Voltage Regulation

NCERT describes a Zener voltage regulator in which an unregulated input is applied through a series resistor to a Zener diode connected in reverse breakdown. When the Zener is regulating, it holds its terminal voltage fixed at the Zener voltage, here 6 V, while the remainder of the input voltage must be dropped across the series resistor. Applying Kirchhoff's voltage law, the resistor voltage equals the input minus the Zener voltage, so $V_R=12-6=6$ V. The value 3 V is wrong because it would only account for half the excess and does not satisfy the voltage loop. The value 12 V is wrong because it ignores the regulated 6 V held by the Zener across the load. The value 18 V is wrong because it adds the voltages instead of subtracting, which violates Kirchhoff's law for a single loop. As stated in NCERT Class 12, Chapter 14, the series resistor absorbs the difference between the input and the Zener voltage. A consistency check confirms that 6 V across the resistor plus 6 V across the Zener sums exactly to the 12 V input, as required for the loop.