Silicon diode in series with a resistor (no diagram given): If the P–N junction is made of silicon and is forward biased from a 12 V source, what voltage will appear across resistor R1 (assume a typical silicon junction drop)?

Introduction / Context:
Many introductory circuits place a single silicon diode in series with a resistor and a DC supply. When the diode is forward biased, it conducts and exhibits an approximate ”barrier” or junction drop. Determining the voltage across the series resistor is a standard, practical application of the diode forward-voltage model used in power rails, indicator LEDs, and rectifier droppers.

Given Data / Assumptions:

• Supply is 12 V DC.
• There is one silicon P–N junction in series with resistor R1.
• The diode is forward biased and in normal conduction.
• Use the common approximation: V_D(silicon) ≈ 0.7 V at modest currents.
• Ignore small variations of V_D with current and temperature for this calculation.

Concept / Approach:
In a simple series circuit, the source voltage divides among series elements. A silicon diode operated forward-biased will drop about 0.7 V, and the remaining voltage appears across the resistor. This is a direct application of Kirchhoff’s Voltage Law with the constant-voltage diode model, which is sufficiently accurate for quick estimates and exam problems.

Step-by-Step Solution:

Verification / Alternative check:
If the diode were germanium (about 0.3 V), the resistor would see roughly 11.7 V. If the diode were open (no conduction), the resistor would see 0 V because no current flows. These boundary checks confirm the 11.3 V result is consistent with a silicon junction in conduction.

Why Other Options Are Wrong:

• 12 V: Would ignore the diode's forward drop.
• 11.7 V: Closer to a germanium-like 0.3 V drop, not silicon.
• 0 V: Would imply an open circuit or reverse bias, not the stated forward-biased condition.

Common Pitfalls:
Forgetting that diode forward voltage is not exactly constant; it varies with current and temperature (typically 0.6–0.8 V for silicon). However, 0.7 V is the accepted exam approximation unless the problem provides a specific value. Also, ensure polarity is such that the diode is indeed forward-biased.

Final Answer:
11.3 V