How to Test a Varistor?(Complete Guide)

A varistor, also known as a voltage-dependent resistor, is an electronic component employed to safeguard circuits against damage resulting from overvoltage. Widely present in power supplies, surge protectors, and various electrical devices, play a crucial role. If you find the need to test a varistor, it is essential to understand the proper procedure. This article delves into the various methods for testing a varistor, offering valuable tips to ensure accurate results. Let's begin exploring the testing process!

 

What is Varistor?

A varistor, also known as a voltage-dependent resistor (VDR), is a semiconductor device designed to safeguard electronic circuits against overvoltage and transient voltage events. The resistance of varistors changes in response to applied voltage.

 

Varistors come in two main types: metal oxide varistors (MOVs) and silicon carbide varistors (SiCs)

 

Metal oxide varistors (MOVs) represent the most prevalent type of varistor. Comprising a blend of metal oxides like zinc oxide (ZnO), manganese dioxide (MnO), and lead dioxide (PbO), MOVs are commonly employed to safeguard against transient voltage events such as lightning strikes or power surges.

 

Silicon carbide varistors (SiCs) are crafted from silicon carbide (SiC), a robust and brittle material known for its resistance to thermal shocks and high breakdown voltage. SiCs find application in protecting against overvoltage events like electrical faults or power outages.

 

Varistors, in general, find widespread use in consumer electronics like televisions, computers, and cell phones. Beyond that, they are integral in industrial controls, telecommunications equipment, automotive electronics, and various other applications. The manufacturing of varistors encompasses a diverse range of shapes and sizes tailored to specific fields of use.

 

Understanding the Mechanism of Varistors

 

Varistors function by reducing their resistance as the voltage across them increases. This property makes them well-suited for safeguarding electronic circuits against overvoltage and transient voltage events. Typically installed in parallel with the circuit to be protected, varistors play a crucial role in limiting current flow during transient events, thereby preventing damage to the electronics within the circuit.

 

Varistors are especially effective in lightning protection. In this application, numerous varistors are connected in parallel and strategically placed on or near electronic devices susceptible to lightning damage. During a lightning strike, the collective varistors limit the current flowing through the device, preventing potential harm.

 

Furthermore, varistors find utility in protecting circuits from voltage transients induced by switching power supplies and inductive loads. In these scenarios, varistors are incorporated in series with the circuit to be protected. Upon the occurrence of a transient voltage event, the varistor's decreased resistance curtails the current flow through the protected circuit, safeguarding the electronics from potential damage.

 

The effectiveness of a varistor in shielding a circuit from transient voltage events is contingent upon its clamping voltage. The clamping voltage is the maximum voltage that a varistor can withstand without undergoing breakdown and short-circuiting. This critical parameter is influenced by factors such as the varistor's material composition, geometry, and rated current.

 

Varistor Testing: Overview and Importance

The testing of a varistor is a crucial process conducted for various reasons. One primary motivation is to uphold quality and adherence to specifications, particularly during the manufacturing phase. Additionally, testing becomes essential when concerns arise regarding the varistor's performance or reliability. Periodic testing is also instrumental in identifying and addressing potential issues before they escalate into more significant problems.

 

Regardless of the testing rationale, it is crucial to comprehend the proper procedures. Let's dive in! The prevalent method for testing a varistor involves resistance or voltage measurement. In this test, the varistor is integrated into a circuit, and its resistance or voltage is gauged using a digital multimeter. The obtained results are instrumental in delineating the varistor's characteristics, including its clamping voltage and rated current.

 

Unplug from the Outlet: A Safety Measure

 

Disconnecting from the Outlet for Varistor Testing

 

The initial step in testing a varistor involves disconnecting it from the outlet. This precautionary measure serves to prevent accidental shocks and ensures that the device is not powered on during the testing process. Upon opening the outlet cover, locate the varistor on the circuit board. It is crucial to identify which side of the device is connected to the ground and which side is linked to the live wire. To minimize potential confusion, consider labeling each side of the varistor before disconnecting it from the outlet.

 

Disconnecting and Desoldering Varistor Leads

 

Identifying the wire connections on each side of the device, proceed to disconnect and desolder the varistor's lead. Employ a soldering iron and solder for this task.

 

Exercise caution during the disconnection and desoldering process, as the leads may be small and delicate. If possible, utilize a magnifying glass to enhance visibility. Use pliers to delicately disconnect the lead, ensuring not to cause any damage.

 

Once the leads have been successfully disconnected and desoldered, set them aside for future use, and proceed to remove the varistor from its position.

 

With the varistor now disconnected from the outlet, the next step involves testing its functionality.

 

Utilize a Multimeter for Varistor Testing

 

Using a Multimeter to Test the Varistor

 

A multimeter, designed for measuring electrical properties like voltage, current, and resistance, gauges resistance in ohms (Ω).

 

When testing a varistor, ensure the multimeter is set to the correct range, typically based on the nominal resistance value you intend to measure.

 

To test the varistor, connect one multimeter probe to each lead, ensuring both probes are connected to the same side of the device (e.g., ground and live wire). A reading on the multimeter indicates the device is functional and allowing current. However, interpret the result carefully; a low-resistance reading may imply functionality but could indicate a fault. Take appropriate measures to address the issue.

 

If no reading is obtained on the multimeter, it signals that the device is non-functional and not conducting current, suggesting potential damage or failure.

 

After completing the varistor test, reattach it to the outlet cover and securely close the circuit board. Reconnect and solder the leads accurately to eliminate the risk of accidental shocks.

 

If the varistor is found to be faulty, prompt replacement is crucial. A defective varistor poses a safety hazard and may lead to damage within your electrical system.

 

(FAQs) about Testing MOV Varistors

 

How can I assess a MOV varistor with a multimeter?

