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A 7 V DC supply is applied across a silicon diode in series with a 1 kΩ resistor. What is the approximate current flowing through the diode?

Difficulty: Easy

Correct Answer: 6.3 mA

Explanation:


Introduction / Context:
Diode bias problems are common in introductory electronics. When a forward bias voltage is applied with a series resistor, the diode drop is approximately constant (≈0.7 V for silicon), and the remainder of the supply drops across the resistor. Current is then easily computed using Ohm’s law.


Given Data / Assumptions:

  • Supply voltage Vs = 7 V.
  • Series resistor R = 1 kΩ.
  • Forward diode drop Vd ≈ 0.7 V for silicon at normal currents.


Concept / Approach:
Current through the circuit is set by the voltage across the resistor: I = (Vs − Vd)/R. Diode drop is assumed constant for estimation.


Step-by-Step Solution:

Vs = 7 V, Vd = 0.7 V.Voltage across resistor = 7 − 0.7 = 6.3 V.I = 6.3 / 1000 = 0.0063 A = 6.3 mA.


Verification / Alternative check:

Checking: If diode drop were taken as 0.65 V, current would be 6.35 mA, still ≈ 6.3 mA. The approximation is valid.


Why Other Options Are Wrong:

7 mA: ignores diode drop.0.7 mA: divides incorrectly by 10.0: false, since diode is forward biased.10 mA: requires different values of resistor/supply.


Common Pitfalls:

Forgetting to subtract diode drop; mixing forward drop of Si vs Ge diodes.


Final Answer:

6.3 mA
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