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Proper way to use 2 power supplies in one rig?

post #1 of 7
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Hi all, I'm looking at all my options in crossfiring 2-3 7970's and already have a Seasonic x750 and a older unused XFX Core 550. I am wondering if there is a "proper" way to use these 2 in one rig. Obviously I could jump the atx connector and turn the secondary(550) on and off at the back of the psu, but am wondering if there is a better way. This would save me the expense of a new psu and the hassle of selling the others or have them lying around. Thanks!
    
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post #2 of 7

I'm personally not sure, but I know that you'd have more than enough power from the X-750 if you had two in Crossfire.

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post #3 of 7
Its far better and far less trouble just buying a bigger PSU if you want 3 video cards

Your 750 is more then for two
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post #4 of 7
Actually, if you really were to do this (with 3 cards; X750 is plenty for 2 unless lots and lots of overvolting) and insisted on not buying a different power supply, you would be technically better off using the XFX Core 550W as the primary. It's a group-regulated design, so you don't want it powering only +12V on graphics cards or something like that. Seasonic X Series wouldn't mind either way. A reasonable distribution would be mobo + CPU + one graphics card on the XFX, drives + two graphics cards on the Seasonic.

There's an adapter (really, you just need to make the wires go the right places) to allow both power supplies to be turned on at the same time when pressing the power button. You just need to have the case power button pull PS_ON# to ground (it's low active) on both.


But really, it's not worth the hassle.
post #5 of 7
on your 24 pin adapter is 1 green wire. If you jump this to any brown ground wire the PSU will turn on. A paper clip works for this.
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post #6 of 7
Quote:
Originally Posted by shilka View Post

Its far better and far less trouble just buying a bigger PSU if you want 3 video cards

Your 750 is more then for two

 

What worries me is this:

 

http://www.guru3d.com/articles_pages/amd_radeon_hd_7970_review,8.html

 

They calculated that the 7970 pulled 195W by itself. Multiply that by 2 in order to add 2 more 7970s, and the result is 390W. Now, their system pulled 355W while the one 7970 was under full load. Adding 390W to that makes it pull 745W from the wall outlet. This isn't taking into account the likely increase of 10-20W while idling on top of their idle wattage of 163W. So, let's just call it 175W, which is just 12W more making for a total of 757W being pulled from the wall outlet. Their CPU was idling during their power consumption test, so I have to add up to 150W to cover a heavy overclock of the 3770K. This makes the PSU pulling 907W from the wall outlet. This means that if the PSU is 90% efficient while pulling 907W from the wall outlet, then the computer is pulling 816W from the PSU. Although, the gaming power draw will be a bit lower, like in the 780s or 790s.

 

So for 3 x 7970s at stock for gaming, I  would recommend a good 850W-950W PSU.

It's a computer!
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CPUMotherboardGraphicsRAM
i5-2500K @ 4.5GHz (1.400-1.424V in Manual Mode) ASUS P8P67 EVO B3 (UEFI ver. 1850) GTX 780 ASUS DirectCU II (1228 / 6300, 1.180V) G.SKILL Ripjaws X 8GB (2 x 4GB) 1866MHz, CL9 
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250 GB Samsung 840 EVO (C:) 250 GB Samsung 840 EVO (D:) 150 GB WD VelociRaptor Samsung SH-S243N 24x DVD Burner 
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Samsung SH-S203N 20X DVD Burner Thermaltake Frio Win 7 Home Premium x64 SP1 Retail AOC G2460PG 
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Filco Majestouch 104-key Cherry MX Blues w/NKRO Corsair HX650 (Bronze, ordered on 12-12-2009) CM 690 Intellimouse Optical (1.1A) 1000Hz polling rate 
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Basic, but premium round X-Fi Titanium HD Klipsch ProMedia 2.1 (with 16 AWG Monster Cable... 
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It's a computer!
(19 items)
 
  
CPUMotherboardGraphicsRAM
i5-2500K @ 4.5GHz (1.400-1.424V in Manual Mode) ASUS P8P67 EVO B3 (UEFI ver. 1850) GTX 780 ASUS DirectCU II (1228 / 6300, 1.180V) G.SKILL Ripjaws X 8GB (2 x 4GB) 1866MHz, CL9 
Hard DriveHard DriveHard DriveOptical Drive
250 GB Samsung 840 EVO (C:) 250 GB Samsung 840 EVO (D:) 150 GB WD VelociRaptor Samsung SH-S243N 24x DVD Burner 
Optical DriveCoolingOSMonitor
Samsung SH-S203N 20X DVD Burner Thermaltake Frio Win 7 Home Premium x64 SP1 Retail AOC G2460PG 
KeyboardPowerCaseMouse
Filco Majestouch 104-key Cherry MX Blues w/NKRO Corsair HX650 (Bronze, ordered on 12-12-2009) CM 690 Intellimouse Optical (1.1A) 1000Hz polling rate 
Mouse PadAudioAudio
Basic, but premium round X-Fi Titanium HD Klipsch ProMedia 2.1 (with 16 AWG Monster Cable... 
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post #7 of 7
Thread Starter 
Thanks for all the great responses! I've crossfired with 2 - 7970's before and had some issues. However, both were Matrix and @ 1.35v. This time around, I'm adding reference cards and will initially undervolt the Matrix. The 750 should suffice for that, but when a third card comes into play, I agree it would be better to get a higher wattage PSU. But, nice to know how I could set this up initially to test.
    
CPUMotherboardGraphicsRAM
Intel 5820k Asrock x99 OCF V3.1 One of many Kingston HyperX Predator 
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steelseries M-Audio BX5a DLX 
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CPUMotherboardGraphicsRAM
Intel 5820k Asrock x99 OCF V3.1 One of many Kingston HyperX Predator 
Hard DriveOptical DriveCoolingOS
Samsung 840 EVO Asus Bluray Custom Water Win7 Ultimate 
MonitorPowerCaseMouse
Asus VG248QE EVGA P1200 Mountain Mods Extended Custom U2-UFO  Mad Catz R.A.T 5 
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steelseries M-Audio BX5a DLX 
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