Basic knowledge and detection method of thermal po

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Basic knowledge of capacitors and detection methods

I. Basic knowledge

capacitors are energy storage elements used in circuits for tuning, filtering, coupling, bypass, energy conversion and time delay. Capacitors are usually called capacitors. According to its structure, it can be divided into fixed capacitor, semi variable capacitor and variable capacitor

1. structure and characteristics of common capacitors

common capacitors can be divided into electrolytic capacitors, mica capacitors, porcelain capacitors, glass glaze capacitors, etc. according to their dielectric materials

aluminum electrolytic capacitor

it is made of an aluminum cylinder as the negative pole, which is filled with liquid electrolyte and inserted with a curved aluminum strip as the positive pole. It also needs to be treated with DC voltage, so that a layer of oxide film can be formed on the electrode sheet as the medium for domestic experimental machine manufacturers. It is characterized by large capacity, but large leakage, large error and poor stability. It is often used as AC bypass and filtering, and also used for signal coupling when the requirements are not high. Electrolytic capacitors can be divided into positive and negative electrodes, and cannot be connected reversely when used. There are positive and negative polarities. When using, do not connect the positive and negative polarities reversely

paper dielectric capacitance

two pieces of metal foil are used as electrodes, clamped in extremely thin capacitance paper, rolled into a cylindrical or flat cylindrical core, and then sealed in a metal shell or insulating material (such as fire paint, ceramics, glass glaze, etc.) shell. It is characterized by small volume and large capacity. However, the inherent inductance and loss are relatively large, so it is more suitable for low frequency

metallized paper capacitor

structure is basically the same as paper capacitor. It is coated with a layer of metal film on the capacitor paper to replace the metal foil. It has small volume and large capacity, and is generally used in low-frequency circuits

oil immersed paper capacitor

it immerses the paper capacitor in specially treated oil, which can enhance its withstand voltage. It is characterized by large capacitance, high voltage resistance, but large volume

glass glaze capacitor

with glass glaze as the medium, it has the advantages of ceramic capacitor, smaller volume and high temperature resistance

ceramic capacitors are made by using ceramics as the medium, spraying silver layers on both sides of the ceramic matrix, and then firing silver films as plates. It is characterized by small size, good heat resistance, low loss, high insulation resistance, but small capacity, suitable for high-frequency circuits

ferroelectric ceramics have large capacitance, but large loss and temperature coefficient, which is suitable for low-frequency circuits

film capacitor

structure is the same as paper capacitor, and the medium is polyester or polystyrene. Polyester film capacitor, with high dielectric constant, small volume, large capacity and good stability, is suitable for bypass capacitor

polystyrene film capacitor, with low dielectric loss and high insulation resistance, but high temperature coefficient, can be used in high-frequency circuits

mica capacitor

use metal foil or spray silver layer on mica sheet as electrode plate. After the electrode plate and mica are laminated layer by layer, it is made by die-casting in bakelite powder or sealing in epoxy resin. It is characterized by low dielectric loss, large insulation resistance and low temperature coefficient, which is suitable for high-frequency circuits

tantalum and niobium electrolytic capacitor

it is made of metal tantalum or niobium as the positive pole, dilute sulfuric acid and other solutions as the negative pole, and the oxide film generated on the surface of tantalum or niobium as the medium

it is characterized by small volume, large capacity, stable performance, long service life, large insulation resistance and good temperature characteristics. It is used in equipment with high requirements

semi variable capacitance

is also called fine tuning capacitance. It is made of two or two groups of small metal shrapnel with medium sandwiched between them. When adjusting, change the distance or area between the two pieces. Its medium includes air, ceramics, mica, film, etc. Exterior inorganic architectural coating jg/t 26 ⑵ 002

variable capacitance

it is composed of a group of fixed pieces and a group of moving pieces, and its capacity can be continuously changed with the rotation of moving pieces. Two groups of variable capacitors are installed together and rotate coaxially, which is called double connection. The medium of variable capacitance includes air and polystyrene. The air medium variable capacitor is large in volume and small in loss, which is often used in electronic tube radios. The polystyrene dielectric variable capacitor is made into a sealed type, which is small in size and is mostly used in transistor radios

2. Main performance indicators

nominal capacity and allowable error: the capacity of the capacitor to store charge, commonly used units are f, UF, PF. The capacitance marked on the capacitor is the nominal capacity of the capacitor. There will be an error between the nominal capacity of the capacitor and its actual capacity. Generally, there is resin on the capacitor to directly write its capacity, and there are also numbers to mark the capacity. Generally, when the capacity is less than 10000pf, PF is used as the unit, and when it is greater than 10000pf, UF is used as the unit. For simplicity, capacitors greater than 100pF and less than 1uF often do not note units. If there is no decimal point, its unit is PF. If there is a decimal point, its unit is UF. If some capacitors are marked with "332" (3300pf) three significant digits, the first and second digits of the capacitance are given from the left, and the third digit represents the number added with 0, and the unit is PF

rated working voltage: within the specified working temperature range, the capacitor can work reliably for a long time. The maximum DC voltage it can withstand is the withstand voltage of the capacitor, also known as the DC working voltage of the capacitor. If in the AC circuit, it should be noted that the maximum AC voltage applied cannot exceed the DC working voltage of the capacitor. The commonly used working voltages of fixed capacitors are 6.3V, 10V, 16V, 25V, 50V, 63v, 100V, 2500V, 400V, 500V, 630V, 1000V

insulation resistance: because the medium between the two poles of the capacitor is not an absolute insulator, its resistance is not infinite, but a limited value, generally above 1000 megohm. The resistance between the two poles of the capacitor is called insulation resistance, or leakage resistance, which is the ratio of the DC voltage under the rated working voltage to the leakage current through the capacitor. The smaller the leakage resistance is, the more serious the leakage is. Capacitor leakage will cause energy loss, which will not only affect the life of the capacitor, but also affect the work of the circuit. Therefore, the greater the leakage resistance, the better

dielectric loss: the energy consumed by a capacitor under the action of an electric field is usually expressed by the ratio of the lost power to the reactive power of the capacitor, that is, the tangent of the loss angle. The larger the loss angle is, the greater the loss of the capacitor is. The capacitor with large loss angle is not suitable for working at high frequency

