Dual 12V rail power supplies

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bgc99

Senior member
Aug 13, 2004
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Is one rail for the 24pin mobo connector and the other for all the drives/video card power connectors or what?

BGC
 
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KBTuning

Senior member
Mar 22, 2005
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its so you can devide up the load.... like if you have 2 hdd and 2 video cards.... put one on each rail and split up everything like that.... it also allows for more things that pull on the 12v rail... like i have a 420watt Thermaltake PSU that only has a crappy 19a rail and when i go into games i can here the fans spin down because im running 2 160GB 7200RPM SATA Maxtor drives, a x800GTO2 at X850XTPE and beyond, and a Winchester 3000+ that i have sitting at 2.25Ghz.... if i had a dual rail psu i wouldnt have that problem because i would have each HDD on a rail and the like.....
 
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boyRacer

Lifer
Oct 1, 2001
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Originally posted by: KBTuning
its so you can devide up the load.... like if you have 2 hdd and 2 video cards.... put one on each rail and split up everything like that.... it also allows for more things that pull on the 12v rail... like i have a 420watt Thermaltake PSU that only has a crappy 19a rail and when i go into games i can here the fans spin down because im running 2 160GB 7200RPM SATA Maxtor drives, a x800GTO2 at X850XTPE and beyond, and a Winchester 3000+ that i have sitting at 2.25Ghz.... if i had a dual rail psu i wouldnt have that problem because i would have each HDD on a rail and the like.....
Click to expand...

No
 
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GalvanizedYankee

Diamond Member
Oct 27, 2003
6,986
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Dual 12V lines was a spec Intel demanded. They have backed away from it in silence.

At http://www.formfactors.org/ The fifth link down, in the 45+page pdf it is explained,
to a degree.

Generally the 12V2 serves the processor and the 12V1 serves the VGA. However, some
PSUs, like the EnerMax Liberty, have 22A on both lines but total output will never exceed
32A. So with this unit it would make no differance which line served the proc. This would
be determined by the way the Molex connectors are set-up anyway.

I have read that some folks are not happy with the limits of dual rails because one side
can starve for the juice. So they just combine both rails at the PSU for sigle rail operation.
This has been done with the lower wattage PSUs. I don't reccomend it, I'm just reporting ;)

My 400 watt single rail Zippy will supply 12V@30A@40C...I'm content with that :D


...Galvanized
 
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Painman

Diamond Member
Feb 27, 2000
3,728
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A *good* single rail PSU won't suffer voltage dips when there'a a load on the 12v line. I've got an Enermax 550W server PSU with 36A per rail. Running an OCed A64 3700 and an X1900XT both @ load, I get a voltage drop of .06 on the 12V rail, down to 12.11 from 12.17. That's a DMM measurement, not software. Dual rail PSUs are crap for gaming rigs IMO, esp. now that video cards are such juice hogs. Put 2 of them in SLi or Crossfire, how can you be 100% sure that the power distribution across the 2 rails is within specs? No one needs that headache. Just avoid it.
 
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jlbenedict

Banned
Jul 10, 2005
3,724
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0
Originally posted by: KBTuning
its so you can devide up the load.... like if you have 2 hdd and 2 video cards.... put one on each rail and split up everything like that.... it also allows for more things that pull on the 12v rail... like i have a 420watt Thermaltake PSU that only has a crappy 19a rail and when i go into games i can here the fans spin down because im running 2 160GB 7200RPM SATA Maxtor drives, a x800GTO2 at X850XTPE and beyond, and a Winchester 3000+ that i have sitting at 2.25Ghz.... if i had a dual rail psu i wouldnt have that problem because i would have each HDD on a rail and the like.....
Click to expand...


You cannot determine what devices you want to specifiy on whichever rail you want..
The dual rails are internal and I'm quite sure the load distribution is something the power supply manages..
 
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bgc99

Senior member
Aug 13, 2004
472
0
71
You cannot determine what devices you want to specifiy on whichever rail you want..

The dual rails are internal and I'm quite sure the load distribution is something the power supply manages..
Click to expand...

The reason I'm interested is that my Asus A8N Sli Premium bios says the 12V is at 11.64-11.71V. I tested a spare drive power connector and a spare PCIex power connector and they both showed 12.00-12.01V, right on the money. The 5V reading from the drive connector was about 5.12V, in the bios it shows 4.94V.

My concern is if these power connectors are on one rail and the mobo 24pin connector on another rail, I can't assume that the mobo is in fact getting the same voltage as the connectors I tested are getting.

BGC
 
Z

Zepper

Elite Member
May 1, 2001
18,998
0
0
Software readings whether in the BIOS or with a hardware monitor utility program under windwoes, are unreliable. Use a DMM for accurate measurements. 11.4 (read with an accurate meter) is within spec.

OP: "..."

bR: "No."

Har, har!

That's correct. One rail powers the CPU(s) and perhaps SLI vid cards. The other powers everything else that needs +12V power (mobo, drives, add-on cards, fans, etc.). I haven't written this in some time so it bears repeating: "A stout single 12V rail PSU beats split rail PSUs for flexibility any day." Check the Sparkle FSP550-60PLG or the FSP550PLG-SLI or many high-power Zippys for good examples.

.bh.

Hey, somehow I slid right past 10k and never even noticed until now - so intent on answering the insipid questions of the K-Ds. ;)
 
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