Question about power requirements

Ub3rn00b

[H]ard|Gawd
Joined
Sep 18, 2006
Messages
1,064
I've seen some video cards that say they require a power supply with 12v 22amps for single card and 12v 36 amps for two cards to run SLI.

This is on the evga 7900gto.

I've only ever seen a few power supplies that can run provide 36amps. I mean I looked through the power supplies on newegg and saw a couple that were 36 amps but they were very expensive ones.

The 600w OCZ shows 18amps per 12v rail. So this power supply isn't enough to run the 7900gto? WTH? But on the reviews, there are people that say they are running all kinds of stuff plus 2 7900GTX cards.

So as you can see, my knowlege is lacking. I have no idea what I'm looking at when I read the ratings because I must be wrong about this junk. Please help!
 

JackAshley

Gawd
Joined
Aug 21, 2006
Messages
729
Two 18A rails makes a 36A power supply. For example, my FSP 450w power supply has dual 18A rails, which totals to 36A, and it cost about 60 dollars Canadian...but, a power supply with a beefy single 36A rail would be much more desireable than dual-rail.

-Ashley
 

ZenOps

2[H]4U
Joined
Jun 11, 2000
Messages
2,914
Double rail has some advantages and one disadvantage.

On a double rail, your CPU is on seperate circuitry than the rest of the 12 volt devices. When the CPU idles or runs full speed, or when it is running in the throttled mode, it can fluctuate the 12V rail up and down and all over the place. Since its seperate from the other devices, CPU power fluctuations will not affect the stability of your other components. Intel recommends a double rail for several processors, and highly recommends it for larger power supplies with power hungry videocards.

The main disadavange is that you cannot simply add-up the two 12volt rails, one supplies the CPU, the other one supplies the rest of your components. If you've got an ultra efficient CPU, you cannot move the excess amperage to your other components on the secondary 12V without breaking out the soldering iron. Vice versa as well..

BTW: Intel seems to be pushing for unifying the voltages and voltage rails in upcoming years. It means simpler power supplies, but more complex motherboards. Personally I think its just a cash grab, as Intel does not make powersupplies. The only reason they can do this is that they are finally past the Prescott era, which would draw so much power difference between idle and load that it tended to screw up the other components.
 

Ub3rn00b

[H]ard|Gawd
Joined
Sep 18, 2006
Messages
1,064
Ok. The PSU i have is a quad rail 18a each so it should be PLENTY of power to power two 7900 GTOs. I understand that now. Thanks alot. Now when someone said "12v rail at 18amps per rail" how can you tell if a power dongle is on the same 12v rail as another power dongle? For example, if I want to have each of my 7900 GTOs on separate 12v rails, how do I make sure that they are indeed on different rails?

It's things like this that confuse me. This is simple knowledge to those that know. I simply don't know. W3rd...
 

JackAshley

Gawd
Joined
Aug 21, 2006
Messages
729
4 12v rails? Don't mind my skepticism, but I've never heard of that...but I havent heard of much, so it doesnt mean you're wrong, but look into that :)

-Ashley
 
Joined
Jul 7, 2006
Messages
527
PC Power and Cooling 1KW Quad has 4 12V rails at ~18A.........Its a baaaaaad bitch too ;) (Still way too expensive for me though :( )
 
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