How to Select a Varistor?

Introduction

Selecting the right varistor is crucial for protecting electronic circuits from transient overvoltages. Varistors, or voltage-dependent resistors, offer effective surge protection by clamping excessive voltages. This guide dives into the key parameters and considerations for choosing a varistor, ensuring robust and reliable circuit protection for advanced hardware designs.

Table of Contents

1. Understanding Varistors

2. Key Parameters for Selecting a Varistor

2.1 Maximum Continuous Operating Voltage

2.2 Breakdown Voltage

2.3 Clamping Voltage

3. Power Dissipation and Energy Absorption

4. Worst-Case Scenario Calculations

4.1 Peak Pulse Current

4.2 Energy Absorption

5. Package Selection

6. Additional Considerations

6.1 Response Time

6.2 Capacitance

6.3 Operating Temperature

Conclusion

1. Understanding Varistors

Varistors, primarily made from metal oxide (MOV), exhibit nonlinear resistance, decreasing sharply when voltage exceeds a certain threshold. They protect circuits by absorbing transient overvoltages, converting excess energy into heat. Their role in safeguarding sensitive electronic components makes them essential in power electronics.

2. Key Parameters for Selecting a Varistor

2.1 Maximum Continuous Operating Voltage

Definition

The Maximum Continuous Operating Voltage (MCOV) is the highest voltage a varistor can continuously withstand without degrading. It's the threshold before the varistor starts clamping.

Selection Criteria

Choose a varistor with an MCOV slightly higher than the highest expected normal operating voltage of your circuit to avoid unnecessary clamping and ensure longevity.

Example

For a circuit operating at 230V AC, a varistor with an MCOV of around 250V AC would be appropriate.

2.2 Breakdown Voltage

Definition

Breakdown Voltage is the voltage at which the varistor starts to conduct significantly and begins to protect the circuit by clamping.

Selection Criteria

Select a varistor with a breakdown voltage just above the maximum operating voltage but below the voltage that could damage your circuit.

Example

In a 24V DC system, a varistor with a breakdown voltage of 30V might be suitable.

2.3 Clamping Voltage

Definition

Clamping Voltage is the voltage at which the varistor limits the transient overvoltage. It’s higher than the breakdown voltage but ensures protection by clamping the surge.

Selection Criteria

Select based on the maximum voltage your components can handle during transient conditions.

Example

For protecting a circuit sensitive to voltages above 40V, choose a varistor with a clamping voltage near 40V.

3. Power Dissipation and Energy Absorption

Definition

Power Dissipation is the ability of the varistor to absorb and dissipate energy from voltage transients without failing.

Selection Criteria

Choose based on the energy and power levels of anticipated surges to ensure the varistor can handle the load without damage.

Example

For surges of 100J, select a varistor with an energy absorption capacity above 100J.

4. Worst-Case Scenario Calculations

4.1 Peak Pulse Current

Definition

Peak Pulse Current is the maximum current the varistor can handle in a single surge without damage.

Selection Criteria

Base your choice on the highest surge current expected in your application.

Example

If expecting surges of up to 100A, a varistor with a peak pulse current rating of 120A would be a safe choice.

4.2 Energy Absorption

Definition

Energy Absorption is the total energy the varistor can absorb from a surge over a specific time.

Selection Criteria

Select a varistor that can absorb energy levels beyond what your application might encounter during transient events.

Example Calculation

For an energy requirement of 150J during a surge, a varistor with an energy absorption rating of at least 150J should be selected.

5. Package Selection

Definition

The package of a varistor refers to its physical form and mounting type, such as surface mount or through-hole.

Selection Criteria

Choose based on your design’s space constraints, mounting method, and environmental factors.

Common Packages

Common varistor packages include radial lead, axial lead, and surface mount.

6. Additional Considerations

6.1 Response Time

Definition

Response Time is the time it takes for a varistor to start clamping after a transient is detected.

Selection Criteria

Select a varistor with a quick response time to ensure rapid protection.

6.2 Capacitance

Definition

Capacitance in a varistor affects the circuit’s AC performance, potentially impacting high-frequency signals.

Selection Criteria

Consider the capacitance and its impact on your circuit, especially for high-frequency applications.

6.3 Operating Temperature

Definition

Operating Temperature is the range within which the varistor can operate effectively without performance degradation.

Selection Criteria

Choose based on the temperature conditions of your application to ensure reliability.

Conclusion

Selecting the right varistor involves understanding your application's voltage and energy requirements, considering worst-case scenarios, and choosing based on key parameters like MCOV, breakdown voltage, and clamping voltage. Packaging, response time, capacitance, and operating temperature also play crucial roles in ensuring optimal performance and protection in your designs.

Varistors are indispensable for protecting sensitive electronics, and their correct selection can greatly enhance the reliability and longevity of your circuits. By considering the factors discussed in this guide, you can make informed decisions and design robust electronic systems capable of withstanding transient overvoltage challenges.