A varistor is a special type of electronic component. Despite the word “pressure” in its name, the “pressure” in varistor refers to voltage, not physical pressure.
Varistors are highly sensitive to voltage changes. Under normal voltage conditions, a varistor’s resistance is very high. When the voltage is below the varistor’s rated voltage, it remains in a high-resistance state, allowing little to no current to pass. When the voltage exceeds the varistor’s rated voltage, its resistance rapidly drops to a low-resistance state, absorbing excess current and clamping the voltage within a safe range. This helps protect the circuit and other electronic components from voltage and current damage.
However, over prolonged use or repeated surge impacts, varistors can become short-circuited. Here are some signs that a varistor may have short-circuited in your electronic device:
1. Device Fails to Power On or Breaks the Circuit
When a varistor short-circuits, it effectively creates a low-resistance path in parallel with the power input. As a result, when the device is powered on, the current increases rapidly, which may blow a fuse or trip a circuit breaker.
If you find that your device trips the breaker or blows a fuse as soon as it is plugged in, and no other issues are present, the most likely cause is a short-circuited varistor.
2. Varistor Surface Is Burned or Charred
If your electronic product fails to power on, and there is no visible issue from the outside, technicians will often open the device for inspection.
A properly functioning varistor will appear intact, with no visible stains, cracks, or damage. However, if you notice signs of surface damage such as cracks, bulging, or smoke, it is a strong indication that the varistor has burned out and short-circuited.
3. Measuring Resistance Close to Zero with a Multimeter
When measuring the resistance of a varistor with a digital multimeter, it should show a high resistance value under normal conditions. If the resistance is very low, it means the varistor has short-circuited internally and is no longer functioning correctly.
This is a useful diagnostic tool, especially when no issues are found with other components in the circuit. If the varistor’s appearance is intact but its resistance is low, it’s likely the cause of the problem.
Conclusion
If you encounter any of these issues, it’s possible that the varistor has short-circuited. It is crucial to replace it with a varistor of the same model to avoid further damage and prevent future short-circuiting.
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: Nov-14-2025