Ⅰ. The definition and development of capacitors
Ⅱ. The main parameters of the capacitor
Ⅲ. Types of capacitors
Ⅳ. The role of capacitors
Ⅴ. Troubleshooting of capacitors
Ⅰ. The definition and development of capacitors
1. Definition
Two conductors that are close to each other, with a layer of non-conductive insulating medium in between, constitute a capacitor. A capacitor stores charge when a voltage is applied between its two plates. The capacitance of a capacitor is numerically equal to the ratio of the amount of charge on a conductive plate to the voltage between the two plates. The basic unit of capacitance of a capacitor is farad (F). Capacitive elements are usually represented by the letter C in circuit diagrams.
2. Development
The first recorded capacitor in history was invented by Bishop Craster in October 1745. It is a glass bottle whose inner and outer layers are coated with a metal film. There is a metal rod inside the glass bottle, one end is connected with the inner metal film, and the other end is connected with a metal sphere. By using glass as an insulator in a two-layer metal film, Bishop Klaster achieved a significant increase in charge density.
In January 1746, the Dutch physicist Peter van Mussenbroek also independently invented a capacitor with a very similar structure. Bishop Craster's invention was not widely known at the time. Because Mason Brook was teaching at Leiden University at the time, he named it the Leiden bottle.
At the time it was believed that the charge was stored in the water in the Leiden jar. However, American scientist Franklin studied the Leiden bottle and proved that the charge is stored on the glass, not in the water in the Leiden bottle.
Ⅱ. The main parameters of the capacitor
1. Quality factor
The quality factor Q refers to the ratio of the ability of a circuit element to store energy and the energy it dissipates in each cycle.
2. Insulation resistance
Insulation resistance is the ratio of the DC voltage applied to a capacitor to the leakage current flowing through the capacitor.
3. Dielectric absorption
The dielectric absorption effect of a capacitor can be expressed as a percentage of the remaining voltage and charging voltage. This percentage is determined by charging the capacitor to a rated voltage during one specified time interval and then discharging the capacitor during a second time interval. Finally the capacitor is opened and the remaining voltage across the capacitor is measured after the third time interval.
4. Temperature coefficient
Changes in temperature will cause small changes in capacitor capacity, and the temperature coefficient is often used to express the degree of this change.
5. Equivalent series inductance (ESL)
The equivalent series inductance of a capacitor is composed of the capacitor's lead inductance and the equivalent inductance of the two plates of the capacitor in series.
6. Dissipation coefficient
The dissipation factor is equivalent to the reciprocal of the quality factor Q value of the capacitor.
7. Leakage current
In an ideal situation, when the capacitor is connected with a DC voltage, no current can flow between the two plates of the capacitor. In fact, when a DC working voltage is applied to the capacitor, it will be observed that the charging current changes greatly at the beginning, and decreases with time, and reaches a relatively stable state at a certain final value. This final value current is called leakage current.
8. Ripple current
Ripple current is the alternating current flowing through the capacitor. Due to the ESR in the capacitor, the ripple current generates heat which affects the capacitor life and function. Excessive heat may cause the capacitor's maximum allowable core temperature to be exceeded, thereby damaging the capacitor.
Specifications for some types of capacitors, including aluminum and tantalum electrolytics, and some film capacitors include maximum ripple current:
(1) If solid manganese dioxide is used as a tantalum capacitor as a dielectric, the allowable ripple current is limited, and the ESR in the capacitor is also the highest. If the ripple current exceeds the rated value, it will cause a short circuit and damage other devices.
(2) Aluminum electrolytic capacitors are the most commonly used electrolytic capacitors. If the ripple current is high, its life will be reduced. If the ripple current exceeds the rated value, the capacitor will burst.
(3) Ceramic capacitors generally have no ripple current limitation, and their ESR is the smallest.
(4) The ESR of the film capacitor is also very low, but if the ripple current exceeds the rated value, it will cause the degradation of the capacitor.
9. Equivalent Series Resistance (ESR)
An ideal capacitor would not lose any energy by itself. But in fact, since the material that makes the capacitor has resistance, the loss that the capacitor will generate can be equivalently expressed as a single resistor in series with the capacitor. When high AC current flows, the large equivalent series resistance will cause the capacitor to dissipate more power.
10. Breakdown voltage and rated DC working voltage
When the voltage across the capacitor rises to a certain value, the insulating medium between the two plates will be broken down and conduct electricity. This voltage value is called the breakdown voltage of the capacitor. The highest DC voltage that the capacitor can work normally for a long time is called the rated DC working voltage of the capacitor, which is smaller than the breakdown voltage. When a capacitor is used in an AC circuit, the maximum value of the AC voltage applied across it should be less than or equal to the rated DC working voltage.
