Circuit Design Review: Optocoupler-Based Power Measurement Circuit

Human existence is based upon two pillars: Compassion and knowledge. Compassion without knowledge is ineffective; knowledge without compassion is inhuman.

— Victor Weisskopf. I attended a public lecture he gave and found him the most charming physicist I have ever encountered.

I just got back home after attending the Optical Fiber Communications conference in Los Angeles. This is my favorite trade show and I learned quite a bit. The flight home from Los Angeles to Minneapolis (I live near Minneapolis) takes over 3 hours, so I had time to relax on the flight and do a little bit of circuit analysis -- yes, that is my idea of relaxation.

Recently, I have been analyzing a number of circuits that measure power because my company has become very interested in having our products measure their own power usage. This interest is being driven by the Smart Grid activities of the US government and the power companies. The current focus of Smart Grid is on making all power usage measurable and I am busily working on putting that feature into our products. The program's ultimate focus will be to reduce peak usage, but that is a topic for another post.

I have been collecting circuit designs that measure power simply and cheaply. I found a W. Stephen Woodward design in EDN magazine that was interesting, and I thought I would spend some time reviewing it. Figure 1 shows the best image of the schematic I could find.

Figure 1: Woodward Schematic.

When I review a circuit, I usually begin by starting a new Mathcad worksheet and working my way through the circuit. This particular circuit proved to be clever, but probably not accurate enough for what I need. I thought some folks might find the analysis interesting to read. It was all done in Mathcad and I PDFed my worksheet to make it generally available. This is the same template that I use for most of my personal analysis work.

AC Power Measurement Using Optocouplers

You will often see a little dinosaur symbol on my work. I am often asked about it. I use that symbol because some of the young engineers refer to us old analog guys as dinosaurs. Some days I feel like one!