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Different types of capacitors have different capacities to store charge. The amount of charge stored when a 1 volt DC voltage is applied to a capacitor is called the capacitor's capacitance. The basic unit of capacitance is Farad (F). But in fact, Farad is a very uncommon unit, because the capacity of a capacitor is often much smaller than 1 Farad. Commonly used capacitor units are microfarad (μF), nano farad (nF), and picofarad (pF). The relationship is: 1 farad (F) = 1000000 microfarads (μF) 1 microfarad (μF) = 1000 nanofarads (nF) = 1000000 picofarads (pF).
In electronic circuits, capacitors are used to block direct current. They are also used to store and release electric charges to act as filters to smooth out pulsating signals. Small-capacity capacitors are commonly used in high-frequency circuits such as radios, transmitters, and oscillators. Large-capacity capacitors are often used to filter and store charges. Generally, capacitors above 1μF are electrolytic capacitors, while capacitors below 1μF are mostly ceramic capacitors. The electrolytic capacitor has an aluminum case, which is filled with an electrolyte, and two electrodes are drawn out as positive (+) and negative (-) electrodes. Unlike other capacitors, their polarity in the circuit must not be wrong, while other capacitors have no polarity.
When connecting the two electrodes of the capacitor to the positive and negative poles of the power supply for a while, even if the power supply is disconnected, there will still be a residual voltage between the two pins. We can say that the capacitor stores the charge. The voltage builds up between the capacitor plates and accumulates electrical energy. This process is called capacitor charging. There is a certain voltage across the charged capacitor. The process of discharging the stored charge of a capacitor to a circuit is called the discharge of a capacitor.
As an example in real life, we see that a rectifier power supply will continue to light for a while after the plug is unplugged, and then gradually turn off. This is because the capacitor inside stores the power in advance and then releases it. Of course, this capacitor was originally used for filtering. As for capacitor filtering, I wonder if you have any experience using Walkman with rectified power. Due to the cost-saving of manufacturers, generally, low-quality power supplies use smaller-capacity filter capacitors, causing a buzz in the headphones. At this time, a large-capacity electrolytic capacitor (1000 μF) can be connected in parallel at both ends of the power supply, and the buzz problem can generally be solved. To make HiFi audio, you need to use a capacitor of at least 10,000 microfarads for filtering. The larger the filtering capacitor, the closer the output voltage waveform is to DC. And because of the energy storage effect of the large capacitor, when a sudden large-signal arrives, the circuit has enough energy.
In electronic circuits, capacitors can only pass by alternating current, not direct current. In the circuit, the capacitor is often used for coupling, bypassing, filtering, etc., all of which use the principle of "pass AC, block DC". So why can alternating current pass through capacitors? Let's first look at the characteristics of alternating current. The alternating current not only alternate in direction, but its size also changes according to regularity. The capacitor is connected to the AC power source, and the capacitor is continuously charged and discharged. And a charging current consistent with the change of the alternating current will flow in the circuit.
Capacitors with fixed capacitance are called fixed capacitors. According to the different dielectric, it can be divided into ceramic, mica, paper, film, electrolytic.
Figure1 Ceramic capacitor
Ceramic capacitors are made of high-dielectric constant ceramics (barium titanate-titanium oxide). As a dielectric of the ceramic capacitor, high-dielectric constant ceramics are extruded into round tubes, wafers, or discs. And then silver is plated on the ceramics as an electrode by the infiltration method. It is divided into high-frequency porcelain and low-frequency porcelain.
High-frequency ceramic capacitors are suitable for high-frequency circuits of radio and electronic equipment. Capacitors with a small positive capacitance temperature coefficient are used in highly stable oscillation circuits as circuit capacitors and pad capacitors. Low-frequency ceramic dielectric capacitors are limited to bypass or DC-blocking in circuits with lower operating frequencies, or where stability and loss requirements are not high (including high frequency). Such capacitors are not suitable for use in pulsed circuits because they are prone to breakdown by pulsed voltages. Common ceramic dielectric capacitors are through-core or pillar structure ceramic dielectric capacitors. One of its electrodes is a mounting screw. The lead inductance is extremely small, especially suitable for high-frequency bypass.
Monolithic capacitors, that is, multilayer ceramic capacitors, are covered with electrode paddle material on several ceramic thin film blanks, stacked and wound into a whole, and then encapsulated with resin on the outside. It is a new type of capacitor with a small volume, large capacity, high reliability, and high-temperature resistance. Low-dielectric monolithic capacitors with high dielectric constant also have stable performance and a small body.
Figure2 Mica capacitor
Mica capacitors can be divided into foil type and silver type. The silver-type electrode is formed by directly plating a silver layer on the mica sheet by a vacuum evaporation method or a fire infiltration method. Because the air gap is eliminated, the temperature coefficient is greatly reduced, and the capacitance stability is also higher than the foil type. Mica capacitors are widely used in high-frequency appliances and can be used as standard capacitors.
