More or less rails on a PSU?

[Reloadron]

Originally Posted by Panda Man

That just tells me why its not necessary, not why one option outweighs the other If Intel sets the standard to one rail though, so be it

EDIT - Ty, you know dual rails used to be more stable than single rails :D

Actually Intel sort of set the standard for multi-rail design:

1.2.1. Increased +12 VDC output capability

System components that use 12V are continuing to increase in power. In cases where
expected current requirements is greater than 18A a second 12 V rail should be made
available. ATX12V power supplies should be designed to accommodate these increased
+12 VDC currents.

The above is but a small slice of the ATX 12V Power Supply Design Guide. For those who may be having trouble getting to sleep, the design guide is great reading. The design guide is a constantly changing document as well and only actually suggested practice.

1.2.3. Separate current limit for 12V2 on the 2x2 connector:

The 12V rail on the 2 x 2 power connector should be a separate current limited output to meet
the requirements of UL and EN 60950.

The original intent was the 12V2 connector would serve to supply the CPU. The UL (Underwriter Labs) and EN (European) requirments revolve around safety. More the safety of fire hazzard than shock.

When looking at a PSU one must look at the gauge of the wire used for the rails. Looking at for example an original 20 pin motherboard connector there was only a single 12 Volt line. The 24 pin brought another 12 Volt line but only for the added 75 watt requirement of the PCI-E specification. Thus additional 12, 3.3 and 5 Volt lines were added.

Uh Oh, have to get to work.... Later.

Ron

[Dread]

I thought the original reason (besides marketing hype) was to help prevent ripples from crossing over to another rail during heavy demand?

If I had the money, would buy nothing but PCP&C power supplies.

[Elysium]

Its not that I'm going to get a 1000W PSU, I just used that as an example.

I am deciding between the Silverstone Olympia 600W (1 rail) and the OCZ GameXStream 600W (4 rails).

Which one would provide the cleanest, most stable power for overclocking and such?

[Panda Man]

I can't seem to find a 600W Olympia unit, can you link me? Also, the OCZ GameXStream is a VERY good PSU, I'd highly recommend it for your system The Olympia series is also a good line and would serve you well, but I don't see a 600W anywhere, just 650+. Are you sure its not a Strider?

[screwballl]

I prefer 2 bigger rails (2x 30-50A) than several smaller ones (ie: 5x 17A) or a single huge one (1x 50-80A)

[gvblake22]

Originally Posted by Panda Man

I can't seem to find a 600W Olympia unit, can you link me? Also, the OCZ GameXStream is a VERY good PSU, I'd highly recommend it for your system The Olympia series is also a good line and would serve you well, but I don't see a 600W anywhere, just 650+. Are you sure its not a Strider?

SilverStone Olympia OP650 650w Power Supply Review

[Panda Man]

Originally Posted by gvblake22

SilverStone Olympia OP650 650w Power Supply Review

When did this happen!? I'm just going to assume he meant the 650W version, I never could find a 600W Olympia.

[Reloadron]

The following is probably one of the better explenations I have seen regarding PSUs:

This is to clear up the enormous confusion I've noticed in the last few months about dual 12V line current specs on PSUs.

SPECS

Version 2.0 of Intel's ATX12V Power Supply Design Guide began recommending dual 12V lines for PSUs that can deliver more than 18A at 12V. Why? To abide by safety requirements of UL and EM 60950, which stipulates not more than 240VA on any wires or exposed traces. Intel's PSU Guide calls for a current limiter that keeps current to under 20A on each of the 12V rails: 12V x 20A = 240VA.

What is the safety reason for this 240VA maximum? It's the maximum recommended for an electronic device that a consumer will have reasonable likelihood of access. In plain terms, it might be to keep people from zapping themselves inside a PC, or more likely, accidentally creating a fire risk. This safety "rule" does not apply to any electronic or electrical devices where the chance of consumer exposure is low, such as a TV or CRT monitor, for example.

It's important to remember that even though there are two "independent" 12V lines, they still draw from the same main source. It's highly unlikely that there are two separate 120VAC:12VDC power conversion devices in a PSU; this would be much too costly and inefficient. There is only one 12VDC source, and the two lines draw from the same transformer. Each line is coming from the same 12VDC source, but through its own "controlled gateway".

PSU makers' specs are misleading in that thay rate the current capacity of each 12V rail independently. What really matters is the total 12V current: Generally, up to 20A is available on any one 12V line assuming the total 12V current capacity of the PSU is not exceed.

What the above means is that you don't need to worry about imbalances in power draw on the 12V lines ?as long as no single rail is asked to deliver more than 20A. PSU makers seem to mark each line for max current on a purely arbitrary basis, probably more for marketing reasons than any other. A PSU rated for 32A max on the 12V lines can be labelled many different ways:

12V1: 18A, 12V2: 14A
12V1: 17A, 12V2: 15A
12V1: 16A, 12V2: 16A
12V1: 15A, 12V2: 17A
12V1: 14A, 12V2: 18A

It could be marked 20A + 12A, but being a cautious bunch, the engineers will probably not specify more than 18A on any one line. This gives 2A headroom to allow some room for error for the current limiting circuit.

REALITIES

Note that 12V2 is supposed to supply only the AUX12V (2x12V) 4-pin plug, which feeds only the CPU. With PSUs that adhere strictly to the ATX 12V v2.xx Guide, 12V1 supplies 12V to all the other components that require it. This might lead to a problem with very high power gaming systems that utilize two high power video cards. Current high end VGA cards by themselves can draw >90VA each. Much of this comes from the 12V line via the 6-pin PCIe connector for the VGA card. If you add several hard drives and optical drives, the 240VA limit may be too low.

