A Zener diode is a specially designed diode mainly operating in reverse biased conditions. They are more doped than regular diodes, as they have narrow depletion zones. Regular diodes are damaged when the voltage across them exceeds the reverse breakdown voltage, while Zener diodes only operate in this region. The depletion zone in the zener diode returns to its normal state when the reverse voltage is removed. This special feature of Zener diodes is a Voltage regulator . Let’s see how this happens.
Let’s first understand how a Zener diode works before understanding it. Zener diode as voltage regulator .
When we apply reverse voltage to a Zener diode, negligible amount of current flows through the circuit. Zener failure occurs when a voltage higher than the Zener breakdown voltage is applied. Zener breakdown is a phenomenon in which a significant amount of current flows through the diode with a negligible drop in voltage. When we increase the reverse voltage further, the voltage across the diode remains at the same Zener breakdown voltage value while the current flowing through it continues to rise as seen in the graph above. Here in the graph V z indicates the Zener breakdown voltage. Zener breakdown voltage can typically range from 1.2 V to 200 V depending on its application.
The exciting part of this diode can be that we choose the Zener diode with a suitable breakdown voltage to work as a voltage regulator on our circuit. For example, we want the voltage across a load in our circuit to be no more than 12 volts. Then we can select a Zener diode with a breakdown voltage of 12 volts and connect it to the load. Then, even if the input voltage exceeds this value, the voltage on the load will never exceed 12 volts.
Let’s try to understand this with a circuit diagram.
Wherein the Zener diode is connected to the load R L . We want to set the voltage on the load and not exceed the value V z . Depending on our needs, we choose the suitable Zener diode with Zener breakdown voltage close to the voltage we need across the load. We connect the Zener diode in the case of reverse bias. When the voltage on the diode exceeds the Zener breakdown voltage, a significant amount of current begins to flow through the diode. Since the load is in parallel with the diode, the voltage drop across the load is also equal to the Zener breakdown voltage. The Zener diode provides a path for current to flow and hence the load is protected from overcurrent. Thus, the Zener diode serves two purposes here: As a voltage regulator, the Zener diode protects the load from excessive current.