Node power supply

[Robert G. Brown]

On Tue, 22 May 2001 MAHRF at de.ibm.com wrote:

>
>
> Hi everyone,
>
> I'm planning to build a home beowulf of nodes with an Athlon 900 in a mini
> tower.
> How many watts should the power supply have at least when the only other
> parts in the nodes are a NIC, a simple graphics card and maybe a small HDD.
> Maybe I'm going to upgrade the nodes to, say, 1,3 or 1,4GHz Athlons in the
> future when these are as cheap as the 900MHz models are.
> I'm asking because less watts means less heat and it's also cheaper.

Do you mean how many watts does it draw in this configuration (only way
to know for sure is to measure)?  Or how much current must it be able to
deliver transiently (to switch on the motherboard)?  Or what AMD expects
and requires to certify correct and proper operation?

I haven't looked it up myself (although it ought to be on the AMD
website and/or available within the technical specs of the motherboard
you are considering) but my vendor tells me that AMD requires a 250W
"certified" power supply for at least the 1.33 GHz systems -- not just
any old 250W power supply will do, and I couldn't get my favorite case
as a consequence (no biggie, they had a certified case that appears just
as good and actually a bit cheaper).

As to what you could get to work -- the only way I can imagine to find
out, especially since you'll be working outside the "spec" for the
motherboard/CPU, is to try various power supplies and see what works.  I
don't >>think<< that you can hurt the chip by underpowering it, the most
likely consequence is that it won't work at all or will work
erratically.

HOWEVER, you might think about the logic of what you are doing.  It is
by no means clear that putting in a smaller power supply is going to
make the system run cooler or run cheaper, although the power supply
itself might be cheaper.

The current draw of the running system is determined by the voltage
applied (which had better be fixed within a fairly narrow tolerance or
the board won't work at all and might indeed break) and the operating
load.  The power is determined by the voltage times the power.  The
power consumed by the motherboard is more or less independent of the
power supply used to provide it. You could have one the size of your
desk that could provide ten kilowatts of power if asked nicely (with the
appropriate voltages on the appropriate lines, of course) and if you
plug in the motherboard it will trundle right along drawing its 80W or
130W or whatever its operational load and hardware configuration
requires.  The only real differences are that the 10000W power supply
could probably drive 50-100 motherboards or so at once (instead of just
the one you could drive with a normal one) and that it would make BIG
sparks if you short it out before blasting the wires into copper vapor.

SO, you won't save power on the operational side by using a small
supply, you will just risk it not being able to draw enough peak
current/power and running erratically or unreliably.

What about the power drawn in the power supply itself (in idle mode)?
Hard to say, as it depends greatly on the quality and design of the
supply.  If the transformers and components inside the supply were
"perfect" there would be no draw at all beyond the idle current provided
to the motherboard and peripherals and the power supply itself would not
heat up at all.  They're not perfect -- eddy currents are generated in
the flux coupling, some flux escapes as 60 Hz radiation energy, the
wires and components all get a bit warm even at the idle load.

With a power supply that is too small, I'd expect that it would get much
hotter at even the idle load, because one of the things that MAKES it a
low-wattage design is a relatively small (physically) transformer with
relatively thin primary and secondary wires.  Those thin wires have a
higher resistance and get hotter than the thicker wires of a bigger
transformer at any load.  I know that the power supply of my laptop is
always hot, even when the laptop is idle or off and just trickle
charging.  As the transformer gets bigger, provided that the flux core
lamination remains high quality I'd expect the transformer to run COOLER
at any given load, not hotter.  There is probably a sanity-check-point
here -- the desk-sized transformer might well generate more heat than a
simply "big" desktop power supply -- but overall I'd expect a big supply
to run cooler than a small one at equivalent load.

Put all this together and you might conclude that your system will run
COOLER if you use a BIGGER power supply than you really need, and won't
run at all if you use one that is too small.  You'll pay more up front
for the larger supply, but you will actually pay for a bit less
electricity during its lifetime of operation (the heating of the
transformer especially is utter waste heat that you pay for in the
electricity and pay for again in the cooling bill and reduced life of
the components sharing the enclosure).

This might be why AMD insists on a certified 250W supply or better for
their systems -- it might well be twice the power or more that the
system actually draws in operation (except possibly during peaks when
all the peripherals in a loaded system run at once) but a 250W supply
runs cooler and heats the case less under load than a 200W supply that
would nominally suffice.  A 300W supply would probably heat the case
even less, especially given that it will typically have an even larger
cooling fan to get rid of the waste heat generated in the transformer
under full load.

  rgb

-- 
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