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Rail balancing with dual 12v rail PSU - need help!

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September 9, 2008 3:15:20 AM

I have an hec Ace Power 580W psu that "supports SLI," and I really need help in figuring out if it has enough 12v amps to power this system:

Crossfired pair of Sapphire Radeon HD 3870s
Intel E7200 wolfdale cpu
1 SATA hdd
1 DVD-RW/CD-RW drive
4 ddr2800 dimms (voltage- 1.9v to 2.1v)
4 non-LED fans
1 PCI network card

From the specs on the psu box, the company only lists the combined power of the 3.3v and 5v rails (@240W). Following the same calculations in this thread for estimating the 12v combined wattage (http://www.tomshardware.com/forum/234600-28-calculate-r...), my psu should provide around 37.96 amps:

580W minus (-12v*1A) minus (+5Vsb*2.5A) minus (100W of combined 3.3v and 5v power) divided by 12v = 37.96 amps @ +12v.
*the steps from the thread stated that the 3.3v and 5v combined typical load should be around 100W, so 100W was subtracted from the total wattage instead of the listed 240W - the psu in the thread had the same combined 3.3v and 5v power as mine)*

Unfortunately, the psu box doesn't show which 12v connectors are from which 12v rail. How can I tell which of my two PCI-express 6pin connectors are attached to which rail, and if my dual rail psu follows the ATX standard? If my dual 12v rail psu does follow the ATX standard (with +12V2 dedicated only to the CPU), is there any possible way I could balance the load of my cards evenly onto both rails so as not to reach the current limit?

One last thing (sorry!) - according to my video card box, a 550W psu with 38 amps on the 12v rail is "recommended". Should the system I'm planning work alright with my 550W psu with 37.96 amps?

Thanks for your help, and I apologize in advance if I'm not making any sense - I've been desperately combing the net for info for the last couple of hours.
September 9, 2008 8:45:45 AM

A what powersupply?? :crazy: 

Hope that equipment you're hooking up to it isn't valuable... :sarcastic: 

Bob
a c 83 ) Power supply
September 9, 2008 9:27:38 AM

Hec, they aren't bad. Not the best, but better then most.

I say don't sweat it. Most multi rail PSUs are actually single rail. They use "tricks" inside the unit to make them appear to be multirail units. If you plug in more then what the limiters allow, it simply turns off/won't turn on.

Seeing as you don't have any choice, just use the plugs that you have. There isn't any way for you to "balance" the load. Plug everything in, it will either work or won't. Your right on the edge of what you need, don't try to overclock much.

You could also consider selling your CF setup. If you find someone dumb enough, you could possibly sell them and get enough for the 4850, which should be faster. Selling a 3870 for ~$80 might be doable, two of them for $160 is enough for the 4850. Good luck.
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September 9, 2008 9:46:43 AM

My bad!! :oops: 

It sounds actually quite decent (having read a review of the 480W model). A decent PSU like this will allow you to plug in equipment that is potentially going to draw too much power and overload it. So just plug in your gear and see what happens!! It does appear to have a slight glitch that the PSU drops the rail voltages rather than shutting down (could cause problems for system stability if they go too low). Also you really want to look out for powersupplies that concentrate the power on the 12V rail (for future reference).

+1 to the 4850 swapout and 3870 sell off!! The 38xx series was a bit of a failure...

Bob
September 9, 2008 11:49:33 AM

Max fron CF 3870s is 14A, so should be fine.

Mike
September 10, 2008 12:33:05 AM

thanks for your fast replies guys!
I guess I will just have to plug everything in, and hope it works.

But if my psu can't deliver all the power I need (even if the system idles okay), will any of my hardware, i.e. video cards, suffer any permanent damage? I've waited almost all my life for this one computer - to see it "explode" from being underpowered would be devastating :( 
a c 83 ) Power supply
September 10, 2008 1:39:17 AM

If you lack the power, it should either tell you the video cards aren't getting the power they need, or it should work, but provide lower frame rates. The danger is if you use a low end power supply, and it tries to supply the power you need. If that happens, the power supply can output "dirty power", and it can load to product death. As I said however, HEC is probably a brand you can risk it with. They aren't PCP&C, but they aren't Athena power either.
September 10, 2008 2:05:48 AM

Thanks, I'm really glad that hec is an alright brand :D 

If I may ask a question on behalf my brother (we got our computers at the same time :) ), he has a 500W raidmax psu (w/ 22amps@12v) and I'm sure his system needs more than it could provide - he's got an MSI Radeon HD 3870 X2, an Intel e7200, 1 sata hdd, 1 dvd-rw, 4 fans, and 4 ddr2800s.

I've read on this forum thread (http://www.ocforums.com/showthread.php?t=537077&highlig...) that raidmax is a terrible brand. Is there a way I could tell if any of his components (i.e. video card) have suffered permanent damage from "dirty power"? His computer has been running fine for about a week and a half (playing pc gears of war and oblivion at max settings), but just now started to mysteriously shut down several times.

thanks again for all your help everyone!
a c 83 ) Power supply
September 10, 2008 2:20:30 AM

If he is using a 500W raidmax PSU with a total of 22A on the 12V rail, he most certainly does not have enough power to run the 3870x2. If the shutdowns happen while gaming/GPU stress, then that would be the issue. You don't say when it happens however, so I/we can only guess.

Get some software that can read the sensors on the motherboard. (or grab a multimeter for accurate measurements.) Try to see if the rails are still in spec while gaming. Seeing as he has a raidmax, and only 22A, I'd just get a different PSU. Have him return it and get something better.
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