Why Do Varistors Fail & How Do They Lose Protection?
From lianglite • Electronics Classroom
A varistor is a voltage-clamping protection device. Using its nonlinear characteristic, when an overvoltage appears across its two terminals, the varistor clamps the voltage to a relatively fixed level, protecting downstream circuits. Key parameters include varistor voltage, surge current rating, junction capacitance, response time, etc.
The typical failure mode of a varistor is short circuit. To prevent power short circuits or fire caused by varistor failure, a thermal fuse or thermal disconnect device can be connected in series with each varistor. The thermal fuse must have good thermal coupling with the varistor. When the varistor fails (high-impedance short circuit), the heat it generates melts the thermal fuse, disconnecting the failed varistor from the circuit and ensuring equipment safety.
Working Principle
When subjected to high temporary AC overvoltage, a varistor may break down and short-circuit instantly (low-impedance short circuit). In such cases, the thermal fuse may not blow in time, leading to fire risk.
To avoid this danger, a surge-rated AC fuse can also be added in series (ordinary AC fuses may not blow during aging failure).
Another effective solution is to use a varistor in series with a ceramic gas discharge tube:
- Under normal operation, the gas discharge tube is non-conductive, resulting in zero leakage current through the varistor, greatly extending its service life.
- During surge impact, the gas discharge tube breaks down first, and the varistor clamps the surge voltage. Total residual voltage is the sum of both, rising only a few tens of volts.
- After the surge, the varistor limits the current, the discharge tube cannot maintain conduction and extinguishes the arc, and the circuit returns to normal.
- If the varistor fails short, the gas discharge tube will quickly fail due to large AC current, but its failure mode is usually open circuit, which is less likely to cause fire.
lianglite specializes in circuit protection components, providing stable and reliable varistors, gas discharge tubes, and integrated protection solutions for safer circuits and more durable equipment.