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: a 20 V source across a un…

Question

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: a 20 V source across a uniform resistive wire PR. A sliding contact Q on the wire. Between P and Q an LED (forward biased) and a Zener diode (reverse biased) are connected in series.]

Accepted Answer

5. To glow, the LED needs 1.8 V and the Zener must break down at 3.2 V: total 5 V across PQ. The wire drops 20 V over 20 cm: 1 V/cm. PQ=5/1=5 cm

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