Last modified 2016-09-18 19:14:20 PDT

Wireless Power Meter

Project Date: 09/2009


wireless_power_meter wpm_closeup
wpm_switching_power_supply_live wpm_wireless_two_meters_live
wpm_switching_power_supply_gnuplot wpm_voltage_gnuplot




The Wireless Power Meter is a simplistic ATmega88p and ZigBee/XBee based true V-I power meter. AC voltage measurement is made from the rectified signal of a step-down transformer, and current measurement is made with the pass-through Allegro ACS712 Hall-Effect sensor. The microcontroller and wireless transceiver are magnetically and optically isolated from the high voltage power circuitry.

This project has some important issues and proposed solutions that are worth pointing out to anyone interested in building such a power meter:

Lastly, I am worried about whether the voltage measurement step-down transformer has any significant parasitic reactive impedance (I’m not terribly familiar with the properties of real transformers) that could cause a phase shift between the actual voltage measurement and the scaled down one on the other side of the step-down transformer. This would be problematic, because very important power information (the power factor) is stored in the relative phase shift between the voltage and current waveforms. An unaccounted phase shift in the voltage waveform would contaminate this measurement.

I should also mention that the more common, commercially accepted way of making these sorts of power measurements seems to be an all-in-one “energy metering IC”. These chips effectively do everything measurement related for you, taking into account all sorts of second-order effects as well, and give you a digital interface to read the results. In application, the energy metering IC itself is typically not isolated from the power line, but the digital interface can be optocoupled, and so you can still isolate the digital data collection circuitry. Here are a few examples of energy metering ICs: – this one is built into an ARM microcontroller!

The easiest path to a custom high-accuracy power meter would probably be a circuit centered around one of those energy metering ICs.



Download the EAGLE schematic here: wireless_power_meter.sch



Download the EAGLE layout here: wireless_power_meter.brd


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