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Optoisolators compute watts and volt-amperes

A decade or so ago, I designed a simple circuit that included a quad optoisolator arranged in a full-wave analog-multiplier bridge (Figure 1). It sensed and calculated watts of acpower consumption and ignored any reactive component in the load. The circuit’s principle of operation relies on the fact that the LEDs of the bridge, like any other device with a semiconductor junction, have a dynamic conductance that’s directly proportional to current: approximately 19 mS (millisiemens)/A at 25°C. Both the line voltage and load-current-proportional sense voltage, which the 0.001Ω copper shunt develops, modulate this current. The approximately 0.4%/°C temperature coefficient of the copper compensates most of the temperature dependence of the LEDs’ conductances.

The circuit in this Design Idea is an elaboration on that older circuit. It acquires not only watts, but also volt-amperes and so makes possible an estimation of power factor—watts divided by volt-amperes. The right-hand side of the circuit in Figure 2 is simply a halfwave version of the older circuit. The left-hand side is similar but substitutes rectified-dc excitation of its half-wave bridge for the ac excitation of the lefthand side. The analog product of instantaneous load current times the average voltage optically couples to phototransistor Q₄/D₄, which A₂ amplifies and the Q₅ through Q₈ transistor array rectifies to provide an analog voltage proportional to load volt-amperes.