What is a Varistor?

A varistor, also known as a voltage-dependent resistor (VDR), is an electronic component with a non-linear resistance that varies with the applied voltage. Its primary function is to protect electronic circuits from excessive voltage spikes by diverting the surge current away from sensitive components. Varistors are widely used in various applications, such as power supply protection, surge protection devices, and transient voltage suppression.

How Does a Varistor Work?

A varistor is made of a ceramic material, typically zinc oxide (ZnO), sandwiched between two metal electrodes. The ZnO grains are sintered together, forming a polycrystalline structure with a complex network of grain boundaries. These grain boundaries act as potential barriers, preventing current flow at low voltages.

When the applied voltage exceeds a certain threshold, known as the varistor voltage, the potential barriers break down, allowing current to flow through the device. This behavior is similar to that of a Zener Diode, but varistors can handle much higher energy levels.

The current-voltage (I-V) characteristic of a varistor is highly non-linear, as shown in the table below:

Voltage (V)	Current (A)
0.1	1E-6
1	1E-5
10	1E-4
100	1E-3
1000	1

As the voltage increases, the current through the varistor increases exponentially. This non-linear behavior allows varistors to conduct very little current at normal operating voltages but quickly switch to a low-resistance state when the voltage exceeds the threshold.

Varistor Symbols

Varistors are represented by various symbols in electronic schematics. The most common varistor symbols are:

1. Basic Varistor Symbol

The basic varistor symbol consists of a rectangular box with two terminals. Inside the box, there is a curved line representing the non-linear resistance characteristic of the varistor.

2. Bidirectional Varistor Symbol

A bidirectional varistor symbol is similar to the basic symbol but with an additional curved line in the opposite direction. This symbol represents a varistor that can conduct current in both directions when the voltage exceeds the threshold.

3. Polarized Varistor Symbol

A polarized varistor symbol has a diode-like arrow inside the rectangular box, indicating that the varistor has a specified polarity. These varistors are designed to protect against voltage spikes in a specific direction.

Varistor Applications

Varistors are used in a wide range of applications to protect electronic circuits from voltage spikes and transients. Some common applications include:

1. Power Supply Protection

Varistors are often used in power supplies to protect against voltage surges caused by lightning strikes, switching transients, or other disturbances on the power line. They are connected in parallel with the input of the power supply, absorbing the excess energy and limiting the voltage to a safe level.

2. Surge Protection Devices (SPDs)

Surge protection devices (SPDs) are designed to protect electronic equipment from voltage spikes and transients. These devices typically incorporate varistors as the main protective component, along with other components such as gas discharge tubes and thermal fuses.

3. Transient Voltage Suppression (TVS)

Varistors are used in transient voltage suppression (TVS) devices to protect sensitive electronic components from electrostatic discharge (ESD), induced voltages, and other transient events. TVS devices are commonly used in communication lines, data lines, and other signal transmission applications.

4. Motor Protection

Varistors can be used to protect electric motors from voltage spikes caused by switching, starting, or stopping the motor. They are connected in parallel with the motor windings to absorb the excess energy and prevent damage to the insulation.

5. Automotive Electronics

In automotive applications, varistors are used to protect electronic control units (ECUs), sensors, and other sensitive components from voltage spikes caused by load dumps, alternator transients, and other electrical disturbances in the vehicle’s electrical system.

Frequently Asked Questions (FAQ)

1. What is the difference between a varistor and a Zener diode?

While both varistors and Zener diodes are used for voltage regulation and protection, they have some key differences:

• Varistors are made of a ceramic material (usually zinc oxide), while Zener diodes are made of semiconductor materials (silicon or germanium).
• Varistors have a symmetrical I-V characteristic and can conduct current in both directions, while Zener diodes have an asymmetrical I-V characteristic and conduct current only in the reverse direction.
• Varistors can handle much higher energy levels and surge currents compared to Zener diodes.

2. How do I choose the right varistor for my application?

When selecting a varistor for your application, consider the following factors:

• Varistor voltage: Choose a varistor with a voltage rating slightly higher than the maximum operating voltage of your circuit.
• Energy rating: Ensure that the varistor can absorb the expected energy of the voltage spikes in your application.
• Response time: Select a varistor with a fast response time to ensure effective protection against transient events.
• Package size and type: Consider the physical constraints of your circuit and choose a varistor package that fits your design.

3. Can varistors be used in series or parallel?

Varistors can be connected in series or parallel to achieve higher voltage or energy ratings:

• Series connection: Connecting varistors in series increases the overall varistor voltage, allowing protection against higher voltage spikes. However, it is essential to ensure that the voltage is evenly distributed across the varistors.
• Parallel connection: Connecting varistors in parallel increases the energy handling capability of the protection circuit. This is useful when dealing with high-energy transients or when redundancy is required.

4. How does a varistor fail?

Varistors can fail due to various reasons, including:

• Repeated exposure to high-energy transients, leading to gradual degradation of the zinc oxide material.
• Excessive current flow through the varistor, causing overheating and thermal runaway.
• Manufacturing defects or mechanical damage, resulting in shorts or opens within the varistor.

When a varistor fails, it typically enters a short-circuit state, which can lead to further damage to the protected circuit if not properly addressed.

5. Can varistors be used for overvoltage protection in AC circuits?

Yes, varistors can be used for overvoltage protection in both AC and DC circuits. In AC applications, varistors are often connected in parallel with the protected load, across the line and neutral conductors. It is important to select a varistor with an appropriate voltage rating and energy handling capability for the specific AC application.

Conclusion

Varistors are essential components in modern electronic circuits, providing critical protection against voltage spikes and transients. By understanding varistor symbols, working principles, and applications, engineers and technicians can effectively design and troubleshoot circuits that incorporate these devices. When selecting varistors for a specific application, it is crucial to consider factors such as varistor voltage, energy rating, response time, and package type to ensure optimal performance and reliability.