In PV grid connected inverter, in order to inject the current at unity power factor we need to know the polarity of the grid . To do so various solutions exists. Among these solutions is the use of optocoupler. I have verified the use of 4N26/4N37 for the said purpose. The circuit is very simple and its working is given below.

Pin Configuration of 4N26/4N37

The circuit fabrication is shown in Fig 2 and its working is as follows

1. Connect  a step down AC voltage (preferably to 9 V) via a 1k ohm resistor to pin #1 and 2.
2. Connect the biasing voltage V_cc= 5V to pin #5  via 1k ohm resistor and ground with pin #4.
3. During the positive half cycle of the AC waveform, the LED will be forward biased and hence the current will flow in the base of the phototransistor. This means that all the voltage will be dropped across the pull up resistor and no voltage will appear at the output. However, during the negative half cycle the LED becomes reverse biased and hence no current will flow in the base of the phototransistor and hence the biasing voltage V_cc will apear on the output. Thus, for each one complete cycle a square waveform will appear.

Circuit for AC voltage polarity detection