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How do you find out required amps

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October 12, 2006 10:19:58 PM

How can I find out the amperage draw for the CPU in my PC? I want to make sure I get enough amps on the 12V rail(s) of a new PSU.

I've used several wattage calculators, which are helpful, but do not give any details on how much current is needed for the CPU. I've got an AMD Athlon 64 3200+ 2.2 GHz.

I checked AMD online tech documents, but they just list max current at 59A. That seems awfully high, given that the old PSU had 16A on a single 12V and the Antec SmartPowers I'm considering have between 15A and 19A on 12V2.

Are there any places that list the current needed for specific CPUs?

More about : find required amps

a b ) Power supply
a c 479 à CPUs
a c 121 À AMD
October 13, 2006 1:22:50 AM

I generally rely on www.xbitlabs.com for CPU and GPU power measurements.

I can't find a chart for the A64 3200, but here's one that has the 3500+:

http://www.xbitlabs.com/articles/cpu/display/athlon64-x...

I would say under max load the 3200+ will use about 40w. That translates to about 3.33 amps (40w / 12v).

Here are some more recent power measurements for Core 2 Duo and AM2 Athlons:

http://www.xbitlabs.com/articles/cpu/display/core2duo-s...

Here's power measurements for AMD's energy efficient Athlons:

http://www.xbitlabs.com/articles/cpu/display/amd-energy...

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Wanna see the power consumption of some GPUs? Then click on the following which divides the GPUs into performance categories:

http://www.xbitlabs.com/articles/video/display/power-no...


Just remember amps = watts/volts. Both CPU and GPU feeds off of the 12v rails.
a b ) Power supply
a c 479 à CPUs
a c 121 À AMD
October 13, 2006 1:38:35 AM

Some additional info:

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Hard Drives and DVD/CD Drives feeds off the 12v and 5v rails.

-The read/writes heads draws about 4w-5w from the 5v rails.

-I estimate a typical 120GB Hard Drive draws about 18w from the 12v rail.
-250GB HDD about 22w.
-500GB HDD about 28w.
-Western Digital 74GB Raptor about 36w.

-DVD Burners draws about 14w from the 12v rail.

-----------------------------------------

RAM draws power from the 3.3v rails. I think PC Power & Cooling estimates 10w - 12w per stick of 1GB RAM.

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Motherboards draw power from the 3.3v and 5v rails to power the various chip on the motherboard itself.

-I'm guessing a basic board draws about 30w - 35w combined.

-A premium board (lots of bells & whistles) draws about 45w - 50w combined.

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PCI cards like sound cards, network cards, and TV tuner cards draws power from the 3.3v rails. They probably don't draw more 10w each.

------------------------------------------

Floppies probably draws power from the 3.3v rail. But how often do you use your floppy?

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Hope that helps.

Note: The about estimates represents the highest possible load (worse possible situation), not typical load.
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October 13, 2006 7:14:02 PM

Thank you for the replies; all very helpful. I found reviews for the 3200 on the xbitlabs site which will help me make my decision. The PSU101 page helped me figure out some other questions too.

Some more general follow-up questions:

I understand how to calculate the max wattage and actual current for a particular rail, based on the info from a psu label or spec. But why would the max wattage output for any particular rail not be equal to the listed V*A? Is this determined just by the amount of resistance on the line?

Also, what if the label or specs don't list the max wattage per rail? How does one find this out? Or is that a good sign to stay away from those PSUs?

Thanks again for the previous information.
October 13, 2006 7:15:57 PM

Very nice post, a great introduction and good explanations. Thanks.
October 13, 2006 9:11:02 PM

Yes, I plan to get a PSU that gives me room for future growth. Thanks for the additional thoughts.
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