Electronic beginners understand the basic knowledge of capacitors - Solutions - Huaqiang

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First: the characteristics of the capacitor (straight through)

A capacitor is a container that can store electric charge. It consists of two pieces of metal that are placed closer together and separated by an insulating material. Depending on the insulating material, a variety of capacitors can be made. Mica. Porcelain. Paper, electrolytic capacitors, etc. In structure, it is divided into fixed capacitors and variable capacitors. Capacitors have infinite DC resistance, that is, capacitors have DC blocking effect. The resistance of capacitors to AC is affected by AC frequency, ie Capacitors of the same capacity exhibit different capacitive reactances for ACs of different frequencies. These phenomena occur because they are open. "This is because the capacitors rely on its charge and discharge function to operate. When the power switch s is not closed, the capacitors are two. The sheet metal plate and other common metal plates are uncharged. When the switch S is closed, the free electrons on the positive plate of the capacitor are attracted by the power source and pushed onto the negative plate. Insulation material, so the free electrons running from the positive plate are piled up on the negative plate. The positive plate is positively charged due to the reduction of electrons, and the negative plate is

There is a potential difference between the two plates of the capacitor. When this potential difference is equal to the power supply voltage, the charging of the capacitor is stopped. At this time, if the power is cut off, the capacitor can still maintain the charging voltage. For a charged capacitor, if we connect the two plates with wires, due to the potential difference between the plates, the electrons will pass through the wires and return to the positive plate until the potential difference between the plates is zero. Returning to the neutral state without electricity, there is no current in the wire. The discharge process of the capacitor is shown in Figure 3. The frequency of the alternating current applied to the two plates of the capacitor is high, and the number of times of charging and discharging of the capacitor is increased; charging and discharging The current is also enhanced; that is to say, the capacitor has a reduced effect on the high-frequency alternating current, that is, the capacitive reactance is small, whereas the capacitive reactance of the capacitor to the low-frequency alternating current is large. For the same frequency of the alternating current, the capacity of the capacitor The larger, the smaller the capacitive reactance, the smaller the capacity, the larger the capacitive reactance.

Second, the parameters and classification of capacitors

In electronic products, capacitors are indispensable electronic devices, which act as smoothing filters for rectifiers, decoupling of power supplies, bypassing of AC signals, and AC coupling of AC and DC circuits in electronic equipment. Due to the variety of types and types of capacitors, we need to understand not only the performance specifications and general characteristics of various types of capacitors, but also the advantages and disadvantages of various components for a given application, as well as mechanical or environmental constraints. . The main parameters of the capacitor and its application will be briefly explained here.

1. Nominal capacitance (CR). Capacitance value indicated by the capacitor product. Mica and ceramic dielectric capacitors have low capacitance (approximately 5000 pF or less); paper, plastic, and some ceramic dielectric capacitors are centered (approximately 0.005 uF to 1.0 uF); typically electrolytic capacitors have larger capacities. This is a rough classification.

2. Category temperature range. The ambient temperature range determined by the capacitor design for continuous operation. This range depends on the temperature limits of its respective category, such as the upper category temperature, the lower category temperature, the rated temperature (the highest ambient temperature at which the rated voltage can be continuously applied), and the like.

3. Rated voltage (UR). At any temperature between the lower category temperature and the rated temperature, the maximum DC voltage or the maximum value of the maximum AC voltage or the peak value of the pulse voltage may be continuously applied to the capacitor. When capacitors are used in high voltage fields, attention must be paid to the effects of corona. Corona is caused by the presence of a gap between the dielectric/electrode layers, which can cause breakdown of the capacitor dielectric in addition to spurious signals that can damage the device. Corona is particularly prone to occur under alternating or pulsating conditions. For all capacitors, ensure that the sum of the DC voltage and the AC peak voltage does not exceed the rated voltage of the capacitor.

4. Loss tangent (tg δ ). At a sinusoidal voltage of a specified frequency, the power loss of the capacitor divided by the reactive power of the capacitor is the loss tangent. In practical applications, the capacitor is not a pure capacitor, and there is an equivalent resistance inside. Its simplified equivalent circuit is shown in the drawing. For electronic equipment, the smaller the RS, the better, that is, the required power loss is small, and the angle between the power and the power of the capacitor is small.

5. Temperature characteristics of the capacitor. It is usually expressed as a percentage of the capacitance at the reference temperature of 20 °C and the capacitance of the relevant temperature.

6. Service life. The life of a capacitor decreases with increasing temperature. The main reason is that the temperature accelerates the chemical reaction and the medium degrades over time.

