Subject: Logic Design

Topic: Biasing of BJT

Difficulty: Medium

The exciting part of this diode is that we can choose the Zener diode with a suitable breakdown voltage to work as a voltage regulator in our circuit. For example, we want that the voltage across a load in our circuit does not exceed, let’s say, 12 volts. Then we can select a Zener diode with a breakdown voltage of 12 volts and connect it across the load. Then even if the input voltage exceeds that value, the voltage across the load will never exceed 12 volts. Let us try to understand that with a circuit diagram:

Here the Zener diode is connected across the load RL. We want the voltage across the load to be regulated and not cross the value of Vz. Depending on our requirement, we choose the suitable Zener diode with a Zener breakdown voltage near to the voltage we require across the load. We connect the Zener diode in reverse bias condition. When the voltage across the diode exceeds the Zener breakdown voltage, a significant amount of current starts flowing through the diode. As the load is in parallel to the diode, the voltage drop across the load is also equal to the Zener breakdown voltage. The Zener diode provides a path for the current to flow and hence the load gets protected from excessive currents. Thus the Zener diode serves two purposes here: Zener diode as a voltage regulator as well as it protects the load from excessive current.