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Power supplies are hurting my brain

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August 12, 2012 7:01:17 AM

I've been reading about volts watts amps and rails since last night and I just don't know enough to figure this out.

I have an i7 2600 (3.4ghz), 6gb ram, 1tb hdd.

I was comparing video cards last night and settled on getting a GTX 570 for my price range. I've read that it should have 38a on the 12v rail.

This is where I get confused, and please believe I have tried to figure this out myself before posting a thread here. I've had dozens of browser windows open researching and for some reason I just can't get the info to click in my brain.

I've read it's okay to have separate 12v rails with less than 38a each as long as they add up to being enough for the card. How can you possibly feed one device off of two separate rails? That card takes 2 six pin connectors, would I literally have to get a wiring diagram and make sure each connector is on a separate rail? Or if it's hooked up to one rail with less than 38a, does power get drawn from the other rail anyway? That wouldn't make much sense, how would the 12v1 rail be rated at 20a if it's just going to take extra power anyway, it would be surpassing it's own current rating. And I know you don't just add up the rails, the PSU should have a separate "combined" rating. Well even if the combined rating is over 38a, how would two rails ever "combine" to power one card?

I just don't get it. Seems to me whether I buy a PSU with a single rail or multiple 12v rails, one of them has to be more than 38a by itself. And then I have to factor in the processor, which a quick google search tells me at 100% load the i7 would be 7.2amps.

Also in my video card thread someone told me I could run a HD7770 on my stock Gateway power supply. That I can understand. I only have a 10a and 13a rail available on this PSU, but the 7770 tops at 80 watts according this review I looked at, which means less than 7 amps. I can understand that 7 is less than 10 or 13 lol. I don't understand when I start talking about the GTX 570 and rails less than 38a.

I'd like to know what power supplies would work but I'd also like to find someone with enough patience to help me understand why they would work.
a c 87 ) Power supply
August 12, 2012 12:44:13 PM

The 38A recommendation is for the whole system. A GTX 570 has a maximum TDP of 219W (18.25A), so you would need a +12V rail with a minimum rating of ~18A to power a GTX 570.

BTW an AMD 7850 is the better choice at the same price as a GTX 570 because if anything it outperforms the GTX 570, but is less power hungry, less hot and less noisy.
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a b ) Power supply
August 13, 2012 6:07:37 AM

POWERBOMB said:
How can you possibly feed one device off of two separate rails? That card takes 2 six pin connectors, would I literally have to get a wiring diagram and make sure each connector is on a separate rail?

PSUs with multiple rails usually have a colored stripe on yellow wires to indicate which rail it feeds from. On my PSU, I have some connectors with solid yellow color (12v1), yellow with black stripe (12v2) and yellow with blue stripe (12v3).

If you want to feed a GPU from both rails, simply check your PCIe power connectors and pick a connector pair that has different markings.

If you have a modular PSU, the PSU should have indications of which connectors are on which rail silkscreened onto it. Worst case, instructions should say it.
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