What is the function of zener in this circuit

[Murugesh_89]

Hi,
In the attached circuit what is the function of the zener diode and what will be its value?
Also is there any other output protection is necessary for this NPN NO type circuit?



Thanks,
Murugesh

 

[godfreyl]

The Zener diode limits how high the output voltage can rise when the transistor switches off with an inductive load. It's value will be higher than the supply voltage, but lower than the transistor's breakdown voltage.