7. Insulation resistance. As the temperature rise causes an increase in electronic activity, an increase in temperature will lower the insulation resistance. Capacitors include two types of fixed capacitors and variable capacitors. The fixed capacitors can be divided into mica capacitors, ceramic capacitors, paper/plastic film capacitors according to their dielectric materials.

Third, the category and symbol of the capacitor

There are also many types of capacitors. In order to distinguish them, several Latin letters are often used to indicate the type of capacitor. The first letter C indicates the capacitance, the second letter indicates the dielectric material, and the third letter indicates the shape, structure, and the like. The figure above is a small paper capacitor. The figure below shows the vertical moment sealing paper capacitor. Table 1 lists the categories and symbols of the capacitors. Table 2 shows several characteristics of common capacitors.

Fourth: Discrimination of the polarity of electrolytic capacitor

Electrolytic capacitors that do not know polarity can be measured for their polarity using the multimeter's electrical barrier. We know that only the positive electrode of the electrolytic capacitor is connected to the power supply (the black test pen when the power is blocked), and the negative terminal power supply is negative (the red test pen when the power is blocked), the leakage current of the electrolytic capacitor is small (the leakage resistance is large). On the contrary, the leakage current of the electrolytic capacitor increases (the leakage resistance decreases).

When measuring, first assume that a very "+" pole is connected to the black test lead of the multimeter, and the other electrode is connected with the red test lead of the multimeter, and the scale of the stop of the lower needle is stopped (the needle has a large left resistance value), and then The capacitor is discharged (the two leads are touched), the two test leads are reversed, and the measurement is repeated. In the two measurements, the last position of the hand is left (the resistance is large), and the black pen is connected to the positive electrode of the electrolytic capacitor. It is best to use R*100 or R*1K gears for measurement. Use a multimeter to judge the quality of the capacitor

Fifth, use a multimeter to judge the quality of the capacitor

Depending on the capacity of the electrolytic capacitor, the multimeter's R×10, R×100, and R×1K blocks are usually used for testing and judgment. The red and black test leads are respectively connected to the negative pole of the capacitor (the capacitor needs to be discharged before each test), and the yaw of the hands is used to judge the quality of the capacitor. If the hands swing quickly to the right and then slowly return to the left, the capacitor is generally good. If the hands do not turn after swinging, the capacitor has broken down. If the hands are gradually returned to a certain position after the pen is swung, the capacitor has leaked. If the hands are not able to swing, the capacitor electrolyte has dried up and lost capacity.

Some leakage capacitors are not easy to accurately judge by the above method. When the withstand voltage of the capacitor is greater than the voltage value of the battery in the multimeter, according to the characteristics that the leakage current is small when the electrolytic capacitor is being charged and the leakage current is large during the reverse charging, the R×10K block can be used to reversely charge the capacitor. Whether the needle stays stable (ie, whether the reverse leakage current is constant), thereby judging the quality of the capacitor, and the accuracy is high. The black pen is connected to the negative pole of the capacitor, the red pen is connected to the positive pole of the capacitor, the hands are quickly swung up, and then gradually retreat to somewhere to stay still, indicating that the capacitor is good, and the hand is unstable after a certain position or stays. The capacitor moving slowly to the right has leaked and cannot be used any more. The hands are typically stuck and stable over the 50 - 200K scale.

Sixth, talk about electrolytic capacitors

First, the role of electrolytic capacitors in the circuit

1, filtering, in the power circuit, the rectifier circuit turns the alternating current into a pulsating direct current, and after the rectifier circuit is connected to a larger capacity electrolytic capacitor, using its charge and discharge characteristics, the rectified pulsating DC voltage becomes Relatively stable DC voltage. In practice, in order to prevent the supply voltage of each part of the circuit from changing due to load changes, there are typically tens to hundreds of microfarads of electrolytic capacitors at the output of the power supply and the power input of the load. Due to the large capacity of the electrolytic capacitor Generally, it has a certain inductance, and the high frequency and pulse interference signals cannot be effectively filtered out. Therefore, a capacitor with a capacity of 0.001--0.lpF is connected in parallel at both ends to filter out high frequency and pulse interference.

2, coupling effect: In the process of transmission and amplification of low-frequency signals, in order to prevent the static working points of the two-stage circuit from interacting with each other, capacitor coupling is often used. In order to prevent the loss of low-frequency components in the signal, the total capacity is generally used. Large electrolytic capacitor.

