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Difficulty: Very hardDiodes

A circuit is constructed with a 10 V10\text{ V} DC power supply connected in series with a 220 Ω220\ \Omega current-limiting resistor. After the resistor, the circuit splits into two parallel branches that eventually reconnect to the negative terminal of the power supply. The first parallel branch contains a Zener diode (rated at VZ=6.2 VV_Z = 6.2\text{ V}) installed in a reverse-biased orientation. The second parallel branch contains a standard silicon diode installed in a forward-biased orientation. Based on this configuration, what is the approximate voltage measured across the parallel diode combination, and which component dictates this voltage?

  1. 0.7 V0.7\text{ V}, dictated by the forward-biased standard silicon diode clamping the voltage.Answer
  2. B
    6.2 V6.2\text{ V}, dictated by the reverse-biased Zener diode regulating the branch voltage.
  3. C
    6.9 V6.9\text{ V}, dictated by the combined additive voltage drops of both diodes in the circuit.
  4. D
    10 V10\text{ V}, dictated by the power supply because the reverse-biased Zener diode acts as an open circuit.

Answer

The correct answer is 0.7 V0.7\text{ V}, dictated by the forward-biased standard silicon diode clamping the voltage.
The correct answer accurately identifies that in a parallel circuit, the component with the lowest forward voltage drop or breakdown voltage dictates the voltage across the entire parallel block. Since the standard silicon diode is forward-biased, it drops approximately 0.7 V. This prevents the voltage across the parallel branches from ever reaching the 6.2 V required to cause the Zener diode to break down and conduct. The Zener diode remains effectively an open circuit, and the standard diode clamps the branch voltage at 0.7 V.

Step-by-Step Solution

1
Determine the biasing state of each diode in the parallel branches based on the power supply orientation.
The Zener diode is reverse-biased (requires 6.2 V to reach breakdown), and the standard silicon diode is forward-biased (requires ~0.7 V to conduct).
A diode's behavior depends fundamentally on whether it is forward or reverse-biased.
2
Analyze the voltage rules for parallel circuits.
The voltage across all components connected in parallel must be identical.
This fundamental circuit law means both the Zener diode branch and the standard diode branch must share the exact same voltage drop.
3
Determine the clamping voltage of the parallel combination.
The forward-biased silicon diode begins conducting heavily at 0.7 V, clamping the voltage across both branches at this level.
Because the silicon diode provides a low-resistance path at 0.7 V, the voltage cannot rise high enough to reach the Zener diode's 6.2 V breakdown threshold. The Zener diode remains effectively off (open circuit).

Key Concept

Diode Biasing and Parallel Circuit Voltage Clamping
Estimated Time:1m 30s
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