In devices such as power adapters and surge-protected sockets, there is a small but critical component known as the varistor.
A varistor is an overvoltage protection device with nonlinear voltage-current characteristics, meaning its resistance changes according to the applied voltage.
The main function of a varistor is to clamp voltage and absorb excess current when abnormal voltage rises occur, such as lightning strikes or power grid surges, thereby protecting downstream circuits and other electronic components.
However, when the energy of a lightning strike or surge is too high or lasts too long, the varistor may overheat and permanently fail in a short-circuit or open-circuit state. This condition is commonly referred to as varistor breakdown.
Varistor failure not only affects normal circuit operation but may also pose safety risks. So what should be done in this situation?
1. Immediately Cut Off the Power
Unplug the device or turn off the main power switch immediately. This prevents a failed varistor from causing a short circuit, circuit damage, or potential electric shock hazards.
2. Inspect the Varistor Appearance
Open the device enclosure and visually inspect the varistor. A normal varistor has a smooth surface with no visible damage. A failed varistor may show bulging, cracks, charring, or burn marks.
If no obvious damage is visible, a multimeter can be used for testing. Under normal conditions, a varistor’s resistance should be close to infinity. If the measured resistance is zero or very low, the varistor has failed.
3. Replace with the Same Specification Varistor
Once failure is confirmed, replace the varistor with a new one of the same model and specifications. After replacement, reconnect the power and test the circuit to ensure proper operation.
4. Perform Regular Inspection and Maintenance
Regularly inspect electronic devices and the condition of varistors. If aging or degradation is detected, replace the varistor in time with the same specification to prevent future breakdown and safety risks.
This article is provided by JYH HSU (JEC) Electronics. JEC is a research, development, production, and sales-oriented company specializing in manufacturing and selling various electronic components such as capacitors and resistors.
Post time: Dec-19-2025