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Which rail goes to what? (In a four 12V rail PSU)

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February 29, 2008 11:24:50 PM

I was wondering what hardware goes on each rail in a power supply with four 12 volt rails? All I know is that the processor gets it's own rail, I believe. Thanks.

More about : rail 12v rail psu

March 1, 2008 12:20:12 AM

It would help to know which power supply you are talking about since designs differ.

-mcg
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March 1, 2008 1:11:04 AM

Here is a review from www.hardwaresecrets.com . There is really only one +12 volt rail.

What immediately caught our eye were the four separated +12 V lines listed on the label (see Figure 20). As it happens to all high-power units nowadays, Thermaltake uses a “virtual rail” concept, where they label their power supplies as having separated +12 V rails but inside the unit they are all connected together to a single +12 V rail on the power supply printed circuit board. Unfortunately all manufacturers seem to be doing like this to match the ATX12V 2.x and EPS12V specifications, which require the power supply to have separated +12V rails.

From page 5.


http://www.hardwaresecrets.com/article/369/1
March 1, 2008 5:19:31 AM

I understand how multiple rails actually work, that they really are only one rail, but I was wondering which rails go to which pieces of hardware? For example, which rail goes to the CPU, which rail goes to hard drives, etc.
March 1, 2008 6:38:28 AM

i believe the way he described it : all of the rails in that particular item are connected together inside of the psu, and all lines go into the common load center. they will actually be combined into say 20Ax4 would be 80 amps to draw from wherever it is needed, technically opening the door for overloading anywhere you want as well. There is no independent throttle on some power supplies that says this wire only gets 20.
March 1, 2008 7:09:05 AM

totakeke said:
I understand how multiple rails actually work, that they really are only one rail, but I was wondering which rails go to which pieces of hardware? For example, which rail goes to the CPU, which rail goes to hard drives, etc.


Seperate out your usages such that each rail uses no more than 18A, as that is the current-limited amount provided to each rail. (or get a pcp&c monster, with no current limiting on any rails. Useful for minor arc welding....)

Hmm.... Does a CPU actually use 12 VDC?
March 1, 2008 2:28:16 PM

"Does a CPU actually use 12 VDC?"
no directly , around cpu socket have few dc regulatora, it generate the different dc voltage for cpu, it needs multi regulator since cpu required the high current, I remember abit motherboard has the option to monitor the current draw, it top out 120 A total when cpu is loading.

pc power and coiling use one rail only design.
March 1, 2008 6:58:20 PM

I meant which rail goes to what from the power supply? For example, which rails go to video cards, which rails go to CD drives, which rails go to fans, etc.
March 1, 2008 7:18:39 PM

I have the same PSU.

My understanding (mostly from a JonnyGuru review of this PSU) is that they are 4 'virtual' rails, but each has OCP limiting the draw to 18Amps max on each (though together the max draw is 60A).

As for how they are split up, one rail does go to cpu (per ATX12V spec), one is for main board supply (which has an optional pcie off it), another is for the two 6pin pcie connectors (not the pcie or pci slots, but the modular connectors on the supply), and the last is for the molex and sata connectors
(this PSU doesn't have the molex on the same rail as the main power - again, assuming I read and understood the Jonnyguru review properly).

later,
Sdrac
a c 138 ) Power supply
March 1, 2008 7:37:47 PM

There are specific cables for specific devices. Use the proper cable to each device. The cables each have different plugs so it should be impossible to use the wrong cable. Where there are multiple options, it does not make any difference since there is really only one rail or source of power.
March 1, 2008 7:51:22 PM

What geofelt says makes sense. I guess it would be pretty hard to mix up cables for stuff if they only fit one way.
March 1, 2008 8:27:13 PM

Not to complete things to much, since it appears totakeke was looking for a simpler answer,
but I think it does matter a bit as to what goes on each rail - depending on whats in your system.

Its true there are all off the same source, but each virutal rail is limited to 18A. If everything is running off molex and/or sata, then all your perheripals are running off the same rail. Your limited to 18A. Most likely more then enough - but if you have an old sys like me and your video card has extra molex connectors on it too - well, then you might want to do what I did and get molex to pcie adaptors so that you can use the 18A on the pcie rails and not have everything on that one limited virtual rail.

It really depends on your system build/components.

sdrac

-- but I do agree that the connector themselves are all pretty full proof; you won't be able to hook a pcie to a molex, etc.
a b ) Power supply
March 1, 2008 8:27:13 PM

Just a minor clarification. The +12 V Rail that goes to the MB provides power to MB circutry, And to PCI/PCI(e) slots. Primary (Majority) current draw being the GPU. Power for the CPU is primarily derived from the 3.3V Rail.

Reason is the CPU voltage is Approx 1 V -> 2 V. The on board regulaters have to "Drop" the difference. With a 3.3 V source and a CPU E of 1.2 V the regulaterd will have a 2.1 V drop (Loss = E x I).

To drop the +12 to 1 or 2 Volt, the Regulater would have to Disipate 11 to 12V X I.

Editted - In refernce to the +12 V rails
The Only real requirement- is to limit the current to less than the Max per Rail, If posible a balanced (Not Req) is best.

Note - For this PS, While each rail is rated at 18A, You can not draw that at the same time (4 x 18 = 76 A, But max total is only 60A) Don't worry - don't think you be pulling 60 amps on +12V rails.
March 1, 2008 8:31:50 PM

@rRetiredChief: yes, I'll edit my other note to clarify. I was talking about the seperate modular 6pin pcie connectors on the supply being on a seperate rail, not the pci/pci(e) slots on the mobo (thats from main supply).
a b ) Power supply
March 1, 2008 8:33:33 PM

Good call sdrac
!