Rails on PSU

AaronW · 19 Apr 2006 at 17:27

Can someone tell me what a rail is on a PSU? Do you need a raid just for a graphics card?

Steve-O 2006 · 19 Apr 2006 at 18:12

a raid??? I have a graphics card and dont think I have a raid

... or did you mean Rail?

AaronW · 19 Apr 2006 at 18:16

Steve-O 2006 said:
a raid??? I have a graphics card and dont think I have a raid ... or did you mean Rail?

Sorry, as per title I meant rails.

Amoeba · 19 Apr 2006 at 19:44

No, a rail is just a source of power in your PSU. Cheaper PSUs only have one rail, whereas more expensive ones have many. If one rail is having a lot of power drawn from it, it shouldn't affect the other rails... so yeh it could be nice to have a rail just for your graphics card (esp if it's a very powerful one) but it's not vital by any means.

fornowagain · 19 Apr 2006 at 23:35

Er nope. Most PSU have one voltage source or "line" per voltage output. That consists of the usual transformers regulators smoothing etc. Although they may share a primary transformer with different tappings. Now a rail is in most cases just a connection to that source. E.g. A dual rail PSU, has one 12v source, but two 12v rails. Each rail has a separate over current limiter set to a maximum of 240VA. The better ones will have some form of voltage regulation and smoothing, zener probably, stopping over voltage. All it really does is stop the user over loading one set of cables. All the loads pull from the same place, so voltages can still drop with other large rail loads. Potentially not as badly because they're current limited and the loads are split to different cables.

If you want to know more:

http://www.silentpcreview.com/article28-page1.html

It's important to remember that even though there are two "independent" 12V lines, they still draw from the same main source. It's highly unlikely that there are two separate 120VAC:12VDC power conversion devices in a PSU; this would be much too costly and inefficient. There is only one 12VDC source, and the two lines draw from the same transformer. Each line is coming from the same 12VDC source, but through its own "controlled gateway".

PSU makers' specs are misleading in that thay rate the current capacity of each 12V rail independently. What really matters is the total 12V current: Generally, up to 20A is available on any one 12V line assuming the total 12V current capacity of the PSU is not exceed.

xirokx · 20 Apr 2006 at 11:12

so if even on a dual rail PSU there is one main rail is the difference that power is supplied more efficiently through diff rails? Therefore not overloading the cables?

How does someone know if the rails are 20a give or take when purchasing a new psu?

R B Customs · 20 Apr 2006 at 12:47

the PSU rail thing is one of the things remaining on my 'things i need to learn about PSUs'
is this somthing we can see?

fornowagain · 20 Apr 2006 at 15:09

xirokx said:
so if even on a dual rail PSU there is one main rail is the difference that power is supplied more efficiently through diff rails? Therefore not overloading the cables?

How does someone know if the rails are 20a give or take when purchasing a new psu?

Yes one source. No not more efficiently, probably less. But it does stop the overload of any one connection (and any associated voltage drop), which is very unlikely anyway. When you buy the PSU look for the combined maximum wattage for the 12v. Then divide it by 12, gives you the maximum 12v current. Don't just add the rails together.

Example. in most cases 12v2 (12 volt rail two) goes to the CPU, which will take what it takes. Everything else is on 12v1. Think of it like this, you can't get out more than you put in. Say a PSU has 360W maximum at 12v, that's 30A. With two rails, if say 12v2 uses 15A, then 15A is left for 12v1. But if 12v2 uses 5A, then 12v1 has a potential 25A, unfortunately it has a current limit of 20A(240VA). And that's the dual rail drawback and the reason multiple rails are appearing.