4. choose common sense

the voltage actually to be borne by the capacitor in the circuit cannot exceed its withstand voltage value. In the filter circuit, the withstand voltage value of the capacitor shall not be less than 1.42 times of the AC effective value. When using electrolytic capacitors, pay attention not to connect the positive and negative electrodes reversely

different kinds of capacitors should be selected for different circuits. Mica and high-frequency ceramic capacitors can be selected for wiping circuit, paper dielectric, polyester, mica, electrolysis, ceramics and other capacitors can be selected for DC isolation, electrolytic capacitors can be selected for filtering, and polyester, paper dielectric, ceramics, electrolysis and other capacitors can be selected for bypass

before installing the capacitor into the circuit, check whether it has short circuit, open circuit, leakage and other phenomena, and check its capacitance value

when installing, make the category, capacity, withstand voltage and other symbols of capacitance easy to see for verification

II. General methods of capacitor detection

1 Detection of fixed capacitor

a ﹐ detect small capacitance below 10PF. Because the capacity of fixed capacitors below 10PF is too small, use a multimeter to measure, and only qualitatively check whether there is leakage, internal short circuit or breakdown. When measuring, multimeter R can be selected × 10K gear, use two probes to connect the two pins of the capacitor randomly, and the resistance value should be infinite. If the measured resistance value (the pointer swings to the right) is zero, it indicates that the capacitor has leakage damage or internal breakdown

b ﹐ detection 10PF ~ 0.01 μ F check whether the fixed capacitor is charged, and then judge whether it is good or bad. R is selected for multimeter × 1K gear. Two triodes β The values are above 100, and the penetration current is small. 3DG6 and other models of silicon triode can be selected to form a composite tube. The red and black probes of the multimeter are connected with the emitter E and collector C of the composite tube respectively. Due to the amplification effect of the composite triode, the charging and discharging process of the measured capacitance is amplified, so that the swing range of the multimeter pointer is increased, which is convenient for observation. It should be noted that during the test operation, especially when measuring the capacitance of small capacity, it is necessary to repeatedly exchange the pins of the measured capacitance to contact two points a and B, so as to clearly see the swing of the multimeter pointer. C for 0 01 μ For fixed capacitance above F, R of multimeter can be used × 10K gear directly tests whether the capacitor has charging process and whether there is internal short circuit or leakage, and the capacity of the capacitor can be estimated according to the amplitude of the pointer swinging to the right

2. Detection of electrolytic capacitor

a. because the capacity of electrolytic capacitor is much larger than that of general fixed capacitor, appropriate measuring range should be selected for different capacities during measurement. According to experience, generally, 1 ~ 47 μ Capacitance between F, R can be used × 1K block measurement, greater than 47 μ The capacitance of F can be used as R × 100 gear measurement

b ﹐ connect the red lead of the multimeter to the negative pole and the black lead to the positive pole. At the moment of contact, the pointer of the multimeter will deflect to the right with a large deflection (for the same electrical barrier, the larger the capacity, the larger the swing), and then gradually turn to the left until it stops at a certain position. At this time, the resistance value is the forward leakage resistance of the electrolytic capacitor, which is slightly larger than the reverse leakage resistance. Practical experience shows that the leakage resistance of electrolytic capacitors should generally be more than hundreds of K Ω, otherwise, they will not work normally. In the test, if there is no charging phenomenon in the forward and reverse directions, that is, the gauge needle does not move, it indicates that the capacity disappears or the internal circuit is open; If the measured resistance value is very small or zero, it indicates that the capacitor has large leakage or has been broken down and damaged, and cannot be used again. C. for electrolytic capacitors with unclear positive and negative signs, the above method of measuring leakage resistance can be used to distinguish. That is, measure the leakage resistance at will, remember its size, and then exchange the probes to measure a resistance value. Lifting holes or lifting rings shall be set in the two measurements; The one with high resistance is the positive connection method, that is, the black probe is connected to the positive electrode, and the red probe is connected to the negative electrode. D. the capacity of electrolytic capacitor can be estimated and measured according to the amplitude of pointer swinging to the right by using the resistance block of multimeter and the method of charging electrolytic capacitor in the forward and reverse directions

3. Detection of variable capacitor

a. rotate the rotating shaft gently by hand, and it should feel very smooth. It should not feel that it is sometimes loose, sometimes tight, or even stuck. When the load shaft is pushed forward, backward, up, down, left, right and other directions, the rotating shaft should not be loose. B. rotate the rotating shaft with one hand and gently touch the outer edge of the moving piece group with the other hand, and you should not feel any looseness. The variable capacitor with poor contact between the rotating shaft and the moving blade can no longer be used. C put the multimeter in R × At 10K gear, with one hand, connect the two probes to the leading out ends of the moving piece and the fixed piece of the variable capacitor respectively, and with the other hand, slowly rotate the rotating shaft several times, and the multimeter pointer should remain at the infinite position. In the process of rotating the shaft, if the pointer sometimes points to zero, it indicates that there is a short-circuit point between the moving piece and the fixed piece; If you encounter a certain angle, the multimeter reading is not infinite, but a certain resistance value, indicating that there is leakage between the moving piece and the fixed piece of the variable capacitor. (end)

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