Guide to Test a Varistor

Varistors, commonly referred to as voltage-dependent resistors (VDRs), are required components in electronic circuits, specifically designed to protect against overvoltage. These devices play an integral role in safeguarding electronics by altering their resistance based on the applied voltage, effectively limiting the current and preventing damage during unexpected voltage spikes. This article provides a comprehensive look at the types of varistors, their operational mechanisms, and a step-by-step guide on testing them to ensure functionality and reliability.

Catalog

Exploring the Varistor

A varistor, or voltage-dependent resistor (VDR), is a semiconductor device that protects electronic circuits from overvoltage and transient voltage spikes. Its resistance varies with the voltage applied across it. Varistor, or voltage-dependent resistor, is a semiconductor device used to guard electronic circuits from overvoltage or transient voltage events. The resistance in a varistor changes in response to the applied voltage, making it an ideal component for protecting sensitive electronics from surges.

Different Types of Varistors

Varistors mainly fall into two categories: metal oxide varistors (MOVs) and silicon carbide varistors (SiCs). MOVs, the more common type, consist of metal oxides such as zinc oxide (ZnO) and manganese dioxide (MnO). They are widely used to protect against electrical surges from events like lightning strikes. SiCs are made from silicon carbide (SiC), known for its toughness and ability to withstand high temperatures and voltages. These varistors are effective in guarding against overvoltage from electrical faults or outages. Varistors are commonly integrated into consumer electronics including TVs, computers, and smartphones, as well as finding roles in industrial controls, telecommunications, and automotive systems. They are manufactured in a variety of shapes and sizes to cater to specific applications.

Metal Oxide Varistors (MOVs)

These are the most common type, made from materials like zinc oxide and manganese dioxide. MOVs are widely used in power supplies and surge protectors to absorb and dissipate high-voltage surges, including those from lightning strikes or power spikes.

Silicon Carbide Varistors (SiCs)

Crafted from silicon carbide, these varistors are known for their high breakdown voltage and resistance to thermal shock. SiCs are primarily used in industrial settings to protect circuits from severe overvoltage events, such as those caused by electrical faults. Both types find applications across consumer electronics, industrial controls, and telecommunications, tailored in various shapes and sizes to suit specific needs.

How Varistors Work?

Varistors function by decreasing their resistance when voltage increases, making them ideal for protecting circuits from overvoltage and transient events. Typically placed in parallel with the circuit they protect, varistors limit current flow during voltage spikes, preventing potential damage to electronic components. They are useful for lightning protection when multiple units are used in parallel to divert excess current away from vulnerable devices. In addition to lighting events, varistors also guard against voltage transients caused by inductive loads or switching power supplies. Here, they are placed in series, allowing them to absorb surges by reducing current flow through the protected circuit during transients. The effectiveness of a varistor is determined by its clamping voltage the upper voltage it can withstand before breaking down and short-circuiting shaped by its materials, dimensions, and rated current. Varistors are highly effective in scenarios such as:

• Lightning Protection: Many varistors are often connected in parallel to devices vulnerable to lightning. During a surge, these varistors limit the current passing through, protecting the device from damage.
• Protecting Against Voltage Transients: Transients, caused by switching power supplies or inductive loads, pose risks to electronics. Varistors handle these spikes by reducing their resistance, thus safeguarding the components from harm.

In a varistor's protective role is its clamping voltage the maximum voltage it can endure without breaking down. This threshold is influenced by factors such as material composition, size, and rated current, and is vital for ensuring the varistor's reliability in high-stress environments.

Testing Varistors is Important

Testing a varistor is required for several reasons, primarily to ensure it meets quality standards during manufacturing, assess its functionality, and catch any potential reliability issues before they escalate. Proper testing protocols help understand the varistor’s characteristics, including its clamping voltage and rated current. Regular testing is required for maintaining electrical safety.Testing varistors is determining to verify their performance, especially during manufacturing, or to troubleshoot reliability issues in an electronic device. Regular testing helps detect potential problems early, ensuring reliable performance over time. The most common testing method involves measuring the varistor’s resistance or voltage using a digital multimeter. Proper testing allows for insights into the varistor's clamping voltage and rated current, helping you to confirm if the component meets its required specifications.

Safety Before Testing a Varistor

Before testing, always disconnect the varistor from any power source to avoid electrical shocks. Open the outlet cover to locate the varistor on the circuit board, noting which side connects to the ground and which to the live wire. For clarity, label each side before removing the varistor from the circuit.

Disconnecting Power

Before conducting tests, ensure that the varistor is completely disconnected from the power source to avoid shocks. Identify the varistor on the circuit board, taking note of which lead is grounded and which is live; marking these connections can help prevent confusion during disassembly.

Desoldering Varistor Leads

Carefully remove the varistor by desoldering its leads, paying close attention to avoid damaging components. Use a soldering iron and pliers as necessary, and maintain visibility throughout the process to ensure precision. Once desoldered, the varistor can be safely tested away from the circuit.

Testing a Varistor with a Multimeter

Utilize a multimeter to check the varistor’s resistance, which is generally measured in ohms (Ω). Connect one probe to each lead of the varistor, ensuring both leads touch the same side. A successful reading indicates the varistor is operational. However, a low resistance reading could imply damage. Conversely, a lack of any reading suggests the varistor is faulty, necessitating replacement to ensure circuit safety.

