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Why more than one 12V rail?

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May 12, 2007 12:20:04 PM

I'm pretty new to PSU's and I've asked for what PSU to use with my graphics card all over the internet and I've found a PSU to use. But my question is now why do most PSU's have 2 or 3 12V rails while others only have one? I've seen PSU's on sale with higher amperage on a single 12V rail than on PSU's with 2 12V rails, so what's the whole point about it?

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a b U Graphics card
May 12, 2007 1:34:01 PM

Quote:
I'm pretty new to PSU's and I've asked for what PSU to use with my graphics card all over the internet and I've found a PSU to use. But my question is now why do most PSU's have 2 or 3 12V rails while others only have one? I've seen PSU's on sale with higher amperage on a single 12V rail than on PSU's with 2 12V rails, so what's the whole point about it?


Quote:
I've seen PSU's on sale with higher amperage on a single 12V rail than on PSU's with 2 12V rails, so what's the whole point about it?


With one 12v rail the single rail would have to supply 12v power to the entire computer including the processor. With dual or multiple 12v rails, one 12v rail could be dedicated to the CPU while the remaining multiple rails could supply power to the rest of the system's 12v needs.
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July 25, 2007 1:47:33 AM

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With one 12v rail the single rail would have to supply 12v power to the entire computer including the processor. With dual or multiple 12v rails, one 12v rail could be dedicated to the CPU while the remaining multiple rails could supply power to the rest of the system's 12v needs.


im getting Corsair 620 instead of antec truepower trio 650 as most of you guys say its better and i visited a few sites saying IT is Better. my question is, do i have to assign w/c 12v goes to?ie processor/gfx card?something like that or it does its thing on its own...nothing to worry?sorry for the newbie question... :cry:  :cry:  :cry: 
July 25, 2007 2:32:38 AM

I think most of them have 'dynamic' distribution of power, it's more of a safety situation in splitting to rails. It's like if you try to push more water through a pipe than it can take, it will snap.

I'm no expert, but what I have deferred is that you use the Volts * Amps = Watts formula. If a 12v rail has 20A then that's 12 * 20 = 240 watts on that single rail. Then each component uses so many watts itself. Like a dvd burner might use 16 watts and the CPU use 95 watts, etc. So, you can basically look at how the rails are physically designed and determine what you'll put on which. But then there's capacitor aging and loads to factor in, so you should get a power supply that exceeds your needs to compensate for this.

Someone please correct me if I'm wrong.
July 25, 2007 8:07:16 AM

LordOfTheFlies said:
But my question is now why do most PSU's have 2 or 3 12V rails while others only have one? I've seen PSU's on sale with higher amperage on a single 12V rail than on PSU's with 2 12V rails, so what's the whole point about it?
Ok, here is the information you are looking for. Originally the regulating bodies, most prominently UL, decided that there should be a maximum on the amount of current that could be delivered on the 12 volt rail and limited it to 240VA (20A). This was to minimize any concerns about the computer catching on fire do to a failure causing too much current flow and hence heat.
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3.2.4. Power Limit / Hazardous Energy Levels
Under normal or overload conditions, no output shall continuously provide more than 240VA under any conditions of load including output short circuit, per the requirement of UL1950/CSA 950 / EN 60950/IEC 950.
These rails are generally only separated by over current protection and do not have separate voltage regulators for each. Therefore, they are really like having one rail in all respects other than the maximum current available on each separate output (rail).

The additional loads put on power supplies with more power hungry components gave rise to the increased popularity of single rail power supplies in desktops. This was in order to avoid the concerns about load distribution across the different rails. These power supplies don't conform to the strict safety specifications, but apparently they are not required to. Here is a quote from PC Power & Cooling's myth page on their website. I tend to agree with it, however if you don't actually need the wattage on a single rail, you might be satisfied with multiple rails and lower any risk of fire, as unlikely as it may be.
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Power Supply Myths Exposed!
8. ARE MULTIPLE 12-VOLT RAILS BETTER THAN A SINGLE 12-VOLT RAIL?
With all the hype about multiple 12-volt rails (ads claim that two rails is better than one, five is better than four, etc.), you%u2019d think it was a better design. Unfortunately, it%u2019s not!

Here are the facts: A large, single 12-volt rail (without a 240VA limit) can transfer 100% of the 12-volt output from the PSU to the computer, while a multi-rail 12-volt design has distribution losses of up to 30% of the power supply%u2019s rating. Those losses occur because power literally gets %u201Ctrapped%u201D on under-utilized rails. For example, if the 12-volt rail that powers the CPU is rated for 17 amps and the CPU only uses 7A, the remaining 10A is unusable, since it is isolated from the rest of the system.

Since the maximum current from any one 12-volt rail of a multiple-rail PSU is limited to 20 amps (240VA / 12 volts = 20 amps), PCs with high-performance components that draw over 20 amps from the same rail are subject to over-current shutdowns. With power requirements for multiple processors and graphics cards continuing to grow, the multiple-rail design, with its 240VA limit per rail, is basically obsolete.

PC Power and Cooling is once again leading the industry. All of our power supplies now feature a large, single 12-volt rail. The design is favored by major processor and graphics companies, complies with EPS12V specs (the 240VA limit is not a requirement) and is approved by all major safety agencies such as UL and TUV.

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