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# PSU rail count question?

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September 20, 2011 2:47:34 AM

Hello,

The explanation:
I have a 850 W power supply made by Rosewill, linked below:

same as this:
http://www.rosewill.com/products/699/productDetail.htm

My situation is I have a 12Volt connector to my Motherboard and, 4X 6 pin connectors for a PCI-E GPU

By my count that is 5 connections using 12Volt rails. But if you look at my power supply (referring to the sticker on the side of the PSU it self, also found in the link attacked it lists the following:
12V1@20A,12V2@20A,12V3@20A,12V4@20A

By my count that is 4 rails.

The question:
Does my motherboard and 1 PCIE 6-pin share one rail or are all 5 connectors divided between each rails, or how does the 5 connections share only 4 rails?
a b ) Power supply
September 20, 2011 1:04:13 PM

From my understanding of "rails" and the number a psu has, it has nothing to do with the actual cables. Reason being, if a single rail PSU supports your 24-pin, and 4 PCI-e cables, how does THAT work? I believe the rails come from inside the psu itself, starting from the power cable connection, into the actual transformer in the psu. Then, the transformer splits to all the psu's cables. So, if there are 4 rails, then from the power cable connection in the rear of your psu, 4 rails travel to the transformers. Does this help?
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a b ) Power supply
September 20, 2011 1:04:35 PM

**If i am way off, please correct me.
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September 20, 2011 4:44:56 PM

OK,

So after a little research I found that you are right in the sense that the rails are before the transformer and that I don't need to be concerned what each 12V is labeled as 20A. Realistically I should just look at the value labeled "combined power" for my PSU's 12Vs, which are 768 W@64 A. I'm not saying that the rating of 20A on each 12V does not matter, I was just concerned because my GPU says it recommends 40A on the 12V (and I have 64 combined.)

I found a nice article regarding the general workings of a PSU linked below.

http://www.dansdata.com/psus.htm
(feel free to mark as solved)

Thanks,

Zzx
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a b ) Power supply
September 20, 2011 4:53:05 PM

No, thank you. the webpage was quite interesting! Also, you can mark solve by clicking best answer. Best of luck!
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a b ) Power supply
a b V Motherboard
September 20, 2011 5:21:14 PM

Im not sure you are 100% correct, everything should be fine but if you connected the graphics card via molex adapters to 1 molex cable then it would overload that rail and cause the power supply to shut down.
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a b ) Power supply
September 20, 2011 5:33:29 PM

about the 40A recommendation, yes you are correct simon12. You will overload your rails if 1 card requires 40A. I find that hard to believe considering my 560 requires only 24A. However, if you do plan on putting more than the 20A, or maybe some change, say 22A, you have a high chance of over loading the rail. what is your gpu that it takes 40A?
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a b V Motherboard
September 20, 2011 5:54:15 PM

zzx said:

I found a nice article regarding the general workings of a PSU linked below.

http://www.dansdata.com/psus.htm

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Best solution

a c 87 ) Power supply
a b V Motherboard
September 20, 2011 7:35:53 PM

40A would be for the full system, not just the graphics card. The only cards that would consume anywhere near 40A by themselves would be the 6990 or GTX 590 and you'd have to deliberately overclock them to make them consume that much.

http://www.jonnyguru.com/modules.php?name=NDReviews&op=...
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a b ) Power supply
a b V Motherboard
September 20, 2011 9:32:02 PM

Read accurate, objective PSU reviews at reputable sites such as www.jonnyguru.com or www.hardwaresecrets.com on the EXACT model PSU that you are interested in as some brands have good and poor quality PSUs.

You can also get an accurate rating of how much PSU power is required for your current or future system at the PSU calculator link below. Once you know the total PSU watts required then you need to confirm that the 12v rail has enough amps. to support your Vid card(s) and the rest of the PC system.

There are several websites that show the Vid card power consumption in watts. Divide the watts by 12 to determine the amps. required on the 12v rail(s). Add 15 amps for the rest of the PC on the 12v rail and you now know the Minimum total 12v rail amps required under full load. It's best to have at least 5-10 amps. reserve on the 12v rail available under full load so the PSU is not loaded to 100%.

http://extreme.outervision.com/psucalculatorlite.jsp

http://www.guru3d.com/article/gefo [...] -review/14

http://www.techpowerup.com/reviews [...] Ti/25.html

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September 20, 2011 9:46:23 PM

the mobo connector does not count as "rail"
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a b ) Power supply
a b V Motherboard
September 20, 2011 10:29:01 PM

zzx said:
Hello,

The explanation:
I have a 850 W power supply made by Rosewill, linked below:

same as this:
http://www.rosewill.com/products/699/productDetail.htm

My situation is I have a 12Volt connector to my Motherboard and, 4X 6 pin connectors for a PCI-E GPU

By my count that is 5 connections using 12Volt rails. But if you look at my power supply (referring to the sticker on the side of the PSU it self, also found in the link attacked it lists the following:
12V1@20A,12V2@20A,12V3@20A,12V4@20A

By my count that is 4 rails.

The question:
Does my motherboard and 1 PCIE 6-pin share one rail or are all 5 connectors divided between each rails, or how does the 5 connections share only 4 rails?

you actually have a PSU with a single 12v rail that has been split 4 times, your download manual does not state what split powers what but a common set up would be

12v1 mobo + HDD
12v2 CPU
12v3 PCI-E
12v4 PCI-E

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September 21, 2011 4:06:14 AM

Silvune said:
40A would be for the full system, not just the graphics card. The only cards that would consume anywhere near 40A by themselves would be the 6990 or GTX 590 and you'd have to deliberately overclock them to make them consume that much.

http://www.jonnyguru.com/modules.php?name=NDReviews&op=...

I miss spoke when I said that it requires 40A, you are correct when you say "full system,"
Here is a link to where i got the value of 40A:
http://www3.pny.com/font-color999999GTX-285-1024MB-PCIe...

Also Silvune, Thank you so much for your link, I was looking for that exact page type. I was trying to figure out the rail layout, and it is extremely well explained here.
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September 29, 2011 4:00:35 AM