# Measuring Power Factor with an oscilloscope?

I would think it is a simple matter of a current probe and a voltage probe, one on each channel and measuring the offset to calculate
the phase angle.
Is this a reasonable method given that your probes are properly calibrated?
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Yes. You'll probably need to take a picture of a steady waveform so you can go off and measure it accurately. Measuring the offset is only useful if both are sine waves. For many loads nowadays, the current waveform can look very strange, and on many mains supplies the voltage waveform is noticably flattened at the peaks, in which case you will need to integrate by hand the product of the voltage and current waveforms over a cycle to work it out.
OTOH, true power meters are the tool designed for the job, and you can just read off the answer.
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Andrew Gabriel

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Well...yes....in the sense that you can measure *anything* with an oscilloscope ("How long is that football field? About 300 Tek 2213s between the goal posts") but in practice you'd use a meter designed to measure power factor and display it directly. It gets kind of tricky, counting graticule divisions, converting to angles, and then looking up the cosine on your slip-stick.
Bill
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If the two are sine waves and not some chopped up waveform caused by non-linear devices.
If your scope has some nice feature for measuring period/time-delay, you could measure the period of the voltage waveform to find frequency (if it's not known), then measure the time delay between voltage and current peaks. Figure out the phase displacement and you're practically home free.
daestrom
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Frank White wrote:

Put the "voltage" on the vertical and the "current" on the horizontal. You'll get a 1:1 Lissajou figure. If the scope's axis gains are equal, two sine waves in phase will show a line at 45 degrees. At 90 degreees they will form a circle. Phases in between will form an ellipse, from which you can calculate the phase angle.
As has been noted, this will only work correctly with sine waves.
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Virg Wall, P.E.

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| Frank White wrote: |> I would think it is a simple matter of a current probe and a voltage |> probe, one on each channel and measuring the offset to calculate |> the phase angle. |> |> Is this a reasonable method given that your probes are properly |> calibrated? | | Put the "voltage" on the vertical and the "current" on the horizontal. | You'll get a 1:1 Lissajou figure. If the scope's axis gains are equal, | two sine waves in phase will show a line at 45 degrees. At 90 degreees | they will form a circle. Phases in between will form an ellipse, from | which you can calculate the phase angle. | | As has been noted, this will only work correctly with sine waves.
But you can still see the variations between voltage and current with this method. It just won't be directly easy to figure out specifically what is happening is your voltage is not a sine wave. But with a voltage sine wave and harmonic currents, it should be rather obvious.
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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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On 10/3/06 12:17 AM, in article snipped-for-privacy@4ax.com,

There is something to be said for using a true rms voltmeter and a true rms ammeter along with a real wattmeter. There are times when electronic equipment is the problem--not the solution!
Bill -- Fermez le Bush
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