 

Reviewing a MOV varistor using a multimeter is a fairly straightforward process. Initially, configure the multimeter to measure resistance (Ω). Then, link the multimeter leads to the two terminals of the MOV varistor. Subsequently, observe the reading on the multimeter. If the meter indicates a value other than zero, the MOV varistor is operational, providing a certain degree of defense against electrical surges.

 

What occurs when a varistor malfunctions?

 

Should a varistor malfunction, it loses its ability to shield against electrical surges. Additionally, a malfunctioned varistor may generate excessive heat, posing a potential fire risk. Consequently, regular testing of MOV varistors is crucial, and prompt replacement is advised in case of failure.

 

Do varistors malfunction by opening or shorting?

 

Varistors can malfunction either by opening or shorting, with a higher likelihood of failing in a shorted state. When a varistor fails in a shorted state, it triggers an immediate power surge, potentially harming electronic devices. On the other hand, if a varistor fails in an open state, it may not cause an immediate power surge, but the defective component could still become excessively hot, presenting a fire hazard. Regular testing and replacement of malfunctioning MOV varistors are essential precautions.

 

How do I check a MOV varistor with a multimeter?

 

Evaluating a MOV varistor using a multimeter is a relatively straightforward process. Start by adjusting the multimeter to measure resistance (Ω). Then, connect the multimeter leads to the two terminals of the MOV varistor. Finally, observe the reading on the multimeter. If the meter displays a non-zero value, the MOV varistor is operational and provides some protection against electrical surges.

 

What occurs in the event of a varistor failure?

 

When a varistor fails, it loses its ability to safeguard against electrical surges. Additionally, the failed varistor may generate excessive heat, posing a potential fire risk. Regular testing of MOV varistors is essential, with prompt replacement recommended upon failure.

 

Do varistors fail by opening or shorting?

 

Varistors can fail either by opening or shorting, with a higher likelihood of failing in a shorted state. A varistor failing in a shorted state can cause an immediate power surge, potentially damaging electronic equipment. If a varistor fails in an open state, it may not trigger an immediate power surge, but the defective component could still become excessively hot, presenting a fire hazard. Regular testing and replacement of malfunctioning MOV varistors are necessary precautions.

 

How can I determine if a varistor is functioning correctly?

 

The most common test for a varistor is the resistance test, measuring the device's ability to resist electrical current. A functional varistor should show a multimeter reading of infinity or at least 100 Ohms. Consult the datasheet for the specific varistor being tested if uncertain.

 

Are there precautions when testing a varistor?

 

Yes, when testing a varistor, take specific precautions. Always use a multimeter/ohmmeter set to the highest resistance range, ensuring that the ohmmeter leads do not touch each other or any other object during measurement. Avoid exceeding the varistor's maximum voltage rating to prevent potential damage and fire hazards.

 

What are the advantages of using a varistor?

 

Using a varistor offers several benefits, such as protecting homes or offices from electrical surges and prolonging electronics' lifespan by absorbing excess voltage. Varistors find applications in various industries, including telecommunications, automotive, and consumer electronics, safeguarding sensitive electronic components from damage caused by electrical spikes.

 

Are there drawbacks to testing a varistor?

 

Potential downsides to testing a varistor include the risk of damaging the component, employing incorrect testing methods, and lacking the necessary equipment. Adhering to proper procedures and using the right tools minimizes these risks. Some varistors may be designed for one-time use, so consult the manufacturer's instructions before testing.

 

What are common mistakes when testing a varistor?

 

Common mistakes include inadequate preparation, such as forgetting to remove protective covers or not properly attaching leads. Additionally, overlooking the voltage rating of the multimeter during testing can lead to damage. Allowing sufficient time for the varistor to settle after applying power is crucial to obtaining accurate readings.

 

Do varistors have polarity?

 

No, varistors lack polarity, allowing them to be connected in either direction without affecting functionality. This is due to their symmetrical construction. Varistors are nonlinear devices, and their resistance changes with applied voltage, eliminating concerns about proper orientation.

 

Why might a varistor burn out?

 

A varistor might burn out due to an electrical surge too strong for it to handle, physical damage, or manufacturing defects. Checking for visible damage is recommended if burnout is suspected, followed by testing to confirm the defect.

 

Is a varistor a capacitor?

 

No, a varistor is not a capacitor. Unlike capacitors that store energy in an electric field between conducting plates, varistors dissipate energy as heat when encountering sudden increases in voltage (voltage spikes).

 

Is a varistor a resistor?

 

No, a varistor is not a resistor. Varistors are electronic components used to protect against voltage spikes, while resistors create voltage drops in circuits. Both serve distinct purposes in controlling the flow of electricity.

 

Is a varistor a diode?

 

No, a varistor is not a diode. Varistors, made of semiconductor material, exhibit very high resistance to electricity. Unlike diodes, varistors allow bidirectional flow of current and possess higher dynamic resistance.

 

What is a varistor made of?

 

Varistors are crafted from materials with high resistance to electricity, capable of conducting under specific conditions. Common materials include semiconductors, insulators, silicon, carbon, and metal oxides like zinc oxide in the case of metal oxide varistors. The material selection depends on the varistor's intended application.

Varistors may also consist of alternative materials like ceramics or glass. Nevertheless, these substances are less prevalent compared to semiconductors and insulators.

 

Final Thoughts on Varistor Testing

 

Having gained additional insights into varistors and their significance, we trust that this article has dissipated any uncertainties, leaving you with confidence in your capacity to test them. So, what's the procedure for testing a varistor? The answer lies in utilizing a multimeter! A multimeter serves as your primary tool for gauging the resistance of your varistor. It aids in revealing the resistance of your varistor and determining its operational status. Ensure you undertake essential actions based on the outcomes to avert potential severe and hazardous issues!