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04-08-2008

Optoisolators Compute Watts and Volt-Amperes

Relying on the dynamic conductance of optoisolators' junctions, simple optoisolator bridges compute the watts and volt-amperes of an ac load.

W Stephen Woodward, Chapel Hill

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 ac-power 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 0.001 Ω 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.

Optoisolators Compute Watts and Volt-Amperes

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 half-wave 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 left-hand 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.

The right-hand side of the circuit

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