Dingfeng Capacitor ---What consequences are expected when the voltage of reverse polarity is applied
What consequences are expected when the voltage of reverse polarity is applied on an aluminum electrolytic capacitor?
The anode foil of polar aluminum electrolytic capacitor is subjected to chemical conversion treatment forcedly so as to have withstand voltage corresponding to the rated voltage of capacitor, while a cathode foil has substantially no withstand voltage because it is not subjected to such chemical conversion treatment. However, since aluminum is an active metal, it may react with oxygen in the air to form oxide film naturally on the cathode foil. Owing to this film, it is said that the cathode foil will have the withstand voltage of about 1 to 1.5 V at room temperature. Since this film is not uniform, but unstable, and shows dispersion partly or for each lot, no guarantee is given to the withstand voltage of cathode. In the circuit in which polarity is reversed, it is recommended to use a non-polarized aluminum electrolytic capacitor.

In case when the voltage higher than the withstand voltage is applied on the cathode foil of polar aluminum electrolytic capacitor, a current flows to electrolyze the water content in the liquid electrolyte. Then, oxygen generated by such electrolysis will react with the cathode foil to form oxide film on the cathode foil surface (chemical conversion coating of cathode foil). Owing to this reaction, the cathode foil capacity will lower. Since the capacitor capacity is the combination of anode and cathode foil capacities, this may lead to the decreased capacity of capacitor and further to the increase in tangent of loss angle (tan δ).
Moreover, owing to this reaction, gases will be generated in the capacitor, thus leading to the internal pressure buildup. The higher the applied voltage is and the higher the ambient temperature is, the more the gases are generated. Depending on the applied voltage and temperature, this may sometimes lead to the phenomena such as swelling of rubber packing of the capacitor and sometimes to activation of safety device or slipping out of rubber packing in the products with no safety device. Therefore, avoid the use of capacitor in the circuit where reverse polarity connection of capacitor or application of reverse voltage may occur.

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