SMPS Zener diode replacement
It is working as Voltage Regulation for UC3842
You should use 24V zener
So let's understand how it is working When your tapped pin of transformer gets &gt; 24V Zener start conducting and the 1K resistor sees Vtapped-24V on one of its end. The resistor sees 0.7V on its other leg because of its transistor base. The voltage drop on the resistor is (Vtapped-24V - 0.7V). If this voltage is &gt;0v transistor starts conducting and the COMP pin gets 0V which disable the output signal for the Mosfet driver of your SMPS. If the voltage comes below 24 Volts it again starts the signal. This way it regulates the voltage.