Comparing Varistors and TVS Diodes

Electronic circuits are becoming increasingly sensitive, but regardless, any circuit needs components that can guard against transient spikes to maintain optimal functionality. Two of the most commonly used protection devices in this pursuit are varistors and Transient Voltage Suppressor (TVS) diodes. Although they share a core function, their construction and application suitability are distinct. This guide will help clarify the differences between varistors and TVS diodes so you can readily determine the most appropriate option for circuit upgrades or repairs.

What Is a Varistor?

A varistor is a voltage-dependent resistor that changes its resistance in response to the applied voltage. Under normal operating conditions, the varistor exhibits high resistance, limiting current flow through the circuit. However, when a voltage surge occurs, the resistance rapidly drops, allowing the varistor to absorb and divert excess energy away from sensitive components.

Earlier generations of varistors utilized Silicon Carbide (SiC) material, but these suffered from higher standby leakage current and energy inefficiencies. Metal Oxide Varistors (MOVs) are now the most utilized type, largely resolving the previous issues. Generally appearing as flat, disc-shaped components with two axial or radial leads, they contain zinc oxide grains compressed into a ceramic disc and placed between two metal electrodes. Additionally, the disc is typically coated with epoxy resin for superior insulation and environmental protection.

Benefits

• Effective Transient Protection: Varistors are well-suited for protecting circuits from short-duration voltage spikes.
• Versatility: Most varistors have the aforementioned two-lead design that is easily incorporated into various circuit types without complex circuitry. Moreover, they are available across a broad voltage spectrum and can be used in both AC and DC circuits.
• Bidirectional Operation: Varistors exhibit symmetrical, bidirectional voltage-current characteristics, allowing them to respond to surges of either polarity without requiring a specific orientation.

Limitations

• Non-Repetitive Use: Varistors are generally not built for repeated surge events. Over time, cumulative exposure can degrade their performance, resulting in increased leakage current and reduced clamping effectiveness.
• No Continuous Surge Protection: Varistors are designed for transient suppression only, not to provide protection against sustained overvoltage conditions.
• Thermal Runaway Risk: As a varistor degrades, internal heating can accelerate failure. To mitigate the potential for fire hazards or circuit damage, many MOVs now include thermal disconnect features that remove the varistor from the circuit when excessive temperature is detected.

What Is a TVS Diode?

TVS diodes are solid-state devices that respond quickly to overvoltage conditions. When the voltage across the diode exceeds its breakdown threshold, the device switches to a low-impedance state and clamps the voltage to a predefined safe level. Once the transient subsides, the diode returns to its high-impedance, non-conductive state without requiring a reset or replacement.

Physically, TVS diodes are compact and easy to integrate, found in surface-mount, axial-lead glass, or plastic packages that allow them to be placed directly across input lines, power rails, or high-speed data paths. Furthermore, these diodes are available in both unidirectional and bidirectional configurations. Unidirectional diodes are frequently used in DC circuits to protect against positive voltage spikes, while bidirectional types are suitable for AC signals or environments where voltage polarity may reverse.

Benefits

• Fast Response Times: TVS diodes tend to react within less than 1 nanosecond, providing nearly instantaneous protection against fast electrical transients.
• Precise Clamping Voltage: TVS diodes offer well-defined clamping thresholds that prevent overvoltage from exceeding the limits of protected components.
• Repeatable Performance: Unlike varistors, TVS diodes are capable of withstanding multiple transient events with minimal performance degradation.
• Low Capacitance Options: Many TVS diode models exhibit low capacitance to lessen signal distortion.
• Compact Design: Their small footprint and broad packaging options make TVS diodes perfect for space-constrained circuit boards.

Limitations

• Limited Energy Handling: Compared to devices like MOVs, TVS diodes have lower peak energy absorption capabilities, being better suited for low- to moderate-energy transients.
• Polarity Sensitivity: Unidirectional TVS diodes have to be oriented correctly in DC circuits to avoid circuit malfunctions.
• Voltage Margin Constraints: The working voltage of a TVS diode must be carefully matched to the protected circuit's normal operating voltage.
• Thermal Considerations: While not as susceptible to thermal runaway as varistors, repeated or prolonged exposure to transients near the diode’s maximum ratings can cause gradual thermal degradation.

Comparing Varistors and TVS Diodes

Purchase Reliable Varistors and TVS Diodes on ASAP Supply Chain

In summary, both protection devices have their place in circuits, with varistors shining in high-energy, high-voltage environments despite their finite operational lifespans. On the other hand, TVS diodes offer superior accuracy and long-term durability for sensitive and high-performance electronic systems. Whether you determine your assembly needs varistors or TVS diodes, be sure to turn to ASAP Supply Chain for quality options.