Power Supplies: Single Rails vs Multiple Rails

By Hurriken ยท 4 replies
Jun 24, 2008
  1. Where does this argument stand now? I'm putting a medium level system together for gaming and am dumbfounded by the division over this topic. I'm beginning to think the pros can't even prove which is better.

    I'm not trying to start a heated debate here. I just want to buy a good power supply for the right price. I'm beginning to think both can be good but I'm thinking the single rails would be more reliable.

    Maybe a better question is what is the most important thing to look for in a PS?
  2. Rage_3K_Moiz

    Rage_3K_Moiz Sith Lord Posts: 5,443   +38

    Multiple rail PSUs cause problems when one rail gets overloaded and shuts the PSU down. For example, if a video card pulls 18A from a PSU that has two 15A +12V rails, it may overload one of the rails and destabilize the PSU.

    Thus, in most cases, a single-rail PSU is almost always better than a multiple-rail one. But understand that all single-rail PSUs are not guaranteed to be great. Only high-end PSUs that deliver large amounts of power on a single +12V rail can be classified as excellent PSUs, since they typically have better internal parts such as high-quality Japanese capacitors from Rubycon or Hitachi and sturdy casings and fans that do not vibrate dangerously. PC Power & Cooling, Seasonic and Corsair are among the best companies that produce units rated as low as 450W with single +12V rails.

    The ultimate thing to look for in any PSU is the total power supplied on the +12V rail(s). If it's a multiple rail PSU, divide the power value by 12 to get the amps provided in total across all the rails. A PSU with any documentation that does not provide this information is to be avoided. The same goes for a PSU with no documentation at all.

    Lastly, look here for more pointers on how to buy the right PSU for your system.
  3. Hurriken

    Hurriken TS Booster Topic Starter Posts: 231

    This was really helpful, thanks for taking the time to post this.
  4. Tmagic650

    Tmagic650 TS Ambassador Posts: 17,244   +234

    Hey Rage,
    I always thought that the individual rails would share the loads between them, so overloads would be rare or nonexistant
  5. Rage_3K_Moiz

    Rage_3K_Moiz Sith Lord Posts: 5,443   +38

    The rail overload would only be typical on PSUs with two or maybe three +12V rails, since the video cards would be forced to share power with peripherals. PSUs with four or more +12V rails will not exhibit the problem since the video cards already have their own rails. Also, some good-quality PSUs have regulators to regulate the voltages across all the rails, which helps avoid an overload.

    As for the "isolated power" comment, I just reread it and it suddenly sounded like a bunch of crap. I read that comment on the PC Power & Cooing website and just stated it here without actually thinking properly about what it meant. Some electrical engineer I am, eh? ;)
    Isolated power is only possible in superconductor materials. There is minimal wastage of power with multi-rail PSUs, since there are rarely PSUs that have two or more truly separate rails fed by separate transformers. Usually, most PSUs have a single transformer that generates a certain number of amps. This is then broken up between two rails, and these rails are rated by how much current each rail can carry independently. Marketing creates confusion in that when you have 30A being produced by the transformer and being fed to two +12V rails rated at 17A each, most people will think that this means that 34A is being produced. This is not the case, since while one rail may be loaded to 17A, the other can only carry 13A since that is the maximum power being generated by the transformer. This is why you cannot add PSU rails together and is the reason most good PSU manufacturers supply a maximum wattage rating for the +12V rails combined.

    Fact is, a good-quality multi-rail PSU can be more energy-efficient than a single rail one, due to the fact that a multi-rail PSU has a secondary transformer coil with extra MOSFET switches. So as long as the MOSFETs used for each rail are of high-quality (>95% efficient, so as to give at least 85% idle power efficiency) and you don't load the extra rails, the switches stay closed. So while a single-rail PSU with a high-quality MOSFET may start with an idle efficiency of 85%, a multi-rail PSU starts with each rail efficiency at 95%.
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