Ⅲ. Types of capacitors
In production, we can manufacture various capacitors based on the materials and structures used. In addition, each type of capacitor has its own characteristics, and these characteristics may also vary. When designing, we need to choose the most suitable capacitor type based on these characteristics. Let's take a look at the main types of capacitors and understand their differences.
1. Fixed capacitors
Fixed capacitors are broadly classified into non-polarized capacitors and polarized capacitors.
A non-polarized capacitor in which there is no restriction on the polarity of the voltage applied to the capacitor terminals. In other words, it does not matter which terminal of this capacitor is positive. As long as it is a non-polar capacitor, a voltage rising from zero potential can be applied, so it can be directly used even in an AC circuit. The mainstream of non-polar capacitors are ceramic capacitors and film capacitors, in addition to mica capacitors, paper capacitors, and air capacitors.
On the other hand, a polarized capacitor is a capacitor in which one of the two terminals is determined to be positive. If the polarity is incorrectly used, the capacitor will fail. Therefore, polarized capacitors are restricted in that they must be used with a DC voltage or a voltage that varies only on the positive side. However, polarized capacitors are widely used because they are small in shape and easy to obtain capacitors with large capacitance. Aluminum electrolytic capacitors, tantalum electrolytic capacitors, conductive polymer (electrolytic) capacitors, and electric double layer capacitors are examples of such capacitors.
The mainstream of capacitors is fixed capacitors, but variable capacitors whose electrostatic capacitance can be changed within a certain range are also included.
Variable capacitors are common capacitors that change the area of the opposing electrode to change the electrostatic capacitance. In addition, there are some variable capacitors whose capacitance values are frequently changed in radio station selection etc. (variable capacitors), while others (trimming capacitors) are changed only once during circuit assembly.
Capacitance can be changed with a knob or a screwdriver, but it is difficult to manufacture small capacitors with large capacitance and pF (picofarad) level due to the structure of changing it mechanically.
Ceramic capacitors are capacitors whose dielectric uses high dielectric constant ceramics and has the following characteristics:
(1) Non-polar
(2) Excellent high frequency characteristics (low ESR)
(3) High heat resistance
(4) Long life
Ceramic capacitors are originally single-plate capacitors with high withstand voltage/low capacitance. However, with the multilayer ceramic capacitors that have realized small and large capacitance due to the thin film laminated structure, and the temperature characteristics that overcome the past shortcomings (capacitance change rate due to temperature Large) capacitors for temperature compensation, its application range has been greatly expanded, and it has become the most commonly used capacitor in capacitors. In addition, capacitors for temperature compensation are larger than conventional high-dielectric constant systems, and it is not easy to achieve a large capacity. Therefore, they should be used differently according to the application.
However, ceramic capacitors have disadvantages such as DC bias characteristics (large change in capacitance due to applied voltage), howling (abnormal noise due to vibration due to high frequency), and cracking easily due to temperature/mechanical shock, so we need to pay attention when using it.
Film capacitors are capacitors in which a plastic film is used as a dielectric, and have the following characteristics.
(1) Non-polar
(2) Excellent high frequency characteristics (low ESR)
(3) Excellent temperature characteristics (small change rate of electrostatic capacitance caused by temperature)
(4) Corresponds to electrostatic capacitance with high precision
(5) Long life
Compared with ceramic capacitors, film capacitors have better temperature change performance and more accurate electrostatic capacitance matching performance, while avoiding problems such as cracks caused by DC bias voltage and temperature/mechanical shock. Therefore, although film capacitors have high performance, their large volume and high cost limit their application range.
Depending on the dielectric to be used, film capacitors also have the following characteristics, and are used according to the application.
PET and PP are lead-type dielectrics. In the past, small, low-priced PET and PP with excellent high-frequency characteristics (low ESR) were often used for high-frequency/high-current, but because PP also has high safety and high moisture resistance. characteristics, and with the advancement of miniaturization technology of PP film capacitors, PP is now used for many purposes. PPS and PEN are used for surface mount film capacitors because of their high heat resistance. Their electrical characteristics have the characteristics that PEN is close to PET and PPS is close to PP.
5. Aluminum electrolytic capacitors
Aluminum electrolytic capacitors have a structure in which an aluminum oxide film as a dielectric is formed on the surface of an aluminum foil of the anode, and an electrolytic solution (a liquid in which an electrolyte is dissolved in a solvent) is used as an electrolyte (cathode).
The characteristic of aluminum electrolytic capacitors is their large capacitance, which is achieved by etching the surface of the aluminum foil to form unevenness to increase the surface area (S) of the electrode, and then forming the thickness (d) of the oxide film in an extremely thin state of the Angstrom level. of. However, the equivalent series resistance (ESR) increases compared to ceramic capacitors and film capacitors.
