AC_WATTMETER_CIRCUIT

Published:2009/7/16 20:35:00 Author:Jessie | From:SeekIC

A true wattmeter is a complicated instrument. But if all the wattages that you want to measure are at the same voltage (120 Vac) and the loads are resistive (such as lights, heaters, or motors under load), you can just pass the current through at 0.12-Ω resistance (four 0.47-Ω, 5-W resistors in parallel), connect an ac voltmeter across the resistance, and read volts as kilowatts. That is, every volt across the resistance corresponds to 1000 W (equal to 8 A) drawn by the load. Remember that the voltmeter terminals are hot; use insulated pin sockets so that you won't touch the contacts accidentally. On inductive or capacitive loads, such as unloaded motors or transformers and computer power supplies, your wattmeter will read high because the current and voltage are not in phase.



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