how does zener diode act as voltage regulator
The zener diode can be used as a voltage regulator. Once the breakdown occurs, the potential difference across the diode does not increase even if the applied battery potential is increased. The (V-I) graph shows the characteristics of a zener diode. Even if the applied Voltage (Vi) fluctuates, the current through the load resistance 'RL' remains the same. The voltage across the diode practically remains the same.
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similarly if input voltage decreases the current through Rs and zener diode decreases. this decreases the voltage across Rs without any change in voltage across zener diode.
thus zener diode acts as voltage regulator.