A 4N25 optocoupler boasts an isolation test voltage of 5,000 V between the LED and transistor. If you put 500 V across it, you would see an effective resistance of more than 10 12 Ω.

This design uses a low-power ZCVD that consists of a half-wave rectifier feeding into a 4N25 optocoupler (Fig. 2). 2. The zero-crossing voltage-detector (ZCVD) circuit employs an optoisolator.

An improved circuit with an optocoupler type TLP621 is shown in Figure 2. Figure 2 A circuit with an optocoupler (TLP621) which generates higher (even) harmonics that can be added to the original ...