The large capacitance of electrolytic capacitors makes them particularly suitable for passing or bypassing low-frequency capacitance and capacitors pdf, and for storing large amounts of energy. For this reason the anode terminal is marked with a plus sign and the cathode with a minus sign.
In addition they can only tolerate low applied voltages. Applying a reverse polarity voltage, or a voltage exceeding the maximum rated working voltage of as little as 1 or 1. 5 volts, can destroy the dielectric and thus the capacitor. The failure of electrolytic capacitors can be hazardous, resulting in an explosion or fire.
As to the basic construction principles of electrolytic capacitors, there are three different types: aluminum, tantalum, and niobium capacitors. Each of these three capacitor families uses non-solid and solid manganese dioxide or solid polymer electrolytes, so a great spread of different combinations of anode material and solid or non-solid electrolytes is available. To increase their capacitance per unit volume, all anode materials are either etched or sintered and have a rough surface structure with a much higher surface area compared to a smooth surface of the same area or the same volume. This oxide layer acts as dielectric in an electrolytic capacitor. After forming a dielectric oxide on the rough anode structure, a counter electrode has to match the rough insulating oxide surface.
This is accomplished by the electrolyte, which acts as the cathode electrode of an electrolytic capacitor. There are many different electrolytes in use. Comparing the permittivities of the different oxide materials it is seen that tantalum pentoxide has a permittivity approximately three times higher than aluminum oxide. Tantalum electrolytic capacitors of a given CV value theoretically are therefore smaller than aluminium electrolytic capacitors. In practice different safety margins to reach reliable components makes a comparison difficult.
The anodically generated insulating oxide layer is destroyed if the polarity of the applied voltage changes. On the other hand, the voltage strengths of these oxide layers are quite high. With this very thin dielectric oxide layer combined with a sufficient high dielectric strength the electrolytic capacitors can already achieve a high volumetric capacitance. This is one reason for the high capacitance values of electrolytic capacitors compared to conventional capacitors.
All etched or sintered anodes have a much higher surface compared to a smooth surface of the same area or the same volume. That increases the later capacitance value, depending on the rated voltage, by a factor of up to 200 for non-solid aluminium electrolytic capacitors as well as for solid tantalum electrolytic capacitors. The large surface compared to a smooth one is the second reason for the relatively high capacitance values of electrolytic capacitors compared with other capacitor families. All electrolytic capacitors have one special advantage. Because the forming voltage defines the oxide layer thickness, the voltage proof of the later electrolytic capacitor can be produced very simply for the desired rated value. Therefore, the volume of a capacitor is defined by the product of capacitance and voltage, the so-called “CV product”.
Combinations of anode materials for electrolytic capacitors and the electrolytes used have given rise to wide varieties of capacitor types with different properties. An outline of the main characteristics of the different types is shown in the table below. The non-solid or so-called “wet” aluminum electrolytic capacitors were and are the cheapest among all other conventional capacitors. They not only provide the cheapest solutions for high capacitance or voltage values for decoupling and buffering purposes but are also insensitive to low ohmic charging and discharging as well as to low-energy transients. Non-solid electrolytic capacitors can be found in nearly all areas of electronic devices, with the exception of military applications. Tantalum electrolytic capacitors with solid electrolyte as surface-mountable chip capacitors are mainly used in electronic devices in which little space is available or a low profile is required. They operate reliably over a wide temperature range without large parameter deviations.
In military and space applications only tantalum electrolytic capacitors have the necessary approvals. Niobium electrolytic capacitors are in direct competition with industrial tantalum electrolytic capacitors because niobium is more readily available. The electrical properties of aluminum, tantalum and niobium electrolytic capacitors have been greatly improved by the polymer electrolyte. In order to compare the different characteristics of the different electrolytic capacitor types, capacitors with the same dimensions and of similar capacitance and voltage are compared in the following table.
In such a comparison the values for ESR and ripple current load are the most important parameters for the use of electrolytic capacitors in modern electronic equipment. The lower the ESR, the higher the ripple current per volume and better functionality of the capacitor in the circuit. However, better electrical parameters come with higher prices. Aluminum electrolytic capacitors form the bulk of the electrolytic capacitors used in electronics because of the large diversity of sizes and the inexpensive production. Tantalum electrolytic capacitors, usually used in the SMD version, have a higher specific capacitance than the aluminum electrolytic capacitors and are used in devices with limited space or flat design such as laptops. They are also used in military technology, mostly in axial style, hermetically sealed. Niobium electrolytic chip capacitors are a new development in the market and are intended as a replacement for tantalum electrolytic chip capacitors.