Zener Diode
Diode which are designed with adequate power dissipation capabilities to operate in the breakdown region may be employed as voltage – reference and constant voltage devices. Such diode is known as avalanche breakdown or zener diodes.
The voltage across the zener diode is in reverse biased condition. The V/I characteristics of zener diode shows that there is no or even less reverse current until the (below the) zener breakdown voltage. When it is slightly above the zener breakdown voltage, large amount of reverse current flows on the circuit.That means small change in voltage causes the sudden change in current.
The source V and resistor R are selected so that the diode is operating in the breakdown region. Here, diode voltage which is also the voltage across the load RL is VZ and diode current IZ. The diode will now regulate the load voltage against variations in load current and against variations in supply voltage V because, in the breakdown region, large changes in diode current produce only small change in diode voltage. Moreover, as load current or supply voltage changes , the diode current will accommodate itself to these changes to maintain a nearly constant load voltage. The diode will continue to regulate until the circuit operation requires the diode current to fall to IZK , in the neighborhood of the knee of the diode volt-ampere curve. The upper limit on diode current is determined by the power dissipation rating of the diode.