Single 12V rail or multiple 12V rails? The eternal question answered

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Jan 8, 2014
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.

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.
Aug 20, 2016

>> "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.



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