依靠光耦连结点的动态导纳，简单的光耦桥计算了交流负载的瓦特和伏安。
大约十年前，我设计了一个简单电路，包括按全波模拟乘法器电桥排列的四个光耦（图1）。它感受和计算交流能量消耗的瓦特，且忽视任何其反作用的负载器件。电路的工作原理为桥式LED，像其他半导体结点设备一样，具有与电流成正比的动态导纳：25°C时，约19 mS (millisiemens)/A。0.001Ω铜分流器产生的线电压和与负载电流成比例的感应电压，都调节这个电流。铜的约0.4%/°C温度系数补偿了大多数LED电导的温度影响。
本设计方案的电路是以前的经典电路。它不仅获取了瓦特，还得到伏安，使判断功率因数成为可能——瓦特除以伏安。图2中电路的右侧为老电路的简单半波电路。左侧相似，但用交流激励替代其半波的整流直流激励。瞬间负载电流与平均电压的模拟乘积，光耦合到光电晶体管Q4/D4，A2放大，Q5再通过Q8晶体管阵列整流，提供了与负载伏安成比例的模拟电压。
英文原文：
Optoisolators compute watts and volt-amperes
Relying on th
e dynamic conductance of optoisolators' junctions, simple optoisolator bridges compute the watts and volt-amperes of an ac load.
W Stephen Woodward, Chapel Hill, NC; Edited by Charles H Small and Fran Granville -- EDN, 2/21/2008
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 0.001Ω 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 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.
英文原文地址：http://www.edn.com/article/CA6531585.html