Start of a HomePowerMonitor Design

So what does one do when you want to put in a whole home power monitor? Hit Google of course. After many hours of searching, I found numerous consumer power monitor systems and countless industrial systems. The industrial systems where mostly completely outside my league. Most did not even including pricing (“If you have to ask, you can’t afford it”). Of the consumer systems most were lacking.

The most common deficiency was that lack of individual branch circuit monitoring. They would either look at the whole house as one measurement, or at best give you a measurement on each phase. No consumer system had the hooks to instrument each branch and that was what I really want.

But in looking at all these systems, I started to get the idea about how they worked. For the single measurement types, they would often clamp a device on the meter to watch the wheel spin or the light blink. Or they might clamp a sensor on the main power line to measure the magnetic field generated on one phase. The two measurement systems used clip on current transformers (CT).

These doodads are little plastic devices you snap over a single wire. Inside is a metal doughnut and wire windings that generate current in proportion to the amount of current flowing through the wire you instrumented. The current can be transformed into a voltage with a “burden resistor” and monitored using your choice of electronics.

The output of a CT is not an average current, but rather an instantaneous one. Since homes run on AC power, this means the CT generates an AC current and (via the burden resistor) and AC voltage. You therefore need some smarts in your electronics to convert an AC voltage of varying amplitude into a measure of current.

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