sharing a power supply

Jack Wathey wathey at salk.edu
Wed Jul 2 16:43:03 EDT 2003



On Wed, 2 Jul 2003, Alan Ward wrote:

> I would appreciate comments on the following:
>
> a) A 450 W power supply should have ample power for all -
> but can it deliver on the crucial +5V and +3.3V lines? Has anybody
> got real-world intensity measurements on these lines for Athlons
> I can compare to the supply's specs?

I made these measurements for my diskless dual-Athlon nodes.  They are
Gigabyte Technologies GA7DPXDW-P, with MP2200+ processors.  They have
on-board NIC, which I use, but otherwise they are stripped down to the
bare essentials: just motherboard, 2 cpus with coolers, and memory.  No
video card, no pci cards of any kind, no floppy, no cdrom, etc.

They have 2 power connectors: the standard 20-pin ATX connector and a
square 4-pin connector that supplies 12V to the board.  I did the
measurements by putting a 0.005 ohm precision resistor (www.mouser.com,
part #71-WSR-2-0.005) in series with each of the 5v, 3.3V and 12V lines,
and then measuring the voltage across that.  Rather than cut up the wires
of a power supply, I cut up the wires of extension cables:

http://www.cablesamerica.com/product.asp?cat%5Fid=604&sku=22998
http://www.cablesamerica.com/product.asp?cat%5Fid=604&sku=27314

There are multiple wires in these cables for each voltage.  Obviously you
need to be careful to cut and solder together the right ones.  A
motherboard manual should give you the pinout details.

Here are the results I got for my nodes:

cpus          memory installed        voltage line        current drawn
----------   ------------------       ------------        -------------
idle         2GB (2 sticks)               +5V                 13.1A
loaded       2GB (2 sticks)               +5V                 17.1A
idle         2GB (2 sticks)               +3.3V               0.34A
loaded       2GB (2 sticks)               +3.3V               0.34A
idle         2GB (2 sticks)               +12V                4.2A
loaded       2GB (2 sticks)               +12V                5.3A
idle         4GB (4 sticks)               +5V                 15.3A
loaded       4GB (4 sticks)               +5V                 19.7A
idle         4GB (4 sticks)               +3.3V               0.34A
loaded       4GB (4 sticks)               +3.3V               0.34A
idle         4GB (4 sticks)               +12V                4.2A
loaded       4GB (4 sticks)               +12V                5.3A

For my stripped-down nodes, only the +5V line turns out to be crucial.
You might want to repeat the measurements yourself, especially if your
nodes have more hardware plugged into them than mine.

Hope this helps,

Jack

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