Two conductors close to each other sandwich a non-conductive insulating medium between them, which constitutes a capacitor. When a voltage is applied between the two plates of a capacitor, the capacitor will store charge. The capacitance of a capacitor is numerically equal to the ratio of the amount of charge on one conductive plate to the voltage between the two plates. The basic unit of the capacitance of a capacitor is the farad (F). In-circuit diagrams, the letter C is usually used to denote capacitive elements.
Capacitors play an important role in circuits such as tuning, bypassing, coupling, and filtering. It is used in the tuning circuit of the transistor radio and also used in the coupling circuit and bypass circuit of the color TV.
With the rapid development of electronic information technology, the updating speed of digital electronic products is getting faster and faster. The production and sales of consumer electronic products such as flat-panel TVs (LCD and PDP), notebook computers, digital cameras, and other products continue to grow, which has driven The capacitor industry is growing.
Capacitors are components that store electricity and electrical energy (potential energy). A conductor is surrounded by another conductor, or the electric field lines emitted by one conductor all terminate in another conductor's conduction system, which is called a capacitor.
In a DC circuit, a capacitor is equivalent to an open circuit. A capacitor is a component that can store electrical charge, and it is also one of the most commonly used electronic components.
This has to start from the structure of the capacitor. The simplest capacitor is composed of plates at both ends and an insulating dielectric (including air) in the middle. After being energized, the plates are charged to form a voltage (potential difference), but the entire capacitor is non-conductive due to the insulating material in the middle. However, this situation is under the premise that the critical voltage (breakdown voltage) of the capacitor is not exceeded. We know that any substance is relatively insulating. When the voltage across the substance increases to a certain level, the substance can conduct electricity. We call this voltage the breakdown voltage. The capacitor is no exception. After the capacitor is broken down, it is not an insulator. However, in the middle school stage, such a voltage is not visible in the circuit, so all work below the breakdown voltage and can be seen as an insulator.
However, in an AC circuit, the direction of the current changes with time as a certain function. The process of charging and discharging a capacitor takes time. At this time, a changing electric field is formed between the plates, and this electric field is also a function of time. In fact, current passes between capacitors in the form of an electric field.
Examples of types:
Aluminum electrolytic capacitors
It is a capacitor with a thin oxide film as the dielectric, which is wound between two aluminum foils with absorbent paper impregnated with paste electrolyte. Because the oxide film has unidirectional conductivity, the electrolytic capacitor has polarity.
Tantalum Electrolytic Capacitor
A sintered tantalum block is used as the positive electrode, and solid manganese dioxide is used as the electrolyte. The temperature characteristics, frequency characteristics, and reliability are better than ordinary electrolytic capacitors, especially the leakage current is very small, the storage performance is good, the life is long, the capacity error is small, and the volume is small, and the maximum capacitor voltage product can be obtained per unit volume. Its ability to withstand pulsating current is poor, and it is easy to short-circuit if damaged. Often used in ultra-small and highly reliable parts.
Self-healing shunt capacitor
The structure is similar to that of paper capacitors, but low-loss plastic materials such as polyester and polystyrene are used as the medium.
Monolithic capacitors (multilayer ceramic capacitors)
A number of ceramic film blanks are covered with electrode paddle materials, and after being laminated, they are wound into an indivisible whole at a time, and the outside is encapsulated with resin. It is a new type of capacitor with a small size, large capacity, high reliability, and high-temperature resistance. Low-frequency monolithic capacitors with high dielectric constant also have stable performance, small size, and large capacity error. Generally, two aluminum foils are used as electrodes, and capacitor paper with a thickness of 0.008 to 0.012 mm is separated and wound by overlapping in the middle. The manufacturing process is simple, the price is cheap, and a larger capacitance can be obtained.
Metallized Polypropylene Capacitor
Generally in low-frequency circuits, it usually cannot be used at frequencies higher than 3 to 4 MHz. Oil-immersed capacitors have higher voltage resistance and better stability than ordinary paper capacitors. They are suitable for high-voltage circuit trimmer capacitors (semi-variable capacitors). The capacitance can be adjusted within a small range and can be fixed at a certain level after adjustment. Capacitance value.
Porcelain-mediated fine-tuning capacitors have high charge and small size, and can usually be divided into two types: tube type and disc type. Mica and polystyrene media usually use spring-type elements, which are simple in structure but have poor stability. Wirewound ceramic fine-tuning capacitors change the capacitance by removing the copper wire (external electrode), so the capacitance can only be reduced, and it is not suitable for use on occasions that require repeated adjustments.
The capacitor ceramics with a high dielectric constant (barium titanate titanium monoxide) is extruded into circular tubes, discs, or discs as the medium, and silver is plated on the ceramics as electrodes by the sintering method. It is divided into high-frequency porcelain and low-frequency porcelain. Capacitors with small positive capacitance temperature coefficients are used in high-stability oscillation circuits as loop capacitors and bad capacitors.
Low-frequency ceramic capacitors are limited to bypassing or blocking DC in circuits with lower operating frequencies, or occasions where stability and loss are not high (including high frequency). Such capacitors are not suitable for use in pulse circuits because they are susceptible to breakdown by pulse voltage.
High-frequency ceramic capacitor
It is suitable for high-frequency circuit mica capacitors. In terms of structure, it can be divided into foil type and silver type. The silver electrode is formed by directly plating a silver layer on the mica sheet by vacuum evaporation or burning infiltration. Because the air gap is eliminated, the temperature coefficient is greatly reduced, and the capacitance stability is higher than that of the foil type. The frequency characteristic is good, the electric charge value is high, the temperature coefficient is small, and it cannot be made into a large capacity. It is widely used in high-frequency electrical appliances and can be used as a standard capacitor.
Glass glaze capacitor
It is formed by spraying a special mixture with a concentration suitable for spraying into a thin film. The medium is then sintered with a silver layer electrode to form a "monolithic" structure. Its performance is comparable to that of mica capacitors, and it can withstand various climatic environments. Working at 200℃ or higher temperature, the rated working voltage can reach 500V.
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