Best 3 Applications Involving in Zener Diode Working Functionality

Zener diodes are normal PN junction diodes operating in a reverse-biased condition. Working of the Zener diode is similar to a PN junction diode in forwarding biased condition, but the uniqueness lies in the fact that it can also conduct when it is connected in reverse bias above its threshold/breakdown voltage. These are among the basic types of diodes used frequently, apart from the normal diodes.

Semiconductor diode in reverse bias condition

If you can recall, a simple PN junction diode is formed by a combination of p-type semiconductor material with an n-type semiconductor material. When one side of a semiconductor crystal is doped with donor impurities and the other side with acceptor impurities, a PN junction is formed.

Unbiased Semiconductor diode

In normal conditions, holes from the p side tend to diffuse to a low concentration region and the same thing happens for electrons from the n-side.

Thus the holes diffuse to the n-side and the electrons diffuse to the p-side. This results in the accumulation of charges around the junction, forming a depletion region.

An electric polarity or electric dipole is formed across the junction, causing the flow of flux from n side top side. This results in varying negative electric field intensity, generating an electric potential across the junction. This electric potential is actually the threshold voltage of the diode and is around 0.6V for silicon and 0.2V for Germanium. This acts as a potential barrier for the flow of majority charge carriers and the device does not conduct.

Now when a normal diode is biased such that a negative voltage is applied to the n side and positive voltage to the p side, the diode is said to be in forward biasing condition. This applied voltage tends to decrease the potential barrier after it goes beyond the threshold voltage.

At this point and afterward, the majority carriers cross the potential barrier and the device starts conducting with the flow of current through it.

When the diode is biased in reverse condition to above, the applied voltage is such that it adds to the potential barrier and hinders the flow of majority carriers. However, it does allow the flow of minority carriers (holes in n-type and electrons in p-type). As this reverse bias voltage increases, the reverse current tends to increase gradually.

At a certain point, this voltage is such that it causes the breakdown of the depletion region, causing a massive increase in the flow of current. This is where the Zener diode working comes into play.

The principle behind Zener diode Working

As stated above the basic principle behind the working of a Zener diode lies in the cause of breakdown for a diode in reverse biased condition. Normally there are two types of breakdown- Zener and Avalanche.

Zener Breakdown

This type of breakdown occurs for a reverse bias voltage between 2 to 8V.  Even at this low voltage, the electric field intensity is strong enough to exert a force on the valence electrons of the atom such that they are separated from the nuclei. This results in the formation of mobile electron-hole pairs, increasing the flow of current across the device.  The approximate value of this field is about 2*10^7 V/m.

This type of break down occurs normally for a highly doped diode with low breakdown voltage and a larger electric field. As temperature increases, the valence electrons gain more energy to disrupt from the covalent bond and less amount of external voltage is required. Thus Zener breakdown voltage decreases with temperature.

Avalanche breakdown

This type of breakdown occurs at the reverse bias voltage above 8V and higher.  It occurs for a lightly doped diodes with a large breakdown voltage.  As minority charge carriers (electrons) flow across the device, they tend to collide with the electrons in the covalent bond and cause the covalent bond to disrupt. As voltage increases, the kinetic energy (velocity) of the electrons also increases and the covalent bonds are more easily disrupted, causing an increase in electron-hole pairs. The avalanche breakdown voltage increases with temperature.

3 Zener diode applications

1. Zener Diode as a voltage

In a DC circuit, Zener diode can be used as a voltage regulator or to provide voltage reference. The main use of Zener diode lies in the fact that the voltage across a Zener diode remains constant for a larger change in current. This makes it possible to use a Zener diode as a constant voltage device or a voltage regulator.

In any power supply circuit, a regulator is used to provide a constant output (load) voltage irrespective of variation in input voltage or variation in load current. The variation in input voltage is called line regulation, whereas the variation in load current is called load regulation.

A simple circuit involving Zener diode as a regulator requires a resistor of the low value connected in series with the input voltage source. The low value is required so as to allow the maximum flow of current through the diode, connected in parallel. However, the only constraint being, the current through Zener diode should not be less than minimum Zener diode current. Simply put, for a minimum input voltage and a maximum load current, the Zener diode current should always be I_zmin.

While designing a voltage regulator using a Zener diode, the latter is chosen with respect to its maximum power rating. In other words, the maximum current through the device should be:-

I_max = Power/Zener Voltage

Since the input voltage and the required output voltage is known, it is easier to choose a Zener diode with a voltage approximately equal to the load voltage, i.e. Vz ~=V_o.

The value of the series resistor is chosen to be

R =(V_in – V_z)/(I_zmin + I_L), where I_L  = Load Voltage/Load resistance.

Note that for load voltages up to 8V, a single Zener diode can be used. However for load voltages beyond 8V, requiring Zener voltages of higher voltage value, it is advisable to use a forward-biased diode in series with the Zener diode. This is because the Zener diode at higher voltage follows the avalanche breakdown principle, having a positive temperature of the coefficient.

Hence a negative temperature coefficient diode is used for compensation.  Of course, these days, practical temperature compensated Zener diodes are used.

2. Zener Diode as a voltage reference 

In power supplies and many other circuits, Zener diode finds its application as a constant voltage provider or a voltage reference.  The only conditions are that the input voltage should be greater than Zener voltage and the series resistor should have a minimum value such that the maximum current flows through the device.

3. Zener Diode as a voltage clamper 

In a circuit involving the AC input source, different from the normal PN diode clamping circuit, a Zener diode can also be used. The diode can be used to limit the peak of the output voltage to Zener voltage at one side and to about 0V at another side of the sinusoidal waveform.

In the above circuit, during positive half cycle, once the input voltage is such that the zener diode is reverse biased, the output voltage is constant for a certain amount of time till the voltage starts decreasing.

Now during the negative half cycle, the Zener diode is in forwarding biased connection. As the negative voltage increases to the forwarding threshold voltage, the diode starts conducting and the negative side of the output voltage is limited to the threshold voltage.

Note that to get an output voltage in positive range only, use two oppositely biased Zener diodes in series.

Working Applications of Zener Diode 

With the growing popularity of smartphones, android based projects are being preferred these days. These projects involve the use of Bluetooth technology-based devices. These Bluetooth devices require about 3V voltage for operation. In such cases, a Zener diode is used to provide a 3V reference to the Bluetooth device.

Another application involves the use of Zener diode as a voltage regulator. Here the AC voltage is rectified by the diode D1 and filtered by the capacitor. This filtered DC voltage is regulated by the diode to provide a constant reference voltage of 15V. This regulated DC voltage is used to drive the control circuit, used to control the switching of light, as in an automated lighting control system.

We hope we have been able to provide precise yet essential information about Zener diode working and its applications. Here is a simple question for the readers – Why are regulator ICs mostly preferred over Zener diode in regulated DC power supply?

Give your answers and of course your feedback in the comment section below.

Photo Credits

•  Zener diode working by learningaboutelectronics
•  Unbiased semiconductor diode by wikimedia
•  Principle behind zener diode working by what-when-how