12V line power requirements

**rushnrockt** · 03-02-2005, 07:27 AM

Alright, it is 5 in the morning and I am tired of combing through the whole Internet. My guess is that I will wake up to a some flaming responses, but I hope those will be kept to a minimum.

So as I am figuring out what kind of approach to use in building my power supply, I figured (obviously) that the main problem becomes 12V line. I've search through ATX specifications of a 250W PSU (I am trying to build something of that order), and those specifications have two (2) 12V lines. The main one having 1-14 Amp requirements and the secondary having 1-8 Amp requirement. Leaving alone the fact that the numbers do not add up, I came to a thought that might or might not make the building easier.

I am thinking of finding the largest common current consumption device for the 12V line, and based on that create several separate 12V lines (for processor, devices, more devices) as opposed to one big one. What I lack is the understanding of how much current is being consumed on each rail of the newer relatively powerful computers.

From what I see based on my search, 7-8 Amps is the most needed for the top of the line Athlon XP's. I cannot find info for P4's or AMD64's that would break down the consumption by rails, most publications give the number provided by manufacturer plus their own estimate of the power drawn by the whole system. Deriving anything from PSU specs is just about impossible, since they give maximums that cut into the power of other rails. High powered (450W) power supplies have two rails that can continuously provide 10-12 amps... but then again, I have no idea how it is spread among the components.

So, my question is, what is the largest SINGLE 12V current consumption in a computer system? My understanding is that it will be the CPU, but the precise number eludes me. I appreciate your input. Going to sleep now.

**Scouse Monkey** · 03-02-2005, 07:46 AM

I would say the CPU. especially as P4 mobos have an extra 12v line to it. I think some processors do use the 5v - mobile athlons i think.

I take it you have looked into how the sproggy works? that has a dual line system to double up on the power from the old version.

**rushnrockt** · 03-02-2005, 09:07 PM

Lines other than 12V do not seem to be as much of a problem, it is the 12V line that is my concern.

Anyone else out there have any input? At all?

**mushin** · 03-03-2005, 01:57 AM

It clearly depends on the system in question. On a high-end system, I agree that it's going to be CPU. The only possible exception would be a top-end GPU; I think they can draw 70 - 80 watts (don't know how much of this is on 12V tho), which is less than a top-end CPU but might be the biggest draw on a system with a more middling CPU.

Of course, on a system with say a VIA EDEN, a 3.5" HD would be a greater load then the CPU, so again, depends on the system.

**rushnrockt** · 03-03-2005, 03:26 AM

Originally Posted by mushin

It clearly depends on the system in question. On a high-end system, I agree that it's going to be CPU. The only possible exception would be a top-end GPU; I think they can draw 70 - 80 watts (don't know how much of this is on 12V tho), which is less than a top-end CPU but might be the biggest draw on a system with a more middling CPU.

Of course, on a system with say a VIA EDEN, a 3.5" HD would be a greater load then the CPU, so again, depends on the system.

Ok, it pains me that I have to get that detailed into to get a single number, but i will try.

I am not going for the max crazy setup with dual xeons and SLI video cards. The power I am looking at is ~250Watts, so the system definitely not going to have components that can draw ridiculous power. It seems like I have the question answered for myself already (6-7 Amps) but it would be helpful if someone with personal knowledge/experience could say something about this.

**Scouse Monkey** · 03-03-2005, 04:48 AM

Originally Posted by rushnrockt

Lines other than 12V do not seem to be as much of a problem, it is the 12V line that is my concern.

Anyone else out there have any input? At all?

I was talking about the 12V line! only mentioned the 5v cos osme CPUs use that instead.

Not sure how much a P4 CPU runns at but say 65W for a lower end one is like 5.5A, so say 85W and that is 7A or summit. Plus other stuff that needs 12V like 3.5" HDD etc

**rushnrockt** · 03-03-2005, 05:08 AM

Originally Posted by Scouse Monkey

I was talking about the 12V line! only mentioned the 5v cos osme CPUs use that instead.

Not sure how much a P4 CPU runns at but say 65W for a lower end one is like 5.5A, so say 85W and that is 7A or summit. Plus other stuff that needs 12V like 3.5" HDD etc

Yeah, I understand the "other stuff" that needs 12V

Which is why all that other stuff will have a separate 12V rail. Or may be two.

**J187** · 03-03-2005, 09:32 AM

Maybe I'm way off base, but wouldn't you need to list the devices you intend to have on your system. Then figure whch has the highest wattage and devide that by the 12v rail. Thats the single biggest Amp requirement. Problem is, you also have to account for 100% usage too. Plus, it strongly depends on what devices we are talking about. Don't a lot of AMD CPUs run on the 3.3v? So say a 2800+ running on the 3.3 at like 1.65 is 60 watts at 100% use - 36amps. Am I out in left field?

If you are referencing specs from a manufacturer, make sure you find one that gives you the whole picture - 3.3,5,12 the combined ratings for 3.3+5 and 3.3+5+12 with amps for individual rails and the 2 combination rails too.

**rushnrockt** · 03-07-2005, 11:53 PM

Someone might be interested later on.
After doing some more research, I found the top 12V drain for a CPU to be 156W for a Pentium 6xx, which translates to a nice 13A......
As far as run of the mill Athlon XPs and Pentium P4s, at full load they run at up to 7-9Amps (Extreme Edition much more).
So it would seem that for a PSU such as the one I am trying to make (250W) anything beyond P4 2.8GHz is unattainable.