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basic GPU question - Page 2

post #11 of 18
Quote:
Originally Posted by shilka View Post

Sory i dont understand the question

what is the REAL benefit of a single 12v rail, and how do higher capacity psu's with multiple 12v rails get around the old issues with older multiple rail psu's?

I reemmber there were issues with having heavily tilted loads or something of the sort.
post #12 of 18
Quote:
Originally Posted by Xinoxide View Post

what is the REAL benefit of a single 12v rail, and how do higher capacity psu's with multiple 12v rails get around the old issues with older multiple rail psu's?

I reemmber there were issues with having heavily tilted loads or something of the sort.

That is something you should ask phaedrus2129 as he knows far more then i do about that

http://www.overclock.net/u/109663/phaedrus2129
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post #13 of 18
In terms of the actual power generation, 99% of computer PSUs are single rail. That is, they have a single transformer with a single +12V output that is rectified by one set of diodes/mosfets, which all goes to the same wires. As far as the bulk of the electronics, single rail and multi rail are identical.

The difference is in the Over Current Protection implementation. OCP is a protection which shuts down the power supply when the current drawn exceeds a limit set by the design engineer. This is a safety feature, to prevent the PSU being overloaded, or to shut the PSU down in case of a short circuit.

OCP works by a current shunt resistor of a very low value, which the +12V current flows through. This resistor translates that current value into a voltage value, which is read by the OCP pin on the PSU's protections chip. If the voltage exceeds a certain value, this indicates that too much current is being drawn, so the protections chip sends a shut down signal to the primary controller, which shuts down the power supply.


So what's the difference between single rail and multi rail? Single rail uses a single shunt resistor through which all current is monitored. This means that the entire PSU has one maximum value for the +12V rail, which is usually set 10-25% higher than the rail's rated value. This is simple and cheap. But it has a downside: with high wattage PSUs, 1000W and up, the rating of the +12V rail may be 80A, 100A, even 120A. And OCP would be set at 90A, 112A, or even 120-140A! This is an extreme amount of current. Even a dead short circuit to ground may not have low enough impedance to draw >100A of current. Thus if you have a short circuit on your motherboard or graphics card, the power supply will happily deliver almost its whole rated current through the short, which can set the circuit board on fire and melt the PSU cable. I've seen this happen several times, when a motherboard VRM failed. What could have just been a dead motherboard turned into a dead mobo, CPU, graphics card, and power supply, because the PSU only had single rail OCP.

Multi rail OCP does things a little differently. Instead of having one shunt resistor and protections circuit, the multi rail PSU will divide the +12V output current into 2, 4, or 8 channels or "rails", each with its own shunt resistor, and a protections chip with the appropriate number of OCP circuits. On a high wattage PSU these individual rails will only be rated for 25, 30, 40, or 50A, instead of >100A. This is more expensive and complicated than a single rail design, but it is much more resistant to the short circuit fires described above. It does have its downsides, however. When multi-rail first started to be implemented, designers would put the CPU on its own 20A rail, then put every other device in the computer on an 18-22A rail. If used with a high end graphics card, users would overload the second rail and cause the PSU to shut down, which gave multi-rail units a reputation for unreliability, even though the units were technically functionally perfectly. Modern implementations have made issues like this rare by dividing the cables between rails carefully to ensure GPUs and CPUs can draw adequate power without tripping the OCP on their rail. But with extreme overclocked multi-GPU setups you can still accidentally trip OCP, causing the system to shut down.

TLDR: single rail is vulnerable to certain types of short circuits, which can cause fires. But this is rare (one in several hundred thousand). Multi rail protects you from this, but can cause compatibility issues when running extreme overclocked tri/quad GPU systems.
post #14 of 18
Thread Starter 
Damn, I actually understood all that.

Thanks.
post #15 of 18
If you want my opinion, for 750W and under it makes no difference. Personally, I currently use a quad rail 750W.

850-1000W is marginal; multi rail is a little safer, but it's still not that important.

1200W+: I definitely prefer multi rail, especially for an average 2-3 GPU gaming system. Unless I'm doing such a large, powerful, and complex system (minimum 4 GPUs) that I can't guarantee I won't trip the OCP.
post #16 of 18
I like the way this is put, really helps understanding why rails are "seperated". Thanks Phaedrus.
post #17 of 18
Sory for going off topic but i wanted to ask you Phaedrus what is the word on the V1200?

You can send me a PM if thats better

Sory for going off topic
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LG CH12NS30 x5 Noctua NF-A14 FLX Noctua NH-D15S Windows 7 64 Bit 
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post #18 of 18
V1200 is almost here. We had to push the launch date a bit due to production line issues, but it won't be much longer.
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