Varistor Explained: How It Protects Electronics from Voltage Surges
Varistors are essential components that protect electronic devices from damaging voltage spikes. Learn how varistors work, how to select the right one, how they differ from fuses, and how to test or replace them if needed.
Varistor is an electronic component whose resistance changes depending on the applied voltage. Its primary function is to protect electrical devices from brief voltage spikes that can damage sensitive microchips and other components.
Varistors are commonly installed in power supplies, surge protectors, household appliances, and industrial equipment. Let's explore how this component works, why it responds to overvoltage, and how it differs from a standard fuse.
What Is a Varistor and What Is It Used For?
A varistor is a nonlinear resistor whose resistance drops sharply when the voltage exceeds a certain threshold. Under normal conditions, it has a high resistance and barely affects the operation of the electrical circuit.
The main role of a varistor is to limit dangerous voltage surges. Such surges can occur during motor switching, disconnection of inductive loads, lightning strikes, and other transient processes in the power grid.
For example, when a brief overvoltage spike appears, the varistor starts conducting a significant current, clamping the voltage across the protected equipment. This reduces the risk of damage to electronic components.
Varistors are most frequently found in:
- Power supplies - protecting input circuits from surge overvoltages.
- Surge protectors - limiting voltage spikes coming from the mains.
- Household appliances - safeguarding electronic boards in refrigerators, washing machines, and other devices.
- Industrial equipment - suppressing overvoltage during load switching.
It's important to note that varistors are not designed to stabilize voltage or protect against prolonged overvoltage in the network. If the overvoltage persists too long, the component can overheat and fail.
Varistor Construction and Operation
Device Structure and Working Principle
Most modern varistors are made from zinc oxide (ZnO) mixed with other metal oxides. The material is pressed and sintered to form a ceramic element, which is then coated with a protective insulating layer and fitted with metal leads.
The key feature of a varistor is its nonlinear volt-ampere characteristic. At low voltages, its resistance is very high, so only a tiny leakage current flows through.
Once the voltage reaches a certain threshold, the resistance drops sharply. The varistor then passes a much larger current, limiting further voltage increase.
After the brief surge ends, a working varistor returns to its high-resistance state. Thanks to this, it can react to overvoltages multiple times, although strong surges gradually shorten its lifespan.
How a Varistor Protects Against Voltage Surges
In AC circuits, a varistor is typically connected in parallel with the protected load, for example, between phase and neutral at the power supply input.
The protection process can be illustrated as follows:
- Normal operation: Voltage is within acceptable limits, varistor resistance is high.
- Surge occurrence: Voltage spikes sharply, varistor resistance decreases.
- Overvoltage clamping: Surge current passes through the varistor, and the voltage across the load is limited.
- Return to normal: After the surge, varistor resistance increases again.
It's important to understand that a varistor does not absorb a voltage spike without trace. Part of the impulse energy is dissipated as heat, and the level of protection depends on both the varistor's characteristics and the parameters of the electrical circuit.
Additionally, a varistor does not hold the voltage at a strictly constant level. The clamping voltage depends on the surge current and the component's properties.
For safeguarding against prolonged voltage deviations, other devices are used.
Learn more about their construction and capabilities in the article Voltage Stabilizers: How They Work and How to Choose the Best Protection.
Key Characteristics and Varistor Markings
Voltage, Current, and Surge Energy
When choosing a varistor, several parameters must be considered to ensure reliable protection for your electronics:
- Maximum operating voltage - the highest steady or RMS AC voltage the varistor can handle long-term without excessive leakage current.
- Varistor voltage - voltage measured at a specified test current, usually 1 mA. It marks the onset of the component's nonlinear behavior.
- Clamping voltage - the voltage across the varistor when a specified surge current flows.
- Maximum surge current - the peak current the varistor can withstand for a given pulse shape.
- Surge energy - the amount of energy (in joules) the varistor can dissipate under specified test conditions.
The higher the permissible surge current and energy, the greater the impact the varistor can handle. However, an excessively high working voltage may weaken protection for sensitive electronics.
