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SLI and CrossFire, Pushing Power Supplies To The Limit

gonaads

Beware the G-Man
Political User
#1
I thought that I would post this and a link. Seeing as how far and how fast graphics cards have progressed and the amount of power these cutting edge cards need to do the wonderful things they do to make our gaming experience a memerable one.

Most of us when we get our systems they have adequate PSUs (power supply unit) for all our normal needs. These being 350 to 400 watt supplies. Most systems now come with 400W supplies and fairly good video cards. But the serious gamer wants/needs the fastest most bleeding edge piece of hardware out there to play the newest games with rich full blow textures for landscapes, water movement, reflections, smoke, etc... that look so damn real you could forget it's only a game.

500W and 600W PSUs don't even cut it for these serious gamers. Now comes the mega power supplies. Those 700 to 850 watt monsters that could light up a small village with the power they put out. Some of these peak at close to 1000 watts! Hell, AMD recommends 700W power supplies (minimum) for high end SLI or CrossFire setups just so ya won't have any problems. I can see it now ya just bought yer super cool SLI system with 2 512MB 7800 GTX cards and the fastest AMD FX CPU and a 650 watt PSU. 650 watts, that should be enough, rite? Ya go and fire it up ya do your burn it everything is going fine. You have your newest game with graphics so smooth and real you think you're in a movie. You install it and you're ready for an "all-niter". But first ya wanna see what this system can do, so ya do your 3DMark testing and... Holy shiit, it up and dies half way through it. your PSU just shut down. Now you're yelling to the sky why oh why, how can this be? It's a 650 watt PSU for cryin out loud and it cost you $150 dollars. Now you have to go out and spend $300 dollars on one that is big enough for your system, plus nVIDIA/ATi certified.

Anyway, no more blah blah. Here is a link to ExtremeTech that has a very good article on these SLI/CrossFire systems and Power Supplies.

SLI and CrossFire Push Power Supplies to the Limit
 
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#2
Best part is Gonaads, is that it has nothing to do with the amount of power.

You can prevent these shutdowns by using a molex-to-PCI Express graphics card adapter and sharing the load with another 12V rail. Alternatively, just load up the other rails with more hard drives or other power-drawing peripherals.
Tony Ou (Silverstone Power supplies) said:
What happens is that some power supplies are designed with a shared power plane... "I am sure you already know that PC power supplies we have today have three main rails, +3.3V, +5V, and +12V, which are required to power various components. However, what most people don't know is that many power supplies are designed with shared power plane (it is very common to have +5V and +12V rails linked together) to help reduce cost and obtain higher maximum power. If you have seen our retail box for our ST60F 600W model or read the manual, you will see this:

+5V min. load is 10A when +12V output is 30A to 38A
+5V min. load is 15A when +12V output is 38A to 42A

This means that in order for our power supply to generate maximum power for +12V rail optimally, the +5V must also be loaded up. Normally this is not a problem because most systems do draw enough power from both +5V and +12V rails evenly to make cross loading requirement a non-issue. Even when SLI came out in late 2004, the most powerful gaming system at the time would rarely draw more than 30A from the +12V rail..."

In other words, if you balance the loads on the different rails, then you won't have this kind of problem.
 

Sazar

F@H - Is it in you?
Staff member
Political User
#3
This, plus the short life-span of video cards these days is one of the reasons I am not a big advoacte of multi-gpu configs.

However, a good psu like the pc power and cooling 850 or 1kw psu's are perfectly fine :)

Their efficiency is legendary and keep in mind, just coz you have a larger psu by wattage doesn't mean your bills will go up. You are only pulling in as much power as needed (upto the max).

Buy pc power and cooling, forget bout the rest.
 

gonaads

Beware the G-Man
Political User
#4
Well that's why I put this up, so everyone can at least have something to go on and at the same time anyone who has good info and/or links to good info can post. This way everyone benefits from this and doesn't go out and buy too small a PSU or too big a PSU. Well in this case bigger is better. But it really depends on your pocketbook. :)
 

Sazar

F@H - Is it in you?
Staff member
Political User
#5
Bigger is not always better.

A lot of the higher rated psu's have crap rails. They are looking for some ridiculous temps to achieve their wattage and stick that as their rating whereas the truth is that in a real-life (and therefore warmer) scenario, they have far lower power outputs.

Pc power and cooling has much better efficiency and rates its psu's effectively in a real-world environment. This is why people consider their ratings to be conservative.

Antec has decent psu's as well. Save a little money and go antec or, bypass everything and shell out for the best.
 

Sazar

F@H - Is it in you?
Staff member
Political User
#8
Ultra X connect uses a modular setup. Modular psu's are pos's. They look nice but introduce a whole bunch of other problems. There is a reason the best products on the market are not modular :cool:

Even antec's version is crap, even though it looks nice.
 

Son Goku

No lover of dogma
#9
One note of caution, while we're on the subject of larger PSUs, and then I've gotta head out:

When I still lived with my parents, I had a room off to the side which didn't fall under the same central heating as the rest of the house. As such, I had a seperate electrical heater for the room. That thing used about 1,500 watts or so. I bring this up, because the electrical outlet itself has a limit on how much current draw it will provide. Usually everything was fine, but every once in awhile, the circuit breaker for the electrical outlet did trip and had to be reset. At about 1.5 kilo-watts (if I'm remembering correctly), it was comming close to the limit for the plug itself...

The PSUs can only be made so big, for the outlet in most people's houses. Larger appliances (like the stove, dishwasher, drier, etc) are many times given a seperate 220 volt line in the house, to accomodate their needs...

Otherwise, and although I do have an X-connect, which I haven't personally had any problems with, saz does bring up a good point. Check the specs on the individual rails themselves, as well as the amperage applied on each. Don't take the overall wattage as an indication in itself...

Anyhow, gotta head out ;)
 

gonaads

Beware the G-Man
Political User
#10
One of the biggest problems is circuit load. I mean circuit load of the room. Say your room where your computer is has a TV, a DVD/VCR, a Stereo System, maybe even a Shredder. Ah, let's not forget the possibilities of a Room/Space Heater. Now think of the amperage rating of the Circuit Breaker that controls that room. Now remember that some homes have more than one room on a single circuit. Hell my Bedroom is wired with the upstairs Bathroom. When my daughters would have their megawatt hairdryer goin it could cause problems. So you have to take into cosideration the power needs of everything else. See if one of the outlets in your room is on a dedicated line and plug your computer and UPS into that. Also some go as far as using volatage stablizers to make sure you have clean consistent power. Those dips in power can do nasty things.
 

Son Goku

No lover of dogma
#11
That's sorta a problem that my sister ran into. She bought an older house, that had no 220, and a single older 110 volt line comming in. The house was also several decades old, and was probably wired more then 30 years ago...

They used to have to shout around the house to see what everyone was using :eek: They learned there were certain combinations of things they couldn't run together. Also, the house had fuses, not circuit breakers, so a blown fuse took more to replace. For instance if the blow drier and the vacuum were run together, blown fuse. If the microwave was run with the washer, same thing. They had a list of appliances that couldn't be run with each other :eek:

I think my bedroom in that place was actually on 2 seperate circuit breakers... It also took about half of the useable basement underneath the house. I say useable, because part of it was walled off of sorts, and instead of a smooth floor, had a rockey surface covered over. My parent's house in Maine is also built on the side of a rather largish hill, so a fair amount of bed rock and stuff... Still, 1/4 to 1/3 the given story was fairly big.

But yeah, that's also why besides putting stuff on a 220, things like driers, stoves, hot water heaters and the like can get their own circuit... I do think electrical outlets have their own max rating too, but not entirely sure what it is, albeit wasn't going to play by plugging another device into that particular outlet the heater used. As to the wiring itself, that too depends on the gaudge wire they're using. We looked at some of these details in some of the comp engineering classes I've taken, but not wrt the wire used by electricians in houses themselves...
 
