Zener Diode as Voltage Regulator
Semiconductor diodes block current in the reverse direction but will experience premature breakdown or damage if the reverse voltage applied across them is too high.
However, Zener Diodes or “Breakdown Diodes “, are basically the same as standard PN junction diodes but they are specially designed to have a specific, low Reverse Breakdown Voltage which makes use of any reverse voltage applied to them.
The Zener diode behaves like a diode generally consisting of a silicon PN junction and when it is biassed towards the front, i.e. the Anode is positive towards the Cathode, it behaves like an ordinary signal diode passing the rated current.
However, unlike a conventional diode which blocks the flow of any current through itself when it is reverse biased, i.e. the cathode becomes more positive than the anode, as soon as the reverse voltage reaches a predetermined value the Zener diode starts to conduct in the reverse direction.
This is because when the reverse voltage applied across the Zener diode exceeds the rated voltage of the device, a process called diode avalanche breakdown occurs in the semiconductor depletion layer and current begins to flow through the diode to limit this voltage increase.
The current currently flowing through the Zener diode increases dramatically to the maximum circuit value (which is usually limited by the series resistor) and once achieved, this reverse saturation current remains fairly constant over the reverse range of voltages.
The point of voltage at which the voltage across the Zener diode becomes stable is called the “Zener voltage”, (Vz) and for Zener diodes this voltage can range from less than one volt to several hundred volts. The point at which the Zener voltage triggers the current to flow through the diode can be controlled very accurately (to less than 1% tolerance) at the doping stage of a particular semiconductor diode construction resulting in a breakdown zener voltage diode, (Vz) for example, 4.3V or 7.5V . This zener breakdown voltage on the IV curve is almost a vertical straight line.
Table of Contents
Characteristics of Zener IV Diode
The zener diode is used in “reverse bias” or reverse breaking mode, ie the anode diode is connected to the negative supply. From the above IV characteristic curve, we can see that the zener diode has a reverse bias characteristic area of almost constant negative voltage regardless of the value of the current flowing through the diode and remains almost constant even with large changes in current as as long as the zener diode current remains between breakdown currents. I Z (min) and the maximum current level I Z (max) .
The ability to control itself can be used to a large effect to regulate or stabilize a voltage source against variations in supply or load. The fact that the voltage across the diodes in the breakdown region is almost constant turns out to be an important characteristic of zener diodes as they can be used in the simplest types of voltage regulating applications.
The function of the regulator is to provide a constant output voltage to the load connected in parallel with it regardless of the ripple in the supply voltage or variations in the load current and the zener diode will continue to regulate the voltage until the diode current drops below the minimum I Z (min) value in the reverse region. breakdown.
Zener Diode Regulator
Zener diodes can be used to produce a stable, low ripple voltage output under a wide range of load current conditions. By passing a small current through the diode from the voltage source, via a suitable current limiting resistor (R S ), the zener diode will supply sufficient current to maintain the voltage drop Vout.
We remember from the previous tutorial that the DC output voltage of the half wave rectifier or full wave rectifier contains ripples which are superimposed on the DC voltage and that as the load value changes so does the average output voltage.
By connecting a simple zener stabilizer circuit as shown below at the output rectifier a more stable output voltage can be generated.
Zener Diode Regulator Circuit
A resistor connected in series, R S with a zener diode to limit the flow of current through the diode with a voltage source, V S connected in combination. The stable output voltage V out is taken from across the zener diode.
The zener diode is connected to the cathode terminal which is connected to the positive rail of the DC supply so that it is reverse biased and will operate in a damaged condition. Resistor R S is chosen to limit the maximum current flowing in the circuit.
With no load connected to the circuit, the load current will be zero, (I L = 0), and all circuit currents pass through the zener diode which in turn dissipates its maximum power.
Also a small value of the series resistor R S will result in a larger diode current when the load resistance R L is connected and large as this will increase the power dissipation requirement of the diode so care must be taken when selecting the appropriate value of the series resistance so that the maximum zener power level is not exceeded. under these no-load or high impedance conditions.
