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Thread: Why I prefer multi-rail to single-rail PSU design

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    Why I prefer multi-rail to single-rail PSU design

    Hey all, have you ever had one of your PC devices short out only to have your ENTIRE SYSTEM feel the burn? Well, this is where multi-rail PSUs come in. They keep any surges or changes in current isolated to that one rail so that any overloads or fluctuations won't compromise the entire system. For example, you decide to have 4 ATI cards in Crossfire. One of your cards decides to go into meltdown. If you're working on a single-rail system, your entire system's going to take on the short before the PSU kills the power. By time the power's off, you've lost your hard drives, your motherboard, another GPU and who knows what else. Had you used a multi-rail system, only the one GPU would have quit and once the PSU registered that the rail was overloaded, it would kill the power MUCH faster than a single-rail PSU would.
    So ask yourself, is my equipment worth the incorrect rumors that a single-rail system is more powerful? Considering each rail puts out the amount of power that its wires are designed to carry, I think not.
    Besides, some PSUs are designed to allow for a single or multiple rail choice by flipping a switch.

    What are your opinions?

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    Re: Why I prefer multi-rail to single-rail PSU design

    taken from another site!

    The market for power supplies (PSU) is quite confusing. Current offerings include power supplies with single or dual, and a few triple and quadruple, 12V rails. So the question is: do we need multiple 12V rails, or is one 12V sufficient? Single 12V rail power supplies are not a thing of the past: there are SLI certified single 12V rail power supplies, such as the OCZ PowerStream 520W, the PC Power & Cooling Turbo-Cool 510 Express, and the Silverstone SST-ST56ZF 560W. These are power supplies offered in the upper level of the market at prices to match ($134.99, $229.99, and $137.99 respectively, Newegg September 30, 2006).

    Why did multiple 12V rails come about?
    Multiple rails came about because the EU mandated that the power on a single rail should not exceed 240VA (12V x 20A). Intel then stipulated in the ATX12V v2.xx standard that if the power on a 12V rail was likely to exceed 240VA, a second 12V rail needed to be engineered. For practical purposes, a limit of 18A was used, allowing for 2A headroom. Manufacturers replied by mass-producing multiple rail power supplies, using several types of engineering. Some power supplies are well engineered, and the 12V rails are truly separated with separate transformers. However, others draw power from one single transformer and the rails are only separated after transforming takes place. The issue with multiple rails is that the power per rail is relatively low, which unfortunately often results in the use of inferior parts.

    Do we really need multiple 12V rails?
    The answer is simple: no, we do not. As stated above, there are high-end power supplies with one single 12V rail that are SLI certified. These single 12V rails are rated at >30A, and thus likely build with high-quality parts. This is also confirmed by Intel, who unofficially dropped the 240VA standard in spring 2005, actually re-instating single 12V rail power supply units as an accepted standard. It is interesting to note that the Siverstone SST-ST56ZF 560W (single 12V rail, 38A) is SLI certified for Dual GeForce 7900 GTX or Dual GeForce 7800 GTX 512 MB, while its “bigger†brother, the Silverstone Zeus SST-ST65ZF 650W lacks this certification, although it is certified for all other SLI configurations and delivers 42A on the combined 12V rails. The problem likely is that it has a bit of an odd distribution of current along quadruple 12V rails (13A, 18A, 16A, 8A). Thus, this is an example where the single rail PSU is preferable over the multiple-rail PSU. On the other hand, there are many really well engineered and well built multiple-rail PSU. This FAQ is only meant to point out that multiple rails are not a necessity for a good PSU.

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    Re: Why I prefer multi-rail to single-rail PSU design

    Woow-woow-woow. After reading the first line of your statement I felt compelled to reply. No PSUs are 'actually' single rails - i.e. no PSU has a single power converting block from 240/250V to 12V, a 'single rail' PSU is just where the manufacturer has taken lost of smaller rails and combined them to a single power supply.

    Now yes your correct, by combining these rails there is a slightly higher chance of failures, when one fails they can all fail. But they key issue with reliability is not to do with the number of rails, it how they are used and what protections are put in place to protect your components in the case of a short-circuit.

    Top-end PSUs (well actually most produced by a reputable name) have circuit breakers spread throughout the power supply that in the event of a malfunction the entire unit will trip and the shortened circuit will either be either instantly cut-off or sent back down the mains (before being very quickly cut-off). As for your comment that "your entire system's going to take on the short before the PSU kills the power" these circuit breakers are the same as you get in a (modern) fuse box in your house, and unless you cheep-out or miss-wire things a short circuit will not destroy all the things you have plugged in at home...

