JPS54161371A - Wattmeter - Google Patents

Abstract

PURPOSE:To reduce the drift error under a light load condition by generating voltage signals proportional to an instant watt at the multiplying state between the voltage signals proportional to the load voltage of a supply line and the voltage signals proportional to a consumption current and by introducing the same into an integrating circuit. CONSTITUTION:Voltage signals ev proportional to a load voltage are subjected to pulse width modulation 11 thereby to produce signals of duty cycles D. On the other hand, voltage + or -ei signals proportional to a consumption current is supplied to analog switches S1 to S4 so that the signals D and their reversed signals D may alternately open and close the switches S2, S3 and S1 and S4. Thus, DC signals eOP and eON having different polarities but the same absolute values (which are voltage signals proportional to the instant watt) and are selectively integrated at 13 through switches Sa and Sb until they are fed to a comparator 14. As a result, an output frequency f0 takes a value proportional to the consumption current, on which calculations are performed to obtain an integrated watt.