Zener diode

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General:

Zener diodes are Si-diodes, that are able to exceed their breakdown voltage (=Zener voltage Uz) in reverse direction without being destroyed.
Actually this is the desired state: The Zener voltage is distinctive for a particular component. Zenerdiodes are used for limiting voltages and setting exact voltages. By use of high contamination breakdown voltages of <50V are reached (rectifier diodes: 50V-1000V).

Two effects can be observed:

1. Uz<5V: In this range the Zener effect overweighs. The diodes have a positive temperature coefficient.

2. Uz>5V: Here the avalanche effect overweighs. The diodes have a negative temperature coefficient.

In forward direction Zener diodes behave in the same manner as normal diodes.

characteristic of a Zener diode

Applications:

• Voltage regulation in voltage sources
• Protection circuit for electronic components (voltage limiting)

How to calculate the series resistor of a Zener diode?

In order not to destroy the Zener diode in reverse direction it needs a series resistor where the excess voltage can drop. To be able to dimension the series resistor, the reverse operating current Iz has to be set. The rule of thumb says the typical operating current Iz should be one tenth of the maximum operating current Izmax of the Zener diode.

The maximum operating current of the Zener diode Izmax can be calculated via the maximum power dissipation Ptot and the Zener voltage Uz of the component.

To calculate the series resistor its voltage drop has to be known. 

Example:

A 14 volts voltage source has to be limited to 10 volts with the help of a Zener diode. 

component: Zener diode BZX79 10V 0,5W

chosen: 820Ω (E-12 series)


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