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. [figure]
Answer
5
Solution
[figure]
LED (forward) and zener (reverse, breakdown) are in series across PQ.
LED just glows when VPQ = 1.8 + 3.2 = 5 V
Potential gradient of wire = 20 V20 cm = 1 V/cm
PQmin = 51 = 5 cm
Answer: 5