In the design of electronic equipment, the selection of safety capacitors is crucial, especially attention should be paid to their failure mechanisms and voltage withstand indicators. The most common failure mode of safety capacitors is low-voltage failure, which is closely related to the safety margin in their design. From a safety perspective, designers usually tend to choose capacitors with a large safety margin in the hope that design deficiencies will not lead to capacitor failure. However, this approach can have flaws.
The main symptoms of low-voltage failure include: the actual applied voltage is far lower than the rated withstand voltage value of the capacitor, usually no more than 10% of the rated value; the capacitor is prone to failure after hot and humid experiments or moisture pretreatment; the capacitor is prone to failure after high-temperature experiments or circuit board baking. The capacitor will then return to normal; the capacitor performance will also recover after the capacitor is removed from the circuit board and a voltage higher than the normal operating voltage is applied. These phenomena indicate that the low-voltage failure of safety capacitors is a problem worthy of attention.
The reason for low-voltage failure is that the medium in the safety capacitor is encapsulated by the casing, and these casings are not completely sealed, thus providing the possibility for moisture penetration. For example, for a safety capacitor with a rated withstand voltage of 50V, when a voltage of 5V is applied to both ends, the moisture on the medium forms a leakage current channel. Due to the low voltage, the leakage current is not large and the water cannot evaporate, thereby destroying the energy storage characteristics of the capacitor. However, at high temperatures or when high voltage is applied, the water vapor evaporates, causing the leakage current path to disappear and the capacitance to return to normal.
Therefore, when selecting safety capacitors, the withstand voltage index should not be set too high. This is a key point that is prone to problems in selection. Safety capacitors are widely used in various electronic products, and their product quality directly affects our quality of life and safety. When choosing safety capacitors, you should give priority to genuine products from the original factory and ensure that they have safety certifications from various countries to ensure their quality and safety.