Best practice for multi-rail power supplies

[HCHTech]

I had a gaming computer come across my bench last week - it had an Antec 1000 Platinum modular power supply with 4 12v rails - this one. Another thread here got me to thinking about whether it made sense to be more intentional with placing devices on different rails to evenly divide the power draw across all four rails.

This machine had an i7, 32GB of RAM, 2 SSDs in a RAID 1 for the OS, 2 2-TB WD Blacks, 2 DVD-RWs and a beefy graphics card.

Whoever built the machine just plugged all of the cables in so that most everything was on the 1st 12v rail. I was just removing an infection, so I didn't re-jigger with the assembly, but wonder if I should have offered to. I haven't dug into the pros and cons of this idea, but I'm sure some of the more electrically-inclined folks here have. Comments?

 

[Mercenary Roadie]

Reading the manual helps a lot with this type of question. It turns out that each of the 12v output can handle 480watts each. So what they have is pushing it a bit on just one output. You have to remember the PSU can only handle a total of 1000watts.

+3.3V +5V +12V1 +12V2 +12V3 +12V4 -12V +5Vsb
MIN 0A 0A 0A 0A 0A 0A 0A 0A
MAX 25A 25A 40A 40A 40A 40A 0.5A 3A
Total| 130W | 1000W | 6W | 15W |
| 1000W |

As you can see even though the 12v output are rated at 40A each (480W) all for can only supply 1000W total.

Personally, I would spread it out but I wouldn't spread it out so much that you end up with a lite load on an output.

Technically all long as they haven't gone over the outputs rating then it just fine.

 

[HCHTech]

That's helpful. So some questions:

Here is their chart:

If you look at the first total line, it states that the 3.3v and +5v lines can't exceed 130W, while the -12v line can't exceed 6W and the +5v line can't exceed 15W. Like everything the actual power draw on those lines must vary with load, but it's never zero, so in fact - the maximum from all four 12v lines can't exceed (1000 LESS whatever is taken on the 3.3v, 5v, -12v and 5Vsb lines) - is that right?

Also, you said

I wouldn't spread it out so much that you end up with a lite load on an output.

What would be the undesirable result of having a lite load on one of the outputs?

Lastly, some googling turns up these answers:

WD Blacks - spinup current would be the highest draw - about 1.16A on the 12v line (motor) and 0.6A on the 5v line (everything else) (do you total these? If so 16.92W)
DVD-RW - 2.5A on the 12v line and 1.5A on the 5v line (again, if we total these, it would be 37.5W)
Graphics card - obviously depends on the card, but a good gaming card can easily be 300-400W when working hard. at 12V, that would be 25 - 33A!
SSDs - My favorite Samsung 850 500GB is at 1.96W. @ 5volts, this would be .39A
i7 - Depends on which one, but somewhere between 75 and 150W. I read a few paragraphs about variable voltage regulator circuits for processors, and it became apparent that calculating the amperage wouldn't be simple.
Fans - Let's say 3W each for an 80mm fan. @ 12v, that would be .25A

So, 2 WD Blacks, 2 SSDs, 2 DVDs, 4 Case fans, 1 CPU Fan & 2 Graphics Card fans, and an i7 would total somewhere around 684W and 46A plus whatever the amperage for the processor would be.

Everything else driven by the motherboard (sound, usb, etc) would add more, what, 40 watts? 80 watts? If we say 80, that takes us to 764W. So that 1000W power supply doesn't really look too big for this rig.

It doesn't look like the amperage draw is too far out of bounds. An optimal setup would probably put the HDs and SSDs on one rail and the Optical drives on another rail, with the graphics card on it's own rail. Since there are 4 rails, If the connections allow, the 4-pin CPU line could be on a separate rail as well.

Does that sound about right?

 

[Mercenary Roadie]

How an output would act if underloaded will depend on the PSU. This one appears to be able to work correctly down to 0.0A.

Basically when an output is underloaded the PSU isn't able to properly regulate the output and the voltage level will change. It really doesn't look like that would be an issue here but I never like taking that chance.

Yes you add up all of the Amperage (or watts). You number look fine to me and I too would say there is nothing wrong with the PSU load

 

[HCHTech]

This is an interesting line of discussion. Let me ask two more questions since I have your ear, @Mercenary Roadie :

- What rail is the power delivered to the motherboard through the 24-pin connector on?

- On this example power supply, it appears that all of the 1x5-pin connectors intended for drive cables are on the 12v1 rail. The manual doesn't mention this possibility, and I don't see reference to a schematic anywhere, but maybe drive cables could be plugged into the other rail connectors only using part of the connector? The higher rail connectors appear to be 2x8. Squinting at that picture in the manual, I can't see if the shapes of the individual pin connectors are the same:

I have typically always used non-modular power supplies in my builds, so this whole line of thought never came up before. With non-modular, you just buy a good one with enough watts for your stuff, plug things in and go. I like the solving-the-puzzle aspect of modular supplies, I think I'll have to get one for my next build. Thanks for your thoughts!

 

[NETWizz]

I don't even care anymore. My PSU is already oversized and my computer has been shrinking for years.

 

[Mercenary Roadie]

I really couldn't tell you without seeing a schematic or metering the outputs.