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Question
Explain how a Zener diode maintains a constant voltage across a load.
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Solution
Principle: In the breakdown region of a Zener diode, for widely changing Zener current, the voltage across the Zener diode remains almost constant.
Zener diode as a voltage regulator
Rs - current-limiting resistance,
RL - Load resistance,
Iz - Current through the diode;
IL - Load current
Electric circuit: The circuit for regulating or stabilizing the voltage across a load resistance RL against change in load current and supply voltage is shown in the above figure. The Zener diode is connected parallel to load RL such that the current through the Zener diode is from the n to p region. The series resistance Rs limits the current through the diode below the maximum rated value.
From the circuit, I = Iz + IL
and V = IRs + Vz
= (IZ + IL)Rs + VZ
Working: When the input unregulated de voltage V across the Zener diode is greater than the Zener voltage Vz in magnitude, the diode works in the Zener breakdown region. The voltage across the diode and load RL is then Vz. The corresponding current in the diode is lz.
As the load current (I) or supply voltage (V) changes, the diode current (Iz) adjusts itself at constant Vz· The excess voltage V − Vz appears across the series resistance Rs·
For constant supply voltage, the supply current I and the voltage drop across Rs remain constant. If the diode is within its regulating range, an increase in load current is accompanied by a decrease in Iz at constant Vz.
Since the voltage across RL remains constant at Vz, the Zener diode acts as a voltage stabilizer or voltage regulator.
