I'm about to go SLI with an Antec Phantom 350w

[voodoomelon]

Hi all.

I've just ordered two Gainward 7900gs cards for my new SLI setup, and foolishly enough neglected to think of the power requirements that this entails.

However the reason I never thought of checking the power required, is because I have a 350w Antec Phantom PS:

Now some of you may be thinking, pah, it's only a 350w PS, it will never run SLI. Here enlies the question, the Phantom is perhaps the most effecient PC power supply on the market, with a tested and reviewed efficiency of up to 88%, which is unheard of in the consumer field.

I have seen test systems running 7900GSs in SLI and the full PC load is 260w at full load.

The Antec Phantom outputs approx 190w on the 3.3v/5v rail and 144w on the 12v rail.

Can someone do the math for me on this, ie, how much power/what kind of power do the 7900GSs use, 12v @ approx 60w each?

The cards are already ordered, so I would just like to know before they arrive,.
Thanks!

 

[Xtreme Barton]

PSU Specs:
http://www.antec.com/specs/Phantom350_spe.html


"...maximum peak is 82 Watt for the 7900 GS. So for a GeForce 7900 GS NVIDIA recommends a 300-350 watt power supply with 22-23 ampere on the 12 volts rails." guru3d

 

[Shadowknight281]

You might be okay, a guy here: http://www.ocforums.com/showthread.php?t=483076 is running SLI with a 350w PSu, which is made by Seasonic

 

[AC3421]

Hi all.

I've just ordered two Gainward 7900gs cards for my new SLI setup, and foolishly enough neglected to think of the power requirements that this entails.

However the reason I never thought of checking the power required, is because I have a 350w Antec Phantom PS:

Now some of you may be thinking, pah, it's only a 350w PS, it will never run SLI. Here enlies the question, the Phantom is perhaps the most effecient PC power supply on the market, with a tested and reviewed efficiency of up to 88%, which is unheard of in the consumer field.

I have seen test systems running 7900GSs in SLI and the full PC load is 260w at full load.

The Antec Phantom outputs approx 190w on the 3.3v/5v rail and 144w on the 12v rail.

Can someone do the math for me on this, ie, how much power/what kind of power do the 7900GSs use, 12v @ approx 60w each?

The cards are already ordered, so I would just like to know before they arrive,.
Thanks!

My PSU has an 89% efficency rating.....

 

[voodoomelon]

"Perhaps" was the key word in that sentance.

Thanks for the link ShadowKnight, that guy seems to be running a near identical spec to me, however the 7900GSs do use a fair bit more power than his 7800GTs. This should be very interesting!

I'll have the following running off it:

1x 3800+ X2
2x 1GB Geil modules
2x 250GB WD HDs
1x 200GB Maxtor HD
2x 7900GSs 256MB Gainwards
2x optical drives
2x cold cathodes
1x HDA Mystique 7.1
1x Waterchill pump
1x 120mm fan
2x USB devices

Hope it doesn't crash and burn...

 

[FIZZ3]

The efficiency of a power supply has nothing to do with how many components it can power. Efficiency describes how many watts it draws from the wall to put out its power to the rails. So an output of 350 watts may require 400 watts from the socket with PSU X, and 425 with PSU Y, but the 350 at the end of it is not affected.

@AC3421: Quoting a manufacturer's efficiency rating is even less meaningful... if you've read some PSU reviews in reputable places like Jonnyguru or SPCR you'll find that those ratings may or may not correlate with reality.

 

[Oklahoma Wolf]

Hope it doesn't crash and burn...

My instinct is you will likely be tripping the overload and/or overtemp protection before it blows. I don't think it will power the SLI rig in question, and I personally wouldn't even try it without inspecting all the capacitors in there for bulging tops, but I'll wish you good luck anyway

 

[Xtreme Barton]

dont do it man !!

just get a better psu and call it a day ..

 

[voodoomelon]

The efficiency of a power supply has nothing to do with how many components it can power. Efficiency describes how many watts it draws from the wall to put out its power to the rails. So an output of 350 watts may require 400 watts from the socket with PSU X, and 425 with PSU Y, but the 350 at the end of it is not affected.

@AC3421: Quoting a manufacturer's efficiency rating is even less meaningful... if you've read some PSU reviews in reputable places like Jonnyguru or SPCR you'll find that those ratings may or may not correlate with reality.

Efficiency has got plenty to do with how many components the PS can run at any one time.

If you have some piece of crap 350w PS with a 70% efficiency rating, then nearly 1/3 of the power being drawn from the wall is being lost to heat and hence is not going to the PC components.

However, if you have a very high efficiency PS like my Phantom, a mere 12% is being lost to heat. That's a difference of 18% efficiency, which equates to 63w extra output by the Phantom.

Of course there are other issues involved, but there is a sizeable difference.

 

[RangerXLT8]

It's not going to run that system in your sig, plus SLi. Another thing, PSU are SLi Certified for a reason. Atleast get a SLi certified PSU. I suggest you measure rails with a DMM with only 1 card in there, and I'd bet that Phantom is struggling already.

