Electronic Design

Simple digital AC wattmeter

To make useful measurements of “real” power consumption, wattmeters must be accurate despite large reactive and dc components in the current drawn by the monitored load. Moreover, power consumption from multiple ac phases (two in the case of a typical 220-V residential power panel, three in the case of 208-V industrial ac) must sometimes be simultaneously monitored and summed.

This simple, optically isolated wattmeter satisfies these criteria. It converts power readings from one or more phases to (given the circuit values illustrated) a 1-pulse/joule (1 Hz/W) frequency output. Optionally, it can be configured to run from an interface to the COM port of a standard PC. PC COM port hardware then interprets each 1-joule pulse as the “start bit” of a valid (although meaningless) ASCII character. Simple BASIC software (see the listing) running on the PC can monitor and average the frequency of character reception as an accurate measure of power consumption. Somewhat more sophisticated software could capture and store detailed logs of power consumption over arbitrarily long time periods.

The core of the wattmeter circuit is a quad-channel optoisolator consisting of LEDs E1-E4 and phototransistors Q1-Q4 connected in a double-bridge arrangement (see the figure). This arrangement acts as a variant (admittedly peculiar) of the well-known “Gilbert cell” analog multiplier to compute the four-quadrant product of ac line voltage and Z1 load current.

The principle of operation of the optobridge multiplier is explained in an earlier Idea for Design (“Optical Isolator Computes Watts,” Electronic Design, Oct. 14, 1994, p. 102). The multiplier’s output is a current drawn from C1 that’s proportional to the true instantaneous power delivered to the load. Accuracy is maintained even if the line voltage wanders and the load is reactive and nonlinear. If multiple ac power phases are to be monitored, only resistors R1 and R2 and the optoisolator need be duplicated for each additional phase. The phototransistors of the additional isolator(s) should be paralleled with Q1-Q4.

The current-to-frequency converter formed by reference VR1, comparators A1 and A2, and associated discretes, converts the passively summed power-proportional currents to a 0-1200 Hz output appearing at A2 (pin 7). If the wattmeter is to be used with a PC COM port, optional regulator VR2 can be added to develop a sufficient +5-V supply for wattmeter operation from the port “handshake” signals. No other dc power source is necessary. Meanwhile, Q6 will convert A2’s output pulses to RS-232 levels.

In the prototype, the 0.002-O shunt (R2) consisted of 5.8 inches of 16 gauge copper wire. Copper’s 3900 ppm/°C coefficient of resistance serves to temperature compensate most of the scale-factor variation due to LED and phototransistor temperature coefficients. Zero stability (voltage feedthrough) of the wattmeter is very sensitive to temperature gradients in the optoisolators. They should be shielded carefully from drafts.

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