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Zener diode is connected in reverse bias in a circuit, for regulating voltage.  It is also called a shunt regulator.

Zener diode is a special purpose p-n junction semiconductor diode. Zener invented it.  The zener diode operates in the break down region in the reverse bias.  Its main function is to regulate voltage.  It allows a wide range of current at the same voltage.  

It is connected in reverse across the load (electrical appliance like refrigerator ). Its resistance is very high till the breakdown voltage is reached. Zener breaks down near the operating voltage of the appliance and does not allow change in the voltage across the load.  Near the zener breakdown voltage, its resistance is very low.

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