## What is a Zener diode and how does it work?

Table of Contents

Toggle### 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.

### 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 R_{1} serves to limit the díode current to I_{Z(max)}:

The series resistor R_{1} 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

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.

Calculate R_{1} and select it from the E12 series.

- For what power must this R
_{1}be designed? - What are the effects if R
_{1}is rated too small or too large?

V_{Z} = 24 V, I_{Zmax} = 60 mA => I_{Zmin} = 0.1 I_{Zmax} = 0.1 * 60 mA = 6 mA

R_{1min} = (V_{IN(max)} - V_{Z}) / (I_{Z(max)} + I_{L(min)}) = (32V - 24V) / (60mA + 5mA) = 123.07 Ohm

R_{1max} = (V_{IN(min)} - V_{Z}) / (I_{Z(min)} + I_{L(max)}) = (29V - 24V) / (6mA + 22mA) = 178.57 Ohm

chosen: R_{1} = 150 Ohm

If R_{1} too small, I_{Z(max)} can be exceeded, and therefor the zener diode can be destroyed.

If R_{1} too big, Minimum current I_{Z(min)} is undercut. This means that stabilization of the output voltage is no longer guaranteed.