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Measuring actual +12v output

post #1 of 10
Thread Starter 
Have a thermaltake 850W black widow.
The 12v rail is rated at 69 amps.

Currently I have 4x 5830 GPUs connected to it.
The CPU is a low end low power AMD dual core.
Kill-a-watt measures about 750W from the wall socket.

Now, I plan to add one more 5830 to it, which I assume takes 150W at full load.
Since kill-a-watt is already reading 750W from the wall, adding that card will push the power from the wall to at least 900W.

Here comes the real question.
The power supply 12v rail is rated at 69amps, which means 12V x 69A = 828Watt.
Also, the power supply definitely is not 100% efficient. Assuming 90% efficiency, a 850W power supply can actually pull 944Watt from the wall.
Since I am measuring my power from the wall and it is 750W now, I am assuming I would be safe to add another HD 5830?
If 5x GPUs, it should pull 900W from the wall. The +12V rail can handle 828W output. So, 828 / 90% efficiency = 920W from the wall just for the +12V rail. Is this correct?

Is there a way to measure +12v output from the power supply unit?
post #2 of 10
Quote:
Originally Posted by lesterhung View Post
Have a thermaltake 850W black widow.
The 12v rail is rated at 69 amps.

Currently I have 4x 5830 GPUs connected to it.
The CPU is a low end low power AMD dual core.
Kill-a-watt measures about 750W from the wall socket.

Now, I plan to add one more 5830 to it, which I assume takes 150W at full load.
Since kill-a-watt is already reading 750W from the wall, adding that card will push the power from the wall to at least 900W.

Here comes the real question.
The power supply 12v rail is rated at 69amps, which means 12V x 69A = 828Watt.
Also, the power supply definitely is not 100% efficient. Assuming 90% efficiency, a 850W power supply can actually pull 944Watt from the wall.
Since I am measuring my power from the wall and it is 750W now, I am assuming I would be safe to add another HD 5830?
If 5x GPUs, it should pull 900W from the wall. The +12V rail can handle 828W output. So, 828 / 90% efficiency = 920W from the wall just for the +12V rail. Is this correct?

Is there a way to measure +12v output from the power supply unit?
You have a couple problems here. Just to make sure, you know that you can't CF all 5 GPU's together, right? You will still be only able to crossfire 4 of them.

Also, I'm surprised that PSU is holding up. You cannot allow an 850w psu to draw 944w from the wall just because you calculated it that way at 90% efficiency.

Get a new PSU if you need 5 gpus, which of course will not all be able to be crossfired. I think you're getting in over your head.
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The Butthurt Box
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post #3 of 10
Thread Starter 
I am not getting over my head.
Who said GPUs need to be crossfired to be useful?
I run programs that can utilize individual GPU
post #4 of 10
You don't want to be doing on that on a Thermaltake Black Widow though. Also, it's definitely not 90% efficient. It's 80+ Bronze rated so the most you're going to get is 85%, and I doubt it holds up to that rating in the real world.
post #5 of 10
Thread Starter 
I am assuming 90% cause I know hardly any power supply can go over that.
If it is less efficient, that would mean I am ever further away from the cap.
post #6 of 10
The black widow is a confirmed piece of crud. You are pushing your luck running 4 GPUs off it as is, stressing that PSU to the maximum rated wattage is a bad idea waiting to happen. Heat, out of spec ripple, just plain blowing up, who knows. If you're already set on doing this anyway, why ask us? One downside of single rail units is some huge spike on the 12v will wipe everything.

Anyway, if you want to do this anyway, buy a DMM, plug it into a molex, will tell you the 12v output. If you want very accurate readings you will need to spend hundreds of dollars however.
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post #7 of 10
The TR2 RX 750 (Black Widow PSUs are just TR2 RX with fancier packaging) was tested with the ripple going out of spec at 450W load, and it was unable to complete the 750W test. The 850W is based on the same platform and probably has similar power limits. It is not safe to run your power supply over 500W (which you are currently doing) or you risk cumulative damage to your components.
post #8 of 10
Thread Starter 
Quote:
Originally Posted by Phaedrus2129 View Post
The TR2 RX 750 (Black Widow PSUs are just TR2 RX with fancier packaging) was tested with the ripple going out of spec at 450W load, and it was unable to complete the 750W test. The 850W is based on the same platform and probably has similar power limits. It is not safe to run your power supply over 500W (which you are currently doing) or you risk cumulative damage to your components.
Do you have any source that the Black Widow 850w is based on the same TR2 RX 750 platform?
Maybe I just got lucky or the unit they tested was defective.
Right now, it is pulling 750W from the wall, with 4x GPU overclocked.
Been 2 days, rock solid stable.
Will report back after maybe 1 week or 1 month and see if it freezes my system.
post #9 of 10
Cumulative damage. Meaning degradation over time. Not immediate explosion or shut down.

Tomorrow. It's late and I need a break from this tiny keyboard.
post #10 of 10
Thread Starter 
I plan to plug in a secondary PSU to power two GPUs if this power supply can't hold up.
I've already sent in the rebate for it, so I can't return the PS.
But, its got a 5yr warranty, and I can always use that.

BTW, I used to have a thoughpower 775W, and it died on me after 1yr in service.
The load from the wall never exceeded 500W. Good thing Thermaltake gladly replaced it.
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