Understanding the 2 rail deal

M

milling_hordesman

Limp Gawd
Joined
Jul 26, 2004
Messages
164
So I'm reading that the 2 rail system for PSUs is the new ATX standard, right? Why are 2 rails so important? I get the gist that one supplies the motherboard, but does is only supply the mobo, or does it also connect to other devices? And how can you tell which leads connect to which rail on the PSU?

Let's set up a theoretical system:

P4 Prescott 3.4GHz
Abit AA8-DuraMax
1Gb Crucial DDR2 Memory
1x200GB 7200rpm SATA Western Digital HDD
1x120GB 7200rpm ATA Seagate HDD
1x 52x Maxtor CD-RW
5 (yes 5) 80mm case fans (I think 0.3A each is a safe assumption, right?)
BFG 6800 Ultra OC

(Any resemblance between this system and the real system in my sig is purely coincidence)

Now according to takaman's, the motherboard, CPU, and HSF alone are going to take just over 10A. Now I have no idea how much wattage the BFG Ultra would take, but I'd assume it's pretty beefy, considering it has its own connector. So assuming all of that, how do you divide the current between the two rails? Or do they kind of do it for you by isolating 1 rail for the motherboard or something? I know that an Antec Neopower has a 14A and a 16A 12V rail. Do they dedicate one of them for the motherboard?

Also, let's say you added a second video card in SLI some time in the future. It would have to go on the same rail as the first, right? So does it make a difference if it's on the same molex string, or would you have to dedicate one them for each card? And how do you get enough individual strings to handle those plus the drives and fans?

And finally, would an Antec Neopower (the only 2-rail PSU I know of) handle the load from the theoretical system above (either 1 or 2 video cards), or would you need a new PSU by that time that has a larger second (or is it first) v12 rail to handle all the 12V being drawn?

Sorry for all the Q's. I've definately read all of the stickies and other posts on this, but I just can't fathom how you can manage to dedicate all these lines and make sure you're balancing load between the rails and everything else.
 
milling_hordesman said:
So I'm reading that the 2 rail system for PSUs is the new ATX standard, right? Why are 2 rails so important? I get the gist that one supplies the motherboard, but does is only supply the mobo, or does it also connect to other devices? And how can you tell which leads connect to which rail on the PSU?
Click to expand...
2 rails is important for stability. It isolates your devices, so for example the spinning up of your hard drives won't affect the supply of electricty to the CPU. Generally, in a dual rail system you'll have 1 rail that connects to the motherboard/CPU via the ATX connector and P4 connector. Then, the rest of the molex connectors are on a seperate rail. Sometimes, like on the Enermax Noisetaker 470 (which is the only dual rail PSU i've dealt with), you'll have an extra molex that is also connected to the CPU/Mobo rail, and it's supposed to be used on your vid card.
milling_hordesman said:
Let's set up a theoretical system:

P4 Prescott 3.4GHz
Abit AA8-DuraMax
1Gb Crucial DDR2 Memory
1x200GB 7200rpm SATA Western Digital HDD
1x120GB 7200rpm ATA Seagate HDD
1x 52x Maxtor CD-RW
5 (yes 5) 80mm case fans (I think 0.3A each is a safe assumption, right?)
BFG 6800 Ultra OC

(Any resemblance between this system and the real system in my sig is purely coincidence)

Now according to takaman's, the motherboard, CPU, and HSF alone are going to take just over 10A. Now I have no idea how much wattage the BFG Ultra would take, but I'd assume it's pretty beefy, considering it has its own connector. So assuming all of that, how do you divide the current between the two rails? Or do they kind of do it for you by isolating 1 rail for the motherboard or something? I know that an Antec Neopower has a 14A and a 16A 12V rail. Do they dedicate one of them for the motherboard?
Click to expand...
Well, it depends on how much amperage per rail you have. In my Enermax 470 example, you've got 16a, and the BFG ultra is supposed to draw 100w, or 8.3a. So, you'd want to put the BFG on some regular molexes. To tally up so far, we've got 10a on the CPU rail, and 8.3a on the Peripheral rail. Now, to hard drives. If we figure 2a per hard drive on the +12v, we can add a bunch of hard drives and still be under the 16a limit of each of the rails.
milling_hordesman said:
Also, let's say you added a second video card in SLI some time in the future. It would have to go on the same rail as the first, right? So does it make a difference if it's on the same molex string, or would you have to dedicate one them for each card? And how do you get enough individual strings to handle those plus the drives and fans?
Click to expand...
At this point i'd say that my example PSU breaks down for SLI, because you'd need quite a bit of power on one rail for that, more than 16a. The problem with connecting them to different rails is that necessarily the rails will not be at exactly the same voltage, which means you'd have a potential between them, and current flow, which is generally considered a Bad ThingTM. All of the molex strings, except perhaps one that is off to the side and marked differently, will be on the same rail.
milling_hordesman said:
And finally, would an Antec Neopower (the only 2-rail PSU I know of) handle the load from the theoretical system above (either 1 or 2 video cards), or would you need a new PSU by that time that has a larger second (or is it first) v12 rail to handle all the 12V being drawn?

Sorry for all the Q's. I've definately read all of the stickies and other posts on this, but I just can't fathom how you can manage to dedicate all these lines and make sure you're balancing load between the rails and everything else.
Click to expand...
Checking the Antec Specs, you've got 18a on +12v1, and 15a on +12v2. One problem is that i've no idea which rail is which. Hey papa-ming, while your supply was open, did you notice which +12v rail is marked +12v1 on the PCB? :D
I still think that dual rail setups are not really ideal for an SLI system. You'd probably have to go with a single, beefy rail design, or a quad rail of some sort like the EG651AX-VH (W) FM or EG851AX-VH (W) FM. In a 1 video card setup, you'd be fine, but once you add another one I think you'd start to push it.
Hope that helps, and please by all means come back with more questions, it keeps me occupied at my boring job ;)
 
Don't really need a photo, but I definitly would appriciate knowing which was V1 and which was V2. Thanks
 
I couldn't find info on which connectors were which rail in the Neo manual. I'd be very interested in finding out so I can be sure to put the vid card on a different rail from the motherboard.
 
You must log in or register to reply here.
Back
Top