Drain on 12v lines

[mulpsmebeauty]

Apologies if I should know the answer, but I haven't come across it in my readings on this forum. I have an Enermax Noisetaker 600W, with 2 x 12v lines, each capable of supplying 18A. One is dedicated to the motherboard, and is providing juice for:

- AMD 64 3400+ (stock speed)
- 6800Ultra via AGP port
- 1x 80mm case fan.

The other rail is giving juice to:
- 2 Molex connectors into the 6800U
- 1x 74Gb Raptor, 2x 160Gb SATAs
- 2x 120mm fans
- 4x 80mm fans (yes it does sound like a freight train)
- CD and DVD drive

I have another (a fourth) HDD for the second 12v line which isn't plugged in yet, and I want to overclock the GPU. The question - will the two 12v lines pool their resources? The manual quotes a maximum total of 35A. It seems like the second line is getting a bum deal with supplying umpteen drives and everything else if it's limited to 18A.

 

[pandora's box]

if possible move this lot:

- 1x 74Gb Raptor, 2x 160Gb SATAs
- 2x 120mm fans
- 4x 80mm fans (yes it does sound like a freight train)
- CD and DVD drive

to the first rail


the cpu uses the 3.3volt line anyway so thats not drawing any power from the 12volt.

 

[BillR]

if possible move this lot:

- 1x 74Gb Raptor, 2x 160Gb SATAs
- 2x 120mm fans
- 4x 80mm fans (yes it does sound like a freight train)
- CD and DVD drive

to the first rail


the cpu uses the 3.3volt line anyway so thats not drawing any power from the 12volt.

Actually I think you will find that the CPU gets all it's current from the 12 volt rail in this situation. Older boards it was the 5 volt rail.

 

[mulpsmebeauty]

I can't move anything from where it is at the moment - one of the 12v lines is dedicated to the motherboard - it has no Molex connectors or anything - just one line that plugs into the mobo, leaving the other to cope with all the other stuff.

 

[Ice Czar]

to answer the question, no
the rails are truely independent and amps are unavailable
in addition you cant move what is being powered off the rails as you undoubtablly already gleened
+12V1 being just the 24pin main and 4pin +12V auxillary mobo connector

however you can determine the realworld draw on the +12V2 rail
the figures provided for drives and fans is durning spinup, at which point the graphics card will be nowhere near its full draw, the drives and fans will then drop to roughly a quarter of that figure, and the graphics card when thenn have that amperage available to employ

PS all AMD64 mobo VRM (Voltage Regulation Moduals) that Im aware of, and the guidelines from AMD themselves dicate that the power is off the +12V rail which the VRM then generates the high current, low-voltages the CPU(s) need to operate


Power supplies become increasingly expensive

When I initially asked about this on a mailing list, Solaris x86 advocate Al Hopper told me I was drowning in my tea, and that it was "all very simple". I love the simplicity of Unix people.

He explained that the later P4 CPUs take their power from a 12 Volt feed and, using the onboard voltage regulators, generate the high current, low-voltages they need to operate (anywhere from ~ 1.6V to around 2.7V). So the first requirement is a PSU that has plenty of power available from the 12V supply. Since the older ATX compatible PSUs didn't supply much current from the 12V section you have to ensure that your new PS delivers enough current (or power in Watts) from the 12V section. That's why using your old P3 ATX PS is a big "no-no".

He then described the history of the post-P3 power supply mess:

The earlier Athlon motherboard manufacturers decided to solve the 12v problem by using an additional 4-pin square connector to get the extra 12V those CPUs required. However many older PSUs didn't provide the 12V 4-pin square connector.

The motherboard makers then wised up and decided that there was nothing magical about a square 4-pin connector, so they put a normal hard disk type socket on the motherboard and provided the 12v power via a standard hard disk (4-pin inline) connector. Problem solved - you may now use your older PSUs provided they supply sufficient 12V current (many did not).

Further confusion came from PSU manufacturers not specifying the capabilities of the PSU in a way that allowed the end user to verify it's 12V power output rating.

In the meantime the ATX spec was saying "use the new 6-pin" inline connector - and very few motherboard makers implemented it.

Just to be sure, some motherboard makers, implemented *both* the hard disk style 4-pin inline connector and the square 4-pin connector. They said "use either or both in any combination you like".

The spec then evolved to the 24-pin main connector. Again, most motherboard manufacturers did not wish to make their customers mad by mandating that they replace their power supplies. This might cause their customers to avoid motherboard upgrades. Some used a "special" 24-pin connector with the extra 4-pin connection blocked off, or colored so that the user could plugin a 20-pin plug into the correct end of the 24-pin socket on the motherboard. Many others simply ignored the 24-pin requirement in the specs. Again - problem solved - use your older PSU.

 

[mulpsmebeauty]

Thanks for that. Looks like I'll play it safe and not plug anything extra into that line. Seems like a bit of an oddity though and a flaw in my research of the product, but if I wanted to power the majority of my hardware off 18A I'd have bought an ok £40 520W PSU with 30A pooled rather than an £100+ 600W PSU with most of the potential untapped. You lives and you learns.

 

[Ice Czar]

you do get alot more flexability with a single +12V rail
especially in a supply that is rated to maintain a tighter than spec load regulation like the PCP&C 510s, the OCZ Powerstreams ect. The advantage of the dual rails is that in a supply of just basic spec compliance, and especially in a mid to low quality supply, spliting the rail ensures that load changes will not effect the stability of the power to the mobo's Voltage Regulation Modual(s) and thus the core components

in this the ATX12V v2.0 spec followed suit with the Intel developed Server System Infrastructure Guidelines for EPS12V supplies
where the rails are subdivided even more
there are 2 variations, one where each CPU gets its own rail (split)
the other where they are both on a single rail (common)
the other 2 rails are for the mobo, and then additional components

so figuring up the draw on a workstation with a 4 rail EPS12V supply gets quite fun