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Author Topic: Power supply for 2x 7950?  (Read 8213 times)
Sigma
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April 22, 2013, 12:56:32 AM
 #1

This is kind of a silly question, sorry.

I have a spare brand new seasonic platinum 660w psu. Now I am planning on building a little litecoin mining rig with two 7950's.

I do not plan on adding anymore cards to this system...Most psu calculators are suggesting around 800+ for what I'll be getting. I think I could trim it to 650 or so if I
don't use an older 7200 rpm hdd and do some sort of flashdrive or ssd though. Do I need to stay very close to the wattage for mining or am I going to be ok with the seasonic?
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erpbridge
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April 22, 2013, 02:00:49 AM
 #2

Don't skimp on your PSU. I tried running an older system, a CoreQuad with a PCIe 16x and 2 PCIe 1x, on a 500w PSU. Oh, it ran just fine at idle with a single 7950... but when I started Scrypt OR SHA256 mining, the system ended up powering down. 500W was just not enough to run motherboard, processor, RAM (which is a minimal load), and video card. (And I wasn't even trying CPU mining yet.)

Now if 500W wasn't enough, 660W? Well, I'm PRETTY certain it will run the one 7950 just fine. Two, though? You might end up hitting the same wall I did, might not. My two 7950 rig is running on a 720W PSU just fine, and the one thing that might sway my opinion one way or the other is I'm ALSO CPU mining with the QX6700 processor in it, increasing it from whatever idle workload running Windows, up to 130W TDP. So, we're talking that a CoolerMaster 80Plus Bronze 720W PSU can stable support 200+200 (the video cards) + 130 (processor) +8.7 (hard drive at max load, see below)+ 30W (approx load for a bare motherboard)=568.7.

Sure, sounds like a 660W could work out... But I still wonder.

But I'll let someone else who has more experience speak to this... I personally think, just through experience, that you might be ever so slightly tempting fate trying to run two 7950's, full load, on a 660W PSU.

If its brand new, within warranty with receipt and everything, you could just try to do it. If you run for a day without crashing or spontaneous rebooting, great. If you overload and burn out the PSU and release the magic smoke, RMA it saying you "were playing games and all of a sudden system shut down", and now you found out that you can't run both the cards on that size PSU. Worst you can do, if you have the protection of the warranty behind you, is try.

------------
As to cutting your HDD out of the mix to save power, and switching to an SSD or USB:

These posts gives some numbers on hard drive and SSD power usage.
http://www.tomshardware.com/forum/267776-32-hard-drive-power-consumption
http://www.tomshardware.com/reviews/ssd-recommendation-benchmark,3269-7.html

Your typical IDE hard drive (which in today's terms is OLD, but to be honest, is what I have in my mining rig) is 8.7W. A SATA drive, 6.8W-8.7W. (Those are at full load... your drive won't be idle, but it won't be fully thrashing either.) Now, switching to a USB drive WILL indeed reduce that down to about 2.5W, and an SSD is near the same power usage as a USB drive (0.6W-2.6W).

However, in the scope of things... Saving 5 or 6W when comparing it to the 200W load of a 7950 is SMALL in the scope of things. If 5 or 6W is the tipping point between if your system running two cards at 200W each will be stable, or will crash... then you've got too small a margin. Its always better to over-size your power supply for these, than to risk stability issues.

The savings will come in the heat dissipation of the hard drive not being a factor. The reduction in power usage? I don't think its significant enough to take into account.
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April 22, 2013, 02:05:36 AM
 #3

6 core AMD, 990 MB, 8 GB RAM, HD, 2-7950, 4 fans. LTC on GPUs, Boinc on 4 CPU cores, no over clocking=595 watts under load. GPU's are undervolted a bit.

When the subject of buying BTC with Paypal comes up, I often remember this: 

Insanity: doing the same thing over and over again and expecting different results.

Albert Einstein
superfastkyle
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April 22, 2013, 02:22:18 AM
 #4

run a lower voltage bios and you can make it work although its not ideal. at 1.25v this will probably not work. at 1.17 or lower it probably will
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April 22, 2013, 03:55:02 AM
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I think I am having a problem trying to run 2x7950s on a Corsair 650W 80+ Bronze. i can run them but not at max. What happens is that one of the cards runs out - it keeps trying to suck more and more power and heats up past 90C and shuts off. Others have noted this.

I am currently running the two at only 430 kH/s until my 1000W Platinum comes in. I think at max draw the 7950 takes 200W.
Sigma
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April 22, 2013, 04:00:23 AM
 #6

Sounds a bit too risky for my blood. I will just sell this PSU and pick up one that has some more room to breathe.
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April 22, 2013, 08:36:26 AM
 #7

Just buy another 80+ gold 650W and daisy chain them - it will be cheaper. Put the extra money toward another card.
zargon
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April 22, 2013, 02:41:01 PM
 #8

Sounds a bit too risky for my blood. I will just sell this PSU and pick up one that has some more room to breathe.


