Dual 12v rail PSUs

[Azsen]

1) How do dual 12v rail PSUs balance the load between the rails?

2) Lets say we have 18A on each 12v rail. Now say the CPU is pulling a fair amount of power and needs 20A to function properly. Does it borrow another 2A from the other 12v rail or what?

3) Is there any disadvantage when using a ATX 2.0, 24pin, dual 12v rail PSU with a standard 20pin motherboard and 24pin -> 20pin adaptor? Can the PSU still balance the load properly or will something suffer because of the split rails?

4) Can you balance your own rails? Like run each piece of hardware off the rail you want it to?

Cheers. 🙂

 

[Matthias99]

Originally posted by: Azsen
1) How do dual 12v rail PSUs balance the load between the rails?

They don't. That's the whole point of independent rails.

2) Lets say we have 18A on each 12v rail. Now say the CPU is pulling a fair amount of power and needs 20A to function properly. Does it borrow another 2A from the other 12v rail or what?

No; probably your system will get flaky and crash.

3) Is there any disadvantage when using a ATX 2.0, 24pin, dual 12v rail PSU with a standard 20pin motherboard and 24pin -> 20pin adaptor? Can the PSU still balance the load properly or will something suffer because of the split rails?

Having one or two rails makes little difference in this situation. Obviously, you still have to keep each rail below its rated amperage draw. Nothing is balanced automatically.

4) Can you balance your own rails? Like run each piece of hardware off the rail you want it to?

Uh, yes. This is what they make dual-rail PSUs for. 😕

 

[imported_whatever]

Although they will not balance the rails, 20A@12v is more power than any CPU will use, even if you OC it a LOT. That is 240W, roughly twice what the highest-clocked Prescotts use.

 

[Azsen]

4) Can you balance your own rails? Like run each piece of hardware off the rail you want it to?

Uh, yes. This is what they make dual-rail PSUs for. 😕

Right, so how do you run run the CPU off one rail and the video card off another? Is it to do with what molex cable you plug into the video card? If so, how do you know which cables are running which of the dual 12v rails off the PSU?

 

[deveraux]

Well, I have an Antec Neopower 480W with dual 12V rails and from what I have gathered from the manual, they have already set which rail drives which components, i.e. the PCI-e graphics card gets its molex from rail 1 while the CPU gets it from rail 2 IIRC. I'm not sure how its done on other dual/tri 12V rail PSUs.

 

[Azsen]

Thanks deveraux for the hint, I just checked my Enermax manual. They recommend I plug the special molex connector with "Extra" written on it into my AGP 8x high end graphics card. That could be how you run stuff off the second 12v rail there. This would mean I have been running everything off the one 12v rail all along because I haven't used the extra connector before... :shocked:

 

[Azsen]

Well I looked in my Enermax 470W EG475P-SFMA24P manual for details about how to make use of the second 12v rail. They recommend I plug the special molex connector with "Extra" written on it into my AGP 8x high end graphics card to "provide it with stable current".

I assume this is how you run stuff off the second 12v rail. Unfortunately I looked over all the cables coming out of the PSU and none of them have "Extra" written on the molex cable. I think they might have replaced it with the PCI-E 6pin connector? So it would appear I cannot make use of the second 12v rail at all now seeing I have an AGP graphics card??

Here I am thinking I'm getting a good PSU that will make use of the 31A on both 12v rails and in actual fact my whole system is probably running off the one 12v rail rated at 16A? I need my 15A on the other 12v rail! I can buy 12v/16A PSUs for like $30...

 

[Azsen]

How do I make my PSU run stuff off the second 12v rail?

 

[imported_Kensai]

Read the manual. ^_^

 

[Azsen]

Few grapes short of a bunch I think...