What is a dual 12V rail?

[bladephoenix]

Here are the specs from my old 350W Enermax:

3.3V = 32A
5V = 32A
12V = 26A

Here are the specs from my new 420W Enermax:

3.3V = 30A
5V = 30A
*12V1 = 18A
*12V2 = 18A

*There is a line underneath these two saying 29A (Presumably the total, but this doesn't make any sense because the total should be 18+18=36). I am confused about this.

Now, this would mean "upgrading" left me with 2 less A on the 3.3V rail & 2 less on the 5A rail. In return, I get 3A on the 12V rail more. Is this right?

However, I am still having some suspected power issues. Perhaps I am only using one 12V rail instead of 2? Does dual 12V rail mean 2 plugs for 12V? I do not have another 12V plug on my mobo though, so this is rather confusing.

Can some one help me out. Thanks.

 

[biostud]

probably that on their own the can handle a maximum load of 18A but, if you load both the maximum combined load is 29A.

read the 24/20 pin PSU sticked thread at top of General Hardware

 

[Navid]

My understanding is that a dual-rail PSU had two isolated 12V rails. Some of the PSU 12V connections are on one of those two rails and some are on the other.

As an example, you may be able to power up your graphics card with one rail while powering up your motherboard with the other.
I do not have a dual-rail PSU!
But, there should be more information about which connector comes from which rail in your users manual.

Each rail has a maximum rating. It tells you the maximum current that you can draw from that rail. Other voltage rails have maximum current ratings too.
But, often, you cannot draw the maximum current from all the rails at the same time (I am talking about drawing maximum current from 5V and 3.3V and 12V.

Having two 12V rails, it is important to know what the maximum current is that you can draw from both 12V rails. I think that is what the 29A figure is all about. It means that you cannot draw 18A from each 12V rail at the same time.

In other words, I suspect that if you are drawing 18A from one of the 12V rails, you can only draw 11A from the other. If you are drawing 17A from one, you will be able to draw 12A from the other ........

 

[otispunkmeyer]

i have an enermax PSU and i am assuming that the extra molex connector which is cabled on its own and braided is part of or is indeed the 2nd 12v rail. my 6800gt feeds off this, motherboard n stuff feeds off the other one

they are 18 amps each PEAK, the 29 is proabably either 29A combined peak, or 29A combined average, ie it can supply a consistant 29A with both rails combined

 

[bladephoenix]

I just finished reading the sticked thread. It seems rather odd that my PSU unit has both a PCI-Express power connector AND a detachable 4 pin plug on the 24 pin mobo connector.

If the extra 4 pins on the 24 pin connector are solely for the purpose of a PCI-Express card, then it is rather redundant that it includes an external connector for it.

What is the point of that?

anyways...OK...if I have this right, dual 12V rails are NOT actually seperate plugs. They are simply seperate rails. So 1 rail would send power to the CPU/Mobo and another rail would power any other device (Drives, etc)?

I am left to assume, then, the older ATX 1.3 PSUs powered the cpu and drives along the same 12V rail right? (Even though in ATX 1.3 there is a 12V plug specifically for the CPU) Is that why the Amps on my old PSU is 26a, while the new one is 2x18a? 18a is more than enough to handle most CPUs, so the remainer would be channeled into the other rail? I guess that would mean on ATX 1.3, the single 12V rail first powered the CPU and any power remaining would continue along the single 12V to power other devices?

I guess dual rails, in that case, provide a slight, however not really necessary, advantage by seperating the CPU and devices. I still don't think this is really that big of a deal though.

 

[Algere]

Originally posted by: Navid
In other words, I suspect that if you are drawing 18A from one of the 12V rails, you can only draw 11A from the other. If you are drawing 17A from one, you will be able to draw 12A from the other ........

QFT

 

[Algere]

Originally posted by: bladephoenix
I just finished reading the sticked thread. It seems rather odd that my PSU unit has both a PCI-Express power connector AND a detachable 4 pin plug on the 24 pin mobo connector.

If the extra 4 pins on the 24 pin connector are solely for the purpose of a PCI-Express card, then it is rather redundant that it includes an external connector for it.

What is the point of that?

Extra 4 pins on the 24-pin connector are for the 65W (5.5A of 12V) of the 75W PCIe 16X power requirement on the motherboard side. The external PCIe connector exist should a video card need more power than what a motherboard can provide.

Originally posted by: bladephoenix
anyways...OK...if I have this right, dual 12V rails are NOT actually seperate plugs. They are simply seperate rails. So 1 rail would send power to the CPU/Mobo and another rail would power any other device (Drives, etc)?

That's what Enermax CSR told me. In the manual their lower powered models have the CPU on one rail & motherboard, drives, etc. on the other.

Originally posted by: bladephoenix
I am left to assume, then, the older ATX 1.3 PSUs powered the cpu and drives along the same 12V rail right? (Even though in ATX 1.3 there is a 12V plug specifically for the CPU)

If they specifically followed ATX12V 1.3 specs, then yes. Enermax's ATX12V 1.3 PSUs had dual 12V rails (A.K.A. hybrid between ATX12V 1.3 & 2.0 specs).

Originally posted by: bladephoenix
18a is more than enough to handle most CPUs, so the remainer would be channeled into the other rail?

If one rail is dedicated to the CPU only, then any power on that rail not used by the CPU will remain unused.

Originally posted by: bladephoenix
I guess dual rails, in that case, provide a slight, however not really necessary, advantage by seperating the CPU and devices. I still don't think this is really that big of a deal though.

I won't go into it in depth but in general, dual rails are in place for increased stability, safety, power delivery, & efficiency under heavy load - last one is still up in the air IMO.

 

[Navid]

Originally posted by: Algere

Originally posted by: Navid
In other words, I suspect that if you are drawing 18A from one of the 12V rails, you can only draw 11A from the other. If you are drawing 17A from one, you will be able to draw 12A from the other ........

QFT

Qualified Funeral Trust?
Quantitative Feedback Theory?
Quantitative Fluorescence Technique?
Quantum Field Theory?
Quantum Fourier Transform?
Quest For Tech, Inc?
Quoted for Truth?

I hope it is the last one!

 

[Algere]

Aye, the last one.