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Simple question about rails

post #1 of 23
Thread Starter 
Apologies in advance as this is probably a very simple question with a very simple answer.

Some PSUs have a lot of rails, 5 or 6. Some newer PSUs have a single rail with a very large current on it. I assumed that it must be a good idea to have a single rail if a lot of the new ones are doing it. However, some are now coming out with 2 high current rails. I assume this is to have one large current rail for the system and one specifically for the graphics cards using Tri SLI or whatever but (my first question) isn't that just a slippery slope back to the 5 or 6 rails that are already common?

Also, what are the advantages and disadvantages of a single large current rail rather than lots of rails with a individual currents?

I was tempted by an Enermax Galaxy 1000W as I am planning on buying a few new parts such as some 9800s when I know more about them but the Galaxy 1000 uses many rails and I don't know if that is a good thing or not, nor do I know if future generations will switch to a single rail if it is better. Having a more stable power supply than my current (no pun intended) one would probably help my overclock too as I am nowhere near limited by temperatures but I can't get a high stable OC and I've replaced most other things so I assume my current PSU doesn't give out a stable current.
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post #2 of 23
It used to be best to have a single 12V Rail, but it's not necessarily the case any more, as technology has improved. I am not by any means an authority, but it is more important to get a PSU made by a respectable company than it is to compare Specs.

Some of the best PSU brands are Silverstone, PC Power & Cooling, Seasonic, Corsair, OCZ, and Forton Source. If you find something by one of those brands, just look for the wattage you need, and hopefully someone else can clarify the specifics of Multiple 12V rails.
    
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post #3 of 23
Found this on JonnyGuru. Hopefully it helps a little:


h. DC Output “Rails”:

Computer power supplies put out multiple voltages. Each of these separate voltages is called a “rail.” A computer power supply typically has a +3.3V rail, a +5V rail, a +5VSB rail (SB stands for “Stand By” as it is live as long as the unit is receiving any AC current) a -5V rail, a -12V rail and one or more +12V rail. Typically, all leads of the same voltage draw power from the same rail, so it is erroneous to state that each set of wires coming from a power supply represent a rail. Even a power supply with multiple +12V rails tend to distribute the power of a particular +12V rail across multiple wires to multiple connectors. Furthermore, multiple +12V rails are often split off of the same +12V source.

i. Multiple +12V Rails:

Many power supplies on the market have multiple +12V rails. This may be accomplished by having more than one +12V transformer or, more typically, taking the typical +12V output of a power supply and splitting it up into multiple, what are often called "virtual" +12V rails. Note that even units with multiple transformers may have these two individual outputs split further with "virtual" rails, or in some cases these two outputs may be combined to create one larger +12V rail.

The division of the +12V rail is done because a 240VA output (equivalent to 240W DC) may be potentially harmful or fatal or may cause fires due to the overheating of leads burning wire insulation caused by too much current being delivered to a connector, typically due to a short (as per UL and EM 60950 safety requirements.) Because modern PC’s regulate CPU and GPU V-core from a +12V source, a PC can require much power from the +12V rail of a power supply than PC’s of the past that only tend to use the +12V rail for drive and fan motors. It is not unusual for the power demands of the +12V rail to exceed 240VA, thus the need for multiple +12V rails. The assumption that each individual connector with a +12V lead is on its own rail is not uncommon. But the fact of the matter is that there may be several connectors all using the same +12V rail.
    
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post #4 of 23
Thread Starter 
Thanks, though it didn't really provide much more than I already knew.

I know that old PCs had single rails and that more rails were added for the sated reasons. However, single rails seem to be becoming more and more popular. For example the Silverstone DA1200 has a single 90A 12V rail whereas my 2 year old ST60F has 55A over its 4 12Vs. (I know the numbers are not comparable I am just saying how the newer ones tend to be single rails whereas there were pretty much no single rail PSUs a couple of years ago at 600W+)

I assumed there must be something good about having a single rail. Something like devices being able to pull as much power as they need without having to be on a specific rail, or part of a rail being left unused because not enough components are running on that rail. However, then I see that even newer PSUs can sometimes have one large rail for most of the system and then another for just the PCI-E connectors, which is probably how the multiple rails started those years ago.

It seems a bit strange from me, like serial being used many years ago for storage, then PATA, then as the overhead for splitting and joining the signal became too high, back to serial with SATA. 2 rails seems to be the equivalent of 2 SATA cables.. what next, 4 cables, 16? Then we are all the way back to PATA.

To be honest until today I had never heard of PC Power & Cooling but I have read it in quite a few posts in the last hour. Enermax is a very reputable manufacturer as far as I know and the Galaxy are their top series.
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post #5 of 23
The Decathlon is Silverstone's Single Rail Modular line. Their Zeus and Strider lines have 6 Rails totalling 94A, and are also modular. The Olympis is a Non-Modular Single Rail PSU, again with a 90A 12V Rail.

So, even the best make single and multiple 12V Rail PSUs.
    
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post #6 of 23
Thread Starter 
Yes but what are the advantages of each? Single rail seems a lot more popular now than it was a couple of years ago, so what has changed.

I feel like I'm repeating myself a bit here.
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post #7 of 23
And I feel like I'm not helping much, lol.
    
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post #8 of 23
Let me give it a try.Some of it is just a sales pitch to begin with because even some of the ones that claim to have multi rails really don't.A lot have a single rail and are then only divided with max amps rating on that cable.Example would be one that claims 4 18 amp rails but that doesn't mean it has 72 amps it may have 60 amps and the most you can put on any one cable is 18.What you look for is the max watts or amps for the combined 12v.Another thing may be the temp that they claim the max at,if they say at 25c then thats not good but if they say 50c thats much better. Cheap PSU's loose their efficency and stability at lower temps.Not the best explanation but I hope it helps.
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post #9 of 23
Thread Starter 
So single rail is no better or worse than multi rail? If this is the case, why are both sold but single rail becoming increasingly popular in very high end PSUs?
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post #10 of 23
From what i can understand the multilpe rail psu is a marketing gimmic. #1 there is only 1 rectifier in the psu giving you the 12volts. so electrically there is 1 rail no matter how many 12v leads come out of it. They use sensors to know when 1 rail is pulling too much current.

Basically if you buy ba single rail psu = no worries
buy a multiple rail psu you may find yourself in a predicament, lets say your vid card needs 18A and your rails supply 60 amps over 4 rails, guess what yoyu cant run that card cause no single rail is capable of delevering over 15 amps soo youre stuck.

another way is to say you need 5 amps for the vid card ok well guess what you just lost 15 amps cause you cant split them up 1 rail feeds the vid card then you lose the 1 rail.

single= simple

this would never happen with a single rail
    
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