A question about the +12V rail

[dragonfly22588]

Alright,

So if a GPU calls for +28 amps on the +12V and you have a PSU that provides 17 amps on two +12V rails, does the PSU satisfy the requirement for the GPU? Because I’m a little confused by the idea that the +12V rail is separated as opposed to being on 1 line. Could someone explain this to me, thanks.

 

[efeat]

Alright,

So if a GPU calls for +28 amps on the +12V and you have a PSU that provides 17 amps on two +12V rails, does the PSU satisfy the requirement for the GPU? Because I’m a little confused by the idea that the +12V rail is separated as opposed to being on 1 line. Could someone explain this to me, thanks.

http://en.wikipedia.org/wiki/Power_supply_rail#Multiple_.2B12V_Rails

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

Basically, there's only one true 12v rail and only the highest of the high end PSUs have multiple. The rail is separated into 'virtual' rails to limit how much power flows through each individual wire. Too much, and it overheats and melts itself. As far as combining rails, it doesn't quite work additively, but if you add the numbers together and subtract about 20% off, you'll get a rough estimate. 17 +17 = 34 * .8 = ~27 amps, so you'll probably get 27 amps out of the two rails. I'm not why it works like that, but it does. Hopefully someone can clear up why it does that.

As far as your gpu calling for 28 amps - that's probably hogwash. GPUs call for an excessively high amount of amperage and power just to be cheeky. Honestly, 28 amps at 12v would mean your GPU consumes 336 watts of power by itself. 672 watts and 56 amps if you ran it in sli/crossfire, and that's completely ignoring the rest of the system. Take a look at a review Tom did and you'll see how much cards really require. The only cards that actually require 28 amps or more are the quad/tri GPU setups or two really high end cards in sli/crossfire.

According to http://www.playtool.com/pages/psuconnectors/connectors.html the 6 pin PCIE connectors are only officially rated up to 75 watts, which is 6.25 amps. The 8 pin are officially rated 150 watts, which is 12.5 amps. The system board can supply another 75 watts on own. A 28 amp card would need four 6pin PCIE connectors, or two 8pin PCIE connectors, or two 6pins and one 8pin. Unless you have a card that requires that many connectors...you probably don't need 28 amps =)

 

[jsc]

That 28 amp rating you are talking about is the "suggested" requirement of a complete "typical" system.

Tell us what graphics card and the rest of the system specs and we can give you a pretty good estimate of the power capacity you need.

 

[bpogdowz]

I'd use the PSU calcuator.

just google outer vision PSU calculator

It's "the professional's choice".

 

[JAYDEEJOHN]

OK. Having 17 amps per rail means, thats the max available on that rail. Having 2 rails of 17 amps each doesnt mean theres 34 amps total because unless the 12v rail specifies the required watts, or in this case 12x34=408, it wont have 34 amps in total. On the 12v rail, you mulitply the watts available by the volts. Being that anything over 17 amps is too much on a 12v rail, it wont work, or burn up. In a typical read out, your 12 volt rails may only have 320 watts allocated to it on a 500 watt psu. Either rail can use 17 amps, no more, but both cant use 17 amps, or 34 total, cause, as I said, thats 408 watts, with only 320 watts available.
@ OP, that 28 amps should be ok, close to max, but it should work.