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In a circuit what specifically are varistors used for ?

Varistors, also known as voltage-dependent resistors or VDRs, are electronic components used in circuits primarily for voltage regulation and protection against voltage spikes or surges. These components exhibit a nonlinear voltage-current characteristic, meaning their resistance decreases significantly with an increase in voltage. Varistors are employed in a variety of electronic and electrical systems for different purposes. Let’s delve into the specific applications and functions of varistors in a circuit:

1. Surge Protection:

  • Voltage Spikes and Surges:
    • Varistors are commonly used to protect electronic devices from voltage spikes and surges. These transient voltage events can occur due to lightning, switching operations, or other external factors.
  • Clamping Action:
    • Varistors act as voltage clamps. When the voltage across them exceeds a certain threshold, their resistance decreases, allowing them to conduct and divert excess current away from sensitive components, thus protecting the circuit.

2. Overvoltage Protection:

  • Sensitive Electronics:
    • Varistors are employed in circuits with sensitive electronic components, such as integrated circuits and semiconductor devices, to prevent damage caused by overvoltage conditions.
  • Quick Response:
    • Varistors respond rapidly to changes in voltage, offering a quick and effective means of protecting against overvoltage situations.

3. Voltage Regulation:

  • Stabilizing Voltage:
    • Varistors can be used in circuits to stabilize the voltage by providing a controlled resistance that varies with the applied voltage.
    • They are sometimes used in conjunction with other voltage regulation components to achieve a desired output voltage.

4. Suppressing Electromagnetic Interference (EMI):

  • Noise Filtering:
    • Varistors can help suppress electromagnetic interference by absorbing unwanted high-frequency noise generated by other components in the circuit.
    • They provide a shunting path for high-frequency signals, reducing interference in sensitive electronic systems.

5. Power Supply Protection:

  • Power Line Conditioning:
    • Varistors are used in power supply circuits to condition and protect against fluctuations and disturbances in the electrical supply.
    • They enhance the robustness of power supplies by mitigating the impact of voltage variations.

6. Lightning Arrestors:

  • Overhead Power Lines:
    • Varistors are employed in lightning arrestors for protecting overhead power lines and electrical distribution systems from lightning-induced voltage surges.
    • They help prevent damage to transformers, substations, and other critical components.

7. Automotive Applications:

  • Transient Protection:
    • Varistors are used in automotive circuits to protect electronic systems from voltage transients caused by events such as load dumps or sudden changes in battery voltage.
    • They contribute to the reliability of electronic control units (ECUs) in vehicles.

8. Telecommunications:

  • Equipment Protection:
    • Varistors play a crucial role in protecting telecommunications equipment, such as modems and routers, from voltage spikes induced by lightning or other disturbances on communication lines.

9. Inductive Load Protection:

  • Motor Control Circuits:
    • In circuits controlling inductive loads, such as electric motors, varistors are used to suppress voltage spikes generated during the switching of inductive loads.
    • This protects semiconductor devices like transistors and ensures the longevity of the motor control circuit.

10. Gas Discharge Tubes (GDT):

  • Combination with Varistors:
    • In some applications, varistors are used in combination with gas discharge tubes to provide additional protection against transient overvoltages.


Varistors serve as versatile components in electronic circuits, offering valuable protection against voltage spikes, surges, and other transient events. Their nonlinear voltage-current characteristic and quick response make them essential for safeguarding sensitive electronic equipment and ensuring the reliable operation of various electrical systems. The specific application of varistors in a circuit depends on the requirements of the system and the nature of potential voltage-related disturbances.

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