Essential Capacitor Knowledge for Engineers

First: Capacitor characteristics (DC isolation and AC passing)

A capacitor is a container that can store electric charge. It is composed of two metal sheets close to each other, separated by an insulating material. According to different insulating materials, various capacitors can be made, such as mica, porcelain, paper, electrolytic capacitors, etc. In terms of structure, it is divided into fixed capacitors and variable capacitors. The resistance of capacitors to DC is infinite, that is, capacitors have the function of isolating DC. The resistance of capacitors to AC is affected by the frequency of AC, that is, capacitors of the same capacity show different capacitive reactances to AC of different frequencies. Why do these phenomena occur? This is because the capacitor works by its charging and discharging function . When the power switch S is not closed, the two metal plates of the capacitor are uncharged like other ordinary metal plates. When the switch S is closed, the free electrons on the positive plate of the capacitor are attracted by the power supply and pushed to the negative plate. Because there is an insulating material between the two plates of the capacitor, the free electrons running over from the positive plate will accumulate on the negative plate. The positive plate will become positively charged due to the decrease in electrons, and the negative plate will become negatively charged due to the gradual increase in electrons.

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 stops. If the power supply is cut off at this time, the capacitor can still maintain the charging voltage. For a charged capacitor, if we connect the two plates with a wire, due to the potential difference between the two plates, the electrons will pass through the wire and return to the positive plate until the potential difference between the two plates is zero. The capacitor returns to a neutral state without charge, and there is no current in the wire . The discharge process of the capacitor is shown in Figure 3. The higher the frequency of the alternating current applied to the two plates of the capacitor, the more times the capacitor is charged and discharged; the charging and discharging current is also enhanced; that is to say, the resistance of the capacitor to high-frequency alternating current is reduced, that is, the capacitive reactance is small, and conversely, the capacitive reactance generated by the capacitor to low-frequency alternating current is large. For alternating current of the same frequency, the larger the capacity of the capacitor, the smaller the capacitive reactance, and the smaller the capacity, the larger the capacitive reactance.

Second, parameters and classification of capacitors

In electronic products, capacitors are essential electronic devices. They act as smoothing filters for rectifiers, decoupling of power supplies, bypassing of AC signals, and AC coupling of AC and DC circuits . Since there are many types and structures of capacitors, we need to understand not only the performance indicators and general characteristics of various capacitors, but also the advantages and disadvantages of various components for a given purpose, as well as mechanical or environmental restrictions. Here we will briefly explain the main parameters of capacitors and their applications.

1. Nominal capacitance (CR). The capacitance value marked on a capacitor product. Mica and ceramic dielectric capacitors have low capacitance (approximately below 5000pF); paper, plastic and some ceramic dielectric capacitors are in the middle (approximately 0.005uF~1.0uF); electrolytic capacitors usually have larger capacitance. This is a rough classification.

2. Category temperature range. The ambient temperature range in which the capacitor can work continuously is determined by the capacitor design. This range depends on the temperature limit of its corresponding category, such as the upper category temperature, lower category temperature, rated temperature (the highest ambient temperature at which the rated voltage can be applied continuously), etc.

3. Rated voltage (UR). The maximum DC voltage or the maximum AC voltage or the peak value of the pulse voltage that can be continuously applied to the capacitor at any temperature between the lower category temperature and the rated temperature. When the capacitor is used in high voltage fields, the effect of corona must be noted. Corona is caused by the gap between the dielectric/electrode layer. In addition to generating parasitic signals that damage the equipment, it can also cause the capacitor dielectric to break down. Corona is particularly prone to occur under AC or pulsating conditions. For all capacitors, it should be ensured that the sum of the DC voltage and the AC peak voltage does not exceed the rated voltage of the capacitor during use.

4. Loss tangent (tg δ). Under a sinusoidal voltage of a specified frequency, the loss power 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, but also has an equivalent resistor inside. Its simplified equivalent circuit is shown in the attached figure. For electronic equipment, the smaller RS ​​is required, the better, that is, the smaller the loss power is required, and the smaller the angle between it and the power of the capacitor is required.

