4 x 7950 Power Issues

[esenminer]

I'm hoping to get some advice. I was running gigabyte GA-970A-DS2 mobo with 2x7950s and a 650W Cooler Master (80Plus Bronze) PSU and everything was fine. I got 2 more 7950s and an additional PSU (same 650W Cooler Master) to power them. I shorted the on pin on the second psu, installed the new 7950s, fed them power from the second PSU.

The system recognized all 4 GPUs and cgminer recognized all 4 gpus and in fact I can mine (scypt LTC) no problem at 13 or 14 intensity.  The problem happens when I want to mine at 19 or 20 which I did when I only had 2 cards. cgminer starts and just before the stats page loads the whole system goes down - not a crash but powers down.  When it goes down I can't start the computer normally, I have to physically unplug the main PSU, wait a couple of seconds and plug it back in so it seems like the first PSU is getting overloaded.

What can be causing this? My though is that since the 3rd / 4th cards are plugged into the mobo they might be draining a little bit of power from the first PSU as well but that seems unlikely. All 4 cards are Sapphire 7950s OC Dual-X.

A single PSU would have made more sense but I need to work with what I have. Any advice or thoughts?

[derr777]

Your on the right track... in LTC mining, there is more power pulled from the motherboard for the higher memclock frequencies needed.  The motherboard is going to pull probably 75 watts per card or so.  I fear you might have issues trying to put 3 cards on one of those 650s and 1 card + motherboard on the other, so your only option is likely to get another PSU..  You could get an 850 or 900 and put three cards on it and 1 card + motherboard on the 650, but trying to power this setup with 2-650s is likely not going to work, unless you can undervolt/downclock the cards to a point where you can put 3 cards on one PSU.  Can you undervolt your cards or are they voltage locked?

[esenminer]

Thanks derr777. I tried sharing the power connections on the mobo - i.e. plugged the 20pin connector from the first CPU and plugged in the 4pin + the CPU 4 pin connector from the second PSU. That let me mine at intensity 16 for a bit before it all came crashing down I think what I'm going to do it is go and find (or buy) a regular psu for the mobo and use the other psus for the gpus that should give me enough power.

I think these particular Sapphire 7950s are voltage locked because changing their voltage in cgminer doesn't have any effect - they still read at 1.25V.

[derr777]

That should work... the only other thing that will work without buying a different PSU is to switch to BTC mining... I had a similar setup and the movement of the memclocks down from 1250 to 600 freed up enough watts for the entire setup to function.  Its those high memclocks that pull serious wattage from the motherboard.  I since found a decent 850w PSU so that I can switch to LTC on that rig at a future date if I wanted to, but for a while I had same issue.

Good Luck!

edit: Oh, one more thing.. I never got cgminer to adjust voltage on a 7xxx series card.  Have always had to use MSI Afterburner, so you might try that..  Use GPU-Z to monitor the voltage (VDDC), and fiddle with MSI afterburner and see if you can undervolt a bit.  Might solve things without a new PSU..

[sniffinpoprocks]

I have another idea, use the external power supply for 3 cards, use one for the other card, mobo, and everything else. may balance out everything.

[esenminer]

derr777 - since i'm on linux i was going to flash the bios to get a lower voltage but it turns out the 'boost' mode (there is a switch on the cards which i never noticed before, has a nifty blue light ) actually lowers the voltage to 1.16V from 1.25V - this gave me a little bit more leeway but eventually once the cards started heating up and the fans revved up a bit the PSU gave in again or cgminer crashed. i might have to play with the settings to get it running more stable.

sniffinpoprocks - i hadn't consider this because the PSU didn't have enough connectors but I just realized I can get a 4 pin mini molex -> 4 pin regular molex connector which will give me enough connectors on the +12V for 3 gpus. The gpus on the mobo need 75x4=300W + 1 gpu = 150W + mobo is probably another 150W so just around 600W for the 1st PSU and 150x3 = 450W for the second. It's defn worth a shot! Undervolting would also decrease those numbers so seems reasonable...

thanks for the advice guys - i'll let you know how it turns out.

[esenminer]

not having any luck with alternative arrangements of the psu. even with 3 cards it's not letting me run at full intensity. i really need to get a kill-a-watt and see how much power those cards are actually pulling - i have them set at 1075 engine / 1500 memory so maybe it's a lot more than i calculated.

in any case what I think i'm going to do, instead of buying another much stronger PSU is to get another identical set of mobo / cpu /  memory - it's like $175 and just have 2 rigs with 2 x 7950 each - that way i can use the second PSU i already have.

thanks again for the help.

[destruct]

The main problem is that your buying Bronze level power supplies.. they typically don't give you the power they advertise.

Take the new one you got back and invest in a gold coirsar or seasonic unless you want your very expensive video cards to die

[Aragami]

The main problem is that your buying Bronze level power supplies.. they typically don't give you the power they advertise.

Take the new one you got back and invest in a gold coirsar or seasonic unless you want your very expensive video cards to die

I would question your credibility in the area of PSU standards if you think Bronze certified PSU's do not provide the Watts they advertise. Not to sound harsh, it's just that standards such as the 80+ bronze/silver/gold/platinum exist for a reason.

I believe whether a PSU is gold or bronze has absolutely no bearing on the advertised Wattage that the PSU can dish out. If you check out the spec differences between a bronze and gold PSU for example, you'll notice the difference is in fail-overs and safe-guards mainly as well as efficiency levels. Total Watts supplied has absolutely nothing to do with a PSU's level of 80+ certification. A PSU will dish out whatever amount of Wattage it advertises, efficiency in supplying that power is a different story (this is where 80+ comes in).

I highly recommend reading this: http://en.wikipedia.org/wiki/80_Plus

Again, sorry to sound like an ass if i did, i just dislike it when inaccurate information is provided.