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Abstract

A watthour meter, preferably for a wye connected load, includes a current transformer for sensing the load current, a resistive voltage divider network for sensing the line voltage, and a measuring circuit for calculating the AC electrical energy consumption of the load based on the sensed load current and line voltage. The load consumption calculations are performed using a Mark-Space oscillator and amplitude modulator in the watt transducer chip. Reduced voltage from the resistive voltage divider network is fed into the input junction of the Mark-Space oscillator. A feedback resistor is connected from the output junction to the input junction of the oscillator. The feedback resistor and the voltage divider network are mounted on the same thermally conductive substrate so that each are subject to the same thermal effects. Thus, any change, due to thermal effects, in the current through the voltage divider network seen at the input junction to the Mark-Space oscillator are compensated by a current change through the feedback line due to thermal effects on the feedback resistor. The watthour meter also includes a electronic register that receives the load consumption information from the watt transducer chip. The register is electrically isolated from the watt transducer chip and, therefore, from the voltage divider network, by an opto-isolator to reduce the possibility of electrical shock at the register. Power is provided to the watt transducer chip and the electronic register from the split secondary coils of a power transformer.

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