Sunday, January 12, 2014

Wireless Whole House Power Monitor - Part 1 - Hardware

This post is long overdue.  My Wireless Whole House Power Monitor (WWHPM) has been up and running for many months now.  Actually, it's not installed in my circuit-breaker panel yet, but it's been running on the bench, in my PanaVise, and sending measurements to my Raspberry-Pi wireless base station project.  I'll cover that in a future blog, detailing my adventures in Python, with a smart data-logger class that automatically summarizes and reduces data over time.

WARNING and DISCLAIMER: this project is for educational purposes only.  Working in or around AC utility lines is inherently dangerous.  The reader assumes all responsibility for complying with local and national electrical codes.  Consult a licensed  electrician about connecting this or any other project to the AC utility lines.

The hardware design of the WWHPM is pretty straightforward and features transformer coupling for the two voltages and two currents.  The only DC connection to the AC utility lines is the safety ground.  The voltages are measured using small 4.5Vrms transformers, and the currents are measured using AC current clamps.  These signals are attenuated and fed into analog inputs on Arduino-compatible Moteino.  Note, my Moteinos are R2, using the RFM12B.  On the new R4 Moteinos, you can choose the RFM12B or RFM69W.

Everything to left of the dashed line will be inside the main circuit-breaker panel.  The project box is plastic, attached side by side to a  standard wiring box.  The AC voltage lines should be wired using standard Romex cable, to two circuit breakers on each phase.  The AC current clamps are low voltage signals, and per the national electrical code they should run through a separate conduit.  

Each analog input is protected by diode clamps, D1 through D8, in case of line surges, transient spikes, etc.  Likewise, fuses F1 though F6 provide over-current protection.

The current clamps are sold as an accessory to a DMM, so they already have the burden resistor inside, and they output a voltage proportional to the current.  R1 and C1, and R2 and C2, serve as low pass filters to improve SNR, and the resistors also limit the current in case the clamp diodes ever conduct.

Transformers T1 and T2 provide 4.5 Vrms, which is attenuated by R3 and R4, and R5 and R6, respectively.  Note: the analog input impedance is about 100 megohms, so no significant load on the circuit.  A third transformer, T3 provides the DC power supply current and the 60 Hz (or 50 Hz) interrupt signal via Q1.  

The power supply is pretty basic, with bridge rectifier BR1 and filter cap C4 to provide a nominal +4.5Vdc to the Moteino Vin.  R8 forward biases D10 and D11, to create a nominal +1.8Vdc.  This voltage provides a "pedestal" or baseline voltage for the AC voltage and current signals, keeping the analog inputs within their 0 to 3.3V dynamic range.

And that's pretty much it.  The rest as they say, is all software.

YouTube video: