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multiple 12v rails

post #1 of 23
Thread Starter 
hey guys. im in the market for a new psu because i will be upgrading my vga soon. maybe 480 sli or 580 sli. anyway i had my eye on the xfx 850 psu which newegg has a great deal on right now($20 dollar promo brings it to $130shipped).

i noticed the xfx 850w has 1 12v rail rated at 70 amps. i was looking at other power supplies with more than 1 12 v rail, for instance a 850w psu with 4 12 rails rated at 18a 18a 30a 30a respectively.

im just curious, how is this possible? how can 1 850w psu have WAY more amps on the 12v rails than another?
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post #2 of 23
because if you tried to load all of the 12v rails on the second one all at once, it would probably trigger OCP.

Basically, it cannot deliver all of that at once

EDIT: which PSU is the second one?
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post #3 of 23
Thread Starter 
Quote:
Originally Posted by reflex99 View Post
because if you tried to load all of the 12v rails on the second one all at once, it would probably trigger OCP.

Basically, it cannot deliver all of that at once

EDIT: which PSU is the second one?
it was a thermal take tough power

thing is, i dont need the psu just yet. im not even sure when i will be upgrading. just that the xfx deal seems too good to pass up. $130 shipped for a 850w modular 80 silver cert psu is a great deal. not to mention there is a $40 mir. that just makes this even sweeter. and i beleive its a seasonic oem on the inside.

would this xfx 850w psu be enough to power a sandy bridge rig with 2x gtx 580 sli, or 2x gtx 480 sli?
Edited by AliceInChains - 2/4/11 at 10:21pm
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post #4 of 23
agree with reflex. one 12v is better than multiple when sli'ing or for higher graphics cards because individual lines may not be able to supply that many amperes as required by the sli'd and probably oc'd cards.

and on multiple lines u just DONT add up all the amperes each line can give and multiply it by 12 check the specs to see how much the entire 12v line can give(max watt) and then divide by 12. that gives u the sum of the max amperes all the 12v lines put together can give..
Edited by mrcool63 - 2/4/11 at 10:20pm
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post #5 of 23
What is wrong with your TX750?
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post #6 of 23
Thread Starter 
Quote:
Originally Posted by reflex99 View Post
What is wrong with your TX750?
is that enough to power 2x 580's?
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post #7 of 23
850 is cutting it close for 2x580s.

850 will be fine though, about 50w to spare.
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post #8 of 23
At stock on the 580s, and if you don't run furmark, then yes, it's plenty.
Otherwise, that XFX unit is a great one if you want a bit of OC headroom on the 580s.

Multi-rail PSUs normally split up the 12v output from a single source into several rails by using several sets of OCP (Over Current Protection) to divide it up. It's no real advantage for any PSU under 1,000w. Single rail units just skip that splitting part and give you one big rail with all the power on it. Neither offers any real advantages or disadvantages at the power range in question.
Note that the rail capacities do NOT add up to a total power output greater than a single rail unit of the same design. It just shows the trip points for the OCP on each rail, the actual 12v source has a limit for total output which prevents you pulling more than its' maximum. The excess capacity is there to allow flexibility in which rails you use.
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post #9 of 23
Reflex is incorrect.

Two 5850s will run easily on a quality 650W unit, and could scrape by on a quality 550W. And single rail vs. multiple rail does not matter.



Single vs. Multi rail DOES NOT MATTER




End of story, and this myth needs to DIE. Tell your friends.
post #10 of 23
Phaedrus we are talking about Nvidia not ATi lol
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