Dual Rail or not to Dual Rail?

Doppleganger · 30 Mar 2006 at 23:43

Hello all.

I'm in the process of upgrading my PSU. My trusty old FSP (Sparkle) 400-60FPN is doing a great job of powering an 7800GTX SLI rig but the 12v rail (according to the inaccurate MBM) is falling to as low as 11.38v. Also, my core voltage is dropping on full load from 1.66 to 1.58v, meaning my overclock is potentially no longer stable. However, I've only had one crash when running full load and that was a sound-skipping issue in Oblivion. I'm fairly sure it's linked to my struggling PSU and not a bug in that game but I'm not certain..

Anyway, I've been reading reports on the fact that it is better to have a beefy single rail PSU rather than dual rails for an SLI/Crossfire system. In fact some 7900GTX SLI rigs are not stable on dual rail PSUs.

So, should I buy a single rail PSU rather than a dual rail one and if so, what PSU?

Cheers for any help.

BigDom · 31 Mar 2006 at 01:35

Get a Tagan 530W or 580W. Very nice PSU and gives you the option of dual or single on the Modular silent ones.

R B Customs · 31 Mar 2006 at 10:17

sorry i have to hijack this.
been looking for the info in bitesize information but cant find it
what is dual and single rail ?
is it dual PCI-E connectors or something
btw, i have the taga easy con 530... what can i say, its been made by germans ! and boy does it show. best quality gear iv seen

Rick

wam7 · 31 Mar 2006 at 12:02

Yes it is a commonly held belief that I have heard espoused, though I am yet to find any hard data. I do know that people using the dual rail OCZ Powerstream 600W had problems running CF/SLI but once they have changed to a Quad rail supply (FSP Epsilon) there problems were solved.

All this tells me is that the FSP quad rail is better than the OCZ dual rail. Not that a single 12v rail is the best.

barney · 31 Mar 2006 at 13:04

Im running a crossfire setup and have tried a couple of different psu's. Tried a 550w single rail psu to start with and that fell over once i put second card in (wasnt the best of psu's though). Next tried a hiper 580 with duel rail and again it fell over with 2 cards in the system. Im now using a Epsilon 700 which has four 12v rails. All seem to be pretty strong and havent had any problems so far (though it has only been in for 2 days and only 1 day with my new Duel core CPU in). I think as long as they have a strong duel rail it should be fine but i would definatly recommend just getting the epsilon to avoid all the problems i had

Havana_UK · 31 Mar 2006 at 13:30

I'm running 7800GTX SLi with dual rail Enermax Liberty 620W.

Not having any issues, but then I don't know if the issues people are getting with 7900 SLi / X1900 Crossfire apply to the 7800GTX or not...

Hav

Doppleganger · 31 Mar 2006 at 17:04

Ok, after reading some of your replies and doing some further searching I've settled on one of these 2 PSUs to replace my old Sparkle:

Akasa Ultra Quiet 650W PowerPlus Active PFC ATX2.0 PSU - Black Nickel (CA-015-AK)

OR

FSP Sparkle FX600-GLN Epsilon 600W ATX2.0 PSU (CA-002-SK)

Both are 4 rail PSUs and I've heard good things about the Akasa and I'm a fan FSP Sparkle PSUs in any case.

Cheers.

Street · 31 Mar 2006 at 17:20

Doppleganger said:
but the 12v rail (according to the inaccurate MBM) is falling to as low as 11.38v.

Never trust any software measurement programs... Get a multimeter and test it that way.

Doppleganger · 31 Mar 2006 at 17:38

Street said:
Never trust any software measurement programs... Get a multimeter and test it that way.

I know that MBM (or any other voltage measurement programs) isn't entirely accurate. I'm basing my need for a new PSU because basically my 400watt Sparkle is not really up to the job of powering a 7800GTX SLI system with 3 HDDs and 1 optical drive.

It's doing a fine job atm but sooner or later it's gonna fall over.

fornowagain · 31 Mar 2006 at 19:57

Doppleganger said:
Anyway, I've been reading reports on the fact that it is better to have a beefy single rail PSU rather than dual rails for an SLI/Crossfire system. In fact some 7900GTX SLI rigs are not stable on dual rail PSUs.

So, should I buy a single rail PSU rather than a dual rail one and if so, what PSU?

Cheers for any help.

The only really significant difference between most single and dual rail PSU's is that dual rails have a 20A/240VA current limiter on each rail and maybe a voltage clipping zener diode. It's misleading, in 99% of cases there's only a single 12v source. It's the total combined wattage that matters. A crossfire system may be able to reach 20A. Most of the OCL's are set higher than 20A, but not all, so some trip out and the PSU shuts down.

