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Topic: Question re: phase lag on energy measurements (Read 9064 times) previous topic - next topic


Phase matching of the system is another critical issue.
The errors induced in the system at PF = 1 are minimal.
A power factor of 0.5 with a phase error as little as
0.5 degrees will cause a 1.5% error in the power mea-
according to "AN-564 APPLICATION NOTE" from AD.

Put your transformer to rest, go with optical:


Optically isolated measurements re: the voltage signal are an attractive option. The main issue as I see it is that the number of potentially 'hot' components goes up considerably. The transformer, fuse, switch-mode power supply, etc. are all explicitly UL-listed, which should allow me some day to get the whole assembly listed quite easily.

One attractive alternative to the above is the Avago series of precision opto's. Very linear. But the circuits required to make it all work are extensive, making for a much larger PCB than I would like.

Another option is to simply run the resistors straight into the ADC and then use the ADUM5401 series from Analog to provide power and a safe SPI bus interface. If I had to ditch the transformer, I would prefer this solution for its simplicity even if it means adding yet another 3.3V power supply to the design (one for the Analog, the other for the digital power supply of the MCP3911).


Hello everybody ,

Is there anyone completed MCP3911 project?


Oct 22, 2016, 02:13 pm Last Edit: Oct 22, 2016, 02:13 pm by MarkT
Hi guys and gals,

I hooked up the front end of my ADC to my oscilloscope and measured something interesting. Specifically, there appears to be a variable phase lag between the transformer for the voltage signal (ERA 0.08VA) and the current transformer (CR Magnetics CR8348-2500-N) as a function of load. Voltage always is lagging current.

The phase lag ranges from ~ 1.32ms at a 40W load to 0.722ms lag with a 1340W load. Both loads have unitary gain, i.e. a 40W light bulb and a toaster. Is this normal?
Firstly you ideally need to filter out the harmonics before measuring the phase of the fundamental, any
even harmonics will distort the waveform making it hard to distinquish true phase difference from
harmonic distortions.  This is best done in software not by eye from the 'scope screen.

The source impedance of the mains is immaterial here, presuming the current measured is through the load
and the voltage across it.

In real money those times translate to 24 and 13 degrees (for 50Hz) or 28 and 16 degrees (for 60Hz)
which are quite substantial.

A toaster is a resistive load, any subsidiary electronics is miniscule compared to the nichrome heating
element and at mains frequencies that's just a resistor.

"Unitary gain"? Does not make sense here.  'Purely resistive', 'non-reactive' makes sense here.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

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