[SPAM] [Beowulf] powering up 18 motherboards

Jim Lux James.P.Lux at jpl.nasa.gov
Thu Feb 17 16:17:57 PST 2005

At 02:22 PM 2/17/2005, Dean Johnson wrote:
>Correct me if I am wrong. Presumably your cpu's have fan kits and if you
>may have case fans (or something). Do the motors on fan's not draw more
>initial amperage at startup? Thus you would have an initial spike.

The fans probably don't draw a significant amount of startup current over 
their running current.

Disk drives, on the other hand, do draw a lot more current when spinning up.

>         -Dean
>On Wed, 2005-02-16 at 20:10 -0500, Alpay Kasal wrote:
> > Hello all
 I have a question about powering on motherboards
> > simulataneously

> >
> >
> >
> > I have 18 identical mobo’s right now with identical ram, cpu, and hard
> > disk. I hooked one up to a kill-a-watt and found that it draws 140-150
> > watts when powering on, and stays level at about 90-100 watts
> > afterwards. The problem is that I am setting this up at home, where I
> > only have 10 amp circuits (and only a couple of them can be freed up).
> > Correct me if I am wrong here please

> >
> >
> >
> > 1 mobo = 100 watts / 115 volts = .87amps each mobo while steady on
> >
> > 1 mobo = 150 watts / 115 volts = 1.3amps each mobo while turning on

You're basically right.

There's also a very high current spike that your Kill-A-Watt won't see when 
you first turn on the supply, as the capacitors in the input section charge 
up.  This might result in a nuisance trip of your breakers.

This current spike will be somewhat dependent on the phase of the AC line 
voltage when you first close the switch.  Some power supplies have inrush 
current limiting, others don't.

A 10 amp circuit would be highly unusual in the U.S., but might be common 
practice elsewhere.  In the U.S., a 15 amp circuit is standard.

> >
> >
> >
> > I won’t include the rest of the math, but needless to say, it’d be a
> > pain in arse to turn on the room in piecemeal without tripping a
> > circuit breaker. My questions is :
> >
> >
> >
> > Will a heavy duty UPS aid in getting me through powering up the room?
> > I don’t mind splitting up the 18 machines with 6 outlet surge strips.
> > Any advice?

No, the UPS won't help.  It might make things worse, because as you flip on 
all that load, the voltage will sag, causing the UPS to turn on, which then 
might trip from the overcurrent (assuming you're not out buying a 2kW UPS).

What you want is some way to sequence the power, conveniently.

The answer is to use some relays.  You could spend some tens of dollars 
(brand new, much less surplus) and get some time delay relays.  You could 
use the DC power out of the first power supply to turn on to charge a 
capacitor through a resistor that's hooked to a relay (12V coil, 110V 
contacts).. Most DC relays have a much higher pull-in than hold 

You could use the X-10 type (aka Plug n Power) remote controlled relays 
(don't use Lamp modules.. you need Appliance modules, which are relays inside).

You could build a little power sequencing box that sends the appropriate 
signals to the power supplies to turn them on, one by one... I think you 
might be able to do this with the parallel printer port on the first mobo 
to fire up. I haven't looked at the control interface on an ATX power 
supply recently.

> >
> >
> >
> > Thanks.
> >
> > Alpay Kasal
> >
> >
> >

James Lux, P.E.
Spacecraft Radio Frequency Subsystems Group
Flight Communications Systems Section
Jet Propulsion Laboratory, Mail Stop 161-213
4800 Oak Grove Drive
Pasadena CA 91109
tel: (818)354-2075
fax: (818)393-6875

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