Unusual PSU behavior.

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blade061188

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Oct 14, 2011
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Hi Toms,

I'm in the process of troubleshooting an unusual issue I have encountered with my latest upgrade.

First things first:
Ultra X3 1000W Modular PSU
MSI P67A-G45 Mobo
i5-2500k @ 4.6
G.Skill Ripjaws 8GB
OCZ Agility 3 240Gb
2 x Samsung Spinpoint 7200 1TB
Asus sata dvd burner
Sapphire 7970

My computer is shutting off (no errors/blue screen) under heavy GPU load when the GPU is overclocked, which would normally lead me to believe my PSU wasn't cutting it. 7970 at stock speeds seems to run ok, but I haven't tested for more than ~1hr at a time. However here's where it gets unusual. I upgraded from 2 x 6950s crossfired and overclocked which requires more power than a single 7970.

My PSU is older, and doesn't have a native 8 pin GPU connector, but has a 6pin-8pin adapter. What I suspect is happening is the 7970 is attempting to draw more power from the 8 pin connector than the PSU is able to give (since it's really just a 6pin connector).

I looked at this article:
http://www.tomshardware.com/reviews/power-supply-specifications-atx-reference,3061-12.html

There's a box partway down that describes the power output of 6pin vs 8pin PCIe connectors.

So, ordered and in the mail is an adapter that uses two 6pin PCIe connectors as input and outputs to a single 8pin PCIe connector (which was actually pretty hard to find).

Can anyone confirm/deny that my thinking is correct? Obviously when the adapter comes, I'll be able to test, but are there other things I could be checking on in the meantime?

Thanks in advance.
 
Solution
ko888

Sure seems like it.

If the wire gauge is too light/thin the resistance to current flow will be greater and that results in a voltage drop at the graphics card's PCIe Supplementary Power Connectors. Graphics cards don't respond well to insufficient voltage and current.
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What I suspect is happening is the 7970 is attempting to draw more power from the 8 pin connector than the PSU is able to give (since it's really just a 6pin connector).
Click to expand...
Incorrect.

The graphics card knows that a 6-pin PCIe connector is plugged into its 8-pin PCIe socket so it knows that it is only allowed to draw 75 Watts through that specific connector. That's why it's shutting itself down because the lack of the +2 pins portion of the (6+2)-pin PCIe connector cable that you should be using.

Your single +12V rail is able to supply 70 Amps so that is more than enough to handle two Radeon HD 7970 in 2-way CrossFire mode.

Your Ultra X3 ULT40311 1000-Watt Power Supply should have included two modular (6+2)-Pin PCI-Express Connector cables.

Contact Ultra Products and ask for them.
 
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Your TigerDirect link shows: 8-Pin PCI-Express Connector 2

That sure indicates to me that two modular 8-Pin PCI-Express Connector cables were included with your power supply unit.

The adapter you ordered should work.

This one would also work: http://www.amazon.com/6pin-PCI-E-Power-Cable-Adapter/dp/B0055MS6E0
 
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Maybe I'm not understanding, the adapter that came with the PSU has a 6-pin connector (male) on the side that plugs into the PSU and an 8 pin connector (male) that plugs into the GPU. The adapter you sent me is almost the same, with a 6 pin female connection on one end and an 8 pin male connection on the other.

you said:

The graphics card knows that a 6-pin PCIe connector is plugged into its 8-pin PCIe socket so it knows that it is only allowed to draw 75 Watts through that specific connector. That's why it's shutting itself down because the lack of the +2 pins portion of the (6+2)-pin PCIe connector cable that you should be using.
Click to expand...

I took that to mean that even though the connector physically has 8 pins connected to the GPU, the GPU knows that on the other end of the connector, there are only 6 pins, and will only draw 75 watts. I wanted an adapter that took two 6 pin inputs so the power supply would allow twice the power to be used.
 
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You already have the proper cable. You didn't need to order anything.

The two extra pins don't carry any power. They are used for sensing by the graphics card so that it can determine if it is allowed to draw 75 Watts or 150 Watts through the cable.

Monitor/log your +12V rail's voltage and see if it is within spec when the shutdown occurs.

