Polymer Aluminum Electrolytic Capacitors: Technical noteComparison between tantalum and aluminum

Capacitors are generally composed of electrodes (conductors) and an insulator between them. Capacitors, as typified by ceramic capacitors, can use mostly anything as a conductor and do not have polarity. In contrast, the conductor used for the anode in an electrolytic capacitor is made of a type of metal called a "valve metal."

When the valve metal undergoes electrolytic treatment as an anode in an appropriate electrolytic solution, an oxide layer forms on the surface of the metal, making it harder for current to flow. On the other hand, if the valve metal undergoes electrolytic treatment as a cathode, it has characteristics that indicate the direction in which a current flows. An electrolytic capacitor uses the oxide layer generated in the treatment as an insulator (dielectric). Therefore, the electrolytic capacitor gains polarity due to the characteristics of valve metals (rectification behavior), and this requires caution when using it as this behavior differs from that of a standard capacitor. Typical valve metals include tantalum and aluminum.

There are two types of electrolytic capacitors: those that use electrolytic solution for the cathode and those that use solid electrolytes such as conducting polymers. Electrolytic solution-type electrolytic capacitors draw electricity using a conductor made of the same valve metal as the anode.

When voltage is applied in the reverse direction of polarity (reverse application), the same chemical reaction that would occur during excessive voltage occurs because the cathode is covered only by an oxide layer that cannot withstand the rated voltage. The chemical reaction generates heat and gas, which increases its internal pressure. Solid electrolyte-type electrolytic capacitors do not use a valve metal as the cathode, so no chemical reaction occurs. There is, therefore, no risk of this issue occurring.

Solid electrolyte-type tantalum electrolytic capacitors and aluminum electrolytic capacitors have the same rectification behavior characteristics, and reverse application will cause a short circuit. However, when verified on actual products, reverse application exhibited different results for tantalum and aluminum. These results are discussed below.