Second, the method of judging electrolytic capacitor

Common faults of electrolytic capacitors include capacity reduction, capacity loss, breakdown short circuit and leakage. The change in capacity is caused by the internal drying of the electrolytic capacitor during use or placement, and the breakdown and leakage are generally added. The voltage is too high or caused by poor quality. Judging whether the power supply capacitor is good or bad is generally measured by the resistance of the multimeter. The specific method is: short circuit the two pins of the capacitor to discharge, and connect the positive electrode of the electrolytic capacitor with the black meter of the multimeter. The red meter pen is connected to the negative pole (for the pointer type multimeter, when the meter is intermodulated with a digital multimeter), the needle should swing first in the direction of small resistance and then gradually return to infinity. The larger the swing amplitude of the hands or the slower the return speed, the larger the capacity of the capacitor. Otherwise, the smaller the capacity of the capacitor is. If the pointer is not changed somewhere in the middle, the leakage of the capacitor is indicated. If the voltage is small or zero, it indicates that the capacitor has been short-circuited. Because the voltage of the battery used by the multimeter is generally low, it is more accurate when measuring the low-voltage capacitor, and when the voltage withstand voltage is high, the measurement is not possible. Normal, but when high voltage is applied, there may be leakage or breakdown.

Third, the use of electrolytic capacitors

1. Electrolytic capacitors have positive and negative polarities, so they cannot be reversed when used in a circuit. In the power supply circuit, when the positive voltage is output, the positive pole of the electrolytic capacitor is connected to the output end of the power supply, the negative pole is grounded, and when the negative voltage is output, the negative pole is connected to the output end, and the positive pole is grounded. When the polarity of the filter capacitor in the power supply circuit is reversed, the capacitor is The filtering effect is greatly reduced. On the one hand, the output voltage fluctuation of the power supply is caused. On the other hand, the electrolytic capacitor corresponding to a resistor is heated by the reverse energization. When the reverse voltage exceeds a certain value, the reverse leakage resistance of the capacitor will change. It is very small, so that the power is not working soon, the capacitor can be broken due to overheating.

2. The voltage applied to the two ends of the electrolytic capacitor should not exceed the allowable working voltage. When designing the actual circuit, there should be a certain margin according to the specific conditions. When designing the filter capacitor of the regulated power supply, if the AC power supply voltage is 220~ The rectification voltage of the transformer secondary can reach 22V. At this time, the selection of electrolytic capacitor with a withstand voltage of 25V can generally meet the requirements. However, if the AC power supply voltage fluctuates greatly and may rise to above 250V, it is better to select a withstand voltage of 30V or higher. Electrolytic capacitors.

3. The electrolytic capacitor should not be close to the high-power heating element in the circuit to prevent the electrolyte from accelerating due to heat.

4. For the filtering of signals with positive and negative polarity, two electrolytic capacitors can be used in series with the same polarity. As a non-polar capacitor, this chapter concludes: Various capacitors are needed in electronic production. They play different roles in the circuit. Similar to a resistor, it is often referred to simply as a capacitor, denoted by the letter C. As the name suggests, a capacitor is a "container that stores charge." Despite the variety of capacitors, their basic structure and principle are the same. The two closely spaced metals are separated by a substance (solid, gas or liquid) to form a capacitor. Two pieces of metal are called plates, and the middle substance is called a medium. Capacitors are also classified into fixed capacity and variable capacity. But the common ones are fixed-capacity capacitors, the most common being electrolytic capacitors and ceramic capacitors.

The ability of different capacitors to store charge is also different. The amount of charge stored when a capacitor is applied with a 1 volt DC voltage is referred to as the capacitance of the capacitor. The basic unit of capacitance is Farad (F). But in fact, Farah is a very uncommon unit, because the capacity of the capacitor is often much smaller than 1 Farad, commonly used micro method (μF), nano method (nF), skin method (pF) (pale method also known as pico method) ), etc., their relationship is: 1 Farad (F) = 1000000 microfarad (μF) 1 microfarad (μF) = 1000 nanofarads (nF) = 1,000,000 picofarads (pF) In electronic circuits, capacitors are used to communicate Blocking DC is also used to store and release charge to act as a filter to smooth out the output ripple signal. Small-capacity capacitors are commonly used in high-frequency circuits such as radios, transmitters, and oscillators. Large-capacity capacitors are often used for filtering and storing charge. There is also a feature, generally more than 1μF capacitors are electrolytic capacitors, and capacitors below 1μF are mostly ceramic capacitors, of course, there are other, such as monolithic capacitors, polyester capacitors, small-capacity mica capacitors. The electrolytic capacitor has an aluminum shell filled with electrolyte and leads two electrodes as positive (+) and negative (-) poles. Unlike other capacitors, their polarity in the circuit cannot be connected incorrectly, while other capacitors No polarity. #p#分页头#e#

Connect the two electrodes of the capacitor to the positive and negative terminals of the power supply. After a while, even if the power is turned off, there will still be residual voltage between the two pins (after learning the tutorial, you can use a multimeter to observe), we say the capacitor The charge is stored. A voltage is established between the plates of the capacitor to accumulate electrical energy. This process is called charging of the capacitor. There is a certain voltage across the charged capacitor. The process in which the charge stored by the capacitor is released into the circuit is called the discharge of the capacitor.

