How do PSU power rails work *Calling all Tech Heads**

[Reedweaver]

My PSU is 700W Jeantech Storm

12V1 is rated at 16A and powers the motherboard, CPU1 and first PCI-E plug
12V2 is rated at 25A and powers CPU2, the second PCI-E and S-ATA plugs
12V3 is rated at 17A and powers the Molex plugs.

12V1 is a 16A rail - Motherboard 4-6A – half of the i3 processor 3.25A, 4870 11A = 18.25A MAXED OUT
12V2 is a 25A rail - Other half of i3 processor 3.25A, 4870 11A, 3xRaptor HDDs 10A = 24.25 MAXED OUT
12V3 is a 17A rail -DVD Burner, 5xfans, fan controller = 8A so 9A to play with.

I need to take some pressure off the 12V1 and 12V2 rails…if I connect the 2nd pci-e power connector off both the 4870’s up to this rail how does the card know what power to pull from each source?

Can you even do this…

 

[Reedweaver]

Having done some research its not a good idea to connect to two different rails..

By the way, in case you're ever tempted to hook the independent 12 volt rails together (I've seen people on the Internet who think this is a good idea), don't do it. Your 12 volt rails may have different ideas about what voltage they should set their rails to. One may be quite a bit different than another. They're separate rails, after all, and they have their own circuitry which controls the voltage. They're bound to vary a bit. And if they're just a little different then you can draw lots of current when you connect them together because each of the output circuits try to force the voltage on the same wires to a different value. That causes either a nice orderly shutdown from the over-current protection or smoke and sparks. There are some power supplies which have switches which allow you to gang the rails together. Once you've set the switch properly it's okay to connect them.