Aluminum electrolytic capacitors are products with limited lifetimes. This is because the electrolyte vaporizes under the influence of temperature and gradually penetrates into the sealing rubber. As time goes by, the capacitance decreases, the ESR increases, and eventually it becomes an open circuit state (the electrolyte dries up). The life prediction of aluminum electrolytic capacitors generally applies the "law of 2 times at 10°C".
The basic structure of tantalum electrolytic capacitors is roughly the same as that of aluminum electrolytic capacitors. In tantalum electrolytic capacitors, tantalum pentoxide is formed on the surface of the sintered body of tantalum metal powder used as the anode, and the structure of manganese dioxide (solid) is used as the electrolyte.
Tantalum electrolytic capacitors have the characteristics of being smaller in shape than aluminum electrolytic capacitors, excellent in frequency characteristics, and long in life (the electrolyte is solid). However, the failure mode is a short circuit and there is a risk of fire, so safety countermeasures must be taken.
7. Electric Double Layer Capacitors(EDLC)
Electric double-layer capacitors are special capacitors that have the intermediate capacitance of an aluminum electrolytic capacitor and a secondary battery (battery). Its capacitance density is more than 1,000 times that of aluminum electrolytic capacitors and about 1/10 that of secondary batteries.
There is no dielectric in electrical double layer capacitors like electrolytic capacitors. Instead, it utilizes the electric double layer formed at the interface between the electrode and the electrolyte as a dielectric. This is where the name Electrical Double Layer Capacitor comes from. The charging and discharging of electric double-layer capacitors utilizes the adsorption or desorption of ions on the electrode surface of the activated carbon used in the positive and negative electrodes. Compared with secondary batteries, electric double-layer capacitors have the following characteristics.
(1) The number of charge and discharge cycles has little effect on the deterioration of characteristics (maintenance-free)
(2) Simple charging and discharging (can be discharged to 0V, the energy consumption can be determined by the terminal voltage, and small current or large current can be charged)
(3) Not restricted by similar batteries (recycling, disposal, tariffs)
Therefore, electric double-layer capacitors are used as backup power sources such as data protection of IC memory in the event of a power failure.
8. Polarized Capacitors vs Non-Polarized Capacitors
Polarized and non-polarized capacitors behave the same way when it comes to storing and discharging. However, there are various factors that make them different from each other:
(1) Different dielectrics: The dielectric is the material between two capacitor plates. Polarized capacitors use an electrolyte as the dielectric, giving them greater capacitance than other capacitors of the same volume. However, polarized capacitors produced with different electrolyte materials and processes will have different capacitance values. The use of polar and non-polar capacitors depends on the properties of the reversible dielectric.
(2) Different structures: The most commonly used electrolytic capacitors are circular; square capacitors are rare. There are also invisible capacitors, or distributed capacitors, which must not be ignored in high-frequency and intermediate-frequency devices.
(3) Use environment and purpose: The internal material and structure determine the large capacity and high frequency characteristics of polarized capacitors, making them very suitable for power filters, etc. However, there are also some polarized capacitors with good high-frequency characteristics - tantalum electrolytic capacitors, which are not commonly used due to their high cost.
(4) Different performance: Maximum performance is one of the main requirements for selecting capacitors. If the power supply of the TV uses a metal oxide film capacitor as a filter, its capacitance value and withstand voltage should meet the filter requirements; only a power supply can be installed in the chassis. Therefore, the filter can only use polarized capacitors, and polarized capacitors are irreversible. Usually electrolytic capacitors are above 1MF; it is best used in coupling, decoupling, power filtering, etc. Most of the non-polar capacitors are below 1MF, and only involve resonance, coupling, frequency selection, current limiting, etc. However, there are also large-capacity, high-voltage non-polar capacitors, which are mainly used for reactive power compensation, motor phase shifting, and frequency conversion power supply phase shifting.
(5) Different capacities: Capacitors with the same volume have different capacitances according to their dielectrics.
Ⅳ. The role of capacitors
In a DC circuit, a capacitor is equivalent to an open circuit. A capacitor is a component that can store electric charge and is one of the most commonly used electronic components.
1. Frequency division: The capacitor in the frequency division circuit is called a frequency division capacitor. In the speaker frequency division circuit of the speaker, a frequency division capacitor circuit is used to make the high frequency speaker work in the high frequency band, and the intermediate frequency speaker work in the mid frequency band. The woofer operates in the low frequency range.
2. Compensation: The capacitor used in the compensation circuit is called a compensation capacitor. In the bass compensation circuit of the deck, this low-frequency compensation capacitor circuit is used to enhance the low-frequency signal in the playback signal. In addition, there is a high-frequency compensation capacitor circuit.
3. Coupling: The capacitance used in the coupling circuit is called coupling capacitance. This kind of capacitive circuit is widely used in RC coupled amplifiers and other capacitive coupling circuits to block DC and AC.
