Mike - thanks so much for posting so clearly on a such a muddy topic. You do us a big favor.
I have a question about the "independent" 240VA protection for the 12V rails. I understand that the regulation is all done at one point, and that (except for a few supplies) all ATX units source the 12V from a single transformer, but I'm not real clear on the protection circuits for the individual 12V rails. I've been inside maybe half a dozen recent supplies from as many different manufacturers and have yet to see any form of protection on the "independent" 12V rails - no fuses, no PTC's, no FL's, nothing. Every supply had the grounds and the 12V lines brought to a single area of the main PCB and soldered together. On the modular units I looked at, some had separate "zones" in the distribution panel for the different 12V rails, but still no visible method of independent current limiting.
Have you seen an example of independent 20A/12V current limiting on recent ATX supplies? I'm wondering if ALL manufacturers didn't simply omit this part of the "recommended" spec.
You're welcome. I expect that not all points are covered by my piece, simply because different mfgs are taking varying approaches to the problem as they keep increasing the power capacity. I refer to the many >700W PSUs that are now being flogged. IMO, those PSUs (and their marketing depts) should be flogged. Oh, and let's not let the vidcard makers off the hook -- ATI and nVidia need to be lined up as well.
As to whether the 240VA current limiting is really there, yes, it is, in some but certainly not all PSUs. We've had a couple instances in the lab where we managed to exceed 20A on a 12V line and caused the PSU protection circuit to trip. Off the top of my head, I can't recall which make/model. It's certainly not a fuse, it's some kind of auto-reset device. Turning the PSU off, removing or lowering the load and then powering it back up clears the problem.
We've had many more instances where the total 12V load exceeded the total rated 12V current -- ie, generally at near maximum load -- and tripped the current limiter/protection. You can tell it's that because if the load is redistributed to that the 5 and 3.3V lines take more of it and the 12V load is reduced while the total power draw is kept the same, the protection does not trip.
In any case, going back to the safety issue, I doubt very much that you could exceed 240VA off a single wire set. The only ones you might be able to do it with are the AUX12V and PCIe power connectors. And even there, the odds are very low; 240W is a LOT
for any CPU or video card to draw for any length of time -- even in a momentary peak. I don't believe there are any CPUs or GPUs being made today that can pull that kind of power.
What about dual vidcards, you say, or dual CPUs? Well, the number of cable sets automatically doubles -- you need a 4x12V for dual CPU and twp 6-pin (or 8-pin) PCIe connectors, which are always one plug to a lead.