Thinking of buying a diual-rail PSU? Think again...

Richdog · 1 Mar 2006 at 21:04

http://www.theinquirer.net/?article=29940

Single rail PSU solves RD480 Crossfire problems

Xtremesystems, Sampsa, ATI and support guys confirms it

By Fuad Abazovic: Monday 27 February 2006, 21:32

WE have a solution for problems that we have had with A8R-MVP motherboard and two graphic cards. You can check the original post here. We spoke with many people today and they all indicate that it has to be the power problem.
ATI key motherboard designer John Bruno told us that a single rail PSU is better than a multi rail PSU. He reported that ATI had some issues with multi rail PSUs while all of the single rail 500 W + PSUs were running fine. It turns out that both OCZ Power stream 600 W PSU and Akasa 650W PSU are using more than one power rails.

One of the readers an administrator of a motherboard company forum recommended ENERMAX EG651P-VE FM(24P) PSU with 36A and a single12 V rail. Our colleagues at Bit tech managed to test dozen X1900 cards including Crossfire with a single rail 520W OCZ PSU. You can check the article here.

We got a nice link where one of the top overclocker, our good friend Sampsa had the same problem with 600W multi rail PSU and A8R-MVP board and two X1900 cards in Crossfire. He solved its troubles by switching to a single rail 560W PSU. You can check the post here.

This specific problem reminded us on the same problem that we had with two Geforce 7800 GTX 512 cards. Those two cards in SLI were crushing the machine while two 7800 GTX 256 MB cards worked flawlessly. You can check it out here and we have been working for two weeks with Nvidia to solve the problem but failed to make it work. Someone simply could suggest that we should try single rail PSU 500+ W and everything should work. SLI works with two 7800 GTX 512 after all.

Hurray, now we know that Nvidia and ATI should warm people that they need single rail PSU not just any 600 W PSU. That really makes the difference. We are speechless why no one warned the users about this issue.

Despite we have two 600+ W PSU we are back at the spot where we need to get a new PSU for the lab to test those power hungry rigs. Asus A8R-32 MVP Deluxe board powered with RD580 chipset can even work with multi rail OCZ 600 W or Akasa 650W PSU as we guess it is distributing the power better than the previous one. A mystery solved, but beware if you are buying top motherboard SLI or Crossfire and you want to plug two power greedy cards inside. You need a lot of quality single rail power, not just any power.

This is a VERY significant report imo, and one that confirms the fears and opinions I have had for a long while now. Dual-rail PSU's just don't have what it takes to effectively new, power-hungry uber-rigs, confirmed by some of the biggest names in OC'ing.

To me this says if you're thinking of buying a new PSU now or in the near future and don't want to worry about something as dramatic as not being able to power your rig then get a beefy single-core... avoid the duals.

EDIT - That's just my take on it, others may have different opinions of course...

WJA96 · 2 Mar 2006 at 06:22

That's certainly different. There are a small, but significant, number of threads on these forums where people have had show-stopper type problems with very high end SLi systems and they have steered people off certain motherboards because they were having pronlems with 2 graphics cards. If those people were using multi-rail PSUs then quite a few motherboard manufacturers would have cause to be very unhappy.

Dutch Guy · 2 Mar 2006 at 09:04

The problem is that about 98% of PSU for sale these days are dual railed PSU's

TechnoCorruption · 2 Mar 2006 at 10:53

Thats definately an interesting read, what made me wonder though is they said the rigs with dual rails were alright with 2 GTX 256mbs but not the 512mb versions, I know thest new cards demand more power but just how much, will we need to have psu's with 50amps on the 12v line or will all future g-cards need to be plugged into the mains like the dual 7800gt on 1 card monster.

BigDom · 2 Mar 2006 at 10:58

Dutch Guy said:
The problem is that about 98% of PSU for sale these days are dual railed PSU's

Arent' they switchable though? My Tagan is definatly switchable and guess what.. is running in combined mode.. on BOTH my rigs. In fact thinking about it I had to put Project red alert into combined mode to get it stable.

Richdog · 2 Mar 2006 at 11:04

I was talking to a guy called JonnyGURU last night on my home forum, he's a well-known PSU reviewer and expert and he was somewhat disgruntled with the article The Inq posted. He said:

jonnyGURU said:
What stinks about that article is that they don't go into any detail about how different power supplies split loads up differently in different PSU's.

Most PSU's put both of the PCI-e's on the 12V1 with all of the drives, lights, fans, etc.

Others, like the Enermax I thought, put one PCI-e on each rail.

