On Sun, 14 Nov 2010 18:44:27 -0600, Steve Kraus
wrote:

I'm trying to figure power factor for a large transformer-type DC power
supply. This is pretty much just for my own amusement (so I can figure
power draw from current and voltage readings) so there's nothing critical
here.

If all you want are the results, the common Kill-A-Watt power meter
has a PF measurement feature. $25-$40 everywhere.
http://www.p3international.com/products/special/P4400/P4400-CE.html
I have 4 of these (various models).

If you want to do it safely with a scope, it really requires a 4
channel scope (or 2 channels with two differential inputs per
channel). Put a small value non-inductive resistor in series with the
power line. One pair of diffential input probes go across the
resistor for the current waveform. The other pair of probes go across
the line for the voltage waveform.

With a switcher, you're going to have a non-sinusodial current
waveform. Therefore, you can't just use the phase angle as the power
factor and will need to measure the Apparent Power and the RMS powers.
http://www.pge.com/includes/docs/pdfs/mybusiness/customerservice/energystatus/powerquality/nonsinusoidal_power.pdf

There's also quite a bit of work done on power factor correction these
days which probably includes some measurement techniques.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558