μblog: engineering from the trenches

The swell people over at Analog Devices have created an application note that overviews making a power meter that converts average power into a frequency output that then drives the mechanical counter. The idea is that high-side current is measured resistively and then compared with the applied voltage to the load. The two signals are multiplied giving an instantaneous power. Providing that both the applied current and voltage are sinusoids and vary only in amplitude and phase, the DC component of the instantaneous power signal will be an indication of power delivered to the load and will depend on the power factor (the phase between voltage and current). I am thinking of simplifying the design and using a low-side measurement, a pair of differential amplifiers and a four quadrant multiplier to produce the instantaneous power. Depending on the components available, I can try to use a two-pole Sallen-Key low-pass filter to get the DC out with two decades of ripple suppression.

To clarify, I became interested in building a power meter when I found out that the Wii uses 17W of power in operation and 10W in standby (when Connect24 is enabled). I am wondering what other electronics around the house have such a conservative standby mode.

( an679.pdf )

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