Basic knowledge of capacitor selection (III)

Types of capacitors

There are many types of capacitors due to the different materials, dielectrics and structures of the electrodes. At the same time, due to the emergence of new materials and dielectrics, new types of capacitors often appear on the market. Therefore, the identification of capacitor types and their characteristics is indispensable knowledge for those engaged in circuit design, assembly and maintenance.

From the perspective of appearance and structure, capacitors have fixed capacity, adjustable capacity, cylindrical, square, pancake, and lightbulb shapes. It is generally easier to identify the appearance, but some internal structures cannot be distinguished from the appearance unless there are text notes on the package. In addition, since most capacitors are produced for assembly manufacturers, they have some specific specifications that are difficult to mark on the package. In the case of amateur use, it depends on the user to analyze and choose based on his knowledge of capacitors. The following are some common capacitor structures and characteristics.

3.1 Oil impregnated Paper Capacitor

Also referred to as paper capacitors, it is made of metal foil, mostly aluminum foil, alternately wound with insulating thin paper. After winding, it is first vacuum dried to remove moisture, then impregnated with insulating oil and packaged.

The capacity stability of oil-immersed paper capacitors is extremely high, and the withstand voltage is usually above 200, 400 or 600V. They have no polarity and are suitable for use in AC conditions and in vacuum tube machines. The disadvantage is that the volume per unit capacity is very large.

3.2 Metallized Paper Capacitor

Metallization is a technology developed in recent years. It is to vacuum evaporate a very thin layer of metal on one side of the medium to replace the traditional method of using metal foil as electrode. The advantage of metallization technology is that it can reduce the volume of unit capacity and has self-healing effect when the medium is accidentally broken down.

The structure of metallized paper capacitor is that zinc or other metal is evaporated on the insulating paper, and then it is wound, dried and packaged in oil according to the manufacturing method of oil-impregnated paper capacitor. The characteristics are similar to those of paper capacitors, but the volume is smaller. This type of capacitor is noted as.

3.3 Ceramic Capacitor

It is made of a disc-shaped ceramic medium with silver ion leads plated on both sides for packaging.

Due to the different ceramic components, there are two types of ceramic capacitors:

A High K Ceramic Capacitor is a ceramic capacitor with a very high dielectric constant. It can obtain a higher capacitance in a very small area. However, due to the influence of the dielectric constant, the capacitance error may be large. However, this type of capacitor has excellent high-frequency characteristics due to its dielectric properties and non-winding, so it is usually used in high-frequency bypass circuits.

2. Temperature Compensating Ceramic Capacitor Temperature Compensating Ceramic Capacitor uses ceramic with temperature compensation characteristics as the medium. The capacitance is generally small, ranging from a few PF to tens of PF. The top is painted with red, black, yellow and other colors to identify its temperature compensation characteristics. It is usually used for resonance or bypass in extremely high frequency circuits.

3.4 Polyester Film Capacitor

Commonly known as Mylar capacitor, it is one of the common plastic film capacitors. It uses a polyethylene terephthalate ISO or PET thin plastic film as the dielectric and a metal foil as the destination. There are two winding methods: inductive and non-inductive. It is the most common low-capacitance capacitor in solid-state circuits and has a low noise index.

Taking the PEF series made by Dah Sing TSC as an example, the main specifications are as follows

Operating temperature -40°C to +85°C

Capacity range 0.001 microfarad to 0.47 microfarad

The capacity error has three levels: J = plus or minus 5%, K = plus or minus 10%, and M = plus or minus 20%.

Working voltage 50 100V 200V etc.

Dissipation factor 0.8% at 25° to 85° 1KHz

3.5 Metallized Polyester Film Capacitor

The dielectric is the same as the polyester film capacitor described in the section, but it is not wound with metal foil. Instead, it is made by vaporizing aluminum or zinc metal using metallization technology and then winding it. It usually uses a non-inductive winding method and has several shapes such as square, cylindrical, or flat cylinder, as well as the same as polyester capacitors. The capacity is larger.

The following are the main specifications of Daxin's ordinary capacitors:

Capacity range 0.01 microfarad to 10 microfarad

The capacity error is K = plus or minus 10% and M = plus or minus 20%.

Working voltage 100V 250 400V 630 etc.

Temperature range -40°C to +85°C

Dissipation factor 1%

Dah Hsin also has another type of metallized polyethylene capacitor, MEE flat cylinder type and MET cylindrical type packaging. The specifications are similar to the former. Both are mainly used for AC circuit cross-connection, bypass and high-frequency filtering.