4. Neutralization: The capacitors used in the neutralization circuit are called neutralization capacitors. In radio high-frequency and intermediate frequency amplifiers, and TV high-frequency amplifiers, this neutralization capacitor circuit is used to eliminate self-excitation.
5. Filtering: The capacitor used in the filter circuit is called a filter capacitor. This capacitor circuit is used in power filtering and various filter circuits, and the filter capacitor removes signals within a certain frequency band from the total signal.
6. Decoupling: The capacitor used in the decoupling circuit is called a decoupling capacitor. This capacitor circuit is used in the DC voltage supply circuit of the multi-stage amplifier. The decoupling capacitor eliminates the harmful low-frequency cross-connection between the amplifiers of each stage.
7. High-frequency vibration suppression: The capacitor used in the high-frequency vibration suppression circuit is called a high-frequency vibration suppression capacitor. In the audio negative feedback amplifier, in order to eliminate the high-frequency self-excitation that may occur in the vibration, we use this capacitor circuit to eliminate the high-frequency howling that may occur in the amplifier.
8. Resonance: The capacitor used in the LC resonant circuit is called a resonant capacitor, which is required in both LC parallel and series resonant circuits.
9. Bypass: The capacitor used in the bypass circuit is called a bypass capacitor. If a signal of a certain frequency band needs to be removed from the signal in the circuit, a bypass capacitor circuit can be used. According to the frequency of the signal to be removed, there are full-frequency domain (all AC signals) bypass capacitor circuits and high-frequency bypass capacitor circuits.
10. Load capacitance: It refers to the effective external capacitance that determines the load resonance frequency together with the quartz crystal resonator. Commonly used standard values for load capacitance are 16pF, 20pF, 30pF, 50pF, and 100pF. The load capacitance can be adjusted appropriately according to the specific situation. Generally, the operating frequency of the resonator can be adjusted to the nominal value through adjustment.
11. Integral: The capacitor used in the integral circuit is called the integral capacitor. In the synchronous separation circuit for electric potential field scanning, the field synchronous signal can be taken out from the field compound synchronous signal by adopting this integral capacitance circuit.
12. Timing: The capacitors used in timing circuits are called timing capacitors. A timing capacitor circuit is used in a circuit that requires time control through capacitor charging and discharging, and the capacitor plays a role in controlling the time constant.
13. Differential: The capacitor used in the differential circuit is called differential capacitance. In order to obtain the peak trigger signal in the trigger circuit, this differential capacitance circuit is used to obtain the peak pulse trigger signal from various (mainly rectangular pulse) signals.
14. Bootstrap: The capacitor used in the bootstrap circuit is called a bootstrap capacitor. The commonly used OTL power amplifier output stage circuit uses this bootstrap capacitor circuit to slightly increase the positive half-cycle amplitude of the signal through positive feedback.
Ⅴ. Troubleshooting of capacitors
1. Common faults of capacitors
When one of the following conditions of the capacitor is found, the power supply should be cut off immediately.
(1) Abnormal sound inside the capacitor.
(2) The temperature indicator chip falls off when the shell temperature rises above 55°C.
(3) Capacitor shell expansion or oil leakage.
(4) The casing is broken, and flashover occurs with sparks.
2. Troubleshooting of capacitors
(1) When the capacitor explodes and catches fire, immediately disconnect the power supply, and use sand and a dry fire extinguisher to extinguish the fire.
(2) When the fuse of the capacitor is blown, it should be reported to the dispatcher, and the circuit breaker of the capacitor should be opened after obtaining consent. Cut off the power supply and discharge it, first conduct an external inspection, such as whether there is any flashover trace on the outside of the bushing, whether the shell is deformed, whether there is oil leakage and whether there is a short circuit in the grounding device, etc., and shake the insulation resistance between the poles and the pole to the ground Value, check whether the wiring of the capacitor bank is complete and firm, and whether there is a phase loss phenomenon. If no fault phenomenon is found, replace the insurance and put it into operation. If the fuse is still blown after power transmission, the faulty capacitor should be withdrawn, and power transmission to the rest should be resumed. If the circuit breaker trips at the same time as the fuse blows, do not force it at this time. It must be put in after the above-mentioned inspection is completed and the insurance is replaced.
(3) If the circuit breaker of the capacitor trips, but the shunt insurance is not broken, the capacitor should be discharged for three minutes before checking the circuit breaker, current transformer, power cable and capacitor exterior. If no abnormality is found, it may be caused by the voltage fluctuation of the external fault bus. After inspection, you can try to cast; otherwise, you should further conduct a comprehensive power-on test for protection. Through the above inspections and tests, if the reason is still not found, we need to act according to the system and gradually test the capacitors. Do not try to vote until the reason is found out.
标签：Capacitors
