Question about multi-rail PSUs...

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ScotteusMaximus

Limp Gawd
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Oct 2, 2005
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when a PSU has multiple 12V rails, how are components split up between the rails? is it based solely one which leads i use, or is there some sort of "intelligent" load balancing to even out the distribution?
 
I just asked a similar question in the thread I started... I was trying to figure out how the current is divided amongst the rails so I could figure out if a multi-rail PSU would successfully drive my server (planning on dual Opteron, up to 10 drives, DVD-RW, fans, etc.). So I'll just forward to you the response I received:

EPS12V 6.1.1 12V Power Rail Configuration

There are two types of 12V rail configurations for systems: 'Common plane' and "Split plane' processor power delivery. The 'common plane' system has both processors powered from a single 12V rail (+12V1) from the power supply. The 'split plane' system has both processors powered by seperate 12V rails (+12V1 and +12V2) one dedicated to each processor. The system in both cases, has an additional 12V rail to power the rest of the baseboard +12V loads and dc/dc converters. +12V1, +12V2 and +12V3 should not be connected together on the baseboard to ensure that 240VA protection circuits in the power supply operate properly
Table 6: 12V Rail Summary
........................................................................................................................................................................................
Common Plane System........................................................Split Plane System
+12V1........Processors.........................................................+12V1........Processor 1
+12V2........Baseboard components other than processors.......+12V2........Processor 2
+12V3........Drives and peripherals..........................................+12V3........Baseboards components other than processors
...........................................................................................+12V4........Drives and peripherals
Click to expand...

Hope that's what you're looking for.
 
Correct Joe and with just 2 12v's 12v2 powers the CPU while 12v1 powers all other 12v evices.
 
is there a sticky or a good site to read on this, i just got a multi rail psu 380w antec sp2 ( i think) from an rma adn was thinking about putting it in my server with 5 drives, but its only got a 2400 cpu. THat psu is a 400w antec with 28a ?? on 12v line.
Would it be better with teh one in there now? cuz the new one is like 13a and 15a. or something like that.


(sorry to jump in on your post, im learning on this, and just got it in last night)
 
darknite said:
is there a sticky or a good site to read on this, i just got a multi rail psu 380w antec sp2 ( i think) from an rma adn was thinking about putting it in my server with 5 drives, but its only got a 2400 cpu. THat psu is a 400w antec with 28a ?? on 12v line.
Would it be better with teh one in there now? cuz the new one is like 13a and 15a. or something like that.


(sorry to jump in on your post, im learning on this, and just got it in last night)
Click to expand...

That is going to depend on what motherboard you are using. WHat is the motherboard?
 
its nothing fancy, its either teh epox 8kta or 8kta2 with 1gig pc133
Do only new boards really take advantage of the dual rails? I dont even think this one has a p4 connector on it. My main concern is that teh hdd's will get the power they need. Cuz this system is on 24/7 pretty much.
 
darknite said:
its nothing fancy, its either teh epox 8kta or 8kta2 with 1gig pc133
Do only new boards really take advantage of the dual rails? I dont even think this one has a p4 connector on it. My main concern is that teh hdd's will get the power they need. Cuz this system is on 24/7 pretty much.
Click to expand...

This is why I was asking. You do not want to use an atx12v2.0 PSU on a board that uses the 5v for the processor. You can experience sever crossloading issues. You will want to find an older speced PSU for this application.
 
Thanks very much, glad I didnt put it in there, ill wait till i get a better board, etc. etc. or jsut sell it.
 
Actually, along the same lines as what the OP was asking, we now have the breakdown of the rails, but is there some sort of load balancing that takes place amongst the rails at times? The reason I ask, and forgive the n00bieness of the question should there be an easy answer, is that these power supplies are mainly to be put into servers and workstations, some of which have multi-disk RAID arrays along with an assortment of other DVD-ROM drives and whatnot. When you take a look at some quad rail PSUs, the +12V4 rail which supposedly powers all these hard drives and other devices seems really inadequate. In my search for the proper power supply for my rig, I've seen PSUs with as little as 8A on +12V4 and as much as 16A. 8A seems a bit too low if you're powering something more than, say, 4 hard drives...especially at spinup where the power draw is the highest. So, how is it that we can power a desktop system with 8 drives, SLI video cards, fans, etc. on a PSU that has one large beefy +12V rail (like the PCP&C PSUs) and with a multi rail PSU you have to tiptoe around the issue because it appears the rail that's going to be powering your server's 8 or 12 drive RAID array doesn't seem adequate? Are these PSUs able to provide extra current over and above the rated max or borrow from another rail just to allow hard disks to spin up (especially if the system, for some reason, does not support delayed spinup)?

I hope that makes enough sense for you power supply gurus to answer.
 
okay, so in all likelyhood, according to the previously stated information, all the 12V molex plugs on ANY power supply would be coming from the same rail? can someone confirm this?

I realize that some power supplies advertise they can combine rails, I'm wondering about the vast majority that don't advertise that feature.
 
Even that's not always the case....

4-pin Peripheral Molexes? Yeah.. Those are typically on the same rail. But in the case of the Silverstone quad-12V rail units, the SATA is on a different rail than the rest of the connectors.

And in the case of the Enermax Galaxy, the modular connectors are all on 12V5 while the other peripheral connectors are on 12V4 and PCI-e is on 12V3.

And the Coolmax 750/850/1kW units I have sitting here are really whack. On the 8-pin, CPU1 and CPU2 are on 12V1 and 12V2. Everything else is on 12V4. Only the 24-pin uses 12V3. If you use the 4-pin +12V connector, it too is on the 12V4!!!!
 
Okay, so when nvidia says a video card requires X amps on the +12v, do they mean the external pci-express power adapter, the atx motherboard plugs, or a combination of the two? I'm trying to figure out if my aerocool zerodba satisfies the 27 amp requirement of my 7950 gx2 (it doesn't work). The aerocool website and sticker on PSU says it's 20A on +12v1 and 17A on +12v2...
 
Creamy Goodness said:
Okay, so when nvidia says a video card requires X amps on the +12v, do they mean the external pci-express power adapter, the atx motherboard plugs, or a combination of the two? I'm trying to figure out if my aerocool zerodba satisfies the 27 amp requirement of my 7950 gx2 (it doesn't work). The aerocool website and sticker on PSU says it's 20A on +12v1 and 17A on +12v2...
Click to expand...

Unfortunately, they're talking about the combined 12V rail capability of the PSU. Not just what's available on one rail. Major source of confusion.
 
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