What consequences are expected when the voltage exceeding the rated voltage is applied on an aluminum electrolytic capacitor?
On the anode foil of aluminum electrolytic capacitor, an oxide film capable of withstanding the rated voltage even if it is continuously applied at the maximum operating temperature.
In case when the voltage higher than the withstand voltage of this oxide film (overvoltage) is applied, the anode foil of aluminum electrolytic capacitor will form the oxide film equivalent to the applied voltage. Owing to the reaction, gases will be generated, thus leading to the pressure buildup in the capacitor. As the characteristics of capacitor, decrease in electrostatic capacity and increase in tangent of loss angle will be caused. The higher the applied voltage is and the higher the ambient temperature is, the more the gases are generated and the higher the internal pressure. This may sometimes lead to the phenomena such as swelling of sealing material (rubber packing) and further to activation of safety device (slipping out of rubber packing in the products with no safety device). Therefore, avoid the use of capacitor in the circuit where the voltage exceeding the rated voltage may be applied to it.
The structural breakdown modes in case when overvoltage is applied are as follows:
The safety device is activated (or rubber packing slips out), and liquid electrolyte in the capacitor is flown out, thus leading to dryup and finally to open condition.
If the voltage higher than the withstand voltage of anode foil, that of liquid electrolyte and that of separator paper is applied and it is no longer possible to keep insulation, dielectric breakdown will be caused, thus leading to short-circuiting.
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.