Power Monitor

Posted on Feb 5, 2014

The purpose of this project was to be able to monitor the power consumption at my house. In addition to total usage, I wanted to be able to separately monitor & compare usage of major appliances (water heater, heat pump, furnace, etc. ). This would require current transformers on individual circuits. At $75+ each, most split-core current transforme

Power Monitor
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rs I could find were too expense in the quantity I wanted. Fortunately, CR Magnetics released a new product, CR3110-3000. This split-core is a little too small to monitor the main power lines, but just right for individual circuits. And they are reasonably priced, under $13 each, allowing me to complete this project for about $160. The split-core current transformers are clamped around circuit lines in the breaker box. I have (6) 240V circuits and (8) 120V circuits. For the 240V circuits, I put one transformer on one leg only (more on this below). For the 120V circuits, I doubled them up, putting (2) circuits in each transformer. Thus (10) current transformers where required. This fit with the 11-channel A/D I wanted to use, leaving one channel for monitoring line voltage. Each current transformer is connected across a burden resistors, yielding an AC voltage proportional to the AC current. The value of the burden resistor (100 ohms) was selected according to the CR3110-3000 datasheet so as to keep the transformer in its linear region of operation to 50+ amps. The AC voltage is then covered to DC via an interface circuit consisting of a precision half-wave rectifier followed by a low-pass filter w/ 8:1 gain (schematic below). The output is approximately 0-5V DC for 0-50 Amps AC. A 12-bit A/D results in a resolution of ~. 01 amps or 1. 5W for 120V circuits, 3W for 240V. The line voltage is monitored with a similar interface...

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