How many amps on the 12V rail does the motherboard, CPU, and RAM use? And how to find out for specific boards? I imagine this would be a reasonably common question people have when designing a new system as well.
I'm looking at expanding the storage I have on my home server but I'm concerned about the PSU having enough amps on the 12V rail as it only provides 18A. I've found info on the HDD manufacturer's website for peak power requirements for the drives and believe most of that is from the 12V rail. However I can't seem to find info for the motherboard, CPU, or RAM.
I would think that most, if not all, of the 12V rail power going in to the motherboard is provided to the fans, expansion cards and driving the voltage regulator for the CPU, as I would think the rest of the motherboard and ram uses the other lower voltage rails. Can anyone confirm if this is true?
Any tips on where I might look to see how many amps I'm currently using on the 12V rail? particularly what my peak usage could be? As this could help me identify the required numbers.
I have in there
- Motherboard: Asus M4A78LT-M-LE
- CPU: AMD Athlon II X2 235E CPU AM3 45W
- RAM: Kingmax 8G(2x4G) NANO PC-12800 1600MHZ DDR3
- HDD: 6x SATA green disks (each max peak 1.8A from the 12V rail)
- OS: ESXi 5.0, but am going to install debian linux
Existing 12V rail peak usage guesses:
- HDD: 6 * 1.8 = 10.8A
- CPU: 45W / 12V / 0.9 (guess that motherboard voltage converter is about 90% efficient) = 4.17A
- Fans: 2 * 0.125A = 0.25A
Total of peaks: 15.22A.
Adding 2 more HDD at 1.8A each would take that peak value to 18.82A, which is too high for the PSU. Though I am guessing the only time peak power draw from the HDDs is during spin up at power on that the CPU will not be using its peak power draw. The normal power usage of the HDDs during read/write should only be about 0.5A each. So I'm guessing I should be able to do the storage upgrade without upgrading the PSU but it will be close.
Edit: The ability to stagger HDD startup is I believe done with a 5 or 6 wire SATA power cable (pin 11) and requires a HDD that supports this feature. My PSU only uses 4 wires for power (molex to sata converters) so it doesn't support staggered startup. Also the HDDs and MB SATA controller are cheap and seem to spin up everything at once.
Given that it is a cheap PSU, there is no documentation relating to capabilities for short term spike usage maximums for things like inrush currents. So I have to assume that its capabilities are limited in this regard.