3.6 Polystyrene Film Capacitor

Polystyrene, abbreviated as PS, is also a kind of plastic film. It is often wound with metal foil into a tube and is widely used in small-capacity high-frequency circuits.

The following are the main specifications of Dah Sing PSE horizontal and PSA capacitors:

Working temperature:

Capacity range to microfarad

Capacity error has positive and negative, positive and negative, positive and negative, and positive and negative four levels

Voltage range and level

Dissipation Factor

The minimum value is less than

3.7 Polypropylene Film Capacitor

PP capacitors are made of polypropylene film and metal foil. They are available in two winding methods: inductive and non-inductive. They are similar to Mylar capacitors except that their withstand voltage is slightly higher.

PPN capacitors manufactured by Dahsin belong to this category. The main specifications are as follows:

Capacity range 0.001 to 0.47 microfarads

Capacity error is divided into three levels: J = plus or minus 5%, K = plus or minus 10%, and M = plus or minus 20%.

Applicable voltage: 250V 400V and 630 level 3

Applicable temperature -40 degrees to +85 degrees

Dissipation factor 0.1%

3.8 Metallized Polypropylene Film Capacitor

The capacitor made by vaporizing metal on polypropylene film and then winding it has a larger capacity per unit volume and has a self-healing effect.

3.9 Mica Capacitor

Capacitors with mica as dielectric cannot be wound because of its brittleness. Therefore, they can only be manufactured by the lamination method to increase the capacity. This is called a stacked mica capacitor. Its appearance is mostly square.

In addition, silver mica capacitors can be coated on mica. Their appearance is similar to that of plastic capacitors. Mica capacitors have extremely high frequency response and are often used in extremely high frequency circuits.

3.10 Aluminum Electrolytic Capacitor

High-purity aluminum foil is first corroded to form a porous and rough surface to expand the surface area, and then electrolysis is performed to form a non-conductive oxide film on the surface. The capacitor is wound with this oxide film as the medium.

The capacity of electrolytic capacitors per unit volume is larger than that of general capacitors. This is mainly because the effective surface area of ​​aluminum foil can be expanded to 10 to 50 times after corrosion, and the dielectric constant of aluminum oxide film is also higher than that of general dielectrics. It can produce a very large capacitance per unit volume, which has great advantages in circuit application. In particular, the use of capacitors in power circuits seems to be electrolytic capacitors.

However, aluminum foil electrolytic capacitors also have their disadvantages compared to capacitors made of other materials, such as:

• Large internal loss: This is mainly due to the resistance formed by the electrolyte plus the resistance of the aluminum foil and the contact itself relative to the capacity This internal resistance is a series resistance in the equivalent circuit, which is also a factor affecting the dissipation factor. When charging and discharging with large currents, it may cause heating and other phenomena.
• Large capacitance error: Because most of the capacitance of electrolytic capacitors is formed by the uneven surface of aluminum foil and the oxide film formed by electrolysis, and both of these are unstable in nature during processing or use, the capacitance error of many electrolytic capacitors is -20 % to +80 % of the marked value. Therefore, this defect has no effect on the power circuit.
• Large leakage current: This is mainly due to the dielectric properties. Special attention should be paid when using it in places where DC isolation is required, such as in cross-connections.
• After long-term storage, the leakage current tends to increase and the capacity decreases: this is because the aluminum oxide film is immersed in the electrolyte for a long time, which deteriorates the dielectric properties of the aluminum film, but it can be restored after applying voltage for some time.

3.12 Aluminum solid electrolytic capacitor

Usually, aluminum powder is sintered into granules and then chemically processed to form a cathode with a semiconductor as the medium, while the anode is still made of electrolytic aluminum foil. This is an aluminum solid electrolytic capacitor. The volume per unit capacity is relatively large and is generally rare.

3.13 Tantalum electrolytic capacitors

High-purity tantalum is used as the anode plate, and the structure is similar to that of electrolytic capacitors. However, in addition to the aluminum foil commonly used in aluminum electrolytic capacitors, in recent years, tantalum powder has been sintered to form a dielectric surface through chemical formation. Due to the difference in the cathode formation, tantalum electrolytic capacitors are also available in two types, wet and solid, like aluminum electrolytic capacitors. The wet type uses a strong acid electrolyte as the cathode, and the solid type uses manganese dioxide and carbon powder and solder to lead out the cathode. The reliability of tantalum electrolytic capacitors is generally higher than that of aluminum electrolytic capacitors, but their manufacturing costs are also high.