When the Zener diode is given reverse bias feedback, there is a small leakage current until it attains the breakdown voltage or  a constant voltage. At this point,  the current  begins  flowing  seamlessly  without any change in voltage. Therefore, the constant voltage helps Zener Diode as Voltage Regulator.

  There are two types of regulation: 

1. Line regulation 

2. Load regulation 

1. Line Regulation:

In Line Regulation, Load resistance is constant and input voltage changes. Input voltage  must be sufficiently large to turn the Zener Diode ON.   

2. Load Regulation:

        Load Regulation: In this type of regulation, input voltage is fixed and the load resistance is variable. Output voltage  remains same, as long as the load resistance is maintained above a minimum value. 

AC Analysis Diode:

In ac analysis diode can be represented small resistance called as diffusion resistance and capacitance connected in parallel with diode called as diffusion capacitance. 

The IV characteristics of diode looks like linear as shown in fig 

Vt is called as thermal voltage

RDC = VD / ID