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Author Topic: 3rd GPU and 2nd PSU  (Read 900 times)
ahitman
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August 27, 2011, 04:43:57 PM
 #1

I am using a GA-870A-USB3 motherboard, and two Corsair Builder Servies CX600 600W PSU's. I have running two 5850 in the built in 16x pcie slots (powered by cx600 that also powers the motherboard) and one 5850 extreme on a 1x extender cable (powered by second psu that is only connected to this card). This configuration boots up fine, but as soon a cgminer starts up and starts crunching, the screen turns black and the fan on the 5850 extreme goes to 100% and the PSU fan stops (this happens at stock clocks and voltages as well as oc'd). Using only one PSU and two 5850s there are no problems. (running linuxcoin)

Is this a sign of a weak PSU? Is this a software issue? Do the PSU's have to be connected together somehow?

edit: looking at the box of the PSUs I see that they provide 480W on the 12V rail. That should be plenty to run in the above configuration. Any ideas on how to get this to run?

New Info: So it turns out that even if I only run any miner on the 5850 extreme (connected using riser cable) the system crashes. But it doesnt crash right away, it works fine for even up to a couple minutes, until I change something on the screen (like open or close a window) then it crashes. This card is also the card that the monitor is hooked up to. Has anyone else had crashing problems on opening or closing windows?
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Lord F(r)og
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August 27, 2011, 05:12:44 PM
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psu require a minimum load! some even on +5V line

have a look at: http://www.pcguide.com/ref/power/sup/outputLoading-c.html

maybe this is your problem
ahitman
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August 27, 2011, 05:52:19 PM
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Maybe, will I damage anything if I connect the second smaller ATX plug on the motherboard to the secondary PSU? That would give it a load, but is it okay for the motherboard to be connected to two seperate PSUs?
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August 27, 2011, 07:04:21 PM
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can't and will not grant success, cause:

psus are fully functional?
minimum load requirements of 2nd psu?
requirements of you mobo?
"second smaller ATX plug" means "dedicated CPU power supply"-line (assuming you plug it where it belongs)?

not an expert - never did this before, so i recommend investigation

do at your own risk
ahitman
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August 27, 2011, 10:14:05 PM
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So now I have tried using a different card as the extra card, fresh operating system install, changing the PSUs around, and the same thing happens, after about 10-15 seconds of hashing on the extra card the screen goes to black and the fan goes to 100% (and one of the PSU's shuts down). Is it something with the motherboard?

Anyone have any idea as to whats wrong?
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August 28, 2011, 04:45:21 AM
 #6

Try plugging a spare hard disk into the second PSU. It doesn't need to be connected to the mainboard. This will give it some 5V load so you can test if it's shutting down due to no load.

ArtForz
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August 28, 2011, 05:43:45 AM
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Me-too.
CX series is group regulated, needs some load on +5. As plenty other people said HD/DVD makes for a good "dummy load".
Btw, why on earth don't people get a $5.99 multimeter and just measure the output voltage?

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ahitman
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August 29, 2011, 05:08:31 AM
 #8

That worked, thanks a lot! But now I have to run my USB linux system with a spare HD attached? Is there anything that can put in place of the drive to keep these PSUs going?

Instead of the disc, can I connect the second psu to the secondary plug on the motherboard (the CPU power plug)? Will the motherboard have problems getting power from two psus at once?
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August 29, 2011, 05:43:45 AM
 #9

You could just make a molex dummy plug with a resistor.

I believe about 100 mA on 5V is enough load but I'm not certain.

V=IR, R=V/I,  R = 5 /.1 = 50 ohms.

P = VI,  P = 5 * .1 = .5 Watt. So you need at least a 1/2 Watt resistor. 1W is safer.

Or you can use resistors in parallel which is cheaper than buying power resistors but you have to increase the ohm value to make up for how many in parallel.

eg.

4 x 200 ohm resistors in parallel between 5V and GND on a molex. Then they need only be common variety 1/4 Watt resistors. These kind cost a couple pennies each, plus a molex plug,  which most people have a few laying around anyway.

(I believe the secondary mainboard plug is only 12V and won't help you. I don't think you'd want to attach two PSU to the same mainboard.)

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