An article on CF/SLI worth reading.

http://www.extremetech.com/article2/0,1697,1932947,00.asp

512MB gaming cards in SLI/CrossFire will pull anywhere from around 15A to slightly over 20A (taking 120W divided by 12V = 10A per card) meaning that our +12V3 rail was overloaded as it has to share the power draw from the motherboard as well (which typically draws around 2 ~ 3A on its own). Since we rate our power supplies conservatively, the ST60F could actually run Kelt's 7800GTX 512MB SLI system for a while despite being technically overloaded on the +12V3 rail. As soon as the over current protection point of 20A on the +12V3 is reached, the power will automatically shut down.

And a short but useful guide to the PSU requirements for CF/SLI

http://jonnyguru.com/findingtherightpsu.html
http://jonnyguru.com/psu2.html

Confused about Dual Rails? This may help with the usual misconceptions.

http://forums.silentpcreview.com/viewtopic.php?t=23916
http://www.silentpcreview.com/article28-page1.html

It's important to remember that even though there are two "independent" 12V lines, they still draw from the same main source. It's highly unlikely that there are two separate 120VAC:12VDC power conversion devices in a PSU; this would be much too costly and inefficient. There is only one 12VDC source, and the two lines draw from the same transformer. Each line is coming from the same 12VDC source, but through its own "controlled gateway".

PSU makers' specs are misleading in that thay rate the current capacity of each 12V rail independently. What really matters is the total 12V current: Generally, up to 20A is available on any one 12V line assuming the total 12V current capacity of the PSU is not exceed.

Look for the total combined wattage for the 12v outputs. Don't just add the two rails together, you may not be getting as much as you think you are!!!!

ted34 · 31 Mar 2006 at 20:04

Most dual rail psus dont seem up to the job theses days. Most people seem ok on tagans as they are switchale between dual or single rail but when set to dual rail it will auto to single if the going gets tough. Apart from that id recomend the 700w fsp sparkle - if jokester likes it then its gotta be good

fornowagain · 31 Mar 2006 at 20:08

I can recommend a PCP&C 510 SLi. It will handle pretty much anything and it has amazing power regulation. Trouble is, it's as rare as hens teeth and expensive.

Doppleganger · 31 Mar 2006 at 20:33

Cheers for all the help guys!

Flanno · 11 Apr 2006 at 00:59

I am thinking of the 700w Forton as well, but I understand it drops to 600w at 30 degrees ambient. The Antec NeoHE 550w (which if you add up the watts on the rails comes to 705w) and is supposed to provide 550w constant at up to 50 degrees. It also has a healthy 18a on each of the 12v rails. So I wonder would that be a better bet for an Xfire system. It is Xfire certfied like the Forton, but what worries me is my Intel 955 extreme cpu. From what I have read on various sites the 8 pin 12v connector on the psu (which breaks into 2x4pins) is for workstation systems and dual cpu's. So I assume I just go ahead and plug all 8pins into the motherboard ? Anyone ?

By the way, I just read a post on another site from a Forton rep, that the 700w does 18a on each 12v rail, but was labelled as 15a when they went for certification. It also has a 20a overvoltage protection.

Doppleganger · 11 Apr 2006 at 01:17

I went for the Akasa Ultra Quiet 650W PowerPlus in the end. Big hefty psu with 4 12 volt rails and good reviews. Should be more than enough for when I upgrade to a dual-core cpu. The only slight downside is the fan is slightly on the noisy side.

barney · 11 Apr 2006 at 09:43

Flanno said:
I am thinking of the 700w Forton as well, but I understand it drops to 600w at 30 degrees ambient. The Antec NeoHE 550w (which if you add up the watts on the rails comes to 705w) and is supposed to provide 550w constant at up to 50 degrees. It also has a healthy 18a on each of the 12v rails. So I wonder would that be a better bet for an Xfire system. It is Xfire certfied like the Forton, but what worries me is my Intel 955 extreme cpu. From what I have read on various sites the 8 pin 12v connector on the psu (which breaks into 2x4pins) is for workstation systems and dual cpu's. So I assume I just go ahead and plug all 8pins into the motherboard ? Anyone ?

By the way, I just read a post on another site from a Forton rep, that the 700w does 18a on each 12v rail, but was labelled as 15a when they went for certification. It also has a 20a overvoltage protection.

Yeah i have the 700w forton and it powers my crossfire fine, my motherboard is also a lot more power hungry then most and i havent had any problems. I just plugged both sets of 4 pins into my board but i did try with just one 4 pin plug in and tbh i didnt see any difference in any of the voltages (didnt test too much though just incase

).

wam7 · 12 Apr 2006 at 09:40

Doppleganger said:
I went for the Akasa Ultra Quiet 650W PowerPlus in the end. Big hefty psu with 4 12 volt rails and good reviews. Should be more than enough for when I upgrade to a dual-core cpu. The only slight downside is the fan is slightly on the noisy side.

Ah one of the strengths of the Fortron is that it is very quiet, probably only beaten by the Seasonic S12's. Akasa make good PSU's and yours should do fine but if it doesn't then you know what to get.