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What Are Varistors?

Varistors, also known as voltage dependent resistors (VDRs), are crucial components found in a wide range of electronic devices. They play a vital role in protecting these devices from transient voltage spikes and surges that could potentially damage sensitive circuitry. In this blog, we will highlight how varistors work, ensuring you understand why these components are popular choices for many electrical applications.

In essence, a varistor is an electronic component whose resistance varies with changes in voltage. It is primarily used as a voltage-clamping device, meaning it regulates voltage levels to safeguard other components. In particular, varistors are designed to have a high resistance when the voltage across them is below a certain threshold, but as the voltage exceeds that threshold, their resistance drops rapidly. This characteristic makes them highly effective in absorbing excessive voltage and protecting connected devices.

While there are many different types of varistors available on the market, one of the most commonly used types is the metal oxide varistor (MOV). MOVs are composed of zinc oxide grains pressed between two metal plates. These plates serve as the electrical contacts, allowing the varistor to be connected in parallel with the component or circuit to be protected. When the voltage across the MOV exceeds its threshold voltage, it enters a conductive state, diverting excess current away from the protected circuit. As a result of such capabilities, MOVs are widely used in power supplies, surge protectors, voltage regulators, and many other devices where transient voltage protection is critical.

The functionality of varistors can be understood by examining their voltage-current characteristic curve. At low voltages, the varistor behaves as an insulator, exhibiting a high resistance to current flow. However, as the voltage increases beyond the threshold level, the varistor's resistance drops significantly, allowing current to pass through it. This sudden drop in resistance is due to the presence of junctions within the varistor material that start conducting when the voltage exceeds a certain value. By effectively shunting excess voltage, varistors limit the potential damage to sensitive electronic components.

Varistors are also capable of responding quickly to transient voltage spikes because of their inherent nonlinear behavior. Transient voltage spikes are short-duration, high-amplitude voltage fluctuations that can occur due to lightning strikes, power surges, or electromagnetic interference. If not suppressed immediately, these spikes can cause irreparable damage. With regards to this risk, varistors provide an effective defense against such spikes by rapidly clamping the voltage to a safe level, preventing it from reaching sensitive components.

In addition to their protective function, varistors have a limited ability to absorb energy. When a transient voltage spike occurs, the energy associated with it is dissipated as heat by the varistor. This energy absorption capability helps in reducing the stress on the protected circuit, thereby extending its lifespan and enhancing its reliability.

While varistors are highly effective in protecting electronic devices, they have several important limitations. Most notably, varistors are not designed for continuous operation under high-voltage conditions. Prolonged exposure to high voltage can cause varistors to degrade or fail permanently, rendering them ineffective. Therefore, it is important to select varistors with appropriate voltage ratings and ensure that they are adequately sized for the application in question.

In conclusion, varistors or voltage dependent resistors are invaluable components in the realm of electronics. Their ability to regulate voltage levels, absorb transient voltage spikes, and protect sensitive circuitry makes them an essential part of many electronic devices. Metal oxide varistors, in particular, are widely used due to their superior performance and reliability. By incorporating varistors into electronic systems, engineers can ensure the longevity and functionality of these devices, even in the face of unpredictable voltage disturbances.

If you are on the lookout for top-grade electrical components to suit your assembly, there is no better source than NSN 3Sixty. Our inventory of varistors, connectors, printed circuit boards, and other electronic devices are all sourced from vetted manufacturers on our Approved Vendor List (AVL), meaning you can always depend on us for reliable components. Moreover, we are available 24/7x365, meaning you can receive rapid solutions to quote requests in just 15 minutes or less, any time. Get in contact with a representative today to learn more!


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