The current ATX12V v2.2 spec was created before dual VGA card gaming configurations for Intel boards were announced. SLI, being an AMD feature that came many months earlier, may have been ignored by Intel's PSU design guide team.

Not all PSUs with 6-pin PCIe connectors follow ATX12V v2.xx to the letter, as the guide does not cover the 6-pin 12V PCIe outputs. This connector and its current delivery capacity was specified by nVidia, the originator of the SLI concept. nVidia maintains a list of power supplies that they have certified as being suitable for SLI systems. The question is, Where should this 12V come from?

I interviewed a number of engineers from several power supply manufacturers to pose this very question. The answers were surprising. All of the engineers I spoke with wished to remain anonymous. This is a summary of what they told me:

**Some PSU makers are using 12V2 to supply more than just the 2x12V or 4x12V connectors. It is often used to power the 6-pin 12V PCIe outputs.

**Many PSUs marked as having dual 12V lines actually have only a single 12V line ? they do not feature two <240VA power limiters specified by ATX12V v2.xx; they have only one Over Current Protection (current limiter) for the single 12V line.

**The 240VA current limit is considered a high cost, useless annoyance by most PSU makers. If multiple 12V lines are used, because the vast majority of components now use mostly 12V, the 18~20A limit for any line means that the precise power distribution to the various 12V output connectors can become critically important in some cases.

**The engineers point to the many high power pre-V2.xx ATX12V PSUs that had as much as 30A on a single 12V line. As a product class, those have not proven to be any more dangerous in any way than other ATX12V PSUs.

What's really interesting is that Intel has tacitly waived the 240VA limit requirement in its PSU validation program for the better part of a year.

Intel maintains a web page listing all the ATX12V they have tested that "meet MINIMUM electrical, mechanical fit and functional compatibility" with Intel desktop boards and processors: http://www.intel.com/cd/channel/reseller/asmo-na/eng/35815.htm

For the 32 ATX12V v2.2 PSUs tested in 2005 that are on this list, 17 models are identified as having at least one output line that exceeds 240VA. And yet, these 17 models are on Intel's approved list. According to the engineers I spoke with, the majority of these 17 models have just one 12V line. They also point out that there are another 20 or so ATX12V v2.0 PSU models on the Intel list, and none of them were tested for the 240VA current limit conformance. My sources say that if these models had been tested, more than half would not conform to the 240VA current limit because they have only one 12V line.

Now, my sources say, in the last couple of months, Intel has notified the PSU makers verbally that the 240VA current limit has officially been removed. This means a single 12V line is the accepted norm, never mind ATX12V v2.xx.

What does all this mean? Essentially, the only potential benefit of dual 12V lines is improved safety, and this is disputed by the engineers I spoke with. There are many downsides to dual 12V lines, including higher cost and the extra worry of ensuring adequate 12V current for all the components in complex, high power systems. For the consumer who is trying to make a choice among the myriad of PSUs available on the retail market today, the most practical approach regarding dual 12V lines and power capacity is to consider only the combined 12V current capacity.

EDITED Sept 17/05 with much more detailed info.
_________________
Mike Chin
Editor/Publisher, SPCR

Taken from:

silentpcreview.com | View topic - Confused about Dual 12V lines? Here's the FAQ!

There are some very good points brought up. One can also easily see why in many cases the PSU specification needs work or a good update despite all it has had. Also mentioned is how the 240 VA rule leaves a lot to be desired when it comes to PSU design.

In many cases there is a great deal of hype surrounding the marketing of PSUs. This hype for a good part is nonsense. I am hard pressed to see a need for a 1 KW PSU for much of anything other than a very high end server or workstation.

My current home workstation runs dual Xeon processors, six HDDs, seperate RAID Controller card with onboard processor, a commercial 3D Labs Realizm 800 GPU, fans for everything you can immagine, Intel 7525 motherboard, and countless outher things I stuffed in it a few years ago. The system is about two years old now and runs just about 24/7. The power supply is an Intel flavor 600 Watt server quality PSU. That's it a single 600 Watt unit runs that beast.

Ron

[screwballl]

Now with SLI and Crossfire on the market, having dual dedicated rails could prove much more beneficial to the overall system for those that use the SLI/Crossfire setup.. otherwise it is an unneeded expense and can be avoided for single or no video card systems.

[Reloadron]

Originally Posted by screwballl

Now with SLI and Crossfire on the market, having dual dedicated rails could prove much more beneficial to the overall system for those that use the SLI/Crossfire setup.. otherwise it is an unneeded expense and can be avoided for single or no video card systems.

Absolutely! The trick is getting manufacturers to label what is what. Having a PSU with 4 12 Volt rails (12V1, 12V2, 12V3 and 12V4) is of little value unless a user knows which connector uses which rails (The Fanout). If the user is sitting there with a hand full of cables and connectors and is clueless where they actually go, you got nothing.

Additionally, looking at the PCI EXP. X16 Version 1.0A Specification the added 6 pin connector on PCI-E cards is a 72 Watt affair meaning a draw of about 6 Amps. That is the max a card will draw. Thus 2 cards will draw a maximum of 12 Amps. The actual slot is 72 Watts. Most cards do not draw close to that. Meaning few cards would consume 144 Watts of power on a 12 Volt rail.

Ron