do more to emphasize when a unit has current:rail distribution best mated to a particular type of system configuation instead of generic vanilla PC.
     
  25. jonnyGURU

    jonnyGURU Moderator <BR> Power Supplies
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    Some PSU's do use RC circuits, but not all.

    "Jumpers" are resistors, but don't necessarily provide filtering.

    There could be individual inductors coming from the +12V source, but not always.

    But you do have good information and excellent points.

    The primary goal of my post was to point out the degree of FUD on both sides of the single and multiple +12V rail argument. Regardless of the method, components used, etc. you can't blame "system instability" issues on a PSU with split +12V rails (unless "system instability" defines the PC shutting off completely) and with modern day "designs" that put more thought into high end SLI/Crossfire support there's really no "balancing the rails" the way some FUD marketing would have us believe.