How to Select a Varistor?

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Varistors, also known as Voltage Dependent Resistors (VDRs), are essential components used to protect electronic circuits from voltage surges and spikes. Selecting the right varistor for your application involves understanding several key parameters, including breakdown voltage, clamping voltage, energy absorption, and other factors like package and wattage. This guide will provide a detailed approach to selecting a varistor for your circuit protection needs.

1. Understanding Varistors

Varistors are nonlinear resistors whose resistance changes with the applied voltage. They are typically made of metal oxide (hence the common term Metal Oxide Varistor or MOV) and are used to protect circuits by clamping transient voltages and absorbing surge energy.

2. Key Parameters for Selecting a Varistor

Maximum Continuous Operating Voltage (MCOV or V_M)

• Definition: The maximum RMS AC voltage or DC voltage that can be continuously applied to the varistor without causing significant current flow.
• Selection Criteria: Choose a varistor with an MCOV slightly higher than the normal operating voltage of your circuit to ensure it doesn’t conduct during normal operation.
• Example: For a 120V AC circuit, a varistor with an MCOV of around 130V-150V would be appropriate.

Breakdown Voltage (V_B)

• Definition: The voltage at which the varistor starts to conduct significantly and begins to protect the circuit.
• Selection Criteria: Ensure the breakdown voltage is above the normal operating voltage but below the voltage that could damage your circuit.
• Example: For a 12V DC circuit, a breakdown voltage around 14-16V might be suitable.

Clamping Voltage (V_C)

• Definition: The voltage at which the varistor clamps the transient and starts to conduct a large current to protect the circuit.
• Selection Criteria: The clamping voltage should be lower than the maximum voltage your circuit can withstand without damage.
• Example: If your circuit can tolerate up to 30V, choose a varistor with a clamping voltage below this threshold, perhaps around 25-28V.

3. Power Dissipation and Energy Absorption

• Definition: The amount of power the varistor can dissipate when clamping a surge.
• Selection Criteria: Choose a varistor with a power rating sufficient to handle the energy of the expected transient. This is usually specified in joules (J).
• Example Calculation:
  • Estimate the peak pulse current (I_PP) and pulse duration (t_pulse) of the transient.
  • Calculate the energy dissipation using the formula: $E = V_C \times I_{PP} \times t_{pulse}$.
  • Ensure the varistor’s energy rating exceeds this calculated value.

4. Worst-Case Scenario Calculations

Peak Pulse Current (I_PP)

• Definition: The maximum surge current the varistor can handle.
• Selection Criteria: Determine the highest possible transient current your circuit might encounter and choose a varistor rated to handle this current.
• Example: If a 100A surge is the worst-case scenario, select a varistor with an I_PP rating higher than 100A.

Energy Absorption (E_AS)

• Definition: The amount of energy the varistor can absorb during a transient event.
• Selection Criteria: Ensure the varistor can handle the energy of the worst-case transient.
• Example Calculation:
  • Calculate the transient energy: $E = 0.5 \times V_C \times I_{PP} \times t_{pulse}$.
  • Verify the varistor's energy absorption rating exceeds this value.

5. Package Selection

• Definition: The physical form factor of the varistor.
• Selection Criteria: Consider the available space on your PCB, thermal management requirements, and the mechanical robustness needed for your application.
• Common Packages:
  • Radial Leaded: Common for through-hole applications.
  • Surface Mount (SMD): Suitable for compact, high-density PCBs.
  • Disc Type: Often used in high-energy absorption applications.

6. Additional Considerations

Response Time

• Definition: The time it takes for the varistor to respond to a transient event.
• Selection Criteria: Varistors typically have fast response times, but ensure the chosen varistor is quick enough for your application's requirements.

Capacitance

• Definition: The inherent capacitance of the varistor.
• Selection Criteria: Lower capacitance is critical in high-speed data lines to avoid signal degradation.

Operating Temperature

• Definition: The range of temperatures over which the varistor can operate reliably.
• Selection Criteria: Ensure the varistor can operate within the temperature range of your application.

7. Practical Example

Application: 120V AC Power Line Protection

1. Normal Operating Voltage: 120V AC
2. Maximum Tolerable Voltage: 300V
3. Estimated Peak Pulse Current: 50A
4. Pulse Duration: 20µs

Step-by-Step Selection:

1. Maximum Continuous Operating Voltage (MCOV): Choose a varistor with an MCOV of around 130V-150V.
2. Breakdown Voltage: Select a varistor with a breakdown voltage around 140-160V.
3. Clamping Voltage: Ensure the clamping voltage is below 300V, say around 250V.
4. Power Dissipation: Calculate power dissipation: $E = 250V \times 50A \times 20 \times 10^{-6} = 0.25J$. Select a varistor rated for at least 1J to handle multiple surges.
5. Package: A radial leaded package for ease of mounting and sufficient power dissipation.

Conclusion

Selecting the right varistor involves balancing multiple parameters to ensure optimal protection for your circuit. By carefully considering maximum continuous operating voltage, breakdown voltage, clamping voltage, power dissipation, package type, and additional factors like response time and capacitance, you can choose a varistor that offers reliable and effective protection against transients. Always refer to manufacturer datasheets for precise specifications and consult with engineers if needed to ensure the best choice for your specific application.