To give a real-life example, we see that after the commercially available rectified power supply is unplugged, the LED above will continue to be lit for a while, then gradually extinguished because the capacitor inside stores the energy in advance and then releases it. Of course this capacitor was originally used for filtering. As for the capacitive filtering, I wonder if you have experienced the experience of listening to the Walkman with a rectified power supply. Generally, the low-quality power supply uses a small-capacity filter capacitor for cost-saving reasons, causing a buzzing sound in the earphone. At this time, you can connect a large-capacity electrolytic capacitor (1000μF, pay attention to the positive pole to the positive pole) at both ends of the power supply, which can generally improve the effect. Enthusiasts who make HiFi sounds must use at least 10,000 microfarads of capacitors to filter. The larger the filter capacitors, the closer the output voltage waveform is to DC, and the large capacitors store energy, so that when a large burst of signals arrives, the circuit There is enough energy to convert to a powerful audio output. At this time, the function of the large capacitor is somewhat like a reservoir, so that the originally turbulent water flow is smoothly outputted, and the supply of a large amount of water downstream can be ensured.

In the electronic circuit, only during the charging process of the capacitor, current flows, and after the charging process is finished, the capacitor cannot pass the direct current, and plays a role of "DC blocking" in the circuit. In the circuit, the capacitor is often used for coupling, bypassing, filtering, etc., all of which utilize the characteristics of "passing AC, DC blocking". So why can AC power pass through capacitors? Let's take a look at the characteristics of AC. The alternating current is not only reciprocating in direction, but its size is also changing regularly. The capacitor is connected to the AC power source, and the capacitor is continuously charged and discharged, and the charging current and the discharging current which are consistent with the alternating current change law flow in the circuit. The choice of capacitors involves many problems. The first is the problem of pressure resistance. The voltage applied across a capacitor exceeds its rated voltage and the capacitor is broken down. Generally, the breakdown voltage of electrolytic capacitors is 6.3V, 10V, 16V, 25V, 50V, etc.

There are also many types of capacitors. In order to distinguish them, several Latin letters are often used to indicate the type of capacitor. The first letter C indicates the capacitance, the second letter indicates the dielectric material, and the third letter indicates the shape, structure, and the like. The figure above is a small paper capacitor. The figure below shows the vertical moment sealing paper capacitor. Table 1 lists the categories and symbols of the capacitors. Table 2 shows several characteristics of common capacitors.

Fourth: Discrimination of the polarity of electrolytic capacitor

Electrolytic capacitors that do not know polarity can be measured for their polarity using the multimeter's electrical barrier. We know that only the positive electrode of the electrolytic capacitor is connected to the power supply (the black test pen when the power is blocked), and the negative terminal power supply is negative (the red test pen when the power is blocked), the leakage current of the electrolytic capacitor is small (the leakage resistance is large). On the contrary, the leakage current of the electrolytic capacitor increases (the leakage resistance decreases).

When measuring, first assume that a very "+" pole is connected to the black test lead of the multimeter, and the other electrode is connected with the red test lead of the multimeter, and the scale of the stop of the lower needle is stopped (the needle has a large left resistance value), and then The capacitor is discharged (the two leads are touched), the two test leads are reversed, and the measurement is repeated. In the two measurements, the last position of the hand is left (the resistance is large), and the black pen is connected to the positive electrode of the electrolytic capacitor. It is best to use R*100 or R*1K gears for measurement. Use a multimeter to judge the quality of the capacitor

Fifth, use a multimeter to judge the quality of the capacitor

Depending on the capacity of the electrolytic capacitor, the multimeter's R×10, R×100, and R×1K blocks are usually used for testing and judgment. The red and black test leads are respectively connected to the negative pole of the capacitor (the capacitor needs to be discharged before each test), and the yaw of the hands is used to judge the quality of the capacitor. If the hands swing quickly to the right and then slowly return to the left, the capacitor is generally good. If the hands do not turn after swinging, the capacitor has broken down. If the hands are gradually returned to a certain position after the pen is swung, the capacitor has leaked. If the hands are not able to swing, the capacitor electrolyte has dried up and lost capacity.

