Multi rail PSU's

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fsta

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Jan 18, 2011
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Hi everyone,

i m about to buy a HD 6850 and therfore need to change my PSU as well. My current PSU is a 450W but only 16 AMps on the 12 V rail and 5 years old. I m seeking for a PSU that has more than 30 Amps on the 12V rail ,however, most PSU's have multiple rails. So in order to find the total Amps do i sum the Amps on the various rails?? For example a PSU that has two 18 Amps rails has a total of 36 Amps ???Is that how it goes???

Also i m going to overclock my HD6850 just alittle at some point. How much extra power am i going to need???

Thanks
 
Solution
jaguarskx
No, you do not sum up the amps because the amps stated on the back of the PSU are peak amps, not constant amps. Additionally, all lines cannot peak at the same time because all the 12v rail can only deliver a certain maximum power.

Here's the sticker for the OCZ StealthXstream II 600W. Each rail can provide up to 18a peak, if you were to simply add it all up it would be 72a which works out to 864w on the 12v rails alone for a PSU that is rated at 600w. There is a row underneath which states the maximum combined power on the 12v rails is 600w. But that's assuming the other rails (3.3v and 5v) do not draw any power. Therefore, in actuality the maximum available power on the 12v rails will be less than 600w...
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4745454b

4745454b

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Apr 29, 2006
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No. Each rail might be able to handle 18A by itself, but the thing that feeds both (or more) of the rails has a smaller limit. What you need to do is look on the label for the statement that looks like "12v1 + 12v2 = 408W". Take the watts figure and divide by 12 (volts) to get the Amps available. (34A in this case.)

Hopefully this helps.
 
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jaguarskx

jaguarskx

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Apr 19, 2006
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No, you do not sum up the amps because the amps stated on the back of the PSU are peak amps, not constant amps. Additionally, all lines cannot peak at the same time because all the 12v rail can only deliver a certain maximum power.

Here's the sticker for the OCZ StealthXstream II 600W. Each rail can provide up to 18a peak, if you were to simply add it all up it would be 72a which works out to 864w on the 12v rails alone for a PSU that is rated at 600w. There is a row underneath which states the maximum combined power on the 12v rails is 600w. But that's assuming the other rails (3.3v and 5v) do not draw any power. Therefore, in actuality the maximum available power on the 12v rails will be less than 600w.

sxs2_allcharts.jpg



This PSU should be enough for your PC, but you need to list the other components in your PC. You can purchase this PSU for $54 after rebate:

http://www.newegg.com/Product/Product.aspx?Item=N82E16817341039

 
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F

fsta

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I cannot answer your question asheesh but i can't see why not. So in the example of the OCZ that jaguarskx brought up it would be at around: (600-140-15-9.6)/12=36 A if i m getting it right....
 
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RetiredChief

RetiredChief

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Feb 22, 2007
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You can use two PSU in a single computer - one cavet you must insure that no psu rail from one psu get tied to (shorted to the rail of the second PSU. Example you use a 6 pin pci-e connector to a graphics card that has dual 6 pin connectors. If you plug psu 1 pci-e connector in connector 1 and you use PSU 2 pci-e connector to plug in to the 2nd connector and if the two 12 volt lines are shorted to gether on the GPU you will have shorted the +12 volt of psu1 to +12 of PSU 2, a NO-NO. However you could use PSU1 to power the GPU and PSU 2 to power the HDDs and the CD/DVD drives.

Much better and cheaper just to get a single PSU capible of handling total current to the computer.
 
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asheesh1_2000

asheesh1_2000

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right.. I was just thinking on the redundancy lines.

 
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R

RazberyBandit

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Nope, not quite right. What he was pointing out is that you can't just add the max for each rail like this: 18A + 18A + 18A + 18A = 72A. Below their amperage ratings we see a maximum combined output of 600W. Using the Wattage = Voltage * Amperage formula, the maximum Amperage output of the 12V rails of that PSU is: 600W / 12V = 50A. However, since the 3.3V and 5V rails will also have some load on them, and the maximum output of the ENTIRE unit is 600W, the maximum output of the 12V rails is actually less, but will vary according to the 3.3V and 5V loads.

The real kicker with multi-rail PSU's is how it's distributed. On some four-12V-rail PSU's, 12V1 will connect to the main motherboard power cable, while 12V2 powers the ATX12V and EPS12V cables. 12V3 connects to all the SATA and Molex connectors, leaving 12V4 to power the PCIe cable. But, there's no actual standard, so each PSU is different. Here's some links showing so:
http://www.hardwaresecrets.com/article/OCZ-StealthXStream-2-600-W-Power-Supply-Review/1058/6
http://www.hardwaresecrets.com/article/Antec-TruePower-New-750-W-TP-750-Power-Supply-Review/766/6
As you can see, 12V power distribution can really vary on different PSUs that use multiple rails.

Now, what's the advantage to having multiple rails then if you have to figure out which rail is actually powering different components so you don't accidentally overload one of them? The answer's simple: safety - multi-rail have over current protection, but single-rail PSUs do not. Over current protection (OCP) will shutdown a PSU in the event of a short, whereas single-rail units will simply continue to pour power into the short, feeding it until things start to melt and/or catch fire. Each rail of a multi-rail PSU has it's own OCP circuit so that it functions within a pre-defined limit. That is why you'll see each 12V rail have an individual amperage rating, and why they can sometimes vary within a single unit.

For more info, here are some external sources:
http://www.antec.com/Believe_it/PSU/Myth3.php (may want to start with Myth 1)
http://www.jonnyguru.com/forums/showthread.php?t=3990
 
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4745454b

4745454b

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I'd like to try to tackle the myth of "trapped power" which might be the final thing to mention. Some people believe that if 12v1 goes to the CPU and your CPU only needs 8A, that other 10A is "trapped" and not available for use elsewhere in the system. Remember that you have your single 12V source that's powering those 4 rails. If the CPU on 12v1 only wants 8A, that's all that's going to 12v1. Furthermore, if the 12v2 is powering the mobo and it only wants 3A, that's all its getting. Same with the other rails. There is no such thing as trapped power as each rail will only supply what the components ask for. And because all the rails are really fed by the same source it will be able to spread the power around as it needs to.

The only way to really have trapped power is if you had truly separate rails and circuits. If each of those 4 rails had its on power generating circuitry and they weren't tied together then you could have trapped power. There are or were a few 1kW+ PSUs that had separate rails, but nearly all on the market today don't.
 
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