5. Temperature characteristics of capacitors. Usually expressed as the percentage of capacitance at a reference temperature of 20°C to capacitance at a relevant temperature.

6. Service life. The service life of capacitors decreases as the temperature increases. The main reason is that temperature accelerates chemical reactions and causes the dielectric to degrade over time.

7. Insulation resistance. Since temperature rise causes increased electronic activity, the insulation resistance will decrease as the temperature rises. Capacitors include two categories: fixed capacitors and variable capacitors. Fixed capacitors can be divided into mica capacitors, ceramic capacitors, and paper/plastic film capacitors according to their dielectric materials.

Third, the types and symbols of capacitors

There are many types of capacitors. In order to distinguish them, several Latin letters are often used to indicate the types of capacitors. The first letter C indicates capacitance, the second letter indicates dielectric material, and the third letter and later indicate shape, structure, etc. The picture above is a small paper dielectric capacitor, and the picture below is a vertical square sealed paper dielectric capacitor. Table 1 lists the types and symbols of capacitors. Table 2 shows several characteristics of commonly used capacitors.

Fourth: Determination of the polarity of electrolytic capacitors

If the polarity of an electrolytic capacitor is unknown, its polarity can be measured using the resistance range of a multimeter. We know that the leakage current of the electrolytic capacitor is small (the leakage resistance is large) only when the positive terminal of the electrolytic capacitor is connected to the positive power supply (black test lead when the resistance block is selected) and the negative terminal is connected to the negative power supply (red test lead when the resistance block is selected). Otherwise, the leakage current of the electrolytic capacitor increases (the leakage resistance decreases).

When measuring, first assume that one electrode is the "+" electrode, connect it to the black probe of the multimeter, and connect the other electrode to the red probe of the multimeter. Note the scale where the needle stops (the needle is on the left, the resistance is large), then discharge the capacitor (that is, the two leads touch each other), swap the two probes, and measure again. In the two measurements, the black probe is connected to the positive electrode of the electrolytic capacitor when the needle finally stops on the left (the resistance is large). It is best to use the R*100 or R*1K block when measuring. 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 R×10, R×100, and R×1K ranges of the multimeter are usually selected for testing and judgment. The red and black test leads are connected to the negative pole of the capacitor respectively (the capacitor needs to be discharged before each test), and the quality of the capacitor is judged by the deflection of the needle. If the needle swings up quickly to the right and then slowly returns to its original position to the left, the capacitor is generally good. If the needle does not turn back after swinging up, it means that the capacitor has been broken down. If the needle gradually returns to a certain position after swinging up, it means that the capacitor has leaked. If the needle cannot swing up, it means that the electrolyte of the capacitor has dried up and lost capacity.

Some capacitors with leakage are not easy to accurately judge whether they are good or bad using the above method. When the withstand voltage of the capacitor is greater than the battery voltage value in the multimeter, according to the characteristics of electrolytic capacitors with small leakage current when forward charging and large leakage current when reverse charging, the R×10K block can be used to reverse charge the capacitor and observe whether the position where the needle stays is stable (i.e. whether the reverse leakage current is constant). The quality of the capacitor can be judged with high accuracy. The black test pen is connected to the negative pole of the capacitor, and the red test pen is connected to the positive pole of the capacitor. The needle quickly swings up and then gradually retreats to a certain position and stays still. This means that the capacitor is good. Any capacitor whose needle stays unsteadily at a certain position or gradually moves to the right after staying has leakage and cannot be used anymore. The needle generally stays and stabilizes within the scale range of 50-200K.