The load is connected in parallel with the zener diode, so that the voltage across R L always equals the zener voltage, (V R = V Z ). There is a minimum zener current for which voltage stabilization is effective and the zener current must remain above this value operating under load in the breakdown region at all times. The upper limit of current depends of course on the power level of the device. Supply voltage V S must be greater than V Z .
One minor problem with zener diode stabilizer circuits is that the diode can sometimes produce electrical noise over the DC supply when trying to stabilize the voltage. Usually this is not a problem for most applications but the addition of a large value decoupling capacitor at the zener output may be required to provide additional smoothing.
Then to sum up a little. Zener diodes are always operated in reverse biased conditions. Voltage regulating circuits can be designed using a zener diode to maintain a constant DC output voltage at the load regardless of variations in input voltage or changes in load current.
Zener voltage regulator consists of a current limiting resistor R S is connected in series with the input voltage V S with a zener diode connected in parallel with the load R L in a reverse bias condition is. The stable output voltage is always chosen to be equal to the breakdown voltage V Z of the diode.
Example of Zener Diode No.1
A stable power supply or 5.0V power supply is required to be generated from the 12V DC power supply input source. The maximum power level P Z of the zener diode is 2W. Using the zener regulator circuit above, calculate:
a). The maximum current flows through the zener diode.
b). The minimum value of the series resistor, R S
c). Load current I L if a 1kΩ load resistor is connected across the zener diode.
d). Zener current I Z at full load.
I Z = I S – I L = 400mA – 5mA = 395mA
Zener Diode Voltage
Apart from producing a single stable voltage output, zener diodes can also be connected together in series with normal silicon signal diodes to produce a wide range of different reference voltage output values as shown below.
Zener Diode Circuit in Series
The values of individual Zener diodes can be selected according to the application while a silicon diode will always drop around 0.6 – 0.7V under forward bias conditions. The supply voltage, Vin of course must be higher than the largest output reference voltage and in our example above this is 19v.
A typical zener diode for general electronic circuits is the 500mW series, BZX55 or greater, BZX85 1.3W series when the zener voltage is given, for example, C7V5 for a 7.5V diode giving the diode reference number BZX55C7V5.
The 500mW series zener diodes are available from about 2.4 to about 100 volts and usually have the same order of values as used for the 5% series resistor (E24) with the individual voltage ratings for these small but very useful diodes are given in the table below.
Zener Diode Voltage Standard Table
Diod a Zener BZX55 500mW Power Rate
Diod a Zener BZX85 Power Level 1.3W
Zener Diode Clipping Circuit
So far we have seen how a zener diode can be used to set a constant DC source but what if the input signal is not a stable DC state but an alternating AC waveform how the zener diode reacts to a constantly changing signal.
A diode clipping and clamping circuit is a circuit used to shape or modify an input AC waveform (or any sinusoid) producing a different output waveform depending on the circuit setup.
The diode clipper circuit is also called a limiter because it limits or cuts off the positive (or negative) part of the input AC signal. Since zener clipper circuits limit or cut off the part of the wave above them, they are mainly used for circuit protection or in waveforming circuits.
For example, if we wanted to cut the output waveform at + 7.5V, we would use a 7.5V zener diode. If the output waveform tries to exceed the 7.5V limit, the zener diode will “cut” the excess voltage from the input producing a flat top waveform while keeping the output constant at + 7.5V.
Note that under forward bias conditions the zener diode is still a diode and when the negative AC waveform output is below -0.7V, the zener diode turns “ON” just like any normal silicon diode and the output clips at -0.7V as shown below.
Zener Diode Square Wave Signal
A back-to-back zener diode can be used as an AC regulator producing what is commonly referred to as a “less capable square wave generator”.
Using this setup we can cut the waveform between a positive value of + 8.2V and a negative value of -8.2V for a 7.5V zener diode.
So for example, if we wanted to cut the output waveform between two different minimum and maximum values, + 8V and -6V, we would only use two zener diodes of different values.
Note that the actual output will cut the AC waveform between + 8.7V and -6.7V due to the added forward bias diode voltage.
