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?
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Ato limit the current and to protect LED against over voltage
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Bto limit the voltage and to protect LED against over current
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Cto limit the current and protect LED against reverse breakdown voltage.
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Dnone of the above.
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Eto boost forward current and to clamp forward voltage
Answer
Correct Answer: to limit the current and protect LED against reverse breakdown voltage.
Explanation
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.
Step-by-Step Solution:
Forward conduction: R sets I_LED to a safe value by Ohm’s law.Reverse polarity or transient: D conducts first, keeping Vreverse across LED ≈ 0.7–1 V rather than several volts.Therefore, R limits current; D protects against reverse breakdown.Verification / Alternative check:
Check LED datasheet: maximum reverse voltage ratings are low; external diode is recommended in AC drive or inductive environments.Why Other Options Are Wrong:
R does not “limit voltage” directly; it is a current-limiting element.D does not protect primarily against forward “over-current”; that is handled by R.“None” and “boost current” options contradict safe design practice.Common Pitfalls:
Omitting the antiparallel diode when driving LEDs from AC or when inductive kick can occur.Final Answer:
to limit the current and protect LED against reverse breakdown voltage.