The circuitry in Fig 1 allows the ICL7106/7107 digital-voltmeter IC, IC4, to measure power factor to 0.1% resolution and operate from a single 9Vdc supply. An important feature of the circuit is that variations in load current, line voltage, and line frequency do not affect measurements. For inductive loads, power-factor measurements are accurate to 1% in the range of 0.85 to 1.
The circuit derives two dc voltages, VM and VMcos[theta], that go to the REF HI and IN HI inputs of IC4, yielding the power factor (VMcos[theta]/VM). A unity-gain follower, a precision rectifier (IC1A and IC1B), and an RC filter obtain VM by rectifying the waveform from the secondary winding of the voltage transformer, T1.
A synchronous detector (IC2 and IC3) derives VMcos[theta]. The current transformer, T2, provides the waveform, v(t)=kIOsin(2[pi]ft±[theta]). Buffer IC1B squares this waveform, deriving control waveforms for the synchronous detector. The synchronous detector operates on the output of the input-voltage follower, IC1A, yielding a dc output equal to VMcos[theta]. (DI #1403)