Hello, electronics enthusiasts!
If you've ever wondered how your electronic devices survive voltage spikes, the answer may lie in a small component called a Varistor. In this complete guide, we'll explore everything about these silent protectors that save our devices daily.
A Varistor is a variable resistor whose resistance depends on the applied voltage. The name is a linguistic amalgam of two words: "Variable" and "Resistor". These components are true guardians of electronic circuits, acting as shields against overvoltages that could permanently damage sensitive equipment.
- VDR (Voltage Dependent Resistor) - Voltage Dependent Resistor
- MOV (Metal Oxide Varistor) - Metal Oxide Varistor
Unlike conventional resistors, varistors have non-ohmic characteristics and are classified as non-linear resistors. This unique characteristic makes them indispensable in protection applications.
🧐 How Varistors Work: The Protection Mechanism
To understand how a varistor works, imagine an intelligent traffic guard that only allows cars (electric current) to pass when the speed (voltage) exceeds a safe limit. Similarly, a varistor remains with high resistance (practically insulating) under normal operating conditions, but its resistance drops drastically when the voltage reaches a specific threshold value.
Unlike potentiometers and rheostats, where the resistance changes gradually from a minimum to a maximum value, the operation of a Varistor is more abrupt: when subjected to a certain voltage (threshold voltage), the differential resistance suddenly drops to a very low value.
Varistors are composed of semiconductor elements that provide overvoltage protection in a circuit. Their functionality is similar to a Zener diode, allowing voltages below the limit to pass unchanged, but diverting excess energy when the voltage exceeds the safe value.
In practice, the main function of the Varistor is to protect against excessive transient voltages in the circuit. These transient voltages are usually caused by electrostatic discharges, lightning surges, or switching of inductive loads, phenomena that we will detail later.
🔧 Types of Varistors: Know the Variations
There are several types of varistors, each with specific characteristics for different applications. Let's explore the main types:
1️⃣ Zinc Oxide Varistor (MOV)
The zinc oxide varistor is the most common type of varistor. It is composed of a zinc oxide ceramic material that has high resistance under normal conditions, but its resistance decreases significantly when a surge voltage is applied.
Zinc oxide varistors are widely used for surge protection in electronic equipment and power systems. Their popularity is due to the excellent cost-benefit ratio and efficiency in protecting against a wide range of overvoltages.
2️⃣ Zinc Oxide Varistor with Bismuth
This is a variation of the traditional zinc oxide varistor that contains a small amount of bismuth in its composition. This addition aims to improve the electrical and performance characteristics of the varistor.
Bismuth is added to zinc oxide to promote the formation of a more stable and uniform semiconductor grain boundary layer, resulting in a varistor with improved electrical characteristics.
🔽 The main advantages of the zinc oxide varistor with bismuth addition include:
- Lower breakdown voltage: Bismuth reduces the breakdown voltage of the varistor, allowing it to start conducting at a lower voltage compared to the conventional zinc oxide varistor.
- More stable temperature coefficient: The addition of bismuth helps to stabilize the temperature coefficient of the varistor, making it less sensitive to temperature variations.
- Better performance at high temperatures: The zinc oxide varistor with bismuth addition has better thermal resistance, allowing it to maintain its performance even in high temperature conditions.
3️⃣ Metal Oxide Varistor (MO)
The metal oxide varistor uses a metal oxide instead of zinc oxide. It offers surge response characteristics similar to zinc oxide varistors, but with faster response properties and greater energy absorption capacity.
These varistors are often found in high-power applications and industrial voltage surge protection systems, where fast response is crucial.
4️⃣ Silicon Carbide Varistor (SiC)
The silicon carbide varistor is used for high voltage and high power applications. It has a fast response to voltage surges and is capable of handling high energy levels.
Silicon carbide varistors are often found in power systems, power transmission, and other high-power applications, where energy dissipation capability is a critical factor.
💡How Voltage Change Affects the Varistor's Resistance?
This characteristic is determined by the semiconductor composition of the varistor. When manufactured with semiconductor material, its intrinsic characteristics cause the Varistor to change its resistance with high operating voltages.
When the voltage applied to a Varistor is greater than its nominal value (threshold voltage), the effective resistance of the Varistor drops drastically and continues to decrease as the applied voltage increases.
The curve representing the static resistance of the Varistor in relation to its applied voltage is shown in Figure 2 below:
🔌 Varistor Symbol
In circuit diagrams, varistors are represented by specific symbols that differentiate them from other components. Knowing these symbols is essential to correctly identify varistors in electrical schematics.
👉 How Does a Varistor Help Suppress Voltage Transients?
To understand this, let's first identify the sources of transient voltage. The origin of these transients in electrical circuits is independent of whether they operate from an AC or DC source, and can come from the circuit itself or from external sources.
These transients result in an increase in voltage to several thousand volts, which can be catastrophic for the circuit. Imagine a small river that suddenly turns into a devastating flood - that's how voltage transients can affect a delicate electronic circuit.