6. A brief discussion on electrolytic capacitors

1. The role of electrolytic capacitors in circuits

1. Filtering. In the power supply circuit, the rectifier circuit converts AC into pulsating DC, and a large-capacity electrolytic capacitor is connected after the rectifier circuit. Using its charging and discharging characteristics, the rectified pulsating DC voltage becomes a relatively stable DC voltage. In practice, in order to prevent the power supply voltage of each part of the circuit from changing due to load changes, electrolytic capacitors of tens to hundreds of microfarads are generally connected to the output end of the power supply and the power input end of the load. Since large-capacity electrolytic capacitors generally have a certain inductance, they cannot effectively filter out high-frequency and pulse interference signals. Therefore, a capacitor with a capacity of 0.001--0.1pF is connected in parallel at both ends to filter out high-frequency and pulse interference.

2. Coupling effect: In the process of low-frequency signal transmission and amplification, capacitor coupling is often used to prevent the static operating points of the front and rear circuits from influencing each other. In order to prevent excessive loss of low-frequency components in the signal, electrolytic capacitors with larger capacity are generally used.

2. Judgment method of electrolytic capacitor

Common faults of electrolytic capacitors include capacity reduction, capacity loss, breakdown short circuit and leakage. The capacity change is caused by the gradual drying of the electrolyte inside the electrolytic capacitor during use or storage, while breakdown and leakage are generally caused by excessive voltage or poor quality. The quality of power supply capacitors is generally measured using the resistance range of a multimeter. The specific method is: short-circuit the two pins of the capacitor to discharge, and connect the black probe of the multimeter to the positive electrode of the electrolytic capacitor. The red probe is connected to the negative electrode (for pointer multimeters, the probes are inter-adjusted when measuring with a digital multimeter). Under normal circumstances, the needle should first swing in the direction of small resistance, and then gradually return to infinity. The greater the swing amplitude of the needle or the slower the return speed, the larger the capacity of the capacitor, and vice versa. If the needle stops changing at a certain point in the middle, it means that the capacitor is leaking. If the resistance indication value is very small or zero, it means that the capacitor has broken down and short-circuited. Because the battery voltage used by the multimeter is generally very low, it is more accurate when measuring capacitors with low withstand voltage. When the withstand voltage of the capacitor is high, although the measurement is normal during the test, leakage or breakdown may occur when high voltage is added.

3. Precautions for using electrolytic capacitors

1. Electrolytic capacitors have positive and negative polarities, so they cannot be connected in reverse when used in a circuit . In a power supply circuit, when outputting a positive voltage , the positive pole of the electrolytic capacitor is connected to the power supply output terminal and the negative pole is grounded. When outputting a negative voltage, the negative pole is connected to the output terminal and the positive pole is grounded. When the polarity of the filter capacitor in the power supply circuit is reversed, the filtering effect of the capacitor is greatly reduced, which causes the power supply output voltage to fluctuate on the one hand, and the electrolytic capacitor, which is equivalent to a resistor, heats up on the other hand due to reverse power supply. When the reverse voltage exceeds a certain value, the reverse leakage resistance of the capacitor will become very small, so that the capacitor will explode and be damaged due to overheating soon after power-on.

2. The voltage applied to the two ends of the electrolytic capacitor cannot exceed its allowable working voltage. When designing the actual circuit, a certain margin should be left according to the specific situation. When designing the filter capacitor of the voltage-stabilized power supply, if the AC power supply voltage is 220~, the rectified voltage of the secondary of the transformer can reach 22V. At this time, a 25V electrolytic capacitor can generally meet the requirements. However, if the AC power supply voltage fluctuates greatly and may rise to more than 250V, it is best to choose an electrolytic capacitor with a withstand voltage of more than 30V.

3. Electrolytic capacitors should not be placed near high-power heating elements in the circuit to prevent the electrolyte from drying up due to heat.

4. For filtering of signals with positive and negative polarity, two electrolytic capacitors with the same polarity can be connected in series as a non-polar capacitor. Summary of this chapter: Various capacitors are needed in electronic production, and they play different roles in the circuit. Similar to resistors, they are usually referred to as capacitors and represented by the letter C. As the name suggests, capacitors are "containers for storing charge." Although there are many varieties of capacitors, their basic structure and principle are the same. Two pieces of metal that are very close to each other are separated by a substance (solid, gas or liquid) to form a capacitor. The two pieces of metal are called plates, and the substance in the middle is called a dielectric. Capacitors are also divided into fixed capacity and variable capacity. But the most common are fixed capacity capacitors, the most common of which are electrolytic capacitors and ceramic capacitors.

