How to Use VARISTOR: Examples, Pinouts, and Specs

• A varistor is a voltage-dependent resistor whose resistance decreases as the applied voltage increases. It is primarily used to protect electronic circuits from voltage spikes and surges by clamping excessive voltage to a safe level.
• Common applications include surge protection in power supplies, lightning protection in telecommunications, and transient suppression in automotive electronics.

• Key Technical Details:

  • Voltage Rating: Typically ranges from 18V to 1,000V (varies by model)
  • Energy Absorption: 0.1J to 10,000J (joules)
  • Response Time: Less than 25 nanoseconds
  • Operating Temperature: -40°C to +85°C (typical)
  • Leakage Current: <1mA (at rated voltage)

• Pin Configuration and Descriptions: Varistors are typically two-terminal devices. Below is a table describing the pins:

1. How to Use the Component in a Circuit:

   • Identify the voltage rating of the varistor that matches the operating voltage of your circuit.
   • Connect the varistor in parallel with the load or circuit to be protected. This ensures that when a voltage spike occurs, the varistor will clamp the voltage and divert the excess current away from the load.
   • For AC circuits, ensure the varistor's voltage rating exceeds the peak AC voltage.

2. Important Considerations and Best Practices:

   • Always select a varistor with a voltage rating slightly higher than the normal operating voltage of your circuit to avoid premature activation.
   • Use a fuse in series with the varistor to protect against catastrophic failure in case of prolonged overvoltage conditions.
   • Avoid exposing the varistor to continuous overvoltage, as this can degrade its performance over time.
   • For high-energy applications, ensure the varistor's energy absorption rating is sufficient to handle the expected surge energy.

3. Example: Connecting a Varistor to an Arduino UNO:

   • A varistor can be used to protect an Arduino UNO from voltage spikes on its power supply line. Below is an example circuit and code:

   Circuit:

   • Connect a 14V varistor (e.g., MOV-14D471K) across the 5V and GND pins of the Arduino UNO's power supply input.

   Code:

   // Example code for Arduino UNO with varistor protection
   // This code demonstrates a simple LED blink program.
   // The varistor protects the Arduino from voltage spikes on the power supply.
   
   void setup() {
     pinMode(13, OUTPUT); // Set pin 13 as an output for the onboard LED
   }
   
   void loop() {
     digitalWrite(13, HIGH); // Turn the LED on
     delay(1000);            // Wait for 1 second
     digitalWrite(13, LOW);  // Turn the LED off
     delay(1000);            // Wait for 1 second
   }
   

• Common Issues:

  1. Varistor Overheating:

     • Cause: Continuous overvoltage or excessive energy absorption.
     • Solution: Verify the varistor's voltage and energy ratings. Replace with a higher-rated varistor if necessary.

  2. Circuit Not Protected from Spikes:

     • Cause: Incorrect varistor placement or insufficient voltage rating.
     • Solution: Ensure the varistor is connected in parallel with the load and has an appropriate voltage rating.

  3. Varistor Fails to Clamp Voltage:

     • Cause: Degraded varistor due to repeated surges.
     • Solution: Replace the varistor and consider adding additional surge protection components.

• FAQs:

  1. Can a varistor be used in DC circuits?

     • Yes, varistors can be used in both AC and DC circuits. Ensure the voltage rating matches the circuit's operating voltage.

  2. How do I know if a varistor is damaged?

     • A damaged varistor may show physical signs like discoloration or cracking. It may also fail to clamp voltage effectively, leading to unprotected circuits.

  3. What happens if I use a varistor with a lower voltage rating than required?

     • The varistor may activate prematurely during normal operation, leading to overheating and potential failure.

By following these guidelines, you can effectively use a varistor to protect your electronic circuits from voltage surges and spikes.