The varistor acts as an intelligent containment system, detecting the abnormal voltage increase and providing a low resistance path to divert the excess energy, thus protecting the most sensitive components of the circuit.
📊 Voltage-Current (V-I) Characteristics
According to Ohm's law, the current-voltage characteristic curve of a resistor is a straight line, assuming the resistor value is kept constant. In this case, the current flowing through a resistor is directly proportional to the applied voltage.
However, in the case of a Varistor, the current-voltage characteristic curve is not a straight line. This is due to the unusual resistance behavior of the Varistor, where a small change in the applied voltage can cause a significant change in the current flowing through it.
The voltage-current characteristic curve of a Varistor is shown in Figure 4 below:
The characteristic current-voltage curve of a Varistor shows a linear relationship between current and voltage when the Varistor is not conducting. This occurs because the current flowing through the Varistor will remain constant and the value is very low - this is called the leakage current, generally on the order of a few milliamperes.
This small current will remain constant until the voltage applied across the Varistor reaches its nominal voltage, also called the clamping voltage. The nominal voltage of a Varistor is the voltage across it, measured with a specified DC current of 1mA.
When the applied voltage of the Varistor reaches its nominal voltage, the behavior of the Varistor changes from the insulation state to the conduction state. The resistance of the Varistor becomes very small when the applied transient voltage is greater than or equal to the nominal voltage of the Varistor.
This occurs due to a phenomenon called avalanche breakdown in semiconductor materials, a form of current multiplication that allows large currents in materials that previously acted as insulators. Due to this situation, the small leakage current increases rapidly.
Even though the current flowing through the Varistor increases, the voltage across it is limited to a value close to the nominal voltage. This means that the Varistor acts as a self-regulator for the transient voltages applied across it, allowing more current to flow through the Varistor.
Thus, after crossing the nominal voltage of the Varistor, the current-voltage curve becomes a steep non-linear curve. Due to this characteristic, a Varistor can pass currents varying extensively over a very narrow range of voltages, cutting off any voltage spikes.
🛡️ Practical Applications of Varistors
Now that we understand how varistors work, let's explore where they are used in the real world:
- Power supplies: Protect against voltage spikes in the power grid
- Surge protectors: In power strips and voltage stabilizers
- Telecommunications equipment: Protect telephone and data lines
- Industrial systems: In control panels and automation
- Home appliances: In TVs, computers, and other sensitive devices
- Power systems: In transformers and substations
⚠️ How to Select the Appropriate Varistor
To select the correct varistor for your application, consider the following factors:
- Nominal voltage: Must be higher than the maximum normal operating voltage of the circuit
- Energy absorption capacity: Determines how much energy the varistor can dissipate during a surge
- Maximum surge current: The maximum current that the varistor can withstand without damage
- Response time: How quickly the varistor responds to a voltage spike
- Physical size: Must be compatible with the available space in the project
🔗 Related Content
If you liked this project, you might also be interested in these other articles:
🧾Conclusion: The Invisible Guardian of Electronic Circuits
Varistors are essential electronic components for protecting equipment against voltage surges. They function as a kind of "relief valve" for excessive electrical current, dissipating it safely and protecting the connected devices.
Furthermore, varistors are widely used in various applications, from telecommunications equipment to renewable energy systems. Understanding the functioning and importance of varistors is fundamental to ensure the safety and efficiency of electronic equipment in our daily lives.
When designing or repairing electronic circuits, never underestimate the importance of including appropriate varistors for overvoltage protection. They may seem like simple components, but they are true anonymous heroes that save valuable equipment from irreparable damage.
🤔 Frequently Asked Questions (FAQ)
To ensure your project is a success, we've compiled some of the most common questions about varistors. Check it out!
What is the difference between a varistor and a Zener diode? 🔽
Both protect against overvoltage, but work in different ways. A Zener diode is polarized and protects only in one direction, while a varistor is bipolar and protects against voltage spikes in both directions. Additionally, varistors can dissipate much more energy than Zener diodes.
Can a varistor be used multiple times or does it need to be replaced after a surge? 🔽
It depends on the magnitude and duration of the surge. For small surges, a varistor can operate repeatedly without significant degradation. However, after a significant surge that exceeds its energy absorption capacity, the varistor may be damaged and need replacement.
How to correctly install a varistor in a circuit? 🔽
A varistor should be installed in parallel with the circuit or component to be protected, as close as possible to the power entry point. In AC circuits, it is connected between phase and neutral (or ground), while in DC circuits, it is connected between positive and negative.
What is the lifespan of a varistor? 🔽
The lifespan of a varistor depends on several factors, including the frequency and magnitude of voltage surges it faces. Under normal conditions, a varistor can last many years, but each voltage surge it absorbs slightly reduces its lifespan. Varistors in areas with many electrical storms or grid instability may need more frequent replacement.
Original article published on FVML (Portuguese) – january 11, 2019
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