A few PSU questions

[Pez D Spencer]

I've got a few questions about PSU's that maybe you guys can clear up for me. I figured that instead of digging around on Google that maybe you people can just lay it out for me real quick. I'll try to make it short and simple...

1 - What's the difference between a 20 and 24 pin main power connector? What do the extra four pins do? I know that the 24 pins are typically used on newer boards, but aside from that I'm not real sure what the extra pins do exactly.

2 - What defines an EPS12V PSU? Is it those extra four pins on the main connector or is an EPS12V PSU one that uses an 8 pin aux connector instead of the older 4-pin?

3 - What types of CPU's require an 8-pin aux connector? I have an EVGA 680i board with an 8-pin aux connector but until I upgrade I'm still using my old Pentium 4. Before I got a PSU with an 8-pin aux connector I used my old PSU with a 4-pin and it worked fine. Would a Duo or Quad not work with a 4-pin?

4 - My mobo has an auxillary 4-pin molex connector on it. The maual says it's to provide extra power to the PCI-E bus. When would someone need to plug in a molex here? I plugged a molex into it just to be safe, but since my video card (X1950 PRO) already uses auxillary power via a 6-pin, that 4-pin molex on the board should be needed should it? Say I was using my old X600 that doesn't require extra power like the X1950, logically I still shoudln't need that molex because the X600 should be able to pull enough power through the PCI-E bus shoudln't it?

5 - What's better, mulitple 12V rails or one single 12V rail? Wouldn't a huge single 12V rail be better? The way I see it if you have a 60A 12V rail versus two 30A 12V rails, then the 60A is better because you wouldn't ever have to worry about the 12V pulling too many amps. I'm no expert, but this makes the most sense to me.

Thanks for any info.

 

[mpilchfamily]

1) The 20pin is the older connector. New motherboards will take the 20 pin but they have a 24 pin connector. The extra 4 pins adds an additional +3.3v, +5v, +12v and ground wire to the motherboard to help support the PCI-e bus. Like i said the 24 pin isn't required but its best that you have a Current PSU. Those with the 20 pin are older ATV Version 1.3 PSUs that are not designed to accommodate the heavy 12v requirements of todays systems. Older systems relied more on the +5v rail.

2) Yes that is part of it. Mainly it defines the distribution of power primarily across the +12v rail(s).

3) While not required for single or dual core it is best to have the 8 pin or at least an adapter on your 4 pin when using higher end duals and the quad core CPUs.

4) The Aux is generally used to support dual video cards. But it doesn't hurt to have it plugged in.

5) This is a heavily debated topic with equal arguments for both sides. In the end there is very little difference between a single large rail and 3 or more rails. Most often those PSUs labeled as 3 or more rails are actually single railed PSUs.

Now i may have missed a few things but I'm sure John will come along and correct me on these is someone else doesn't beet him to it.

 

[Pez D Spencer]

Thanks a lot for the info.

 

[John]

mpilchfamily did a nice job summarizing things. Here are some of my thoughts.

1. The extra 4-pins add voltage to the entire mainboard.
2. EPS12V is the separate 8-pin or 4+4-pin 12V power lead for high end mainboards and multi-cpu socket boards.
3. See above
4. Some boards have multiple aux power connections. A prime example is my DFI nF4 Ultra-D which has both a 4-pin molex and 4-pin 'floppy' power. I recall DFI stating that they wouldn't help you troubleshoot if you didn't have all of the aux power connections populated.
5. IMO a single 12V rail is better since you have access to all of the available power instead of separate rails with OCP and reserved power that could go unused (wasted). In some cases the multi-rail psu's aren't labeled properly (or at all) so you have no idea which rail is being utilized. Most Seasonic built (Antec EW/Trio/NeoHE & Corsair) multi-rail psu's are in fact a single rail.

http://www.corsair.com/products/hx.aspx

Triple 12V Rails provide independent reliable power to the CPU, video card and other components with a combined rating of 50A (40A on 520W) maximum! Advanced circuitry design that automatically enables power sharing between the triple +12V rails in an event of overload on any single +12V rail.

 

[Aluvus]

1. The extra pins provide parallel conductors from the supply to the motherboard, for the main rails. This effectively lowers the resistance in the current path and allows more current to be conducted with less voltage drop (also somewhat better efficiency, and better safety).

2. EPS supplies should have both the 24-pin main connector and the 8-pin 12 V connector; the 8-pin connector is unique to EPS.

3. The 8-pin connector was introduced for the benefit of multi-processor servers, which needed to deliver more current to their processors. The concept is exactly the same: more wires in parallel means more current can be conducted without issue. No current consumer processors "need" the extra 4 pins, per se, though as a best practice any new power supply I bought would have the universal 4+4 connector.

4. The Molex connector may be there to cover for an inadequate motherboard implementation. It may be the case that that particular board cannot deliver as much current to the PCI-Express slots as the standard specifies, under some conditions. I would imagine this is a somewhat more difficult spec for SLI boards to hit safely, and they wanted to be sure.

5. One large rail, unless you are concerned about the theoretical safety gains of having multiple rails. Given the way most multi-rail supplies are actually implemented, this is one instance where I would encourage people to buy supplies that openly ignore part of the ATX spec. Because most of the supplies that claim to follow the 20 A current limit guidance... don't.

Originally posted by: mpilchfamily
Those with the 20 pin are older ATV Version 1.3 PSUs that are not designed to accommodate the heavy 12v requirements of todays systems. Older systems relied more on the +5v rail.

Many of them are designed for use in systems that lean primarily on the 12 V rail. Nearly any supply sporting the 4-pin "ATX12V"/"P4" connector (which predates the 24-pin main connector by a fair amount) should be designed to support significant 12 V draw (plenty of bottom-of-the-barrel supplies will, of course, not). Supplies older than that will often choke on substantial 12 V loads.

 

[Zepper]

1- the four extra wires spreads out the current load per wire and set of contacts and allows for higher total current capacity on the main voltages. Not really needed any more as almost anything that needs extra current these days has its own dedicated power connector.

5- Single rail has always been the better design if you want to readily be able to access all available power. Many supposedly split-rail PSUs now are really single rail under the skin - read some of the reviews on http://www.jonnyguru.com. The numbers for the different rails on the label are a marketing fiction. Split-rails, yet another flawed idea from your friends at Intel (they are the ones that maintain the ATX spec)...

.bh.