Friday, April 1, 2011

Adding the Two Phases

Found a fellow out in Arizona who's come up with his own means for measuring his home AC power consumption:

He's employing a neat trick: "add" the two legs on the input of US 2-phase power feeds by wiring the two current transducers in series.

My original approach was to sample both legs separately, dedicating an A/D input for each leg. That would let me sample only two circuits using an Arduino Deumilanove (which has six total analog inputs...well, two-and-a-half, but that half-circuit wouldn't be useful.) While this is a precise and powerful way to do it, the phase-locked nature of the analog power waveforms allows you to "multiplex" the voltage one on top of the other by connecting one leg of each current sensor together.

The payoff is that you can use only one A/D input to measure power draw on a two-leg, two-phase US-configured power circuit. That would enable the basic Arduino Deumilanove/UNO to measure up to FIVE circuits (A0 for the voltage reference and A1-5 for the current in each circuit to be measured) instead of only TWO circuits. Excellent!

The disadvantage to this approach is that you've got to hack up your current sensor: wiring them in series like this means you ought to cut off one of the leads from the sensors and join them together. And, you can get the connection wrong.

Note to self: do more research to satisfy myself that this actually works.

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