I am working on a project which require to activate a 24V Rated relay using a 3.3V rated IO. To provide adequate isolation I have added a Opto-isolater stage.
In order to increase the current output capacity I have added a NPN Transistor Switch and controlling it using the Opto-Isolator output.
The main issue I have encountered in past (Using only Opto-isolator to drive relay) is the output is accidentally connected to the 120V AC supply which potentially blow up the Opto-isolator.
I have following questions related to the over-voltage protection concept:
The transistor's reverse breakdown voltage (VEBO) is 6V, even if I use a over-voltage protection diode the Emitter will observer at least -25V upon AC application (if Zener is 25V rated), In this case how can I ensure that the transistor switch will tolerate the reverse bias (By adding addition components?)
Is there any other method can be used to provide the isolation with 120V AC protection?
Here is the circuit-
Option A and B can be used to activate the Transistor switch
The relay I am using is Omron -G7SA-2A2B, rated 24V, Coil resistance 1600 ohm, Min vol = 75% of rated 24V, Max Vol =110% of rated 24V
The circuit is for the failure case when the 24 V/80mA rated output get connected to the 120V supply. In this case the transistor output gets +/- 120V.