Tutorial 2 - Zener Diodes

We are familiar with the p-n junction diode from the tutorials in Electricity.  You may wish to revise about it HERE.   We also looked at the Light Emitting Diode (LED).  In this tutorial, we will look at the Zener diode.

Zener Diode

The Zener diode was named after an American Physicist, Clarence Melvin Zener (1905 - 1993).  It is an unspectacular component that looks like this:


Image by Teravolt.  Wikimedia Commons


There are several symbols that are used for the Zener diode.  This is the one I have always used:



The ordinary diode prevents a reverse-biased voltage until a particular voltage when there is a sudden avalanche of electrons.  This voltage is about - 30 V, and when the avalanche happens, a large current, about 500 mA flows, giving a heating effect that will burn the diode out very quickly.  The Zener diode behaves in a similar way, but the breakdown voltage is much lower and the avalanche current is much less. 


We can investigate the characteristics of a Zener diode using the circuit below:



This circuit allows positive and negative voltages to be applied to the diode.   Its behaviour is very like an ordinary diode, but a typical reverse-biased breakdown voltage is  5.6 V, and there is a very rapid rise in current.  This is shown in the graph:



In forward bias, the zener diode behaves just like an ordinary diode, with a forward bias voltage of about 0.7 V.   In reverse bias, the current is 0 mA until the breakdown voltage of (-)5.6 V is reached.  If the reverse voltage is above 5.6 V, a current will flow through the diode.  The higher the voltage, the greater the current.  There will be a limit to the current that the diode can conduct.  The zener diode is designed to be used in a reverse biased configuration. 


In its reverse biased configuration, it will hold the output voltage at a constant 5.6 V.   It can be described as a voltage clamp.  Zener diodes are found in voltage regulators



The circuit above shows a zener diode being used as a very simple voltage regulator.


The graph shows how diode limits the voltage, which remains steady at 5.6 volts while a small current is taken. However the diode can only take a limited current, and the output voltage will fall if the current taken is excessive.  The 100 W resistor in the circuit above limits the current to 10 mA which will stop the diode being overloaded.

Question 1

A Zener diode can carry a maximum current of 10 mA at its breakdown voltage of 5.6 V. It is connected to a 12 volt supply. Show that the current limiting resistor has a resistance of 640 W



The simple diode clamp is rather limited.  The maximum current of 10 mA occurs when there is no load.  Let's put a load of 1000 W across the output terminals:


A current of 10 mA still flows through the 640 W resistor.  However there is less current flowing through the zener diode.


Question 2

Calculate the current flowing through the zener diode now



The minimum current through the zener diode is 0 mA.


Question 3

What is the load resistance for this condition?



For a lower resistance than that calculated in Question 3, the voltage is no longer locked at 5.6 V.  The voltage is now determined using the voltage divider equation:



Question 4

What is the load voltage if the load is 300 W?



For a low resistance load, a more sophisticated voltage regulator is needed.


5.6 V is not the only voltage you can get.  There are standard zener diode voltages other than 5.6 V.  They include: 2.4 V; 3.3 V; 4.7 V; 11 V; etc.  The highest voltage is 62 V.


Zener diodes can placed in series to give a regulated voltage that is the sum of the voltages:



The voltage across the load is 11.2 V.


Zener diodes can be used to make a very simple square wave generator:


The sine wave is clipped like this:

Note that the voltage across the forward biased zener diode is 0.7 V (the normal diode biasing voltage).  Therefore the voltage will be 0.7 + 5.6 V = 6.3 V.  This circuit is sometimes called a diode clipper, or even a poor man's square wave generator.