In the LED protection circuit (series resistor R with the LED, and a diode D connected to protect the LED), what are the functions of R and D respectively?

Introduction / Context:
LEDs are current-driven devices with a relatively fixed forward drop over a small current range. Practical LED circuits therefore include a series resistor to set current and a protective diode across the LED (usually antiparallel) to guard against reverse voltage during polarity reversals or inductive transients.

Given Data / Assumptions:

• Series resistor R is present with the LED for current control.
• A diode D is wired in antiparallel (or an equivalent protective orientation) to the LED.
• Reverse breakdown of typical indicator LEDs is low (often 5 V or less), so reverse stress must be limited.

Concept / Approach:
The LED must be protected from excessive forward current and from reverse breakdown. The resistor limits forward current: I ≈ (Vs − Vf)/R. The protection diode D provides a safe reverse path with low drop, clamping the reverse voltage across the LED to approximately a diode drop, thereby preventing reverse breakdown of the LED junction.

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