Power supplies (addendum)

Robert G. Brown rgb at phy.duke.edu
Wed May 23 09:58:35 PDT 2001

Dear List,

Mark Hahn and I have been continuing the PS discussion offline, as I was
a decade or so off in assuming that PC power supplies have big
transformers.  The incoming voltage is converted to high frequency and
stepped down with relatively small transformers before being cleaned up
and regulated and delivered.  My bad.  A useful link on power supplies:


One thing that came up in our discussion that I hadn't really thought
about but that is obvious in retrospect is that a "300 Watt" ATX supply
doesn't deliver 300 Watts to any particular component in a computer --
this is the (approximate) aggregate of the power delivered on the
various voltages provided by the supply, with the bulk of the power
delivered to the +5V and +12V lines.  A typical 250 W supply might
deliver about 120 W to each of these and a handful to the other lines.

Thus the motherboard and electronics in a computer with even a 300 W
power supply might only be able to draw 150 W total, leaving 140 or so
to run the peripherals on the 12V line.

In some sense this is way more than most beowulf node designs "need" for
the peripheral supply -- a diskless design or design with only one disk
might need only 30-50W to run the disk(s) and cooling fans (a useful
table of typical power requirements is on the article linked above).
However, the requirements of the motherboard are not so flexible.
Memory, CPU, the motherboard itself, all of these eat energy to run and
150W could very easily be needed to support a full memory configuration.

You therefore might want to look at the details of how any given supply
distributes and delivers power -- this is probably what gets a power
supply "certified" by a CPU vendor more than the absolute wattage.  The
site above reiterates my earlier post that suggested that it is a bad
idea in general to operate a power supply at 100% of capacity.  Power
supplies do indeed get hot in operation and contain large heat sinks
buffering electronics and they get unhappy if overheated.  A large
supply has more thermal capacity than a smaller one.


Robert G. Brown	                       http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb at phy.duke.edu

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