# Thread: Single vs. Multiple +12V rails: The splitting of the +12V rail

1. 550W Diet Mt. Dew Drinker!
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Originally Posted by jonnyGURU
I might sound like I'm coming off half cocked, but if treshix posted that as his own "this is what I believe, what do you guys think?" post as opposed to posting as a reply to an FAQ that completely thwarts every aspect of his understanding, I wouldn't get so pissed off.
You know, I think his post was a really stupid post.

As the party capable of being offended, I hereby issue this notice officially:

You didn't go off half-cocked. If it was submitted to a FAQ I wrote, I would have responded much the same way.

2. Now, where are we going to get the information on this one?
Max 3.3V and 5V is limited to 250W - or ~21A. Yet, both rails are individually limited to a maximum of 40A. How interesting - You can see how easily this could be misleading, no?

Now, if max sustained is 250W, do we really expect the PSU to be able to handle a short peak load of 960W on the 3.3V and 5V rails? If it cannot, in fact, deliver 960W, or even 480W on either the 3.3V or the 5V rail, then even having them limited at 40A is almost completely useless, is it not? So why are we doing so? My assumption is that it's labelled as such because many people simply go "ooh, big number good!".

But perhaps I am wrong?
I stopped reading your post here. If you do not understand the relationship between current(A), voltage(V) and power(W), it will be difficult to understand the labels on a PSU(The statement "Max 3.3V and 5V is limited to 250W - or ~21A." is what made me stop reading).
40A @ 5V = 200W
40A @ 3.3V = 132W
21A @ 11.9V = 250W

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Originally Posted by sdbardwick
I stopped reading your post here. If you do not understand the relationship between current(A), voltage(V) and power(W), it will be difficult to understand the labels on a PSU(The statement "Max 3.3V and 5V is limited to 250W - or ~21A." is what made me stop reading).
40A @ 5V = 200W
40A @ 3.3V = 132W
21A @ 11.9V = 250W

Okay. Now I'm starting to get angry myself.

I'm sure you all feel that I've proven that I know nothing. However, I continue to believe that what I've proven is that I'm quite human, and I am prone to overstress, mistakes, and making stupid comments - Just like anybody else.

Fallacious information snipped from offending post.
Last edited by treshix; 04-01-2008 at 07:34 AM. Reason: Edited to remove excessive angst.

4. One thing that did confuse with regarding your (both of you) statements about the HX620W, is that you say it's triple rail. I thought the HX series were multi rail on paper and single rail in practice?

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Originally Posted by Smirnoff
One thing that did confuse with regarding your (both of you) statements about the HX620W, is that you say it's triple rail. I thought the HX series were multi rail on paper and single rail in practice?
They are single rail in practice.

However, that wasn't related to the initial point I was trying to make. There was a rather long, disjointed, rambling, and quite frankly idiotic post I had made yesterday that used that particular model as a reference.

It was so bad I just deleted it and used Jonny's reply as a challenge to try and make the point I had originally attempted to make, badly.

Sorry.

6. In many cases the label on the psu's is a "paper" product, The label show the official UL specification. The reality can be different.

If we look on PSU's where the total output on +12V is not specified, in most cases you can find the combined output on the website, in the manual or in a specification document.

Where the combined power is the same as the sum of A on the rails on +12V, we must remember that the OCP's on the rails are set to shut off the psu at a higher amperage than specified on the label.

7. On review, my posting came across snottier than intended. Sorry.

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Originally Posted by Stefan555
In many cases the label on the psu's is a "paper" product, The label show the official UL specification. The reality can be different.

If we look on PSU's where the total output on +12V is not specified, in most cases you can find the combined output on the website, in the manual or in a specification document.

Where the combined power is the same as the sum of A on the rails on +12V, we must remember that the OCP's on the rails are set to shut off the psu at a higher amperage than specified on the label.
Ah. So it may very well be UL certified for performance characteristics very different then actual capability? If I'm understanding you right, I didn't know that.

As for the OCP on the rails being set to shut off the PSU at a higher amperage then what is specified on the labels, then that implies that the amperage limits shown for the rails on the labels are not always actually the limits, which explains why my understanding of this is confused.

Thanks for the information - that helps

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Originally Posted by sdbardwick
On review, my posting came across snottier than intended. Sorry.
Apology accepted. I think that happens to all of us when dealing in a medium that does not allow verbal intonation to pass. I know it's happened to me before.

10. treshix, there's no need for you to apologize to me...but thanks anyway. And I apologize if anything that I posted implied that you are stupid because you obviously aren't stupid at all. You're just making things more complicated then they need to be...we all do that sometimes.

You're correct in that when you add up the individual 12V rating on some units you will get a number equal to the combined rating (although this is the exception rather than the norm). And they aren't under any real obligation to put the actual OCP limits there...some do seem to do nothing more than split up the combined rating between the number of rails (HEC/Compucase units come to mind). But normally these rating numbers are referring to different aspects of the total system. The combined rating is somewhat like how many gallons per minute a water pump can deliver. The individual rating is like the size of the various pipes connected to that pump and how much each of them can deliver due to their size limitations.

So I think you're still missing something when you say things like "fully utilizing all of the rails" or "bleeding rails dry". The fact that the water pump in our analogy can't pump out enough water to max out the flow capacity of all of the pipes on it all at once isn't a bad thing. This could be considered the same kind of "future proofing" that is often touted as a bonus for a single rail system. It just gives you the ability to connect more devices or higher power consuming devices without the PSU reaching it's limits and shutting off.

Another thing that you're confused about is the association between the 12V rails and the 3.3V and 5V combined rail. The basic SMPS design can't deliver the full rated amount on all three of these rails all at once...even if it's a single rail unit. If the combined wattage for all three of these rails is say 800 watts and the 12V rail or rails have a 64A rating then assuming that we actually have a 64A load on the 12V consuming devices in the system (768W) then the maximum amount we can put on the 3.3V and 5V combined is only 32W even if those two rails are rated for 150W. On the other hand if we have a 150W on those two rails then the most we can put on the 12V is 650W. That's just the nature of the design.

So anyway, hopefully that clears up a couple of things for you. By the way, I find it interesting that you like the Gigabyte label because those confuse me. But I understand that it's because of the dual transformer design that is almost like two complete power supplies in one.

I have to say that you might be better off if you just ignore the individual 12V rail ratings completely except in regard to ensuring that the devices installed on the connectors that are wired into an individual rail don't exceed the OCP limit.

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