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Sazar

F@H - Is it in you?
Staff member
Political User
#13
Push your system further, you will find out.

Also read up about the way modular PSU's work and are setup.
 

Son Goku

No lover of dogma
#14
Don't know, though I don't think Saz would just blow smoke up our butts. My A64 has a lot more stuff running in it then the normal PC; and for what's in there, the CPU for instance doesn't get a rest as it's set to always run distributed science apps that suck up 100% CPU time 24/7 (as I never shut down for the night)... I'm sure it's not under full load though, as I for instance don't run an SLI setup, and my gfx card is old (Radeon 9600), not one of these new 7800 GTX's or the like...

My reason for going with it was rather practical though... The molex connector on my old PSU got jammed in a CD changer I used to own since 1999. The thing went bad, and without removing the connector I coudln't get the drive out. However, I couldn't remove the connector with that tiny space, and my rather large fingers, with it still in there... I was also too lazy to rip out the PSU, and have to rewire my whole PC all em years :D

The stupid thing would also randomly reset the SCSI bus (as it was dieing), and cause all kindsa SCSI errors that were affecting the whole drive system (all my drives, except my external USB HD, are all SCSI). The system log in event viewer was litterally getting flooded with error messages from that dieing drive... When I finally unwired the thing, and got it clear, it still took a lot of fource, enough to break the plastic enclosing right off the drive, as it wouldn't lossen up until the stupid thing gave... I didn't care, the stupid drive was dead as a door nail anyway.
 
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gonaads

Beware the G-Man
Political User
#15
Here are some loose figures, but they seem pretty damn close.

Your average everyday PC user probably has what is considered a low end system. The CPU is likely in the 2ghz range (or equivalent in AMD CPU’s) 512mb of ram, and not that many components in all to draw too much power. This kind of user can easily get away with a 300-350w PSU. But if you’re not an average user, if you have greater demands from your system and a lot of power hungry devices, you’re going to want something a little beefier in your PSU requirements.

Nvidia's top cards will happily draw on up to 110w each, with ATI just behind them at 100w. This is in an SLI or Crossfire setup. And that's a lot of power for just 2 components in your system. This is one of the main reasons for wanting a PSU with over 400w of power.
 

Sazar

F@H - Is it in you?
Staff member
Political User
#17
Fyi, this thread pertains to SLI and crossfire. Naturally if you are running a single card solution, this would probably not apply to you. But if you are running a high-end solution and stressing it (oc'ing or running lots of other apps), the logic about the extra resistance introduced with modular psu's is put into play.

Just do the math and use common sense. It doesn't mean you have a crap system NOW. It means that if you are going to stress it, it is prudent to go ahead and get a more efficient setup, like PC P&C rather than xconnect or something else with a modular config.
 

gonaads

Beware the G-Man
Political User
#18
Correct me if I am wrong, but wasn't the whole idea behind "modular" was for ease of install/setup? You don't connect the sections of wiring that you do not need. This way there is less wiring floating around inside your box. So there is a cleaner install, better air flow and all around neatness.

Yes/No?
 

Sazar

F@H - Is it in you?
Staff member
Political User
#20
gonaads said:
Correct me if I am wrong, but wasn't the whole idea behind "modular" was for ease of install/setup? You don't connect the sections of wiring that you do not need. This way there is less wiring floating around inside your box. So there is a cleaner install, better air flow and all around neatness.

Yes/No?
It looks nicer, less cables, potentially better air flow (if you don't care about doing your own cabling clean-up and all round neatness.

But you are introducing additional resistances and therefore making the supply of power less "clean".

Efficiency therefore is reduced. It should not make a difference for most systems, but crossfire/Sli systems are not most systems :cool:
 

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