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-   -   HD 7850 power limit (http://www.rage3d.com/board/showthread.php?t=33990031)

crankTT May 19, 2012 10:15 AM

HD 7850 power limit
I recently purchasing a MSI HD 7850 OC and have been experimenting with overclocking the card. First I used CCC, then Asus GPU Tweak and now I settled on Sapphire Trixx. I was not able to use MSI Afterburner due to system instability when starting it up.

I am able to overclock the core past 1200MHz with a little voltage increase, however anything past ~ 1100MHz at 1.14V gives me decreased performance. The max power slider is set to +20%. I've tried both Furmark and Crysis Warhead benchmarks and the results are consistent. The only thing that makes sense is that the power draw of the card is limited, either by hardware or software.

I realize that the PCIe and 6-pin are technically not designed to provide more than 150W, which is supposedly what the card draws. It's too bad the card only came with one 6-pin power connector. There's no point in overclocking the reference HD 7850 further than the CCC limits because it's power-limited. If there is any way around it, I haven't found it yet.

VW_Factor May 19, 2012 10:30 AM

The slot also provides power.

The Asus and MSI applications are identical, they are merely skinned to look different. I don't understand why you'd have problems with one but not the other.

L4dd May 21, 2012 01:44 AM

P.C.I.-E. cable power
I had read a test that had shown that a P.C.I.-E. cable was able to draw 180 watts, which might not have been the cable's limit, so I doubt that your card is limited due to it having a single P.C.I.-E. cable, and the P.C.I.-E. slot is rated for up to 75 watts.

Rage 3D's following linked article states that the 7850's PowerTune is limited to 155 watts, so I blame that:

crankTT May 21, 2012 09:04 AM

Hmm, that makes sense that Powertune is the culprit. Has anyone figured out how to disable or bypass Powertune?

As I mentioned earlier, the total 'official' power available is about 150W (75W from PCIe and 75W from 6-pin). I'm sure this solution can provide more power, but I wouldn't go TOO much higher for reliability's sake, personally.

It would also be nice to be able to re-flash the card to a 7870 and run at a reduced clock rate, but it's too early to tell if this is possible. It seems the hardware is there on the chip (same # of transistors) but somehow disabled.

VW_Factor May 21, 2012 11:33 AM

PCI Express cards are allowed a maximum power consumption of 25W (1: 10W for power-up). Low profile cards are limited to 10W (16 to 25W). PCI Express Graphics 1.0 (PEG) cards may increase power (from slot) to 75W after configuration (3.3V/3A + 12V/5.5A).[9] PCI Express 2.1 increased the power output from an x16 slot to 150W so that some high-performance graphics cards can be run from the slot power alone.[10] Optional connectors add 75W (6-pin) or 150W (8-pin) power for up to 300W total.

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