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. Numerical answer type. Work it out, then open the answer.

Answer: 5. The LED is forward biased and the Zener is reverse biased in series between P and Q. To make the LED glow: VPQ ≥ 1.8 + 3.2 = 5 V. Along the uniform wire, potential drop per cm = (20 V)/(20 cm) = 1 V/cm. PQmin = (5 V)/(1 V/cm) = 5 cm

A potential divider circuit is connected with a dc source of 20 V, a… | JEE Main 2024 (8 Apr, Shift 2)

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Special Purpose Diodes

A potential divider circuit is connected with a dc source of $20\text{ V}$ , a light emitting diode of glow in voltage $1.8\text{ V}$ and a zener diode of breakdown voltage of $3.2\text{ V}$ . The length (PR) of the resistive wire is $20\text{ cm}$ . The minimum length of PQ to just glow the LED is _________ $\text{cm}$ .

Numerical answer type. Work it out, then open the answer.

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Answer: 5

The LED is forward biased and the Zener is reverse biased in series between P and Q.
To make the LED glow: $V_{PQ} \geq 1.8 + 3.2 = 5$ V.
Along the uniform wire, potential drop per cm $= \dfrac{20\text{ V}}{20\text{ cm}} = 1$ V/cm.
$PQ_{min} = \dfrac{5\text{ V}}{1\text{ V/cm}} = 5$ cm

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