14.
A circuit consists of a battery, a resistor and a reverse-biased Zener diode in series. The
battery voltage is greater than the barrier voltage of the diode. Which of the following
statements is correct?
a. Increasing the resistance will increase the voltage drop across the diode
b. Increasing the resistance will decrease the voltage drop across the diode
c. Changing the resistance does not change the circuit current
d. Changing the resistance does not change the voltage drop across the diode
Circuit Operation
A reverse-biased Zener diode in series with a battery (voltage > Zener breakdown voltage Vz) and resistor operates in the breakdown region. The Zener maintains nearly constant voltage Vz across itself, regardless of current changes, as long as minimum breakdown current flows. Total voltage equation: V_battery = V_resistor + V_Zener, where V_Zener ≈ Vz (constant) and V_resistor = I × R.
Option Analysis
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Option a (Incorrect): Increasing resistance reduces total current I = (V_battery – Vz)/R per Ohm’s law, decreasing voltage drop across resistor but keeping V_Zener constant at Vz.
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Option b (Incorrect): Opposite of reality—increasing R lowers I and resistor drop; diode voltage stays fixed.
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Option c (Incorrect): Circuit current changes inversely with R, as I depends on (V_battery – Vz)/R.
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Option d (Correct): Zener’s breakdown characteristic clamps voltage across it at Vz; resistance changes only affect current and resistor drop, not diode voltage.
In a reverse biased Zener diode series circuit with battery and resistor, where battery voltage exceeds the diode’s barrier (breakdown) voltage, the Zener operates as a voltage regulator. This Zener diode reverse biased configuration maintains constant voltage across the diode despite resistance changes, making it ideal for stabilization.
Key Characteristics
The Zener enters breakdown at Vz, conducting to hold voltage steady across wide currents. Equation: V_battery = I × R + Vz, so Vz remains fixed while I adjusts with R.
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Current I decreases as R increases, dropping voltage solely across resistor.
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No impact on diode voltage due to sharp I-V knee in breakdown.
Practical Implications
Reverse biased Zener diode circuits protect loads by clamping voltage. For exams like CSIR NET, recognize: resistance variation alters current, not diode drop. Test with V_battery=12V, Vz=5.6V: R=1kΩ yields I=6.4mA; R=2kΩ yields I=3.2mA—Vz unchanged.