To test a varistor with a multimeter:

•Set the multimeter to measure resistance, typically in ohms (Ω). Set the multimeter to the appropriate range based on the expected resistance value.

•Attach one probe to each lead, ensuring both probes are connected to the appropriate side of the varistor.

•If the multimeter shows a reading, the varistor is functional, though a low-resistance reading could indicate an issue.

• No reading may suggest the varistor is non-functional, indicating possible damage or failure.

After testing, reattach the varistor to the circuit and securely solder the leads. If the varistor is defective, replace it promptly, as a faulty varistor can compromise the safety and reliability of the entire system.

Conclusion

Varistors is important in safeguarding electronic circuits by dynamically adjusting their resistance to counteract overvoltage and transient events. Understanding the types of varistors such as Metal Oxide Varistors (MOVs) and Silicon Carbide Varistors (SiCs) is required for selecting the right component to fit specific application requirements. Proper testing procedures, especially with a multimeter, ensure that these components maintain their protective qualities over time, identifying any potential faults before they can compromise the device’s safety. By following these testing steps and handling procedures, you can confidently verify a varistor’s functionality and reliability, helping to protect and extend the life of valuable electronic systems.

Frequently Asked Questions [FAQ]

1. How can I test a MOV varistor with a multimeter?

Testing a MOV varistor with a multimeter is fairly simple. First, set the multimeter to measure resistance (Ω), then connect the leads to the two terminals of the varistor. If you get a reading that isn’t zero, the varistor is working and able to protect against electrical surges.

2. What happens if a varistor fails?

When a varistor fails, it loses its ability to protect against power surges. A failed varistor may also overheat, which can create a fire risk. Regularly checking and replacing faulty varistors is important for safety.

3. Do varistors fail by becoming open circuits or short circuits?

Varistors can fail in either an open or short-circuit condition, but they’re more likely to fail as a short. If it fails as a short circuit, it may cause a sudden power surge, which can damage electronic devices. In an open state, it won’t immediately cause a surge, but it could still overheat, creating a fire risk. Regular testing and replacement are recommended.

4. How do I use a multimeter to check if a MOV varistor is working?

To check a MOV varistor, set the multimeter to resistance (Ω) mode and connect the leads to the varistor terminals. If the reading shows a non-zero value, the varistor is functional and can help prevent power surges.

5. What happens when a varistor breaks down?

When a varistor breaks down, it loses its ability to absorb surges, and it may overheat, posing a fire risk. Regular testing and prompt replacement of faulty varistors are for safety.

6. Can varistors fail as an open circuit or a short circuit?

Varistors can fail either as an open or short circuit, but they’re more likely to fail as a short circuit. A shorted varistor can lead to a sudden power surge that might damage electronics, while an open failure won’t cause an immediate surge but can still overheat and pose a fire risk. Regular testing and replacement are important.

7. How do I know if a varistor is working properly?

The most common way to check a varistor is with a resistance test. A working varistor should show a reading of infinity or at least 100 Ohms on a multimeter. Check the varistor’s datasheet for exact values if you’re unsure.

8. Are there safety steps to follow when testing a varistor?

Yes, take precautions when testing a varistor. Always set the multimeter to the highest resistance range, make sure the leads don’t touch each other or anything else, and don’t exceed the varistor’s maximum voltage rating to avoid damage or fire hazards.

9. What are the benefits of using a varistor?

Varistors help protect homes and electronics from power surges, extending the lifespan of devices by absorbing excess voltage. They’re used in many industries—like telecommunications, automotive, and consumer electronics—to protect sensitive components from damage.

10. Are there any downsides to testing a varistor?

Testing a varistor can carry risks, such as damaging the component if testing is done incorrectly or without the right tools. Following proper procedures and using the correct equipment helps avoid these risks. Some varistors are single-use, so check the manufacturer’s instructions before testing.

11. What common mistakes are made when testing a varistor?

Common mistakes include forgetting to remove protective covers, not attaching leads properly, and ignoring the multimeter’s voltage rating. It’s also important to wait a few moments after applying power to the varistor for an accurate reading.

12. Do varistors have a specific polarity?

No, varistors don’t have polarity, so they can be connected in either direction. This is because they’re symmetrical, allowing them to work regardless of orientation.

13. Why might a varistor burn out?

A varistor may burn out due to a strong electrical surge, physical damage, or manufacturing defects. If burnout is suspected, look for visible signs of damage, then test the varistor to confirm if it’s defective.

14. Is a varistor the same as a capacitor?

No, a varistor is different from a capacitor. A capacitor stores energy in an electric field, while a varistor dissipates energy as heat when there’s a sudden increase in voltage.

15. Is a varistor the same as a resistor?

No, a varistor is not the same as a resistor. While resistors are used to control current flow by creating voltage drops, varistors protect circuits from voltage spikes.

16. Is a varistor the same as a diode?

No, a varistor is not a diode. Unlike diodes, which only allow current to flow in one direction, varistors allow current to flow both ways and have a high resistance that changes with voltage.

17. What materials are varistors made from?

Varistors are typically made from materials with high electrical resistance, like metal oxides (such as zinc oxide) or semiconductors. They may also be made from ceramics or glass, though these are less common.