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Optoisolators calculate watts and volt-amperes

11 Apr 2016  | Stephen Woodward

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Several years ago, I designed a simple circuit that included a quad optoisolator arranged in a full-wave analogue-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 0.001Ω 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.


Figure 1: A quad optoisolator arranged in a full-wave analogue-multiplier bridge senses and calculates watts of ac-power consumption and ignores any reactive component in the load.


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 analogue 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 analogue voltage proportional to load volt-amperes.


Figure 2: The right-hand side of this circuit is simply a half-wave version of the circuit in Figure 1. The left-hand side is similar but substitutes rectified-dc excitation of its half-wave bridge for the ac excitation of the right-hand circuit.



About the author
Stephen Woodward contributed this article.


This article is a Design Idea selected for re-publication by the editors. It was first published on February 21, 2008 in EDN.com.




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