lehpron
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My psu has 4 rails with 18A on each rail. That's 72A or 864watts yet it says 650w. Why is that?
P.S( I'm only tryng to help you make a great post here) Multiple PSU rails don't add; the 18A spec is just that: Had you drew power from just one rail, the maximum it could take would be 18A at 12v. But because there may be only one AC-DC transformer in the unit, you're actually pulling 12v from each 12v rail evenly, and will continue until you reach whatever is rated as the max combined 12v wattage, for the transformer to allowcate 12v amperage. In your case, it was 650W?
My PSU has two rails of 18A and 20A, but they don't add to 38A. My max combined 12v draw wattage is 360W. So my max 12v draw cannot pass 30A from both rails. In otherwords, I cannot succeed 15A each; but it's not like I can choose which rail, I draw from both.
This isn't a matter of PSU efficiency, per se, as there are other voltages that feed into the computer other than +12v that take up total wattage, that share the same AC-DC transformer.
Here's an example, the website GlobalAmericaninc.com sells server and industrial computer parts, they also list typical usage loads for their systems. While not full load apart from which program they ran to push up power draw, they give a pretty good idea of how much their systems actually draw (note specs on power draw below):
http://www.globalamericaninc.com/Motherboards-Mini-ITX-Desktop_Core_2_Quad_/_Duo/c244_181_183/p2807870/2807870_-_Mini-ITX_Motherboard_with_LGA_775_for_Intel_Core_2_Quad_/_Core_2_Duo_Processor/product_info.html
Notice how all voltages are drawn, and not one voltage line with it's amps adds up to powering up just the CPU (Q6600 is placed in the 95W TDP class), i.e. the CPU draws much more power than just from the 12v line.
For practical purposes and to make the math easier, we can assume TDP equals electrical power and divide by 12v to get a number of amps for the CPU, but it's actually higher that it really is.
FYI: TDP means Thermal Design Power = max heat output for that specific class of processor, i.e. all stock Core 2 Duos don't give off 65W of heat, however the fastest ones will be closer to this value, while the slower ones will be closer to the next TDP class down. For example, all Core i7's are put in the 130W TDP class, but they don't all give off that much heat. To say the 965 is closer to 130W than 920 is correct, but until we measure or guestimate accurately, that's all we can conclude.
Since many microprocessors have low energy efficiency (my old Pentium D 920 was about 8%), it is safe to assume TDP = power used, but if doing power calculations, it should be considered. I think the new Core i7's are probably between 10% efficient, but no higher.
FWIW: also, many if not all PCIe graphics card draw power from the 12v line only, but draw signal from 3.3v (i.e. this is what voltage the bandwidth communicates with the memory controller).
Bottomline, IMO, if all the work you did to see if you can keep your old PSU while continuing to upgrade nets a small margin, if you can afford it, just get a better PSU. Capacitors age and their aging is load dependant, which means eventually that PSU won't put out the rated wattage for much longer. Usually it's 1-2 years, max load.
< Message edited by lehpron -- 1/16/2009 4:49:16 PM >
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