    As for "Had you used a multi-rail system, only the one GPU would have quit and once the PSU registered that the rail was overloaded, it would kill the power MUCH faster than a single-rail PSU would"
    Thats simply not how electricity works... not much more to say...

    As for "Considering each rail puts out the amount of power that its wires are designed to carry, I think not." Again not true, many PSUs have more connectors than they can supply full-power to all at once. When buying a PSU you dont just look at the connectors do you? You look at the Amperage on the various rails, and do the math to see if it will supply enough wattage to your components.

    So all in all, reliability of PSUs has next to nothing to do with the number of rails - multi-rail PSUs may be a smiggen more reliable but as long as your purchase a PSU from a reputable name who has built the PSU to rigorous standards and put in place the various back-up measures to ensure that even when faults occur they dont cause the entire system to die.

    But its worth noting that even the most reliable PSUs have a chance to kill your entire system - for example a tiny-air bubble could be caught in a copper wire, when this heats up enough it could create enough pressure to pop the wire, and this wire could connect with the metal of your case sending an electrical current through the case, then a stray bit frayed wire could transfer the current to your mobo and fry the lot. This scenario would have nothing to do with the PSUs build quality, just bad luck and an unfortunately poor quality batch of copper wire.

    So, d3m0n1q_733rz, no offence intended, but there are quite a few technical issues with your assumptions. I understand, and actually agree with some of your points (mulit-rails are slightly safer), but I disagree that number of rails is the real issue - there are many other aspects to build quality that are far more important, and in short buy from a good name, dont cheep-out, and you should be ok.

    Edit: one finally comment - IMO the benefits of Single-rail PSUs far outweights the possible costs. Single-rail PSUs tend to have much better power delivery, and are much more flexible.

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    Re: Why I prefer multi-rail to single-rail PSU design

    Quote Originally Posted by ladcrooks View Post
    taken from another site!

    The market for power supplies (PSU) is quite confusing. Current offerings include power supplies with single or dual, and a few triple and quadruple, 12V rails. So the question is: do we need multiple 12V rails, or is one 12V sufficient? Single 12V rail power supplies are not a thing of the past: there are SLI certified single 12V rail power supplies, such as the OCZ PowerStream 520W, the PC Power & Cooling Turbo-Cool 510 Express, and the Silverstone SST-ST56ZF 560W. These are power supplies offered in the upper level of the market at prices to match ($134.99, $229.99, and $137.99 respectively, Newegg September 30, 2006).

    Why did multiple 12V rails come about?
    Multiple rails came about because the EU mandated that the power on a single rail should not exceed 240VA (12V x 20A). Intel then stipulated in the ATX12V v2.xx standard that if the power on a 12V rail was likely to exceed 240VA, a second 12V rail needed to be engineered. For practical purposes, a limit of 18A was used, allowing for 2A headroom. Manufacturers replied by mass-producing multiple rail power supplies, using several types of engineering. Some power supplies are well engineered, and the 12V rails are truly separated with separate transformers. However, others draw power from one single transformer and the rails are only separated after transforming takes place. The issue with multiple rails is that the power per rail is relatively low, which unfortunately often results in the use of inferior parts.

    Do we really need multiple 12V rails?
    The answer is simple: no, we do not. As stated above, there are high-end power supplies with one single 12V rail that are SLI certified. These single 12V rails are rated at >30A, and thus likely build with high-quality parts. This is also confirmed by Intel, who unofficially dropped the 240VA standard in spring 2005, actually re-instating single 12V rail power supply units as an accepted standard. It is interesting to note that the Siverstone SST-ST56ZF 560W (single 12V rail, 38A) is SLI certified for Dual GeForce 7900 GTX or Dual GeForce 7800 GTX 512 MB, while its “bigger†brother, the Silverstone Zeus SST-ST65ZF 650W lacks this certification, although it is certified for all other SLI configurations and delivers 42A on the combined 12V rails. The problem likely is that it has a bit of an odd distribution of current along quadruple 12V rails (13A, 18A, 16A, 8A). Thus, this is an example where the single rail PSU is preferable over the multiple-rail PSU. On the other hand, there are many really well engineered and well built multiple-rail PSU. This FAQ is only meant to point out that multiple rails are not a necessity for a good PSU.
    Actually, it was taken from experience. I had a DVD drive that was causing power issues with my hard drives and causing them to drop-out, so I put it on a separate rail. A friend of mine also had her computer totally fry and all of her pictures she's drawn and stories she's written were gone with the drive's components. And I mean three hard drives, the CPU, EVERYTHING all fried because of her single-rail PSU.
    But I do, however, agree with the site you shared.

