What is a "dual rail" power supply?

Many power supplies boast of having two +12V rails (and some even three!),
but what is the benefit? I mean, does it matter whether a p/s has two rails
with max outputs of 15A each, or a single rail having a max output of 30A?

If dual rail is better, how do I know when I'm dedicating one rail for a
power-hungry video card and the other to, say, the hard disk drives? I have
yet to see power supply docs that specify how to connect components so that
one rail isn't overtasked.

Enlighten me, please.

Squigmont wrote:

Many power supplies boast of having two +12V rails (and some even three!),
but what is the benefit? I mean, does it matter whether a p/s has two rails
with max outputs of 15A each, or a single rail having a max output of 30A?

If dual rail is better, how do I know when I'm dedicating one rail for a
power-hungry video card and the other to, say, the hard disk drives?

Dual 12V rails is for improved fire safety, but even better is an
all-metal computer case with no plastic windows. Otherwise dual 12V
rails can be a nuisance when a computer has both a very high-power
video card and several disk drives. Normally, the motherboard's +12V
connector is for 12V rail 2, the other +12V connectors go to 12V rail
1. Some have the SLI video cable wired to the 2nd one, but I don't
know how to tell, except by detecting the tiny differences in voltages.

Squigmont wrote:

Many power supplies boast of having two +12V rails (and some even three!),
but what is the benefit? I mean, does it matter whether a p/s has two rails
with max outputs of 15A each, or a single rail having a max output of 30A?

If dual rail is better, how do I know when I'm dedicating one rail for a
power-hungry video card and the other to, say, the hard disk drives?

Dual 12V rails is for improved fire safety, but even better is an
all-metal computer case with no plastic windows. Otherwise dual 12V
rails can be a nuisance when a computer has both a very high-power
video card and several disk drives. Normally, the motherboard's +12V
connector is for 12V rail 2, the other +12V connectors go to 12V rail
1. Some have the SLI video cable wired to the 2nd one, but I don't
know how to tell, except by detecting the tiny differences in voltages.

On Mon, 1 May 2006 21:53:50 -0400, "Squigmont"
wrote:


If dual rail is better, how do I know when I'm dedicating one rail for a
power-hungry video card and the other to, say, the hard disk drives? I have
yet to see power supply docs that specify how to connect components so that
one rail isn't overtasked.

What's annoying too is so many cables being dedicated to
connectors for SATA drives and SLI power connectors.

What genius doesn't understand a single molex style
connector for everything is more flexible ?

SATA connectors are crap physically.

On Tue, 02 May 2006 06:14:10 -0700, generous electric wrote:


What genius doesn't understand a single molex style
connector for everything is more flexible ?

And while I'm on a rant, make rail 1 black connectors and
rail 2 white connectors.

In article , generous electric
wrote:

On Mon, 1 May 2006 21:53:50 -0400, "Squigmont"
wrote:


If dual rail is better, how do I know when I'm dedicating one rail for a
power-hungry video card and the other to, say, the hard disk drives? I have
yet to see power supply docs that specify how to connect components so that
one rail isn't overtasked.

What's annoying too is so many cables being dedicated to
connectors for SATA drives and SLI power connectors.

What genius doesn't understand a single molex style
connector for everything is more flexible ?

SATA connectors are crap physically.

The SATA connectors were not designed to help retail products.
They are intended to make putting disk drives in large server
cases easier. You can make a "SATA backplane", which is a
motherboard type thing, and it is sprinkled with SATA data/power
connectors. The disk drive then just plugs into the backplane.
It would allow a disk drive enclosure to be designed without
cables, and with the hot insertion feature, the disks can be
unplugged while the server continues to run.

The fact that the SATA committee didn't make a decent
motherboard solution is besides the point :-)

You can see three horizontal connectors in this picture.
The grey/black connectors are SATA backplane connectors,
and allow this disk enclosure to connect SATA disks without
the use of cables. The drives just slide into place.
Server cases with dozens of drives can be made this way.

http://i23.ebayimg.com/04/i/04/97/e4/9b_10.JPG

If they kept just the 1x4 Molex, that wouldn't be a lot
of fun to try to do the same kind of thing. Too much insertion
force.

And I agree with your observations on dual rail supplies. They
make it easier for manufacturers to meet EN60950 spec, which
I gather has something to do with fire safety, but when I look
at the PCPowerandCooling 850W and 1000W supplies, they still
have 12V3 output with more than 20A capacity on it. Which would
violate that spec. So even with new rules, there still seem
to be exceptions.

Dual output supplies do allow the industry to move to a
higher power level, while meeting the "240VA per output" max
power spec, but it also costs the customer more, as you end
up buying more supply than you need, to get the capacity right.

The whole topic doesn't make a lot of sense. Must be an
"industry insider" kind of thing. I don't see as many
reports about visible flames coming from power supplies,
so maybe they've got everything under control.

Paul

On Tue, 02 May 2006 16:35:19 GMT, (Paul)
wrote:

In article , generous electric
wrote:

On Mon, 1 May 2006 21:53:50 -0400, "Squigmont"
wrote:


If dual rail is better, how do I know when I'm dedicating one rail for a
power-hungry video card and the other to, say, the hard disk drives? I have
yet to see power supply docs that specify how to connect components so that
one rail isn't overtasked.

What's annoying too is so many cables being dedicated to
connectors for SATA drives and SLI power connectors.

What genius doesn't understand a single molex style
connector for everything is more flexible ?

SATA connectors are crap physically.

The SATA connectors were not designed to help retail products.
They are intended to make putting disk drives in large server
cases easier. You can make a "SATA backplane", which is a
motherboard type thing, and it is sprinkled with SATA data/power
connectors. The disk drive then just plugs into the backplane.
It would allow a disk drive enclosure to be designed without
cables, and with the hot insertion feature, the disks can be
unplugged while the server continues to run.

Paul

I'd rather see them use simple and cheap adapters for that
application, and leave the basic molex power connection
alone. but... they didn't ask my opinion