July 1998, Issue 96

A PIC-Based AC Power Meter

SYSTEM DESIGN

I want my power meter to measure AC power, instantaneous and average, 0–1200 W, as well as measure AC energy consumption in kilowatt-hours (or watt-hours). Additionally, I want it to provide digital readout of power or energy, and it should be easy to hook up using standard power receptacles and plugs.

This device is to be packaged as a hand-held instrument, and my budget dictates some ultra-low-cost parts.

I designed the power meter around the Woodward power-measurement circuit. The power source is a 9-V battery, instead of stealing power from the AC line. This setup ensures maximum isolation from the AC line and increases safety.

I chose a nonmultiplexed LCD because of its low power consumption. I use a simple four-digit, seven-segment type rather than an alphanumeric LCD module because I only need to display numeric data. It also costs less.

The user operates the meter using two momentary switches. The mode switch causes the display to cycle though the different operational modes. Regardless of what operational mode the power meter is in, energy-consumption accumulation still occurs. The reset switch resets the energy-consumption accumulator in any operation mode.

This power meter has four operational modes. The power mode displays the power consumed in watts. By accumulating power over time, the energy mode displays watt-hours or kilowatt-hours. The average power mode displays energy consumed over time in watts. The remaining mode displays the time elapsed since reset in hours, minutes, and seconds.

Because the power meter toggles through these four modes, the user needs to know the current operational mode. Normally, this would be done with annunciator segments in the LCD. However, I want to use low-cost stock parts, so a custom LCD with annunciator segments isn’t an option.

The LCD I chose is a four-digit display with three decimal points, a colon, and an arrow. Because I have no real use for the arrow, I decided to use it as the mode indicator.

The colon is used in the time mode, so I only need to discriminate among three modes—power, energy, and average power. I decided that no arrow indicates power mode, a slow-flash arrow means energy mode (1-Hz rate), and a fast-flash arrow is used for the average-power mode (2-Hz rate).