According to madmat at TechGage, the X2 actually puts the PCI-e's on 12V2 with the CPU.

And some power supplies, like the NeoHE, actually give you a third rail just for PCI-e. The Silverstone Strider ST60F gives PCI-e their own rail out of the four 12V rails.

The article makes blanket statements that are going to make everybody knee jerk into assuming that all dual 12V rail power supplies are bad for SLI/Crossfire configurations without looking into the how and the why.

It makes me wonder if they even know how multiple 12V rails work.

So basically what that means to me is that some multi-rail GPU's can do it no problem, but seeing as manufacturers are churning out multi-rail PSU's at a rate of knots how can you be assured that the dual-rail you are getting cuts the mustard?

Sure, that £120 dual-rail with 25+ on each rail made by a top brand may be alright with the latest gear but what about the cheaper, more "budget conscious" models of £80 and lower?

I really don't care what the argument for dual-rails is, i'm staying single all the way...

Dutch Guy · 2 Mar 2006 at 11:43

Maybe now all new PSU's they design have a split other than 50% between rails, so the 12V rail that supplies the mobo/CPU gets 14A while the other 12V rail that supplies the PCIe cards etc. gets 30A instead of 22A for both?

daftVader · 2 Mar 2006 at 12:29

I noticed the psus the Inquirer had problems with were not ATI certified for the X1900. It may mean little, but I'd be interested to hear whether anyone has had any problems when using a certified psu.

lay-z-boy · 2 Mar 2006 at 12:29

Dutch Guy said:
The problem is that about 98% of PSU for sale these days are dual railed PSU's

2% of those psu's sold must be tagans then

, i can turn mine to from a dual to a single at a flick of a switch, and if i overload one rail it switches to single anyway.

Dutch Guy · 2 Mar 2006 at 12:32

lay-z-boy said:
2% of those psu's sold must be tagans then , i can turn mine to from a dual to a single at a flick of a switch, and if i overload one rail it switches to single anyway.

Didn't know that, my Enermax Noisetaker doesn't have that but it might swith automatically :confused:

MikeTimbers · 2 Mar 2006 at 12:34

There is undoubtably FUD over this issue but it is clear that the technical specifications of many PSU do not provide enough information to the casual buyer. For my own part, I will be standardising on the Tagans which allow the selection of either single- or dual-rails.

lay-z-boy · 2 Mar 2006 at 12:40

Dutch Guy said:
Didn't know that, my Enermax Noisetaker doesn't have that but it might swith automatically

It could be auto or it might not have it, im using a 580w u15 tagan.

Richdog · 2 Mar 2006 at 12:59

What are the combined rails of the Tagan 580 then?

Major_hangover · 2 Mar 2006 at 13:20

Im waiting for my Crossfire system, it uses the Tagan TG580-U15 580W.

Should I be asking them to change it to a single rail?

MikeTimbers · 2 Mar 2006 at 13:35

TG580-U15 can combine its rails by using the back of PSU switch. 20Ax2 or 35A combined.

http://www.tagan.de/pages/products/easyconS/TG-580-U15_Tagan_UK_mini.pdf

The 430W is 20Ax2 or 26A combined.

mmj_uk · 2 Mar 2006 at 13:54

He reported that ATI had some issues with multi rail PSUs

So lets blame the PSU's instead.

Dual-rails are fine so long as the load is balanced properly, though granted single rails are going to be less hassle if your power demand is close to your power supplies output.

Major_hangover · 2 Mar 2006 at 14:29

From what I gathered from an other article, things were fine, with the exception of a few highly overclocked set ups.

Richdog · 2 Mar 2006 at 14:36

MikeTimbers said:
TG580-U15 can combine its rails by using the back of PSU switch. 20Ax2 or 35A combined.

http://www.tagan.de/pages/products/easyconS/TG-580-U15_Tagan_UK_mini.pdf

So it functions as a true single rail, exactly as any regular single rail would? 35A is impressive, enough to make me buy one...

Major_hangover said:
with the exception of a few highly overclocked set ups.

That's a big enough exception for me!

Xplo · 2 Mar 2006 at 15:01

Well it seems like its not the dual rail psu's fault, or the psu makers, all they are doing is advancing technology.

Sounds more like ATi need to go back to the drawing board.

Richdog · 2 Mar 2006 at 16:30

MikeTimbers said:
TG580-U15 can combine its rails by using the back of PSU switch. 20Ax2 or 35A combined..

Website specs state combined 12v is only 30A.