In other words the peak-to-peak voltage is 15.4 volts instead of the expected 14 volts, because the forward biased volts across the diode add 0.7 volts in each direction.
This type of clipper configuration is common enough to protect electronic circuits from overvoltages. The two zener are generally placed at the input terminals of the power supply and during normal operation one of the zener diodes is “OFF” and the diode has little or no influence. However, if the input voltage waveform exceeds its limit, then turn the zener “ON” and clip the input to protect the circuit.
In the next tutorial about diodes , we will look at using a forward biased PN junction diode of a diode to generate light.
We know from the previous tutorial that when the charge carrier moves across the junction, the electrons join the holes and energy is lost in the form of heat, but also some of this energy is dissipated as photons but we can’t see it.
If we place a transparent lens at the junction, visible light will be visible and the diode becomes the light source. This effect produces another type of diode commonly known as a Light Emitting Diode (LED) which takes advantage of these light-producing characteristics to emit light (photons) of various colors and wavelengths.
Zener diode is basically like an ordinary PN junction diode but normally works in the reverse biased state. However, the ordinary PN junction diode connected in reverse biased state is practically not used as a Zener diode. A Zener diode is a specially designed multi-doped PN junction diode.
Working Principle of Zener Diode
When a PN junction diode is reversed, the depletion layer expands. If this inverted voltage across the diode increases continuously, the depletion layer expands further. At the same time, there will be a constant reverse saturation current due to the minority carriers.
Minority carriers from the crossing after a certain reverse voltage, acquire sufficient kinetic energy due to the strong electric field. Free electrons with sufficient kinetic energy collide with the fixed ions of the depletion layer and scatter more free electrons. These newly created free electrons obtain sufficient kinetic energy due to the same electric field and cumulatively generate more free electrons through collision. Due to this alternating phenomenon, very soon huge free electrons are formed in the depletion layer and the entire diode becomes conductive. This type of breakdown of the depletion layer is known as avalanche breakdown, but this breakdown is not very sharp. There is another type of depression in the depletion layer that is sharper than avalanche spill, and it is called Zener dump. When a PN junction diode is highly doped, the concentration of impurity atoms will be high in the crystal. This higher concentration of impurity atoms results in a higher concentration of ions in the depletion layer, so for the same applied reverse-biased voltage the width of the depletion layer is normally thinner than the value for a doped diode.
Due to this thinning thinning layer, the gradient of electric field strength across the voltage strain layer is quite high. If the reverse voltage continues to increase, after an applied voltage, the electrons from the covalent bonds in the depletion region go out and make the depletion region conductive. This breakdown is called the Zener distribution. The voltage at which this distortion occurs is called the Zener voltage. If the reverse voltage applied across the diode is more than the Zener voltage, it provides a conductive path to the current through the diode, so there is no possibility of further avalanche breakage. Theoretically, Zener breakdown occurs at low voltage level, then folded for avalanche breakdown in a diode, specifically Zener breakdown. Zener destruction is much sharper than avalanche destruction. The Zener voltage of the diode is adjusted during manufacture with the necessary and appropriate doping aid.Zener diode is connected to a voltage source and source voltage is greater than Zener voltage, voltage across Zener diode remains constant regardless of source voltage. Although in this case the current flowing through the diode can be any value depending on the load associated with the diode. Therefore, we mainly use a Zener diode to control voltage in different circuits.
Zener Diode Circuit
We noted that the Zener Diode is nothing more than a single diode connected to a reverse bias. A diode connected to the reverse bias position in a circuit is shown below,
A circuit symbol Zener diode is also shown below.
Features of Zener Diode
Now, when discussing diode circuits, we should review the graphical representation of the device’s operation. zener diode . Normally referred to as VI characteristics of a Zener diode.
The diagram above shows the VI characteristics of a zener diode When the Diode is connected to the forward bias, this diode acts as a normal diode, but a sharp breakdown occurs when the reverse bias voltage is greater than the zener voltage. At the VI specifications above, V z is the Zener voltage. It is also the knee voltage because the current increases very rapidly at this point.