Electronic Design

Easy-To-Build AC Power Meter

If your goal is to save on electricity, you must first measure the ac current flowing through your appliances. The formula for power is the product of the ac line voltage and the measured current. Presented here is a demonstration of how to build an ac power meter using a current transformer (see the figure).

The current transformer is similar to an ordinary voltage transformer, except that a primary wire is generally absent around the core. The primary coil is constructed by winding either one of the wires connecting an ac plug and an outlet around the core (two turns, in this case).

Current ratio flowing through the primary and secondary wires is inversely proportional to the turns ratio of the transformer. The secondary ac current is rectified and smoothed by D1 and C. The R1 and R2 shunt resistors yield a correct full scale for the meter. You may need to combine a few standard value resistors to create the odd-valued R1 and R2. D2 is used to bypass the opposite-phase current.

The transformer may be broken if the secondary circuit is open with a high current on the primary wire. Of course, a voltage transformer doesn't fire when the secondary is short-circuited.

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