### EmonTx V3 - Sample rate for accurate AC power calculation

Hello,

I would like to measure AC power flow at four points in an off-grid solar installation, using 4 CTs and 1 voltage measurement.

An Arduino Mega is currently being used for DC measurement. However I don't believe that the Arduino Mega can sample fast enough to capture 50Hz AC accurately (I gather it samples at around 10kHz?), because we know the power factor will not be unity, and so many samples of voltage and current per cycle will be required.

Could anyone tell me how fast the EmonTx V3 is able to sample this number of signals please? In order to capture some of the lower harmonics on the line, we are aiming to sample 1000 times per cycle, per signal.

Also, will it be reasonably easy to interface the EmonTx V3 through a wired connection to the Arduino Mega?

If I've missed out some key information, please let me know!

Gwilym

### Re: EmonTx V3 - Sample rate for accurate AC power calculation

Sorry, the emonTx cannot sample at that rate. But why do you want to anyway? I don't know the details of the rules where you are, but in the UK and the EU there are very strict limits on the amplitude of harmonic currents, so there is very little energy in the higher harmonics. The emonTx can manage a little over 50 pairs (voltage and current) per cycle at 50 Hz, so it is good to the 25th harmonic. The limit for us there is 90 mA absolute maximum, so unless you have a very small very dirty load, the proportional error will be small. A standard energy meter I think needs to be good only to the 15th harmonic or thereabouts.

### Re: EmonTx V3 - Sample rate for accurate AC power calculation

Thanks for the quick and useful response.

Massive apologies: when I said 1000 samples per cycle, I meant to say 1000 samples per second!

They key thing for me from what you've said is that the emonTx can do 50 pairs per cycle, or 2500 pairs per second. This seems to be perfectly adequate for this application.

Perhaps the Arduino Mega would be able to do this on its own, do you think?

If not, I wonder if the Mega and emonTx will talk nicely to each other...

Thanks!

### Re: EmonTx V3 - Sample rate for accurate AC power calculation

Hmm. There was I thinking your grasp of fundamental principles was way off course, when it was digitus erronous all along ;-)

I don't know the Mega, but if it's got onewire, it should talk. See Martin Roberts's "full-fat" system where he has 3 stacked emonTx V2's talking to each other: http://openenergymonitor.org/emon/node/1170

### Re: EmonTx V3 - Sample rate for accurate AC power calculation

I'm pretty sure the ADC in the Mega2560 is almost identical to the one in the ATmega328, so I don't see why one would be better equipped than the other.  I think the 2560's has an extra bandgap voltage you can use for Aref (2.56V), and a wider input mux, but apart from that I think they are the same resolution and the same speed.

Thank you both