How to Read Varistor Markings and Make the Right Choice
Varistor bodies usually carry an alphanumeric code containing size and electrical ratings.
For example, the common marking 14D471K can be interpreted as:
- 14 - nominal disc diameter of about 14 mm.
- D - disc type.
- 471 - nominal varistor voltage of 470 V.
- K - varistor voltage tolerance of ±10%.
Note: 470 V is not the maximum working voltage or the exact clamping voltage. For such a varistor, the typical allowable AC working voltage is around 300 V, but exact values should always be checked in the manufacturer's datasheet.
When selecting a varistor, consider the nominal grid voltage, possible long-term deviations, anticipated surges, and the specifications of the protected equipment.
For 220-230 V networks, do not select a varistor based solely on the voltage stated on its body. Always check the allowable AC working voltage, clamping voltage, surge endurance, and thermal protection requirements.
How Does a Varistor Differ from a Fuse?
Both varistors and fuses protect electrical devices but respond to different types of hazards.
A varistor limits brief overvoltages by decreasing its resistance sharply when voltage spikes. After the impulse, a working varistor returns to normal.
A fuse protects a circuit from excessive current. When current exceeds its limit for a certain time, the fusible element melts and breaks the circuit.
| Characteristic | Varistor | Fuse |
|---|---|---|
| Protects against | Surge overvoltages | Overloads and short circuits |
| Operating principle | Reduces resistance | Breaks the circuit |
| Connection | Usually parallel to load | In series with load |
| Reusability | Possible after allowable surges | Requires replacement after operation |
| Response | To voltage increase | To current excess |
In electronic devices, both components often work together. The varistor limits dangerous surges, and the fuse can disconnect power if excessive current occurs - including if the varistor is damaged.
However, a standard fuse does not always prevent a varistor from overheating during prolonged overvoltage. That's why power protection devices often use special thermal fuses or varistors with built-in thermal protection.
To learn more about other ways to protect electrical circuits, read the article Electronic Fuses (eFuse): What They Are and How They Work.
How to Test a Varistor and Detect Faults
Testing a Varistor with a Multimeter
You can test a varistor with a standard multimeter, but this method mainly detects short circuits or severe damage to the component.
Before testing, always fully power down the device, unplug it from the mains, and ensure capacitors are safely discharged. For reliable testing, it's best to desolder the varistor from the board.
Testing procedure:
- Inspect the body. Cracks, discoloration, bulging, or burn marks indicate possible damage.
- Switch the multimeter to resistance measurement mode.
- Connect the probes to the two varistor leads. Polarity does not matter.
- Read the measurement. A good mains varistor typically shows very high resistance, often beyond the multimeter's range.
If the device reads near zero resistance, the varistor is likely shorted and needs replacement.
However, high resistance does not guarantee the varistor is good. A standard multimeter cannot check varistor voltage or its clamping ability. Full diagnostics require special measuring equipment.
Why a Varistor Fails and When Replacement Is Needed
A varistor can fail if the surge energy exceeds its rating or if it is exposed to prolonged overvoltage.
Main causes of failure:
- Strong surge overvoltages in the power grid.
- Frequent voltage spikes gradually degrading the component.
- Prolonged exceeding of allowable working voltage.
- Incorrect varistor selection for voltage or surge resistance.
A damaged varistor may become shorted, lose its protective properties, or break apart. If you see cracks, signs of overheating, or measure a short circuit with a multimeter, the component must be replaced.
A new varistor should match the original's specifications, including working and varistor voltage, permissible surge current, and energy. Installing a component with inappropriate ratings is hazardous.
Do not operate a mains device with a damaged varistor or simply remove it from the circuit. Repair of equipment connected to the mains should be entrusted to a specialist.
Conclusion
A varistor protects electronics from brief overvoltage by sharply reducing its resistance during dangerous voltage spikes. Unlike a fuse, it is not meant to disconnect a circuit during overload and can activate multiple times within its limits.
When selecting a varistor, pay close attention to working voltage, clamping voltage, and permissible surge energy. If a varistor is damaged, replace it with a properly rated component after identifying the cause of failure.