I have 2 x 7950 running in a rig on a killawatt

its showing ~550W pulled from the wall


a 650 *should* be fine

I have a 1kw on it though because its what I had


and I couldnt shoehorn the 6870 I have into due to mounting issues
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April 23, 2013, 10:06:10 PM
 #9

Zargon,

Thanks for posting the killawatt info. That's the point at which I didn't want to speak to, because I don't have a killawatt (yet) to measure actual load.


I have 2 x 7950 running in a rig on a killawatt

its showing ~550W pulled from the wall


a 650 *should* be fine

I have a 1kw on it though because its what I had


and I couldnt shoehorn the 6870 I have into due to mounting issues
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April 23, 2013, 10:38:26 PM
 #10

A 660w Seasonic will not break a sweat powering two 7950's. I've been mining for 6+ months with an XFX 650w unit (rebadged Seasonic) and two 7950's at 1075mv vcore, 1000mhz core. Pull from the wall was a bit less than 500w.

PSU is all about getting a quality unit/brand. Crap like OCZ power supplies might not cut it powering two 7950's but a quality single rail 660w Seasonic unit will be fine.
pajak666
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April 23, 2013, 10:45:35 PM
 #11

I am running 5970 & 5850 which equals basically to 3 GPU'scurrently scrypt mining with 600W cooler master silent pro PSU.
Think about well known and tested brand PSU, 600W-650W should be enough.
I got mine from auction, second handed with 1 year warranty for about 70$.

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superfastkyle
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April 23, 2013, 11:18:31 PM
 #12

Alot of 7950's do not ship at 1.075v core but rather 1.25v and some are even voltage locked. He will have a hard time getting those to work without flashing the bios. If he wants to run a dedicated miner bamt linux is one of the easiest to use but offers no software voltage controls. Trust me a 7950's at stock setting can use almost 300w by itself. I've seen it.

A 660w Seasonic will not break a sweat powering two 7950's. I've been mining for 6+ months with an XFX 650w unit (rebadged Seasonic) and two 7950's at 1075mv vcore, 1000mhz core. Pull from the wall was a bit less than 500w.

PSU is all about getting a quality unit/brand. Crap like OCZ power supplies might not cut it powering two 7950's but a quality single rail 660w Seasonic unit will be fine.
bcpokey
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April 24, 2013, 08:16:19 AM
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Alot of 7950's do not ship at 1.075v core but rather 1.25v and some are even voltage locked. He will have a hard time getting those to work without flashing the bios. If he wants to run a dedicated miner bamt linux is one of the easiest to use but offers no software voltage controls. Trust me a 7950's at stock setting can use almost 300w by itself. I've seen it.

A 660w Seasonic will not break a sweat powering two 7950's. I've been mining for 6+ months with an XFX 650w unit (rebadged Seasonic) and two 7950's at 1075mv vcore, 1000mhz core. Pull from the wall was a bit less than 500w.

PSU is all about getting a quality unit/brand. Crap like OCZ power supplies might not cut it powering two 7950's but a quality single rail 660w Seasonic unit will be fine.

You haven't seen a 7950 using 300W by itself, because the max TDP for a 7950 is 200W.

Now, a 7950 will use potentially up to 200W, and your system can use 100-200W (mobo hd CPU etc), so 200+200+200 < 650, which means you'll be fine. If you can undervolt, then even better. Please research before making claims.
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May 14, 2013, 03:00:25 AM
 #14

Alot of 7950's do not ship at 1.075v core but rather 1.25v and some are even voltage locked. He will have a hard time getting those to work without flashing the bios. If he wants to run a dedicated miner bamt linux is one of the easiest to use but offers no software voltage controls. Trust me a 7950's at stock setting can use almost 300w by itself. I've seen it.

A 660w Seasonic will not break a sweat powering two 7950's. I've been mining for 6+ months with an XFX 650w unit (rebadged Seasonic) and two 7950's at 1075mv vcore, 1000mhz core. Pull from the wall was a bit less than 500w.

PSU is all about getting a quality unit/brand. Crap like OCZ power supplies might not cut it powering two 7950's but a quality single rail 660w Seasonic unit will be fine.

You haven't seen a 7950 using 300W by itself, because the max TDP for a 7950 is 200W.

Now, a 7950 will use potentially up to 200W, and your system can use 100-200W (mobo hd CPU etc), so 200+200+200 < 650, which means you'll be fine. If you can undervolt, then even better. Please research before making claims.

My Powercolor 7950 GPUs do get well over 200w overclocked. I am saying this based on comparing reads on a killawatt while computer is, let`s say, compiling vs mining on linux. Compiling will suck 150w but mining with 3 7950s will bring that to 850w with stock clocks which are 975mhz - 1250 mhz if my memory is good. That is a difference of 700 => 233w per card, not even overclocked. PSU is a KingWing 1000w platinum.
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