Zener Diode Failure Modes: Open vs. Shorted

This article explores the common failure modes of zener diodes, specifically focusing on the differences between an open diode and a shorted diode. We’ll examine how to identify these conditions using simple tests.

Introduction to Zener Diodes

• As you probably know, a zener diode is a special type of highly doped p-n junction diode.

• Unlike regular diodes, zener diodes are designed to operate in the reverse-biased region.

• The appearance and symbol of a zener diode are shown below:

  

• The characteristic curve of a zener diode is shown below. These diodes are available with various zener voltage ratings, such as 3.9V, 5.1V, 6.2V, and 9.1V.

  

During zener diode testing, parameters like zener voltage (Vz), current at Vz, and zener resistance (Rz) are typically measured.

Zener Diode Testing: Open Diode

An open zener diode essentially means that the internal connection is broken, preventing current flow. There are two ways to check for an open condition:

Resistance Test for an Open Zener Diode

1. Set your multimeter to resistance test mode.
2. Connect the red (positive) lead of the multimeter to the anode of the diode and the black (negative) lead to the cathode.
3. A healthy zener diode should show a relatively low resistance (around 450 kΩ) in this configuration.
4. If the diode is open, it will show very high resistance in both directions. A healthy diode should never read high resistance when forward-biased.

Voltage Test for an Open Zener Diode

1. Set your multimeter to voltage test mode.

2. Connect the red (positive) lead to the anode and the black (negative) lead to the cathode.

3. A healthy zener diode will show a voltage close to its zener voltage (Vz, say 5.1V).

4. If the diode is open, the multimeter will read a high voltage, close to the supply voltage (e.g., 9V).

   

Zener Diode Testing: Shorted Diode

A shorted zener diode is when the internal components have failed, creating a low-resistance path. Here’s how to test for this:

Resistance Test for a Shorted Zener Diode

1. Set your multimeter to resistance test mode.
2. Connect the red (positive) lead of the multimeter to the cathode and the black (negative) lead to the anode of the diode.
3. A healthy diode will show a high resistance (around 2.2 MΩ) in this configuration (reverse biased).
4. If the diode is shorted, it will show low resistance in both directions. A healthy diode should never show low resistance when reverse biased.

Voltage Test for a Shorted Zener Diode

1. Set your multimeter to voltage test mode.
2. Connect the red lead to the anode and the black lead to the cathode.
3. A healthy diode should show a voltage reading close to its zener voltage (Vz, say 5.1V).
4. If the diode is shorted, the multimeter will read a much lower voltage (around 0V) compared to its rated zener voltage. This indicates a defective diode.

Conclusion

Understanding the difference between open and shorted zener diodes is crucial for troubleshooting electronic circuits. By using simple resistance and voltage tests with a multimeter, you can quickly identify these failure modes and replace the faulty components.