Before I ask my question, yes, I've read the stickies about power supplies, especially this one, but unfortunately, I still don't entirely understand. PSUs are the one part of things under the hood I just don't quite get to this day.
The video card I'm considering buying from a coworker (an EVGA GTX 560 Ti, presumably not the o/c version) requires two PCI-E power cables (which I have from the PSU), and the specs on it say it needs a 500W PSU with 30A on the 12V rail.
I've got an Antec EA 750W, with four 12V rails at 25A each. Antec's manual for the EA-750 makes it sound like the 12V rails really are split: "The EA-750 PSU uses four separate +12-volt power rails. Different connectors are hooked up to separate circuits to aid in the balanced distribution of power between devices in your computer. The engineers have allocated the rails to different connectors to prevent voltage sags in one device due to sudden demands for power in another device."
Sooo, I guess that means they're really split, and that an individual rail would need to put out 30A to do any good for the GTX560 Ti?
When I bought this PSU I bought it especially because I was running a GTS 450 that required "22A on the 12V rail". Just to be safe, I bought one that had over that on a single rail, thinking the rails were truly separate. (And that's part one of what I truly don't understand, there's talk on this thread about them not truly being separate on some PSUs and truly separate on other PSUs.)
Second part of the question: If it takes two PCI-E power cables to run the card, does each put out 25A, totalling 50A?
Again, my apologies for asking, I'm sure you guys get tons of questions about this sort of thing from klutzes like me, but I did honestly read through things. I just felt like I was reading something in Martian.
So all of your issues have come from confusion about what the power recommendations for graphics cards actually mean. When it says you should have 30A on the 12V rail it means you should have that much for the entire system, the CPU will consume 6-12A depending on the CPU, and a graphics card can consume up to 25A(300W) but this is spread across a variety of connectors, 75W from the motherboard and up to 225W from PCI-e connectors, the first 75W will be coming from a different rail than the 225W generally.
As for the EA-750, it actually has two separate 12V sources surprisingly enough, just stumbled across that neat bit in its JG tear down, so it is one of the few units with multiple 12V sources. The only one I had know of off hand prior to that was the corsair HX-1000 which is actually 2 500W PSUs next to each other in a single box as you can see below. http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story4&reid=163
As for their note about the engineers having allocated the rails to different connectors, they are right they did. Your motherboard and peripheral connectors are on 12V1, your CPU is on 12V2, and your PCI-e cables are on 12V3 and 12V4, this spreads the load across all the rails fairly evenly, and puts a maximum load on rails 3 and 4 of 225W or 18.75A, still well below the 25A limit so they did their job properly. You won't overload any single rail on that PSU unless you use adapters and intentionally try to.
Many people are confused by whether a single 12V rail or a multiple 12V rail power supply is better. The confusion is justified as there are myths about regarding both floating around on the internet as well as people who swear by multi rail and others who swear by single rail. There are claims that one way supplies cleaner or more stable voltage than the other, but this has a lot more to do with the platform and line filtering installed than the rail setup. The answer to the question of which is best a single rail unit or a multi rail unit is that it does not make a difference, so lets explore why.
In older versions of the ATX specs Intel had required the CPU to have its own rail separate from the motherboard and other components. It had also specified that no rail have more than 240 VA available(20A @ 12V), this is what lead to multi rail power supplies in the beginning. Both of those requirements have now been lifted allowing for power supplies with a single 12V rail and 12V rails rated for more than 20A which is very useful.
All power supplies convert the AC(alternating current) power that comes out of the wall into DC(direct current), and most units only have a single 12V DC source, in a single rail unit all of the yellow 12V wires are tied directly to this source, in a multi rail unit they are split into bunches(rails) and tied to the source, there is a controller chip monitoring the current going into each rail and if it exceeds a set limit it will shut down the power supply. Some smaller cheaper units claim to have multiple 12V rails to comply with the old ATX specs, but actually just tie all the bunches together with no over current protection so while the label makes it appear as a multi rail unit its actually a single rail unit, this helps cut costs.
One of the big myths about multi rail units is that because of the overcurrent protection you can end up with “trapped power”, XFX uses a power supply with a 200W 12V DC source as an example:
In a multi rail unit they show the 12V source split into two 100W rails, and when loaded up with 75W on each rail from the graphics card adding 50W for the CPU onto either of the rails would cause it to shut down even though it had 50W remaining for the source and a single 200W 12V rail would have been able to handle it all fine.
The lie in this scenario is that multi rail units have their over current protection(OCP) limits set such that you wont end up with trapped power, in this scenario the unit would likely have its OCP limit set at about 130W on each rail so that there would not be an issue of “trapped power”, this is why for most units with multiple 12V rails you cannot simply add the rails together to figure out the total power available from the 12V source, you must look at the total power listed for the 12V rails listed below them. On the units where the sum of the listed max currents for the 12V rails does add up to the stated limit of the 12V source the true current limits are usually set 3-5A(36-60W) higher than the label shows. With a modern power supply you are unlikely to overload any rail without using a large number of adapters and splitters to get more connectors, the manufacturer already spread the connectors across the rails so that it would be very unlikely for you to overload any single rail without trying pretty hard.
In short, its the total capacity of the 12V source that matters, how the rails are laid out does not affect you much.
>> "the unit would likely have its OCP limit set at about 130W" >> Current is measured in Amps not Watts.
A regulated PSU controls one load in a servo loop by monitoring current (amps) not power dissipation (watts)
Connecting more than one load results in the loads fighting each other within the loop.
If each derived supply rail (from 12V) has it's own servo loop and it's own current limiter (which should be before the servo loop
so that the other loads don't get starved) then it can be made to work.
It's hard to beat the beauty of a single control loop because you can design and specify with precision.