What does a 24-pin connector actually do?

[shira]

I know this subject has been beaten to death, but it seems like no one really knows the answers:

My understanding is that the "extra" four pins on the NF4 power connector are intended to supply power directly to one (both?) PCI-e Video slot(s). I can understand that, for a dual-rail PSU, those four pins represent one of the rails (and I assume the other 20 pins represent the other rail).

But what happens when you use a single-rail PSU with a 24-pin connector? Is the connector simply taking whatever power/amperage is needed for the PCI-e slot out of the total amps available from the single rail and shunting it to the extra four pins? Or is something else happening. And if you use only a 20-pin connector, how is power delivered?

In the discussions up to now, it looks like a dual-rail PSU is NOT a good idea for use on SLI boards, since one of the rails can be over-burdened. But if you use a 24-pin connector on a single-rail PSU, does this artificially create a dual-rail PSU, and will you run into the same problems?

Again, sorry. But I don't think these sorts of questions have been specifically addressed.

 

[o1die]

You can get by with using the 20 pin connector on some boards. I would try a 20 to 24 pin adapter. They run about $5.

 

[ribbon13]

http://pcpowercooling.com/products/powe...pplies/dimensions/images/mb_T51ATX.gif
http://pcpowercooling.com/products/power_supplies/dimensions/images/mb_T51EX.gif

 

[ChineseDemocracyGNR]

I would not use an adaptor, it's not needed and some people had serious problems with them.

 

[shira]

The purpose of my post was not to ask whether or not to use an adapter. I'm wondering what happens when a single-rail PSU with a 24-pint connector is used as opposed to one with a 20-pin connector.

I realize that people are having success on NV4 MBs with both types of PSUs. But I'm wondering:

What are the implications of using a single-rail PSU with a 24-pin connector? Does this make the PSU into a "poor-man's" dual-rail PSU?

What is different when you use only a 20-pin PSU? Under what conditions MUST you use a 24-pin PSU?

If you have only a single video card, is there any reason (a 6800U, for example, with very higher power requirements) to choose a 24-pin PSU over a 20-pin PSU?

What are the advantages/disadvantages of each configuration?

 

[ribbon13]

Single Rail 24pin has existed for a while; The 24pin existed before in EPS12V form. All it does is provide seperate 12v for the PCIe bus.

High-end cards usually take power off of a seperate cable anyway. In the 6800U's case, the 6-pin PCIe Video power cable.

 

[shira]

Originally posted by: ribbon13
Single Rail 24pin has existed for a while; The 24pin existed before in EPS12V form. All it does is provide seperate 12v for the PCIe bus.

High-end cards usually take power off of a seperate cable anyway. In the 6800U's case, the 6-pin PCIe Video power cable.

But since a single-rail PSU has only one rail (duuuh! on me), what does it mean to say that the extra pins provide a separate 12V supply for the PCIe bus? What is the maximum power available from this 12V supply? And what happens when you use a 20-pin PSU instead? The PCIe bus will still get power, right? So what's the difference between these two approaches?

 

[coyrls]

I'm not an expert on this (but that's not going to stop me having a go at answering!). I think you're getting two things mixed up, the purpose of dual rail is to use one rail to supply power to the motherboard and the other rail to supply power to disks etc. So with a 24 Pin dual rail power supply all 24 pins will be supplied off the same rail. The V2 power spec does specify both dual rail and 24 pin connectors, but they are not related.

 

[shira]

Originally posted by: coyrls
So with a 24 Pin dual rail power supply all 24 pins will be supplied off the same rail.

I do understand this. But what I am confused by is the fact that both 20-pin and 24-pin single-rail PSUs will work with NF4 MBs. That implies that there's some difference in the way these two configuations supply power to the PCI-e slot(s) (and possibly other components).

What I'm asking is, what is that difference, and what is the practical significance of that difference (in particular, what are the bottom line advantages/disadvantages to each configuration)?