Smart meters

If you have the processing grunt then a complex FFt is the ultimate way out, but it is not so easy in limited processing power

I'd been mulling over the best way to handle it with limited digital processing power - one thought was to go back to the old Analogue Computing techniques and get some linear electronics (mostly op-amps) to do the hard work of calculating the instantaneous I * V and integrate it over time -  leaving the digital side to just do a leisurely A-to-D on the resulting power quantity. Not without it's challenges, not least that the power scale is a lot wider than current or voltage individually, so likely a lot more precision needed (especially in the A-to-D itself), but might be worth a mull over still.

   - Andy.

Probably worth taking this onto a thread on its own - I too have messed a little with this sort of thing, though that was over a decade ago, I was monitoring I and V on a 3 phase genset (and the levels in a sewage tank...) with a modest PIC, and generating Ethernet traffic  a few bytes at a time, and not having enough memory.

As an example of the memory constrained thinking, I do recall a problem that IP packets have the checksum at the front, and the way I created and sent the data almost live, I did not have enough buffer to store the whole packet and post calculate that.  I do recall the 'aha' moment of realizing I could send any known checksum, and then the live data, just keeping a counter running and at the end append a dummy word of the right value to make the error checking work so the checksums  of all the packets the unit ever sent were identical...

I suspect that the analogue way will reveal more, and you will end up compensating for the non idealities of the current transformer. From memory the voltage waveforms are quite well behaved in comparison to the currents even on a genset.

Mike.

I was under the impression that the simplest way to do a watt-hour meter was to sample the I and V as quickly as your ADCs will go, multiply the instantaneous I by the instantaneous V to get instantaneous P, then sum the instantaneous P's all together over a second, and divide by your sampling rate.

It is. Only if the wave-forms are well behaved on the time-scale for the samples, and the bandwdths of the current and voltage probes you use are more or less matched.

Mike

It is. Only if the wave-forms are well behaved on the time-scale for the samples, and the bandwdths of the current and voltage probes you use are more or less matched.

Mike