A potential divider circuit is connected with a dc source of 20 V, a light emitting diode of glow in voltage 1.8 V and a zener diode of breakdown voltage of 3.2 V. The length (PR) of the resistive wire is 20 cm. The minimum length of PQ to just glow the LED is _____ cm.
Answer: 5
Solution
The LED (1.8 V) and Zener (3.2 V breakdown) together need 1.8+3.2=5 V across PQ before conduction/breakdown starts, i.e. the LED just glows once VPQ=5 V. Since the 20 V source is spread uniformly over the 20 cm wire PR, 1 V corresponds to 1 cm, so