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    Re: Why I prefer multi-rail to single-rail PSU design

    Quote Originally Posted by d3m0n1q_733rz View Post
    I had a DVD drive that was causing power issues with my hard drives and causing them to drop-out, so I put it on a separate rail.
    Aren't you confusing the point here then?
    If you have a defective optical drive that's interfering with the power delivery to other devices on the same power cable then surely the issue is not related to the PSU. Or are you saying that the DVD drive was not defective? If you are then it sounds more like the person that built the PC was trying to cut down on the number of cables being used and this resulted in too many devices running off of one cable.

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    Re: Why I prefer multi-rail to single-rail PSU design

    Quote Originally Posted by d3m0n1q_733rz View Post
    A friend of mine also had her computer totally fry and all of her pictures she's drawn and stories she's written were gone with the drive's components. And I mean three hard drives, the CPU, EVERYTHING all fried because of her single-rail PSU.
    Nope. That's because of her crap PSU, nothing to do with it being single-rail or not.

    The link between the hard drive power and CPU power is exactly the same in the case of a single-rail or a multi-rail PSU.

    Multi-rail only applies to the 12V lines, to remain within ATX spec. Has nothing to do with what you're talking about, and has nothing to do with cables.

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    Re: Why I prefer multi-rail to single-rail PSU design

    well said kalniel

    I've seen PSUs go pop, even cheap OEM Dell ones, and the worst that happened was some smoke and the need to replace the PSU.

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    Re: Why I prefer multi-rail to single-rail PSU design

    Quote Originally Posted by ik9000 View Post
    well said kalniel

    I've seen PSUs go pop, even cheap OEM Dell ones, and the worst that happened was some smoke and the need to replace the PSU.
    I've seen them take the whole system out..

    Luck of the draw but you increase your chances with a PSU which offers various forms of protection in case of emergencies..
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    Re: Why I prefer multi-rail to single-rail PSU design

    Quote Originally Posted by dangel View Post
    I've seen them take the whole system out..

    Luck of the draw but you increase your chances with a PSU which offers various forms of protection in case of emergencies..
    yep no question, I was meaning that the OP saying that it was clearly the single rail PSU at fault for bricking the system needs to question it more closely, as Kalniel pointed out. Even with a PSU going bang, it doesn't mean it will always fry the innards. (Nor, as you point out, does it mean it won't either)

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    Re: Why I prefer multi-rail to single-rail PSU design

    One thing I have noticed myself is that I've come across alot more HDD and motherboard failures on systems with cheap and nasty PSU's that are still working than I have on systems with decent PSU's.

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    Re: Why I prefer multi-rail to single-rail PSU design

    The crux of the matter is that you want a quality PSU, number of rails is pretty much irrelevant.

    It could be a 4 rail design like this:
    http://www.tomshardware.com/reviews/...ve,3189-2.html

    Or it could be a single rail design:
    http://www.tomshardware.com/reviews/...ve,3189-4.html

    Both are absolutely superb PSUs, number of rails is almost meaningless.

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    Re: Why I prefer multi-rail to single-rail PSU design

    Seen 2 go recently....one quite spectacularly.

    First was a friends who brought old PC around with AGP card....VGA was dead. on testing it seems that it had taken the AGP slot with it. Gave him an old PCI card to keep it hobbling along.... 2 weeks later everything but the IDE HDD was dead!

    Second was my Mums who hadn't told me she went to PC World a while ago and bought a PSU (Jeantech).....asked me to look at her PC.....plugged the kettle lead in and BOOM - fireworks display out the back of the PSU and......mobo dead.
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    Re: Why I prefer multi-rail to single-rail PSU design

    Quote Originally Posted by shaithis View Post
    Second was my Mums who hadn't told me she went to PC World a while ago and bought a PSU (Jeantech).....asked me to look at her PC.....plugged the kettle lead in and BOOM - fireworks display out the back of the PSU and......mobo dead.
    Eeep

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    Re: Why I prefer multi-rail to single-rail PSU design

    No, what had happened was that the drive was basically in pre-fail and each time I would go to write a DVD, the power would fluctuate drastically between normal and not. I only had this and two hard drives on the same cable. So, I moved my HDDs to another rail and the fluctuations were quarantined to just the one rail which left all of my other components untouched. Once the drive failed, I swapped it out for a new one.

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    Re: Why I prefer multi-rail to single-rail PSU design

    Actually, we believe that it was the HDD that shorted out and caused the short to go through the entire PSU and to the rest of the system where everything went boom. But had the short been contained, then the damage would likely have been much less.

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