What Should Be Noticed When Using Varistors
Release date:
2025-11-26
A varistor is a voltage-limiting protective device that responds sensitively to voltage changes. Leveraging the varistor's nonlinear characteristics, it can clamp the voltage to a relatively fixed level when an overvoltage occurs across its terminals, thereby safeguarding the downstream circuitry. Additionally, varistors play a crucial role in protecting circuits against transient overvoltages. Although its current-carrying capacity [1] Large, but with high energy capacity [2] However, it’s not very effective. Short circuits or burnout failures often occur because its maximum pulse width for surge current is significantly shorter than the actual pulse current width in high- and medium-power semiconductor systems.
（Note: Image source from the internet.）
A commonly available varistor on the market is made of zinc oxide (ZnO), offering excellent nonlinear characteristics, high surge current capacity, fast response speed, a wide voltage range, and an extensive variety of specifications—making it the mainstream choice in the market. When using a varistor, we should pay close attention to the following aspects to prevent damage caused by improper handling.
1) Insufficient voltage resistance
When selecting a varistor, pay attention to its working voltage to avoid exceeding the actual operating voltage, which could lead to the varistor being punctured and compromising its functionality.
2) Excessive Current and Surge
Using components that require frequent plugging and unplugging in a product can accelerate the degradation of varistors, as the surge currents generated during these actions—especially when neither end device is grounded—are particularly high. This significantly reduces both the product's inherent protection capability and the effectiveness of TVS clamping, leading to a higher failure rate.
3) Excessive temperature or aging
Under high-temperature conditions, thermal stress increases, accelerating material aging and leading to higher leakage current or a decline in clamping voltage. After repeated electrical surges or extended usage, the resistive material's performance degrades, potentially causing short circuits and triggering overheating that could lead to fires.
To extend the service life of varistors, we often employ the following overheat protection techniques:
1) Low-melting-point soldering technology
A low-melting-point solder joint is added at the leads of the varistor, and then a spring is used to hold this solder joint in place. When the varistor’s leakage current becomes excessive and its temperature rises to a certain level, the solder at the joint melts. Under the tension of the spring, the solder joint quickly separates, effectively disconnecting the varistor from the circuit. At the same time, the alarm contact is activated, triggering an alert signal. However, solder tends to age over time, which may lead to false disconnections.
2) Potting Technology
To prevent varistors from smoking, catching fire, or even exploding when they fail, some manufacturers use this technology to encapsulate the varistors. However, since arcing occurs internally within the varistor during failure, it can compromise the integrity of the sealing material and generate carbon. This carbon, in turn, helps sustain the arc—but ultimately leads to internal short circuits and blackening inside the device.
3) Isolation Technology
This technology encloses the varistor within a sealed box, isolating it from other circuits to prevent the spread of smoke and flames. Even when all other backup protection measures fail, this isolation technique remains a simple yet effective solution—but it does require significant equipment space and demands careful prevention of smoke and flames escaping through the openings around the box’s lead wires.
4) Temperature Fuse Technology
The varistor is encapsulated in series with a temperature fuse, triggering the fuse to melt via thermal conduction. However, the response speed is relatively slow (as heat must first transfer to the fuse), and its lifespan is limited.
5) Hot-melt Fuse Technology
The low-melting-point metal, protected by wax, melts when the varistor overheats, thereby breaking the circuit. However, this technology has limited reliability—particularly in high-temperature environments—and typically lasts about 5 years, requiring regular replacement.
(Source: Electronic Component Technology; If any copyright issues are found, please contact us for deletion or correction.）
[1] Surge current capacity, also known as rated impulse current or maximum surge current, refers to the maximum instantaneous current peak that a varistor can withstand without experiencing performance degradation or permanent damage, under specified pulse waveforms and number of cycles.
[2] Energy capacity, also known as rated energy, refers to the maximum surge energy that a varistor can absorb and convert into heat before safely failing.
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