Optocoupler simplifies power-line monitoring

The use of a linear optocoupler and a capacitor-based power supply yields a simple, yet precise power-line-monitoring system. The circuit in Figure 1 converts the 110V-ac power-line voltage to an ac output voltage centered at 2.5V, covering 0 to 5V. The circuit isolates the output signal from the power line. You can connect the output directly to an A/D converter. For other power-line voltages, simply change the value of R₁. For a power-line voltage of 220V ac, use a value of 470 kΩ for R₁. The input stage is a nonisolated block that uses the neutral line as a ground reference. This block receives power from a capacitor-based power supply that provides a stabilized 5V-dc voltage and a 3.3V dc reference. The TLC2272 op amp, IC₁, and the TLC2272 linear optocoupler, IC₃, form a feedback amplifier in which the I_P1 current is proportional to the input voltage, V_IN.

Resistor R₂ adds a dc offset current to allow for both polarities in V_IN. The match between the two photodiodes in the IL300, IC₂, ensures that I_P2 is closely proportional to I_P1. The output stage converts I_P2 to a voltage level isolated from the power line. Variable resistor VR₂ trims the overall gain, and VR₁ adjusts the output-voltage offset, which is nominally 2.5V. You can test this circuit using simulation the model in Listing 1 for IC₂. Typical values for K₁ and K₂ (optical transfer ratios) are approximately 0.007. The global optical transfer ratio is K₃=K₂/K₁. After performing the simulation, you can build and test a prototype. The power supply for the isolated block provides 5V dc and a 3.3V reference from an available voltage of 7 to 10V. You do not need the regulated 5V if that voltage is already available in your system.

An important goal in this design is to obtain a stable dc voltage at the output. This property is crucial for dc measurements of V_IN. Even if you suppose the ac power line to be free of dc voltage, some types of loads drain dc currents, thereby introducing a small dc voltage because of voltage drops in the ac lines. Thermal drifts in the output voltage stem principally from drifts in K₃. In tests of the prototype, the K₃ temperature coefficient was 470 ppm/°C. Table 1 shows V_OUT at different temperatures. The TLC2272 op amp has rail-to-rail output, yielding a wide output-voltage range, and low quiescent current, simplifying the capacitor-based power supply. Because the TLC2272 is a dual device, you can connect the unused half as a voltage follower. When you monitor a three-phase power line, you'd use one and one-half TLC2272s. Note that the op amps in the isolated block, IC₃, and the nonisolated block, IC₁, cannot be halves of the same chip; otherwise, you'd lose the isolation.

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