Question
In the circuit shown, the Zener diode is an ideal one with breakdown voltage of 5.0 volt. The values of the resistance are RS = 10 kΩ and RL = 1 kΩ. The current through the resistances, when the supply voltage is 11.0 V, is
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Solution
[Key Concept: A Zener diode conducts in reverse only if the voltage across it, with the diode removed, exceeds its breakdown voltage.]
Step 1: Remove the Zener: RS and RL in series carry I = 11 V/(10 kΩ + 1 kΩ) = 1.0 mA.
Step 2: The voltage across RL (and the Zener) is then 1.0 mA × 1 kΩ = 1.0 V, which is below the 5.0 V breakdown voltage, so the Zener stays off.
Step 3: Hence both resistances carry 1.0 mA. A 5 V clamp would need 5 mA in RL but only 0.6 mA could flow in RS, which is impossible.
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