[Beowulf] Re: Beowulf of bare motherboards

[Alvin Oga]

On Tue, 28 Sep 2004, Andrew Piskorski wrote:

> On Mon, Sep 27, 2004 at 11:16:36PM -0400, Glen Gardner wrote:
> 
> > I'd like to sequence the turnon of the nodes such that the inrush 
> > currents at turnon are lower, allowing the 12V supply to come up to a 
> > stable 12VDC a bit faster.
> 
> How would you do that, if you were using standard ATX power supplies?
> Is there some sort of simple circuit you could place inline which will
> delay the ATX "power good" signal by different amounts to different
> boards?  I'd discarded this idea as too complicated, but you made me
> think of it again.

a simple solution ...
	put a 1uf - 10uf capacitor across the "power on switch"
	of the motherboard
	( use different size caps to vary when the mb powers up )

	a .25 or 1 second delay is good enuff to minimize the surge
	on the poor power suppply

> The Finnish guys powered 3 motherboards per power supply by soldering
> on extra 20 pin connectors.  I'm also going to try running multiple

just have to watch out about reverse load sharing ..
	that the motherboards do not pull ( stress ) the power supply
	with too much surge current during power up

some ps are rated at 5x - 10x the current rating of the motherbord
with cpu with memory with disks ... ( just take a amp meter and measure it
all .. or a digital storage scope )

> nodes off one supply, but without any soldering, by using a
> CB-Z-Y2020-EXT "ZIPPY Power Cable Splitter: ATX 20 pin to Two ATX 20
> pin for ATX Power Supplies".  At $55 shipped for an MGE "600 W" Active
> PFC supply (from Newegg) vs. about $8.50 for each 20 pin ATX Y-cable,
> the choice seems obvious.

you can buy brand new 200W ps for about $20 now days... why spend the
time soldering ?? if space and heat is not an issue to have one 1 ps
per motherboard/system  
	- if the ps is shared and if it dies, all 3 mb will go down

> I've read that maximum current is usually drawn at boot up, so at
> least in theory, it could be better, and might allow running more
> nodes per supply, if the nodes on each supply DIDN'T all try to boot
> at exactly the same time.

i'll be trying to boot 5 - 10 mini-itx off one 600W 2U power supply
in one 4U chassis using only the +12v line ... and one dummy (management)
motherboard to keep the atx power connector signals happy
( to load down the 3.3v and 5v power lines is loaded down by the disk )
 	- the operating mini-itx consumes about 2A each at 12v
	( disk drives is about .5A spin up now days )

	- allow for 5A each motherboard ... and i'd be looking at a 50A
	or 60A at 12v ... and magic ... ( look mah .. no fire (yet) )

	- old rule of thumb ... use 50% of the rated power capacity
	of the power supply and blow cold ice on the ps too

c ya
alvin