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The truth about multi-rail PSUs

post #1 of 19
Thread Starter 
http://www.xbitlabs.com/articles/oth.../atx-psu5.html
Quote:
The ATX12V 1.3 standard recommends a max current of 18A for the +12V rail and this is where a problem occurred. It was about safety regulations rather than about increasing the current load further. According to the EN-60950 standard, the maximum output power on user-accessible connectors must not exceed 240VA. It is thought that higher output power may with a higher probability lead to various disasters like inflammation in case of a short circuit or hardware failure. Obviously, this output power is achieved on the +12V rail at a current of 20A while the PSU connectors are surely user-accessible.

So, when it became necessary to push the allowable current bar higher on the +12V rail, Intel Corporation, the developer of the ATX12V standard, decided to divide that power rail into multiple ones, with a current of 18A on each, the 2A difference being left as a small reserve. Purely out of safety considerations, there was no other reason for that solution. It means that the power supply does not necessarily have to have more than one +12V power rail. It is only required that an attempt to put a load higher than 18A on any of its 12V connectors would trigger off the overcurrent protection. That’s all. This simplest way to implement this is to install a few shunts into the PSU, each of which is responsible for a group of connectors. If there’s a current of over 18A on a shunt, the protection wakes up. As a result, the output power of none of the 12V connectors can exceed 18A*12V=216VA, but the combined power on the different 12V connectors can be higher than that number.

That’s why there are virtually no power supplies existing with two, three or four +12V power rails. Why should the engineer pack additional components into the already overcrowded PSU case when he can do with just a couple of shunts and a simple chip that will be controlling the voltage in them (the resistance of a shunt being a known value, the current passing through the shunt can be known if you know the voltage).

But the marketing folk just couldn’t pass by such an opportunity and now you can read on any PSU box that dual +12V output circuits help increase power and stability, the more so if there are not two but three such lines!

You think they stopped at that? Not at all. The latest trend is power supplies that have and don’t have the splitting of the +12V rail at the same time. How? It’s simple. If the current on any of the +12V output lines exceeds the 18A threshold, the overcurrent protection becomes disabled. As a result, they can still embellish the box with the magical text, “Triple 12V Rails for Unprecedented Power and Stabilityâ€, but can also add there some nonsense that the three rails are united into one when necessary. I call this nonsense because, as I have written above, there have never been separate +12V power rails. It’s impossible to comprehend the depth of that “new technology†from a technical standpoint. In fact, they try to present the lack of one technology as another technology.
Interesting read. Seems multiple 12V rails really arent what they seem.
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post #2 of 19
good read
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post #3 of 19
My forton is Dual 12v is that bad now?. Chould it overload my system,
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post #4 of 19
That's kinda what I thought but I never was quite sure, thanks for finally clearing that up.

Unknownm, the article pretty much says that "Dual 12v rails" are a myth, just marketing mumbo-jumbo basically you just have a single, normal 12v rail but it's "split up" in that there can not be one source using any more than 18A at a time, don't worry, that's how it's supposed to be.
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post #5 of 19
Wow, this will clear up a lot of discussions. Rep +
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post #6 of 19
Single Rail PSU ftw!
    
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post #7 of 19
I hated multiple rails. I had an Antec and didn't like it.

Switched to OCZ PowerStream 520W and everything is great but next I had to have the Bentley of PSUs.......PC Power & Cooling FTW.

Hard to recommend anything else these days.
post #8 of 19
Chozart explained this to us long ago

P & C Silencer 750W = +3.3V@24A,+5V@30A,+12V@60A,-12V@0.8A,+5VSB@3A

I listened to him Even though he might think I didn't.

R
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post #9 of 19
I am honored Ropey
    
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post #10 of 19
Quote:
Originally Posted by Ropey View Post
P & C Silencer 750W = +3.3V@24A,+5V@30A,+12V@60A,-12V@0.8A,+5VSB@3AR
And how do you calculate it if there are multiple 12V rails?
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