Power factor measurement

[Charles Linquis]

I still don't understand why they sample so fast. The power factors of most loads don't change over thousands of cycles. You can sample every single part of a waveform even if you sample it once a second. You just need to take a lot of samples.

I have built two wattmeters over the past few years. One did have to sample fast (about 10KHz). I needed the value of each half-cycle as soon as it reached zero-crossing.
I started the A/D, then did the calcs for the previous sample, then grabbed the result and started the A/D again.

The other sampled at a 2 mSec rate (500Hz). I sampled for 3 seconds (1500 samples). The waveforms I measured were definitely
not sinusoidal. The results were the same in both cases, and I actually used a KILL-A-WATT as a calibration reference.

[ardhuru]

I've seen a PC based utility that does this with an interseting approach; it uses the 2 sound card channels to track the voltage and current readings, does the math and displays all the parameters on the screen. The simple coupling hardware is also described in the help section. The company's since changed its name so it took some searching, but here it is. http://www.infinitespectra.com/freew...400/index.html

Since you can calibrate the software for your own setup, I'm sure the components arent too critical either.

And did I mention its free?!

Regards,

Anand

[dhouston]

Neat!

If you do this, make sure you use an isolation transformer between AC line and your test circuitry. Otherwise, you caan make the case of your PC hot.

[ardhuru]

Dave, the circuit does isolate both the channels already, doesnt it? The voltage sensing is by way of a 12 volt step-down, and the current sense is a coil wound around one of the power carrying cables. This might be the only difficult to procure component for those having no access to Digikey, but since there is an option to calibrate the unit, I suspect a home made hand wound coil should be fine.

[dhouston]

Anand, sorry - I didn't look at the circuit. The transformer is fine.

[Demon]

Moved from Schematics.

Robert