 

[mantralord]

My only concern is that it's a fanless unit, so it might not be able to cool itself for long periods under full load. However, taking into consideration the airflow in your system, and the fact that even a fanless unit requires airflow, it is definitely possible.

The irony here is that the PSU will need lots of (probably loud) airflow, yet it's designed to be silent.

 

[UglyChild]

I dont think your PSU will run SLI at all. You might get lucky and have it run for about 10min of gaming, but thats only if you disconnect everything else from PSU, your lights, you fans, your drives, ets.

Try it.

 

[Neuromancer]

If it does power the SLI unit the PSU will have to be running at or over 350Watts, so it won't last very long.

 

[Alexhk]

Okay let me see if I can clear up some of the errors I've seen in this thread.

Voodoomelon you're up first. - A power supplies efficiency does not have a direct bearing to the load it can handle (indirectly through heat of course it does.)

Do something for me, look at the label on your power supply. Notice that what is being listed is it's rated output wattage and amperage. Not the input. If you have two 350 watt power supplies that can actually handle a 350 watt load then they will both output 350 watts regardless of their efficiency. Efficiency will be seen in how much power each PSU draws from the wall to output 350 watt. If one power supply is 80% Efficient at full load it will draw about 437 watts while one that is only 65% efficient at full load will draw over 535 watts to reach its 350 watt output. So obviously the less efficient power supply is creating truck loads of extra heat which isn't desirable but both power supplies can still output the same 350 watts or power. All that extra heat in a less efficient power supply can lead to an early death if cheap capacitors that can't handle high temperatures are used, ala antec smartpower units.

I guess this section is for everyone - The efficiency rating that the PSU company quotes for a given unit.

The first question you should ask yourself when trying to determine the validity of a quoted rating is "Does this PSU company actually manufacture their own power supplies." In the case of the two above, Antec and Kingwin the answer is both No, and No. You should then look around, is there anyone who actually manufactures power supplies which such a high efficiency rating. If that answer is a no, and you know that the company you bought your PSU from doesn't actually make their own then the rated efficiency must be lower than they say it is. At real system loads no one is hitting 88% efficiency.

AC3421, you have me completely perplexed, kingwin doesn't even pretend their units have such high efficiency. In fact the 600 watt mach 1 is rated at 70% at full load. Where did you read 89% from?


Now to the idea of this system surviving -

Xbit labs report the power consumption of a 7900GT at just under 50 watts and if we assume the 7900GS (i don't actually know about this and will be checking after I post) to have a similar power consumption you're looking a little under 200 watts for your CPU and video cards under load. Unfortunately they all use the 12 volt rail which you said the phantom is only capable of outputting 144 watts on. You're pretty outta spec there, I see little chance for success.

 

[mantralord]

Xbit labs report the power consumption of a 7900GT at just under 50 watts and if we assume the 7900GS (i don't actually know about this and will be checking after I post) to have a similar power consumption you're looking a little under 200 watts for your CPU and video cards under load. Unfortunately they all use the 12 volt rail which you said the phantom is only capable of outputting 144 watts on. You're pretty outta spec there, I see little chance for success.

Here are the specs for the phantom: http://www.silentpcreview.com/article188-page1.html

They don't say what the max 12v output is, but one of the rails is 18A max, so I'm inclined to think that it does at least (and probably) 18A, which is 216 watts.

The guy in that other thread had a Seasonic with 17A powering his cards. Anyway, I still think the biggest issue will be managing the airflow properly.

 

[Maviryk]

If you do try it, put a fan on the PSU, since it doesn't have one. Try it and see what happens, worst that would happen is overcurrent protection. Best is it will work.

 

[RollingThunder]

"A penny wise and a dollar foolish."

 

[nvidiaOCmaster]

It will work fine. That im sure of. I saw a display case at a computer show running quad SLI on a 350W PSU and it was working fine.

 

[praxis1452]

It will work fine. That im sure of. I saw a display case at a computer show running quad SLI on a 350W PSU and it was working fine.

umm... idle maybe?

two 7950gx's are around 220W loaded. It's more but I'm actually under estimating. Maybe they downclocked them as well. Or lowered the vcore.

CPU 65W. It's possible yes. But the psu will be at load and all of the power will be drawn off of the 12v rail. He can only supply 144W. 2x89W 7900gs's are >144W. That's not even including his cpu. Wattage doesn't matter. Amperage does and he doesn't have enough where it needs to be. If they were 7900gt he may barely make it. a 7900gt draws around 60W under load. so 120W gpu and 65W cpu IF you are running a conroe at stock. you'll still be around 25W over the limit. You'll be limited by your power supply.

Efficiency does not matter in how much it can output in this situation. Whoever thinks it does is wrong.

 

[MadMan007]

http://www.silentpcreview.com/article188-page1.html

That certainly clears it up. 144W\12A on a lone 12V rail just didn't sound right, unless your PSU is an older version that really only has one weak 12V rail. With 18A and 16A 12V rails you should be fine, the 16A\192W rail should be enough for the two cards plus some peripherals.

*edit Hmm it does say this too: "* 12V1 line feeds only the main 2x12V connector" So I guess the 16A rail goes to all peripherals and the motherboard ATX connector. Still I think it should be ok. Make sure the airflow is good as already suggested.