The dielectric of the glass glaze capacitor is formed by spraying a special mixture with a suitable concentration for spraying into a thin film. The dielectric is then sintered with a silver layer electrode to form a "monolithic" structure. The performance of glass glaze capacitors is comparable to that of mica capacitors. It can withstand various climatic environments, and can generally work at 200 ° C or higher. The rated operating voltage can reach 500 V, and the loss tanδ = 0.0005 ~ 0.008. Zh
Figure3 Paper capacitor
Paper capacitors are widely used in radio and electronic equipment. Generally, two aluminum foils are used as electrodes, and a capacitor paper with a thickness of 0.008 to 0.012 mm is wound in the middle and overlapped. The manufacturing process is simple and the price is cheap. A large capacitance can be obtained, generally below 0.25 μF, but the capacity error is large and difficult to control. The quality is better than ± 10%, the loss is large (tanδ ≤ 0.015), and the stability of temperature and frequency characteristics is poor. Paper capacitors commonly used in the past are non-sealed, and are only impregnated with earth wax, paraffin, etc., and are easily aged. Its stability is poor and easy to be affected by humidity. The insulation resistance of the paper capacitor decreases after being wet. Paper capacitors with capacitor cores placed in metal or ceramic tubes and sealed are of good quality, with minimal impact from external climatic conditions, and can be used normally in places with a relative humidity of 95 to 98%.
The electrode of the metalized paper dielectric capacitor is directly attached to the capacitor paper by vacuum evaporation, and its volume is only about 1/4 of that of ordinary paper capacitors. Its main feature is that it has a "self-healing" effect, that is, it can "heal" after a breakdown, and is an improved type of paper capacitor. Paper capacitors are dielectric frequency capacitors, which are generally used in low-frequency circuits, and usually cannot be used at frequencies higher than 3 to 4 MHz. Oil-immersed capacitors have higher withstand voltage than ordinary paper capacitors, and also have good stability.
Figure4 Film capacitor
The structure of the film capacitor is similar to that of a paper capacitor, but low-loss plastic materials such as polyester and polystyrene are used as the dielectric. Polystyrene capacitors have excellent performance and can be used as excellent coupling capacitors in low-frequency circuits. It is also particularly suitable for RC time constant circuits because its dielectric absorption is very small and its discharge is fast. High-temperature-resistant film capacitors include polyester capacitors, polytetrafluoroethylene capacitors, and polycarbonate capacitors. The polyester capacitor is also called a polyester capacitor. It has better electrical performance than metalized paper dielectric capacitors. It is mainly used as bypass and DC blocking in circuits to replace paper dielectric capacitors. Polycarbonate capacitors have better electrical performance than polyester capacitors and can work for a long time at +120 ~ 130 ℃.
The electrical properties of polypropylene capacitors (CBB) are similar to those of polystyrene capacitors, but the capacitance per unit volume is large, capable of withstanding high temperatures above + 100 ℃, and the temperature stability is slightly worse.
Electrolytic capacitors are capacitors using a thin oxide film as the dielectric. Because the oxide film has unidirectional conductive properties, the electrolytic capacitor has polarity.
Figure5 Aluminum electrolytic capacitor
It is wound by sandwiching two aluminum foils with water-absorbing paper impregnated with a paste electrolyte. Ordinary aluminum electrolytic capacitors are not suitable for high-frequency and low-temperature applications, and should not be used at frequencies above 25kHz. They are usually used for low-frequency bypass \ coupling and power filtering.
Figure6 Solid Tantalum Electrolytic Capacitor
A sintered tantalum block was used as the positive electrode, and solid manganese dioxide was used as the electrolyte. They have a series of advantages, for example, temperature characteristics and frequency characteristics are superior to ordinary electrolytic capacitors, especially with extremely low leakage current, good storage, long life, and small size. They can get the largest capacitance-voltage product per unit volume, suitable for use in ultra-small and highly reliable parts.
Figure7 Trimmer capacitor
The trimmer capacitor is also called a semi-variable capacitor. Its capacitance can be adjusted within a small range and can be fixed to a certain capacitance value after adjustment.
Porcelain-trimmed capacitors are extremely high-quality and small in size. They are usually divided into two types: round tube and wafer. Mica and polystyrene dielectric trimmer capacitors usually adopt a spring-type structure. This trimmer capacitor has a simple structure but poor stability.
Wire-wound porcelain-trimmed capacitors are made by removing copper wires (external electrodes) to change the capacitance. Therefore, the capacitance can only be reduced, which is not suitable for occasions where repeated debugging is required.
Figure8 Variable capacitor
The variable capacitor means that the capacitance value can be changed in a relatively large range and can be determined as a certain value. Variable capacitors are divided into two forms: film dielectric and air dielectric. Commonly used in coupling and tuning circuits, common double capacitors, ceramic capacitors, etc.
Different types of capacitors play different but important roles in circuits such as tuning, bypassing, coupling, and filtering. It is used in the tuning circuit of the transistor radio, and it is also used in the coupling circuit and bypass circuit of the color TV.
With the rapid development of electronic information technology, the upgrading of digital electronic products is becoming faster and faster. The production and sales of consumer electronics products, including flat-panel TVs (LCD and PDP), notebook computers, and digital cameras, continue to increase, driving the Capacitor industry to grow.
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