How do +12V rails work?

[Raziaar]

Okay well I've determined that it seems my video card works, even though I thought it was dying, on my brothers computer. His computer is 150W more than mine at 650W, and his two 12V rails have more amperage than mine. His are both 20v on the +12v rails, whereas mine are like 17v and 19v on the +12v rails.

So I figured maybe if I remove my video card power supply from the two independent chains of molex connectors as I have being split and converted into a single pci-express cable, I would instead use the modularity of my power supply and plug one pci-express cable directly into the power supply(there's four slots where it can connect into, two being taken up by the two molex chains) and plug it into my video card.

Right now it's working, but I'm almost 100% guaranteed it'll continue with it's crashes like it has been.

The question is, how do +12v rails work? If I have two +12v rails on my card, and my card needs like 35amps combined from those, does that mean I need two cables into my video card like I had with the molex connector chains being split into it? Or does dual +12v rails not have anything to do with the number of chains of connectors coming out of the power supply?

 

[No Limit]

No, the 12V rails are combined together. So if you have 2 12V rails at 20amps each your total 12V output is 40amps on all the cables.

 

[Asus]

How a lot of current PSUs work when they have dual 12v+ rails (some say they do but actually are 1 which IMO is better) is that one rail is usually for the 20/24 pin to the board and molex connectors while the 2nd is to the dedicated PCI-E GPU power 6/8 pin. With your modular PSU cables, do they have specific spots for the different types of cables?
But if you use an adapter to convert a 4pin molex into PCI-E GPU plug then the card is pulling all it's power from the same rail (PCI-Express slot+plug) while the other is untouched. Dual rails are not directly added.


Debunking Power Supply Myths

FYI. The chart showing +3v, +5v and +12v rails shows how many AMPs are on each rather than volts. (20Amps on the +12v)

 

[No Limit]

Asus, dual 12V rails are not truly independent. They are connected together. The only reason they are called dual rails is because each one is fused to blow at a certain current output. This is for safety reasons so incase of a short circuit the wire doesn't melt and start a fire. If you have 2 20amp 12V rails then it doesn't matter which cable you use, each cable will be able to output up to 40 amps of power.

 

[Raziaar]

Hey, thanks guys. And Asus, it seems it doesn't matter where I add the cables on my modular power supply. The one part that isn't modular though is used to power the motherboard.

My computer seems to be running fine, nearly four hours after doing what I have done. I still expect it to start crashing again, but thus far it hasn't.

 

[Asus]

This is the page that talks about +12v rails.
I misquoted it. It mentions most units putting 1 rail for the CPU and the other for PCI-E power plug and PCI-Express slots. Meaning everything using +12v except the CPU is running on 1 rail.

jonnyguru.com is a good place to look.

 

[Raziaar]

Thanks Asus, you're always a fantastic help!