# Ac Watts Calculator Circuit

Posted on Jan 10, 2013

The load`s power factor, which is the cosine of the phase angle between the voltage across the load current, can be calculated simply with this circuit. A1:1 isolation transformer is used to prevent direct contact with the line. By properly adjusting Rr, the vector diagram of voltages Vs, Vd, and Vr forms an isosceles triangle, which simplifies the power calculation. The method basically consists of determining the power factor of the loadthe cosine of the phase angle between the voltage across the load and the load circuit. Using a simple circuit, that angle can be calculated quite simply. This circuit uses a 1:1 isolation transformer to prevent direct contact with the line.

It is wise to proceed with caution whenever voltages of this magnitude are utilized in a test setup, even though the voltages that will be measured are usually below 1 V. Rs is a circuit-sense resistor and Rr is a multi-turn potentiometer. The voltage across Rr is approximately 0.5% of the line voltage, which should be sufficient for most applications, Rr is adjusted so that V = tFj; then Vd is measured. In the vector diagram according to Kirch-hoff`s voltage law, Vs, Vd, and Vr form a triangle, which becomes isosceles by adjusting Rr. Vs is in phase with the load current and Vr is essentially in phase with the load voltage.

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