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Thread: Proper PSU for two R9 295x2 in crossfire

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    Quote Originally Posted by jonnyGURU View Post
    Get something that's either single +12V rail or, at the very least, has a separate OCP for each PCIe connector.
    I am getting a Corsair AX1500i PSU, and i think that it is the one, that has a single +12V rail. I have high hopes for this one, it might work for my build just right. Hell, if it fails to deliver enough power for this, i really don't know what single PSU will, lol (might be the new EVGA G1600, i just haven't read Your review about it yet :C).
    Although, by the time i'll get AX1500i, i will have it AND my current Lepa P1700M, so if single PSU like these two won't be enough to support two R9 295x2's, than using both of them at the same time will certainly do the job.
    What do You think? Do You think buying AX1500i was a good idea in this case? Or maybe i should bought something else? Or maybe it would be better to have 2 PSU's?

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    The AX1500i is a good option, but it isn't single rail. You can make it single rail with the software from Corsair, but I believe that isn't necessary. You can also change the OCP which could be necessary/useful.

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    Shouldn't be necessary since the default OCP is 30A per PCIe cable.

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    Quote Originally Posted by -The_Mask- View Post
    The AX1500i is a good option, but it isn't single rail. You can make it single rail with the software from Corsair, but I believe that isn't necessary. You can also change the OCP which could be necessary/useful.
    Quote Originally Posted by -jonnyGURU- View Post
    Shouldn't be necessary since the default OCP is 30A per PCIe cable.
    This might sound really, really stupid, but: why would i want to change OCP, if it is already 30A per PCIe cable? Wouldn't it be enough, if i would just connect each PCIe cable from each slot on the PSU to each connector on the graphics cards (total of 4x30A needed, which means, that i would use 4 PCIe cables with a total of 120A)? Or if i will make it a single rail, then the OCP will be spread for each PCIe cable depending on how much each one of them needs?
    I've heard that theoretically (or even practically, i don't know) multi-rail configuration is better, since it has better short circuit protection than the single rail one, because multi-rail's OCP is spread out equally for each rail.
    I am sorry, but i don't understand...

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    A short means that there is almost no resistance, so the current goes sky high if you don't have a short circuit protection. But if you make the AX1500i a single rail it means it doesn't shutdown with a load from 125A @ 12V, because it isn't higher then 1500W. So if you have a short which doesn't generate more then 125A @ 12V the PSU "thinks" it is a normal load and doesn't shutdown. But 125A is way to much for a normal connector or wire, so it starts to meld or catches fire. Same goes for the component that has a short it could also catch fire.

    With a OCP in place the PSU will shutdown if the current goes behind the limit, which in this case will be much lower then with no OCP.

    More here: http://www.jonnyguru.com/forums/showthread.php?t=3990

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    Yeah... you really need to read the stick The Mask linked to.

    And nobody said anything about changing the OCP from 30A. 30A should be fine, but in the Link software you could crank it up to as high as 40A.

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    Quote Originally Posted by -The_Mask- View Post
    A short means that there is almost no resistance, so the current goes sky high if you don't have a short circuit protection. But if you make the AX1500i a single rail it means it doesn't shutdown with a load from 125A @ 12V, because it isn't higher then 1500W. So if you have a short which doesn't generate more then 125A @ 12V the PSU "thinks" it is a normal load and doesn't shutdown. But 125A is way to much for a normal connector or wire, so it starts to meld or catches fire. Same goes for the component that has a short it could also catch fire.

    With a OCP in place the PSU will shutdown if the current goes behind the limit, which in this case will be much lower then with no OCP.

    More here: http://www.jonnyguru.com/forums/showthread.php?t=3990
    Well, i hope that Corsair provided good enough cables with this PSU so they won't melt under heavy load...
    After i read the post You've sent and the jonnyGURU's review on EVGA 1600G, i am thinking about buying one, as it is supposed to be single-rail PSU (just like AX1500i), but it can provide, about 130A (i'm not sure) which is a little bit more than 125A and it might be just that little bit i need, for the PSU not to overload. I'm just really afraid, that with all the stuff connected to the PSU, other than the video cards, it will be powering a motherboard and a CPU, a fan controller, an optical drive, an SSD and 2 HDD's.

    I am sorry, but i don't get this part: "With a OCP in place the PSU will shutdown if the current goes behind the limit, which in this case will be much lower then with no OCP". If the current goes behind the limit, OCP will shutdown the PSU (did i understand it right?), but this: "which in this case will be much lower then with no OCP" i just don't understand at all... Would You please be so kind to explain this part to me?

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    Quote Originally Posted by jonnyGURU View Post
    Yeah... you really need to read the stick The Mask linked to.

    And nobody said anything about changing the OCP from 30A. 30A should be fine, but in the Link software you could crank it up to as high as 40A.
    The post The Mask linked to is very informative and pretty easy to understand, even though i really know just a little bit more, than absolutely nothing about this kind of topics. Thank You for writing it!

    Well, in the power consumption part of R9 295x2 it recommends to use 8-pin power connector to each 8-pin on the card (there are 2 of them) from a single rail that is able to support the minimum of 28A to each 8-pin connector. So, yeah, i hope 30 on each connector would do fine.

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    Quote Originally Posted by yifeng3007 View Post

    I am sorry, but i don't get this part: "With a OCP in place the PSU will shutdown if the current goes behind the limit, which in this case will be much lower then with no OCP". If the current goes behind the limit, OCP will shutdown the PSU (did i understand it right?), but this: "which in this case will be much lower then with no OCP" i just don't understand at all... Would You please be so kind to explain this part to me?
    So.. if your OCP is 30A and current exceeds 30A, the PSU shuts off.

    If you have no OCP, whatever the PSU can provide to the +12V rail TOTAL will attempt to go down the wire if there's a short. This amount of current, say 125A in the case of an AX1500i, is WAY MORE than any PSU wire, connector pin, etc. can handle.

    OCP can be important. It's like airbags in a car. You hope to never need them, but you're glad it's there when you DO need them. Just last week, we had a customer with a CX750M. A PSU w/o OCP. His optical drive died on him. Something inside went >>ZAP<< and shorted the +12V. But because the CX750 has 62A available on the +12V, the load just kept increasing and increasing until the connector and wire insulation on the lead going to the optical drive got so hot that they melted. Not pretty. A 1 in 100,000 incident? Probably. But still.... not pretty.

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    OCP is kind of like the circuit breakers in your house. Each circuit in your house gets its own breaker; likewise each cable or group of cables leaving a multi-rail PSU has an OCP "breaker", where if there's a short circuit or an overload condition that could damage the PSU, the PSU will shut off to protect itself.

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