This circuit monitors the input voltage (VIN) and safeguards the output voltage using a series-connected transistor (Q14). In the event of over-voltage conditions, Q14B is activated, which in turn causes Q14A to disconnect the output from VIN. The desired overvoltage limit is established by the combination of D10 and the voltage across the base emitter (VBE) of Q14. For instance, if the required cutout voltage is 12V, the Zener voltage of D10 should be 11.4V (12V - 0.6V).
You can view a simulation of the above circuit in the video below.