If the modular cables, that Ultra provided, don't make good contact with the modular connector on the power supply there will be a voltage drop seen by the graphics card. As the current draw increases, the voltage drop increases, and the graphics card will see an insufficient power condition causing it to shut down.
 
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The purpose of including the dual 6-pin PCIe to 8-pin PCIe adapter is for those that have a power supply that has only 6-pin PCIe connectors and doesn't have any 8-pin PCIe connectors. Also the wire gauge may not be heavy enough on some the power supplies with only the 6-pin PCIe connectors.

Monitor/log using a utility like the freeware CPUID Hardware Monitor or some equivalent.
 
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Ok I will try that.

Also, I don't think I'm asking my question well enough. If the only purpose of the adapter was to allow power supplies with only 6-pin connectors to use 8 pin connectors, why not simply have a single 6 pin to single 8 pin adapter? What is the purpose of have two 6 pin inputs to a single 8 pin output?
 
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If the power supply manufacturer didn't use a heavy enough wire gauge for the 6-pin PCIe connector cables then the current load needs to be distributed across two 6-pin PCIe cables to prevent a voltage drop due to higher resistance with thinner cables.
 
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That's the new 64 million dollar question
After all 1x 6 pin ( 75 watts ) can provide enough power for 2x 6 pin connectors ( 150 watts )
dualpcie.jpg

And with the extra grounds, we get enough juice out of 3 yellow 12v wires for a 6 pin and an 8 pin ( 225 watts ), go figure
 
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Ok. Thanks for the clarification.

Could the voltage drop due to thinner cables be a possible cause of my problem as well? The "flex-force" cables that came with my PSU are indeed quite thin.

I will report back once I've had a chance to test my +12V rail with HWmonitor.

 
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But it's not two extra grounds. It introduces the Sense1 pin with its corresponding ground. Sense pins don't carry any +12V power nor are they grounds.

2x4_PCIe_Power_Connector_Pinout.jpg
 
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The "sensing" is done on the cards pcb, it "sense's" wether or not there's a ground wire connected to the "sense" pin
Wanna test how that works ?
Use old components
Pull pin 5 from a 6 pin connector ( that would be Sense 0, top/center ), try and light up a card, it won't, it can't "sense" a connection to ground
Now pull "ground" pin 4 or 6 and swap it into the pin 5 position, the card will light up, it can "sense" a connection to ground

Is there a difference between the wire used for a ground pin and the wire used for a sense pin ?
No, they are both grounds



 
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I think of the sense pins as acting more like jumpers.
 
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Ok, I tried monitoring my +12V rail and it stayed at 12V or a little more (12.2-12.3) even under load.

i took out the PSU and dusted it out with compressed air and dusted out all the connections, but It's still crashing.

Ideas?
 
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Also, I don't know if it is helpful to know, but the shut-off doesn't happen immediately in game. The higher the GPU clock speed, the less time it takes to shut off, but if it's on the edge, it'll run for up to 10 or 15 minutes before it shuts off.

 
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Wait until you receive your cable, that you've ordered, and see if it makes any difference. If it doesn't make any difference then the power supply may very well be on its last legs.

The fact that the model number has changed over its lifetime for the same product model indicates that they were trying to fix some existing problem.
 
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Alright, the cable should arrive on Tuesday. I'll update at that point.

Hopefully that will solve the issue. Like I said at the beginning of my post, I had been running 2 6950s in crossfire before I upgraded to one 7970, and didn't have any issues power-wise before.

It could just be that the power supply is going out and the problem just happened to start at the same time as the GPU update, but that wasn't my first thought since the power requirement for a pair of 6950s is higher than one 7970
 
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There is one difference in that you are now using an 8-pin PCIe connector and trying to draw theoretically twice the amount of power through that PCIe cable. You did mention something about the wire gauge of that cable being rather light.
 
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Sure seems like it.

If the wire gauge is too light/thin the resistance to current flow will be greater and that results in a voltage drop at the graphics card's PCIe Supplementary Power Connectors. Graphics cards don't respond well to insufficient voltage and current.
 
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Solution
I think it must have been lack of current, because I measured the voltage on the card physically with my meter, and even under load the +12V stayed strong. I am guessing that the one cable just couldn't supply enough current, or the PSU is old enough that it wasn't speced to output that much current on a single PCIe Line.

Thanks for your help.
 
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