Some leakage capacitors are not easy to accurately judge by the above method. When the withstand voltage of the capacitor is greater than the voltage value of the battery in the multimeter, according to the characteristics that the leakage current is small when the electrolytic capacitor is being charged and the leakage current is large during the reverse charging, the R×10K block can be used to reversely charge the capacitor. Whether the needle stays stable (ie, whether the reverse leakage current is constant), thereby judging the quality of the capacitor, and the accuracy is high. The black pen is connected to the negative pole of the capacitor, the red pen is connected to the positive pole of the capacitor, the hands are quickly swung up, and then gradually retreat to somewhere to stay still, indicating that the capacitor is good, and the hand is unstable after a certain position or stays. The capacitor moving slowly to the right has leaked and cannot be used any more. The hands are typically stuck and stable over the 50 - 200K scale.

The ability of different capacitors to store charge is also different. The amount of charge stored when a capacitor is applied with a 1 volt DC voltage is referred to as the capacitance of the capacitor. The basic unit of capacitance is Farad (F). But in fact, Farah is a very uncommon unit, because the capacity of the capacitor is often much smaller than 1 Farad, commonly used micro method (μF), nano method (nF), skin method (pF) (pale method also known as pico method) ), etc., their relationship is: 1 Farad (F) = 1000000 microfarad (μF) 1 microfarad (μF) = 1000 nanofarads (nF) = 1,000,000 picofarads (pF) In electronic circuits, capacitors are used to communicate Blocking DC is also used to store and release charge to act as a filter to smooth out the output ripple signal. Small-capacity capacitors are commonly used in high-frequency circuits such as radios, transmitters, and oscillators. Large-capacity capacitors are often used for filtering and storing charge. There is also a feature, generally more than 1μF capacitors are electrolytic capacitors, and capacitors below 1μF are mostly ceramic capacitors, of course, there are other, such as monolithic capacitors, polyester capacitors, small-capacity mica capacitors. The electrolytic capacitor has an aluminum shell filled with electrolyte and leads two electrodes as positive (+) and negative (-) poles. Unlike other capacitors, their polarity in the circuit cannot be connected incorrectly, while other capacitors No polarity.

Connect the two electrodes of the capacitor to the positive and negative terminals of the power supply. After a while, even if the power is turned off, there will still be residual voltage between the two pins (after learning the tutorial, you can use a multimeter to observe), we say the capacitor The charge is stored. A voltage is established between the plates of the capacitor to accumulate electrical energy. This process is called charging of the capacitor. There is a certain voltage across the charged capacitor. The process in which the charge stored by the capacitor is released into the circuit is called the discharge of the capacitor.

To give a real-life example, we see that after the commercially available rectified power supply is unplugged, the LED above will continue to be lit for a while, then gradually extinguished because the capacitor inside stores the energy in advance and then releases it. Of course this capacitor was originally used for filtering. As for the capacitive filtering, I wonder if you have experienced the experience of listening to the Walkman with a rectified power supply. Generally, the low-quality power supply uses a small-capacity filter capacitor for cost-saving reasons, causing a buzzing sound in the earphone. At this time, you can connect a large-capacity electrolytic capacitor (1000μF, pay attention to the positive pole to the positive pole) at both ends of the power supply, which can generally improve the effect. Enthusiasts who make HiFi sounds must use at least 10,000 microfarads of capacitors to filter. The larger the filter capacitors, the closer the output voltage waveform is to DC, and the large capacitors store energy, so that when a large burst of signals arrives, the circuit There is enough energy to convert to a powerful audio output. At this time, the function of the large capacitor is somewhat like a reservoir, so that the originally turbulent water flow is smoothly outputted, and the supply of a large amount of water downstream can be ensured.

In the electronic circuit, only during the charging process of the capacitor, current flows, and after the charging process is finished, the capacitor cannot pass the direct current, and plays a role of "DC blocking" in the circuit. In the circuit, the capacitor is often used for coupling, bypassing, filtering, etc., all of which utilize the characteristics of "passing AC, DC blocking". So why can AC power pass through capacitors? Let's take a look at the characteristics of AC. The alternating current is not only reciprocating in direction, but its size is also changing regularly. The capacitor is connected to the AC power source, and the capacitor is continuously charged and discharged, and the charging current and the discharging current which are consistent with the alternating current change law flow in the circuit. The choice of capacitors involves many problems. The first is the problem of pressure resistance. The voltage applied across a capacitor exceeds its rated voltage and the capacitor is broken down. Generally, the breakdown voltage of electrolytic capacitors is 6.3V, 10V, 16V, 25V, 50V, etc.

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