What is a Zener diode and how does it work?

Characteristics of Zener Diode

The Zener-diode has a V(I) characteristic curve which is similar to letter "Z". Unlike "normal" diodes, this 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 Zener-diode. In the forward direction, the Zener-diode works like a rectifier 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

The series resistor R₁ serves as protection against excessive current, which could destroy the Zener-diode. If you want to calculate R_1, you have to consider two worst-case scenarios:

Worst-case 1: The input voltage rises to its highest value V_{IN max} and at the same time the circuit is loaded with the minimum output current I_{L min} (so the maximum current flows through the Zener-diode):

Worst-case 2: The input voltage drops to its lowest value V_{IN min} and at the same time the circuit is loaded with the maximum output current I_{L max} (this means that the minimum current flows through the Zener-diode):

Data sheet Zener Diode

Data sheet Zener Diode Exercise

Note: If, as here, the minimum Zener 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 R₁ and select it from the E12 series.

• For what power must this R₁ be designed?
• What are the effects if R₁ is rated too small or too large?