PSU 12V rail question

• Call4ljw

  17 Dec 10, 16:39

  

  Is the above picture a 12V rail from the PSU?

  My PSU comes with 2 12V output, and i heard that using only 1 12V increases the efficiency. I'm planning on get the new sandy bridge processor and a basic motherboard, which will bring the power consumption to be below 200W, far lower than what a single 12V rail can provide.

  So i'm thinking if i can use only 1 12V output instead? and if yes, which one is more recommended to use?(left or right)

   

  Thanks!

   

  Edit:

  I saw on another website that 12V output actually refers to the one on the mainboard(true?). By mainboard i refer to http://en.wikipedia.org/wiki/Power_supply_rail

  So do i actually need to use the 12V to the CPU?(picture at the top)

• Call4ljw

  20 Dec 10, 14:23

  bump.

• SBS7484P

  24 Dec 10, 10:46

  This is not a rail. This is a 4-pin CPU power connector.

  What you heard is false.

  A rail is a split 12V sub-line with an OCP feature on it.

  Take a look at this example.

  A PSU with 4x 12V rails have the following limits of 22A, 22A, 36A and 36A on the 4 lines respectively.

  What does this mean? It shows that the 12V group is split into 4 sub channels that can handle this amount of current each, without tripping the OCP feature on either line, and shutting down the PSU.

  There are no proven advantages with regards to efficiency and/or performance by choosing to use only one 12V rail instead of using.

   

  Furthermore, modern PSU's with two 12V rails are probably low-wattage PSU's with no 6-pin PCI-e support, and will group the two 12V rails accordingly; the first rail for 12V CPU (or EPS 8-pin CPU power) and the remaining rail for S-ATA devices and Molex devices.

  PSU's with four rails like my abovementioned example have them grouped as such:
  1st rail: CPU
  2nd rail: Peripherals (S-ATA, Molex)
  3rd rail: PCI-e 6-pin/8-pin/6+2-pin #1
  4th rail: PCI-e 6-pin/8-pin/6+2-pin #2

• SBS7484P

  24 Dec 10, 10:48

  In some cases, the PSU may actually have only one rail, and the manufacturer decides to place a label on the box that suggests the PSU has multiple rails.

  There is nothing really wrong with using a PSU with a large, single 12V rail.

  
  The only problem here is, you could be overloading your Molex connections to the point that they catch on fire, and the PSU will still keep going.

   

   

• troublemaker2005

  29 Dec 10, 00:06

  from what i know efficiency or rather how much power can the 12V rail draw depends on the current rating in amps. of course for example a 10 amp rating at the 12V rail is better than a 5amp rating.

  also if you have mutiple connections try to use power line from different branches. avoid loading power connections to your other iternal PC hardware from a single line.

   

  the connector shown in the picture looks like one to power graphics card, or similar to the additional power socket to the processor

• SBS7484P

  29 Dec 10, 19:55

  It is true that using power from different branches on a multi-rail PSU is better so as to prevent tripping the OCP on each rail.

   

  But in most modern PSU's, there are 4 rails distributed for Molex/SATA, CPU, GPU0 and GPU1.
  
  That basically means all your peripherals, and things like fan controllers, CCFL's, water pumps, etc. are running on one rail.

  This provided you have a higher end PSU that splits the peripheral (Molex/SATA) group into two different rails.

  In view that modern GPUs have large power demands, therefore each 6-pin/8-pin PCIe connector is allocated its own rail.