Capacitors Blog

Aluminum electrolytic capacitors are comprised of anode and cathode plates separated by an absorbent spacer. As shown in Figure below, metal tabs are attached to the anode and cathode plates, and the assembly is wound into a cylindrical section. The tabs are welded to aluminum terminals installed in a header (top). The section-header assembly is immersed in a bath of hot capacitor electrolyte (significantly different from the formation process electrolyte). In what is called the impregnation process, a vacuum is applied to the electrolyte and sections, causing electrolyte to be drawn into the sections, thoroughly wetting the sections. The sections are placed in aluminum cans, and the headers are sealed to the cans. The capacitor units are slowly brought up to maximum rated voltage at maximum rated temperature during the aging process. The aging process grows oxide on areas on the anode foil which have an insufficient oxide barrier, such as slit edges and places which have been cracked during the winding operation. Inspections and tests occur at several stages of the production process.

When an aluminum electrolytic capacitor is stored under no load conditions for a long period of time, its leakage current tends to increase slightly. This is due to a drop in the withstand voltage of the dielectric caused by the reaction of the anode oxide layer with the electrolyte. When the voltage is applied to the capacitor, the leakage current returns to its initial level because of the re-forming action of the electrolyte (called voltage treatment). If the storage temperature is high, the leakage current will increase substantially. Therefore, it is desirable to store capacitors at normal temperature level with no direct sunlight. A voltage treatment is recommended when using a capacitor stored for a long period of time. The treatment for an individual capacitor is accomplished by charging up to its rated voltage through a resistance of about 1 kΩ and applying the voltage for approximately 30 minutes.

Electrolytic capacitors generally have a positive and a negative terminal. As we said earlier, the plates (foil) of the capacitor are anodized with a DC current. This anodizing process sets up the polarity of the plate material (it determines which side of the plate is positive and which is negative). We also said that part of the electrolyte was to help heal a damaged plate. Since it has the properties to heal a damaged plate, it has the ability to randomize the plate. Since anodizing process can be reversed, the electrolyte has the ability to remove the oxide coating from the foil. This would happen if the capacitor was connected with reverse polarity. Since the electrolyte can conduct electricity, if the aluminum oxide layer is removed, the capacitor would readily pass direct current from one plate to the other (it would basically be a short circuit from one plate to the other). This would, of course, render the cap useless.

An aluminum electrolytic capacitor consists of cathode aluminum foil, capacitor paper (electrolytic paper), electrolyte, and an aluminum oxide film, which acts as the dielectric, formed on the anode foil surface.

A very thin oxide film formed by electrolytic oxidation (formation) offers superior dielectric constant and has rectifying properties. When in contact with an electrolyte, the oxide film possesses an excellent forward direction insulation property.  Together with magnified effective surface area attained by etching the foil, a high capacitance yet small sized capacitor is available.

One way to make a large capacitor is to take two long strips of aluminium foil (=large plates), put strips of isolating materials between them, and make a nice compact roll. Capacitors up to around 1MF can be made this way, but they are physically big, so if we want even higher capacity, we need to look for other things than plate area. It happens that we know of a very thin and very voltage resistant type of isolation material: Aluminium oxide. If we cover a strip of aluminium foil with a thin oxide layer, we have one plate and a very thin dielectric. Problem now is to make the other plate come close enough to the other side of the oxide layer. The thing that comes really close to anything is a liquid, so if we submerge our oxide covered plate in a conducting liquid, the liquid forms the other plate, and we can make a very large capacitor. A conducting liquid is called an electrolyte, see?

Water in the electrolyte plays a big role. It increases conductivity thereby reducing the capacitor’s resistance, but it reduces the boiling point so it interferes with high temperature performance, and it reduces shelf life. A few percent of water is necessary because the electrolyte maintains the integrity of the aluminum oxide dielectric. When leakage current fows, water is broken into hydrogen and oxygen by hydrolysis, and the oxygen is bonded to the anode foil to heal leakage sites by growing more oxide. The hydrogen escapes by passing through the capacitor’s rubber seal.

One of the problems with electrolytic capacitors is that they have a limited frequency response. It is found that their ESR rises with frequency and this generally limits their use to frequencies below about 100 kHz. This is particularly true for large capacitors, and even the smaller electrolytic capacitors should not be relied upon at high frequencies. To gain exact details it is necessary to consult the manufacturers data for a given part.

We started the development of capacitors with large capacitance and for high ripple current, ahead of other companies. Now, our capacitors for mid- to high-voltage have a high market share.

They have the following common features:

• Through the self-development of high-quality anode foil and electrolyte, high reliability and excellent cost performance have been achieved.
• Our products are used for the equipment that requires a large capacitance such as inverter and large power source.
• Lead-free/Conforming to RoHS Directive.

Basic Information:

Atomic Number: 13 
Atomic Mass: 26.981539 amu 
Melting Point: 660.37℃ (933.52 K, 1220.666 °F) 
Boiling Point: 2467.0℃ (2740.15 K, 4472.6 °F) 
Number of Protons/Electrons: 13 
Number of Neutrons: 14 
Classification: Other Metals 
Crystal Structure: Cubic 
Density @ 293 K: 2.702 g/cm3 
Color: Silver 
British Spelling: Aluminium 
IUPAC Spelling: Aluminium

Atomic Structure:

Number of Energy Levels: 3
First Energy Level: 2
Second Energy Level: 8
Third Energy Level: 3

Isotopes:

Facts:

Date of Discovery: 1825
Discoverer: Hans Christian Oersted
Name Origin: From the Latin word alumen
Uses: airplanes, soda cans
Obtained From: bauxite