The effect of over-voltages of thyristor can be minimised by __________. 

Explanation:

Effect of Over-Voltages on Thyristors

Introduction: Thyristors are semiconductor devices used for switching and rectification in various electronic circuits. However, they are sensitive to over-voltages, which can damage the device or affect its performance. Over-voltages can occur due to various reasons, such as lightning strikes, switching operations, or faults in the electrical system. Therefore, it is crucial to protect thyristors from over-voltages to ensure their reliable operation.

Correct Option Analysis:

The correct option for minimizing the effect of over-voltages on thyristors is:

Option 3: Metal Oxide Varistors

Metal oxide varistors (MOVs) are widely used for protecting electronic devices, including thyristors, from over-voltage transients. MOVs are voltage-dependent resistors that exhibit high resistance at normal operating voltages and low resistance when exposed to over-voltage conditions. This property allows them to effectively clamp the voltage to a safe level, thereby protecting the thyristor from damage.

Working Principle of MOVs:

MOVs are made from a mixture of zinc oxide (ZnO) and other metal oxides, which are sintered together to form a polycrystalline ceramic material. The grain boundaries in the ceramic structure act as potential barriers. Under normal operating conditions, these barriers exhibit high resistance, preventing current flow. However, when an over-voltage transient occurs, the barriers break down, and the resistance drops significantly, allowing the MOV to conduct current and clamp the voltage.

Advantages of Using MOVs:

• High Energy Absorption: MOVs can absorb a large amount of energy from over-voltage transients, providing effective protection for thyristors.
• Fast Response Time: MOVs respond quickly to over-voltage transients, ensuring immediate protection for the thyristor.
• Compact Size: MOVs are available in compact sizes, making them suitable for integration into various electronic circuits.
• Cost-Effective: MOVs are relatively inexpensive compared to other over-voltage protection devices, making them a cost-effective solution.

Applications:

MOVs are used in a wide range of applications to protect electronic devices from over-voltages, including:

• Power Supplies: MOVs are used to protect power supplies and other sensitive components from voltage spikes and transients.
• Communication Systems: MOVs protect communication equipment from over-voltages caused by lightning strikes and other sources.
• Industrial Equipment: MOVs are used in industrial applications to protect thyristors and other semiconductor devices from over-voltage conditions.

Analysis of Other Options:

Option 1: Semiconductor Varistor

Semiconductor varistors are also used for over-voltage protection, but they are not as effective as MOVs for protecting thyristors. Semiconductor varistors have a different construction and may not provide the same level of energy absorption and fast response time as MOVs. Therefore, they are not the preferred choice for this application.

Option 2: Insulator Varistor

Insulator varistors are not commonly used for over-voltage protection in electronic circuits. They are primarily used for high-voltage insulation purposes and do not have the same clamping properties as MOVs. Therefore, they are not suitable for protecting thyristors from over-voltages.

Option 4: Capacitor

Capacitors are used for various purposes in electronic circuits, including filtering and energy storage. However, they are not specifically designed for over-voltage protection. While capacitors can absorb some energy from voltage transients, they do not provide the same level of protection as MOVs. Therefore, capacitors are not the best choice for protecting thyristors from over-voltages.

Conclusion:

Protecting thyristors from over-voltages is crucial for their reliable operation. Among the options provided, metal oxide varistors (MOVs) are the most effective solution for minimizing the effect of over-voltages on thyristors. MOVs offer high energy absorption, fast response time, compact size, and cost-effectiveness, making them the preferred choice for this application. Understanding the properties and advantages of MOVs helps in selecting the appropriate protection device for thyristors and ensuring their reliable performance in electronic circuits.