How to use a mixed signal microcontroller in an AC power measurement application

By Ionela May 8th, 2009

The mixed signal microcontroller used is the Microchip PIC16C924. This microcontroller has five A/D channels, two of which are used to digitize voltage and current signals. The microcontroller features of pulse width modulation (PWM) and direct liquid crystal display (LCD) drive are utilized to further reduce cost and parts count. The PWM output feature is used with a single pole RC filter to provide a comparator reference with 10 bits of resolution. The direct LCD drive is used to drive an 8-digit, 7-segment LCD.

The PICREF-3 can accept all worldwide power mains and is calibrated for both. Worldwide power mains can be either polarized or unpolarized. A polarized power main has a grounded neutral connection. Reversal of line and neutral on a polarized power main will create a power line fault. An unpolarized power main does not have a grounded neutral connection. Therefore, line and neutral can be connected at random without causing a power line fault.

Hardware Overview

The PIC16C924 allows for the real-time interleaving of sampling voltage and current waveforms, and power and energy calculations. Also, the PIC16C924 drives the display (LCD) and controls the triac and current sense input (for true RMS current measurements).

AC voltage (90V - 264V) is applied to the AC input and a load is connected to the AC output. AC current flow is from the AC input to the AC output and through the load. AC current continues through triac switch and current sense transformer back to the AC input.

Firmware Overview

The PIC16C924 firmware controls the Watt-Hour Meter operational modes and calculates power forms for display. There are five (5) modes of operation for the PICREF-3:
• Reset
• Entry
• Measurement
• Sleep
• Hibernation
The 8-digit LCD is used to display information in each mode. Each digit has seven segments. Pushbuttons are used to control display output and set parameters.