ZENER DIODE AS VOLTAGE REGULATOR

ever want to use Zener diode but do not know from where to begin. this tutorial will help you to learn the basics about Zener diode.

 THEORY

Zener diode also called as breakdown diodes are the special type of diodes.just like normal diodes they can conduct when forward biased. These diodes are specially designed to conduct even when reversed biased and the reverse bias voltage should be greater than the break over voltage. To generate the milliamperes of current in reverse bias and to avoid the avalanche breakdown, we are going for Zener diode or Zener component. in Zener diode the doping level is 1 X 10^5 that is one doping in 10^5 atoms. this means for every 10^5 silicon or germanium atoms, one trivalent or pentavalent impurity is added.Due to high doping on both sides the depletion width is reduced on both sides therefore electric field is high at the barrier

• in forward bias, electrical behaviour of the Zener diode is same as the electrical behaviour of the P-N junction diode.
• in reverse bias Zener diode acts as a load regulator.

REVERSE BIASING OF ZENER DIODE

In reverse bias, negative terminal of the battery is connected to anode and positive terminal of the battery is connected to cathode. due to high doping, width of the depletion region decreases. due to this reverse biasing zener breakdown will occur. in zener breakdown region , it acts as a load regulator.

ZENER DIODE AS VOLTAGE REGULATOR

The above discussed concept is bookish but to make a practical circuit you need to know how to select the resistor or type of Zener diode to use. the components required to use zener diode as voltage regulator are

• 470 ohms resistor
• 3.3V zener diode
• battery 12V
• multimeter to check the voltage 

WHY 470 OHMS?

To answer this question youe need to know 

• what is the voltage at point A in the above circuit.
• what is the zener current

we are generating a 3.3 V regulated  supply from 12V. Zener current is the minimum current required by zener diode. whenever a zener diode is reversed biased and when the voltage crosses the breakdown voltage then the voltage across the zener diode becomes constant irresepective of the current flowing through the zener diode. this point is very important to make any kind of zener based project. for example in above circuit we are using a zener diode of 3.3V and 76ma zener diode. so whenever input voltage of greater than 3.3V is applied across the zener, then the voltage at A becomes 3.3V AND DOES NOT INCREASE.

CALCULATIONS

we know that the voltage at point B = 12V

VOLTAGE AT POINT A = 3.3V

suppose we require a current of 20 milli amperes at the output

then by the resistance required between point A and B Is given by

                                     R ={(voltage at pt B - voltage at point A)/(20)} X (10^3}

the value comes out to be 435 ohms and we select the closest value available ie. 470 ohms

as shown in figure the voltage across the Zener diode is 3.36 V 
1N4728 is 3.3V and it is available in wide voltage range from 3.3V TO 200V with a power dissipation of 1W

APPLICATIONS

• used to power low voltage devices.
• used to convert 12V OR other voltages to small regulated voltages.
• to power low power chips like BT66 or UM3561 sound generator ic's which require lesser than 5 V Supply.
• acts as a waveform clipper to limit the maximum and minimum voltage range.
• used as a level shifter between TTL and CMOS.

POINTS TO KEEP IN MIND

1. zener diodes are available in particular voltage and current rating. never exceed the wattage or current throught the zener diode.
2. calculate the amount of power dissipated by zener diode and check whether it is in limits.