Tech Tip: Measuring Energy Requirements for Home Appliances

As energy costs continue to rise and as alternative power systems like solar energy become more cost effective, it is important to know how much energy your electrical appliances are using. Since residential and commercial facilities use AC power, the AC voltage and power values must be converted to the equivalent DC values required by electronic instrumentation. This article will explore one way to convert the AC quantities into DC values suitable for measurement by modern electronic circuitry.

There are several methods you can use to convert current to voltage including using a shunt resistor or a current transformer. The AC voltage produced must be converted to DC values. You will not be able to use a simple rectifier circuit due to the low level signal produced by the shunt or transformer and therefore you will need an active circuit that will eliminate the forward voltage drop produced in a simple rectifier arrangement. This will allow you to measure very small AC current values with a small value shunt or standard current transformer.

The simple full-wave active rectifier circuit shown below is only an example circuit and is by no means the only possible implementation. This circuit uses readily available components and is easy to build on a simple prototyping printed circuit board. This circuit should produce a DC output voltage that is proportional to the AC current value being drawn by the electrical appliance whose energy consumption you’re measuring. If you need a different scale factor you may use the unused op-amp in the LM324 quad op-amp to provide additional gain or scaling.

Because electrical power is what we’re interested in, we should also measure the AC voltage powering the appliance. But for a first order approximation we can simply substitute in the known AC voltage of the household wiring, since this value does not change appreciably and is normally maintained at a constant level by the power company at a nominal 120 volts. We can use the Ohm’s law power formula P = V x I (Power = Voltage x Current) to determine the power dissipated by the appliance.

P = V x I = Voltage x Current = Watts

This application note has shown a simple method you can employ to obtain a reasonably accurate indication of the energy consumption of a standard household electrical appliance. The resulting voltage can be suitably scaled and displayed on a digital panel meter or used in any data acquisition system. The circuit diagram for the active rectifier circuit is shown below.

Click to enlarge


George Leger has a Masters in Electrical Engineering from Stanford University, worked in private industry pioneering surface-mount technology and in government research labs for twenty years, published several papers on surface-mount technology, co-authored papers published in national symposiums on accelerator technology, was past president of SMTA and an adjunct professor at the community college level, holds a patent, and is a certified microchip design partner, serving as a consultant to many companies developing electronic circuits.

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