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Ordinary silicon diodes block any current through them when they are reverse biased, and are damaged when the reverse voltage is too high. Therefore, these diodes are never intentionally operated in the breakdown region.
Zener diodes, however, are different. They are specially designed to operate in the breakdown region without failure. For this reason, Zener diodes are sometimes referred to as breakdown diodes.
Zener diodes are the backbone of voltage regulators, and circuits that keep the load voltage almost constant despite large changes in line voltage and load resistance.
The following figures show the schematic symbols of a zener diode. In either symbol, the lines resemble a “Z“, which stands for “Zener“.
A zener diode can operate in any of three regions: forward, leakage, and breakdown. Let’s understand this through the I-V graph of a zener diode.
When forward-biased, Zener diodes behave much the same as ordinary silicon diodes and start conducting at around 0.7V
The leakage region exists between zero current and breakdown.
In the leakage region, a small reverse current flows through the diode. This reverse current is caused by the thermally produced minority carriers.
If you continue increasing the reverse voltage, you will eventually reach the so-called Zener voltage VZ of the diode.
At this point, a process called Avalanche Breakdown occurs in the semiconductor depletion layer and the diode starts conducting heavily in the reverse direction.
You can see from the graph that the breakdown has a very sharp knee, followed by an almost vertical increase in current. Note that the voltage across the zener diode is almost constant and approximately equal to VZ over most of the breakdown region.
The graph also shows the maximum reverse current IZ(Max). As long as the reverse current is less than IZ(Max), the diode operates within its safe range. If the current exceeds IZ(Max), the diode will be destroyed.
Zener Voltage RegulatorThe Zener diode maintains a constant output voltage in the breakdown region, even though the current through it varies. This is an important feature of the zener diode, which can be used in voltage regulator applications. Therefore a zener diode is sometimes called a Voltage-regulator diode.
For example, the output of half-wave, full-wave or bridge rectifiers consists of ripples superimposed on a DC voltage. By connecting a simple zener diode across the output of the rectifier, we can obtain a more stable DC output voltage.
The following figure shows a simple zener voltage regulator (also known as a zener regulator).
To operate the zener diode in its breakdown condition, the zener diode is reverse biased by connecting its cathode to the positive terminal of the input supply.
A series (current-limiting) resistor RS is connected in series with the zener diode so that the current flowing through the diode is less than its maximum current rating. Otherwise, the zener diode will burn out, like any device because of too much power dissipation.
The voltage source VS is connected across the combination. Also, to keep the diode in its breakdown condition, the source voltage VS must be greater than the zener breakdown voltage VZ.
The stabilized output voltage Vout is taken from across the zener diode.
Breakdown OperationTo test whether the zener diode is operating in the breakdown region, we need to calculate how much Thevenin voltage the diode is facing.
Thevenin voltage is the voltage that exists when the zener diode is disconnected from the circuit.
Because of the voltage divider, we can write:
When this voltage exceeds the zener voltage, breakdown occurs.
Series CurrentThe voltage across the series resistor equals the difference between the source voltage and the zener voltage. Therefore, according to the Ohm’s law, the current through the series resistor is:
The series current remains the same whether or not there is a load resistor. Meaning, even if you disconnect the load resistor, the current through the series resistor will be equal to the voltage across the resistor divided by the resistance.
Load Voltage and Load CurrentBecause the load resistor is in parallel with the zener diode, the load voltage is the same as the Zener voltage.
Using the Ohm’s law, we can calculate the load current:
The zener diode and the load resistor are in parallel. The total current is equal to the sum of their currents, which is the same as the current through the series resistor.
This tells us that, the zener current equals the series current minus the load current.
Zener diodes are manufactured in standard voltage ratings listed in Table below. The table lists common voltages for 0.3W and 1.3W parts.
The wattage corresponds to the power that the diode can dissipate without damage.
Zener ApplicationsSo far we have seen how Zener diodes can be used to regulate a continuous DC source. Apart from that, Zener diodes are also used in different applications. Here are some of them.
PreregulatorThe basic idea behind Preregulator is to provide a well-regulated input to the zener regulator so that the final output is extremely well regulated.
Below is an example of a preregulator (the first zener diode) driving a zener regulator (the second zener diode).
In most applications, zener diodes remain in the breakdown region. But there are exceptions such as waveshaping circuits.
In above waveshaping circuit, two zener diodes are connected back-to-back to generate a square wave. This circuit is also jokingly called “The poor man’s square wave generator“.
On the positive half-cycle, the upper diode Z1 conducts and the lower diode Z2 breaks down. Therefore, the output is clipped.
On the negative half-cycle, the action is reversed. The lower diode Z2 conducts, and the upper diode Z1 breaks down. In this way the output is approximately a square wave.
The clipping level equals the zener voltage (broken-down diode) plus 0.7V (forward-biased diode).
Producing Nonstandard Output VoltagesBy combining zener diodes with ordinary silicon diodes, we can produce several nonstandard DC output voltages like this:
As you may know that connecting a 6V relay to a 12V system can cause damage to the relay. You need to drop some of the voltage. Below figure shows one way to accomplish this.
In this circuit, 5.6V zener diode is connected in series with the relay so that only 6.4V appears across the relay, which is within the tolerance of the relay’s voltage rating.
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