Re: hot boxes and power consumption
On Thu, Aug 21, 2003 at 04:11:02AM +0100, Pigeon wrote:
> On Wed, Aug 20, 2003 at 12:33:12PM -0600, Paul E Condon wrote:
> > >
> > Also, Radio Shack sells a "current transformer" which is an add-on device
> > for their digital multimeter. With the multimeter and the current transformer
> > you can measure current and voltage on the AC supply input to the computer.
> > Or to any other electric device about which you are curious.
>
> ...but this arrangement will only give you a useful reading if the
> current waveform is sinusoidal. The current drawn by a PC is most
> definitely not sinusoidal; the rectifiers in the PSU only conduct on
> the peaks of the voltage waveform, so the current waveform looks like
> a series of narrow spikes of alternating polarity. The
> common-or-garden multimeter does not perform a true RMS conversion,
> and won't give a sensible result on this sort of waveform.
>
> (Also, you have to separate the live and neutral conductors in the
> power lead so the current transformer can be fitted round one
> conductor only, which is a pain.)
>
The power consumption values from the current transformer method are
very similar to those from watt meters such as Kill a Watt. To me this
indicates that either both are correct, or both are incorrect, possibly
because of the impulsive current wave form. The right way, I think, is
to examine both current and voltage wave forms with an oscilloscope and
compute the time average of the product of the two wave forms. And be
careful to handle the phase relation between to two correctly!
(Also, the current transformer includes a gizmo for separating the
hot and neutral wires safely.)
Paul
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