Different capacitors have different abilities to store charge. It is stipulated that the amount of charge stored when a 1 volt DC voltage is applied to a capacitor is called the capacitance of the capacitor. The basic unit of capacitance is farad (F). But in fact, farad is a very uncommon unit, because the capacity of capacitors is often much smaller than 1 farad. Microfarad (μF), nanofarad (nF), picofarad (pF) (picofarad is also called microfarad), etc. are commonly used. Their relationship is: 1 farad (F) = 1000000 microfarad (μF) 1 microfarad (μF) = 1000 nanofarad (nF) = 1000000 picofarad (pF) In electronic circuits, capacitors are used to block DC through AC, and are also used to store and release charge to act as filters and smooth out pulsating signals. Small-capacity capacitors are usually used in high-frequency circuits, such as radios, transmitters, and oscillators. Large-capacity capacitors are often used for filtering and storing charge. And there is another feature, generally all capacitors above 1μF are electrolytic capacitors, and capacitors below 1μF are mostly ceramic capacitors, of course there are others, such as monolithic capacitors, polyester capacitors, small-capacity mica capacitors, etc. Electrolytic capacitors have an aluminum shell filled with electrolytes and two electrodes as positive (+) and negative (-) poles. Unlike other capacitors, their polarity in the circuit cannot be connected incorrectly, while other capacitors have no polarity.

Connect the two electrodes of a capacitor to the positive and negative electrodes of a power source. After a while, even if the power source is disconnected, there will still be residual voltage between the two pins (you can use a multimeter to observe this after learning the tutorial). We say that the capacitor stores charge. The voltage is established between the capacitor plates, accumulating electrical energy. This process is called charging the capacitor. There is a certain voltage at both ends of a charged capacitor. The process of releasing the charge stored in the capacitor to the circuit is called discharging the capacitor.

To give an example from real life, we see that after the plug of the commercially available rectifier power supply is unplugged, the light-emitting diode on it will continue to light up for a while, and then gradually go out, because the capacitor inside stores electrical energy in advance and then releases it. Of course, this capacitor was originally used for filtering. As for capacitor filtering, I wonder if you have ever used a rectifier power supply to listen to a walkman. Generally, low-quality power supplies use a small-capacity filter capacitor for cost-saving reasons, which causes buzzing in the headphones. At this time, a large-capacity electrolytic capacitor (1000μF, pay attention to the positive pole to the positive pole) can be connected to both ends of the power supply, which generally improves the effect. Audiophiles who make HiFi audio systems use capacitors of at least 10,000 microfarads for filtering. The larger the filter capacitor, the closer the output voltage waveform is to DC, and the energy storage function of the large capacitor ensures that when a sudden large signal arrives, the circuit has enough energy to convert into a strong and powerful audio output. At this time, the role of the large capacitor is a bit like a reservoir, which allows the original turbulent water flow to be output smoothly and can ensure the supply when a large amount of water is used downstream.

In electronic circuits, current only flows when the capacitor is charging. After the charging process, the capacitor cannot pass direct current and plays the role of "blocking direct current" in the circuit. In circuits, capacitors are often used for coupling, bypassing, filtering, etc., all of which take advantage of its "passing alternating current and blocking direct current" characteristics. So why can alternating current pass through capacitors? Let's first look at the characteristics of alternating current. Not only does alternating current change direction, its size also changes according to a law. When the capacitor is connected to an AC power supply, the capacitor charges and discharges continuously, and charging and discharging currents consistent with the law of alternating current will flow through the circuit. The selection of capacitors involves many issues. The first is the problem of withstand voltage. If the voltage applied to both ends of a capacitor exceeds its rated voltage, the capacitor will be broken down and damaged. The withstand voltage of general electrolytic capacitors is divided into 6.3V, 10V, 16V, 25V, 50V, etc.