[rescue] I'm not an EE Q - 48 vdc followup

[Tim H.]

On Fri, 19 Jul 2002 01:52:41 -0500
lesliec at theplanet.com (Leslie Connally) wrote:

> ((in a DEC cabinet I picked up, it has an DC in / AC out "whatever". I
> cannot imagine how much DC power it takes to be turned into enough AC
> power for a rack full of equipment! Or does this thinking just reflect
> my unappreciation for the 'power'of DC.


Power calculation for DC is real easy, P=IV where P is power in watts, I
is current in amps and V is voltage in volts.  So if you have 2000 watts
of server in the rack (power supplies are listed as maximum, so a
machine with a 400 watt PS will draw something up to that) and you need
to do DC backup to the rack 2000W/48V=41.7 Amps.  

This will be significantly more current than would be required at 110V
AC, but the power is the same.  
In the case of equipment plugged into a voltage source, current (and
likewise power) are a function entirely of the load, the voltage is
determined by the source.  the 110 volts out of the wall can be
considered fixed for any load up to the point that the breaker trips. 
The current is set by the impedance (actual resistance, taking into
account the effects capacitance and inductance have), and is expressed
as P=IZ where Z is impedance, but it's uglier than that because
impedance includes phase shifting, where capacitance and inductance
cause the voltage and power waveforms to be out of step with each other.
 This is why highly inductive or capacitive loads can cause the electric
meter to be innaccurate, and why you can find sites on the internet that
claim to be able to make your  meter run backwards.

Oh, and of course all that waveform stuff has led to labeling AC voltage
by the equivalent DC power capability, and that 117V RMS at the wall
outlet is actually ~165V peak or ~330V peak to peak, and those guys in
Europe/UK are actually dealing with over 600V peak to peak at the wall
outlet.  When you light yourself up with "110v" the buzz you feel is
actually 165 volts, but if it's any consolation it's only warming you up
with the same amount of power as 110v DC. :-)

Tim