How to Detect Zener Diode Based Voltage Regulators?

A Zener diode is helpful for regulating and stabilizing variations in load or supply against a voltage source, so it is widely used in various regulators. As the Zener diode is in parallel with the load, it can be used to detect the failure of the regulators.

This share contains the characteristics of Zener  Diode. Then it assumes a normal Zener regulated DC power supply to explain how to detect the failure of the regulators with two examples. If you are troubled by the failure of the voltage regulator, you can find better solutions in this share. Let's keep exploring!

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Content

● What are the Characteristics of Zener Diodes?

● Assume a Normal Zener Regulated DC Power Supply

● Case 1: Zener Diode Open

● Case 2: Incorrect Zener Voltage

● FAQ

● Conclusion

We know that Zener Diodes are widely used as Shunt Voltage Regulators to regulate voltage across small loads. Zener Diodes can be used to produce a stabilised voltage output with low ripple under varying load current conditions. Bypassing a small current through the diode from a voltage source, via a suitable current limiting resistor (RS), the zener diode will conduct sufficient current to maintain a voltage drop of Vout.  

Zener Diodes have a sharp reverse breakdown voltage and breakdown voltage will be constant for a wide range of currents. Thus we will connect the zener diode parallel to the load such that the applied voltage will reverse bias it. Thus if the reverse bias voltage across the zener diode exceeds the knee voltage, the voltage across the load will be constant. 

According to the characteristics mentioned above, here we are discussing troubleshooting of Zener diode based voltage regulators. This method is applicable to a variety of regulators, including DC / DC Regulators.

The Introduction to Zener Diodes that Explaining How They Work

Assume a Normal Zener Regulated DC Power Supply

The Below Figure shows a filtered DC power supply that produces a constant 24 V before it is regulated down to 15 V by the zener regulator. The 1N4744A zener diode used. A no-load check of the regulated output voltage shows 15.5 V as indicated in part (a). The typical voltage expected at the zener test current for this particular diode is 15 V. 

Fig : Zener-regulated power supply test

In part (b), a potentiometer is connected to provide a variable load resistance. It is adjusted to a minimum value for a full-load test as determined by the following calculations. The full-load test is at minimum zener current (IZK). The meter reading of 14.8 V indicates approximately the expected output voltage of 15.0 V. 

Case 1: Zener Diode Open

If the zener diode fails open, the power supply test gives the approximate results indicated in Figure. 

In the no-load check shown in part (a), the output voltage is 24 V because there is no voltage dropped between the filtered output of the power supply and the output terminal. This definitely indicates an open between the output terminal and ground. In the full-load check, the voltage of 14.8 V results from the voltage-divider action of the 180 ohms series resistor and the 291 ohms load.

In this case, the result is too close to the normal reading to be a reliable fault indication but the no-load check will verify the problem. Also, if RL is varied, VOUT will vary if the zener diode is open.

Fig : Indications of an open zener

Case 2: Incorrect Zener Voltage

As indicated in Below Figure, a no-load check that results in an output voltage greater than the maximum zener voltage but less than the power supply output voltage indicates that the zener has failed such that its internal impedance is more than it should be. 

The 20 V output in this case is 4.5 V higher than the expected value of 15.5 V. That additional voltage indicates the zener is faulty or the wrong type has been installed. A 0 V output, of course, indicates that there is a short. 

Frequently Asked Questions

1. Q: What is Zener Diode as Regulator?

A: Zener diodes are widely used as voltage references and as shunt regulators to regulate the voltage across small circuits. When connected in parallel with a variable voltage source so that the Zener diode is reverse biased, it conducts when the voltage reaches the diode's reverse breakdown voltage. 

2. Q: How Much Voltage Can Zener Diode Adjust?

A: The zener diode works in the forward bias mode as an ordinary diode, and the on voltage is between 0.3 and 0.7 v. 

3. Q: How Does Zener Diode Reduce DC Voltage?

A: A Zener diode's design has the special property of reducing a reverse voltage to a specified value. This makes Zener diodes good, low-cost voltage regulators. To use one in a circuit, you calculate a resistor value, then connect the resistor and Zener across the voltage you want to regulate. 

4. Q: How Does Zener Diode Works? 

A: A Zener diode operates just like a normal diode when it is forward-biased. However, when connected in reverse biased mode, a small leakage current flows through the diode. As the reverse voltage increases to the predetermined breakdown voltage (Vz), current starts flowing through the diode.

Conclusion

In this share, we learn one of the characteristics of Zener diode and how to detect the fault of voltage regulator through two examples. Getting the hang of detecting the regulator failure can help you increase the circuit safety. Do you want more about the methods of detecting the regulators? Leave your comments below and tell us your ideas! If you think it is much helpful for you, please share with your friends!

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