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Why can't aluminum Electrolytic capacitor withstand reverse voltage?


       Aluminum Electrolytic capacitorIt is made by using an aluminum cylinder as the negative electrode, which contains a liquid electrolyte, and inserting a curved aluminum strip as the positive electrode. It also requires direct current voltage treatment to form a layer of oxide film on the positive electrode as the medium. Its characteristics are large capacity, but high leakage, poor stability, and positive and negative polarity, making it suitable for power filtering or low-frequency circuits. When using, do not connect the positive and negative poles in reverse.

Due to the polarity of the Electrolytic capacitor, attention must be paid to the correct connection of the positive and negative poles when using, otherwise not only the capacitor will not work, but also the leakage current will be large, and the internal part of the capacitor will heat up in a short time, damage the oxide film, and then damage it.

It is the basic structure of aluminum Electrolytic capacitor. It consists of an aluminum layer consisting of an anode, aluminum oxide attached to the insulating medium, a cathode aluminum layer of the receiving electrode, and a real cathode consisting of electrolyte. The electrolyte is soaked on paper between two aluminum layers. The aluminum oxide layer is electroplated on the aluminum layer, which is very thin compared to the voltage applied to it and is prone to breakdown, leading to capacitor failure.

The alumina layer can withstand forward DC voltage, but if it is subjected to reverse DC voltage, it is prone to failure within seconds. This phenomenon is called 'Valve Effect', which is why the aluminum Electrolytic capacitor has polarity. If both electrodes of the Electrolytic capacitor have oxide layers, an non-polar capacitor will be formed.

The mechanism of the threshold phenomenon of the reverse voltage of the aluminum Electrolytic capacitor is called the hydrogen ion theory. When the Electrolytic capacitor bears the reverse DC voltage, that is, the cathode of the electrolyte bears the forward voltage and the oxide layer bears the negative voltage, the hydrogen ions gathered in the oxide layer will pass through the medium to the boundary between the medium and the metal layer, and will be converted into hydrogen. The expansion force of hydrogen will make the oxide layer fall off.

Therefore, the current directly flows through the capacitor after breaking down the electrolyte, and the capacitor fails. This DC voltage is very small. Under the action of 1~2V reverse DC voltage, the aluminum Electrolytic capacitor will immediately fail due to the hydrogen ion effect in a few seconds. On the contrary, when the Electrolytic capacitor bears a positive voltage, the negative ions are concentrated between the oxide layers. Because the diameter of the negative ions is very large, it cannot break through the oxide layer, so it can bear a higher voltage.

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