We are using 4N33, optically coupled isolator (datasheet link). The circuit is shown below. In the schematic with R3--> 10K, it was not working so we change to 200K ohm, C2 & ZD1 are not connected. The 4N33 optocoupler is part of the circuit where it is being used for 28V power sensing. The circuit where it is being used has linear voltage regulator of 9V & 5V connected. When the board is being powered ON, we are getting voltage at 3.3V bus, even without connecting 3.3V linear voltage regulator we are getting varying voltage between 0.7V to 2.8V. We are not able to understand even if 28V is not connected, we getting 0.7V to 2.8V in the 3.3V bus. After removing the 4N33 from the IC base, the voltage at 3.3V bus returned to zero.
Then we change the resistor R3 to 6.8M ohm, even with the IC connected the voltage is 0.02V.
Is it EMI noise which is getting amplified. How come without giving 3.3V we are getting voltage.