What is a Zener diode and how does it work?

Characteristics of Zener Diode

The Z-diode has a V(I) characteristic curve which is similar to letter "Z". Unlike "normal" rectifier diodes, the Z-diode is operated in the reverse-bias range. When a certain breakdown voltage is reached, the Zener effect and the avalanche effect take place, in which the current increases suddenly without destroying the Z diode. In the forward direction, the Z diode works like a normal diode.

Symbol and Characteristic Curve of Zener Diode

Zener Diode as a Voltage Regulator

Zener diodes are mainly used as a voltage regulator to maintain a constant DC output voltage. The following circuit converts unregulated input voltage V_In to a regulated output voltage V_Out  since the Zener voltage Vz and the output-regulated voltage V_Out across the load are the same. The series resístor R₁ serves to limit the díode current to I_Z(max):

Zener Diode as a Voltage Regulator - circuit

Data sheet Zener Diode

Data sheet Zener Diode Exercise

Note: If, as here, the minimum Z current is not specified in the data sheet, the following rule of thumb applies:

.          I_Z(min) = 0.1 I_Z(max)

Exercise

The following circuit is given. The measured values are as follows: V_IN varies between 29 and 32 V. The load current varies between 5 mA and 22 mA. The Zener voltage can be assumed to be constant.

Zener Diode as a Voltage Regulator -circuit

Calculate the resístor R1 and select it from the E12 series.

• For what power must this resístor be designed?
• What are the effects if this resistor is rated too small or too large?