This circuit shows how a voltage regulator functions. Voltage regulators are found in numerous devices like power supplies, lamp modules etc. With voltage regulators the input voltage is not one fixed value, for example 12 V. but can vary between 12 and 30 V. In the scenario of this circuit, the voltage regulator limits the output voltage to 12 V.
When the input voltage is going below 12 V. the output voltage will drop also.
