Working principles of ceramic capacitors and electrolytic capacitors, how to use a multimeter to measure capacitance

　　Working Principles of Ceramic Capacitors and Electrolytic Capacitors

　　In the circuit design process, capacitors are used for filtering. Sometimes electrolytic capacitors are used, and sometimes ceramic capacitors are used. Sometimes both are used. I would like to ask: What is the purpose of using electrolytic capacitors? What is the purpose of using ordinary ceramic capacitors? How to calculate their capacity? How to choose and determine the withstand voltage of electrolytic capacitors? In which cases are electrolytic capacitors used, in which cases are ceramic capacitors used, and in which cases are both used? It is mentioned in the old version of the analog electronic book that there is a special formula to calculate the size of the capacitance value, but some ICs and the like have regulations on how to match capacitors in their datasheets. I hope this can help you.

　　Electrolytic capacitors and ceramic capacitors are generally used between the power supply and ground of the IC to play a filtering role. Ceramic capacitors are used alone for decoupling. Its use is generally explained in the IC. The size of its electrolytic value is related to the current required by the IC. The ceramic value is 0.01uf.

　　If I want to replace a capacitor with another one, must both the capacity and the withstand voltage be met? Sometimes, it is difficult to have the best of both worlds. Can I give up one of them at this time?

　　The range of filter capacitors is too wide, so here we will briefly talk about power supply bypass (decoupling) capacitors.

　　The choice of filter capacitor depends on whether you use it for local power supply or global power supply. For local power supply, it is to play the role of transient power supply. Why do we need to add capacitors for power supply? Because the current demand of the device changes rapidly with the demand of the driver (such as DDR controller), and when discussing in the high-frequency range, the distributed parameters of the circuit must be considered. Due to the existence of distributed inductance, the drastic change of current is hindered, causing the voltage on the chip power pin to decrease - that is, noise is formed. In addition, current feedback power supplies have a reaction time - that is, they have to wait until the voltage fluctuation occurs for a period of time (usually ms or us level) before making adjustments. For the current demand change at the ns level, this delay also forms actual noise. Therefore, the role of the capacitor is to provide a low inductance (impedance) route to meet the rapid change of current demand.

　　Based on the above theory, the capacitance should be calculated according to the energy that the capacitor can provide for the current change. When choosing the type of capacitor, it is necessary to consider its parasitic inductance - that is, the parasitic inductance should be smaller than the distributed inductance of the power path.

　　Detailed instructions are available in many books. Here is a reference book: High Speed ​​Digital Design ch8.2.

　　The discussion of the problem must start from the essence. First of all, we may all know that capacitors have an isolation effect on DC, while inductors have the opposite effect. All are based on basic principles. At this time, capacitors have the two most common functions. One is to isolate DC between poles. Some people also call it coupling capacitor because it isolates DC but passes AC signals. The DC path is limited to a few levels, which can simplify the complicated calculation of the working point. The second is filtering. Basically, there are two types. As a coupling, the value of the capacitor is not strict, as long as its impedance is not too large, so as not to attenuate the signal too much.

　　But for the latter, it is required to consider from the perspective of the filter. For example, the power supply filtering at the input end requires filtering out both low-frequency (such as caused by power frequency) noise and high-frequency noise, so it is necessary to use large and small capacitors at the same time. Some people will say, if there is a large capacitor, why do we need a small one? This is because the large capacitor has a large inductance due to the large plate and pin end, so it does not work for high frequencies. The small capacitor is just the opposite. The size can determine the capacitance based on this. As for the withstand voltage, it must be met at all times, otherwise it will explode. Even for non-electrolytic capacitors, sometimes it does not explode, and its performance is also reduced. There are too many things to talk about, so let's talk about this much first. They are all filtering functions. The capacity of aluminum electrolytic capacitors is relatively large, and they are mainly used to filter out low-frequency interference. The capacity is about 1mA current corresponding to 2~3μf. If the requirements are high, 1mA can correspond to 5~6μf. Non-polar capacitors are used to filter out high-frequency signals. When used alone, most of them are used for decoupling. Sometimes they can be used in parallel with electrolytic capacitors. Ceramic capacitors have good high-frequency characteristics, but at a certain frequency (about 6MHz, I can't remember exactly), the capacitance drops rapidly.

　　The role and precautions of 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. By utilizing its charging and discharging characteristics, the pulsating DC voltage after rectification 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: In the process of transmitting and amplifying low-frequency signals, capacitor coupling is often used to prevent the static working points of the front and rear circuits from affecting 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 disappearance, breakdown short circuit and leakage. The capacity change is caused by the gradual drying of the electrolyte inside the electrolytic capacitor during use or placement, while breakdown and leakage are generally caused by excessively high applied voltage or poor quality. The resistance range of the multimeter is generally used to judge the quality of the power supply capacitor. The specific method is: short-circuit the two pins of the capacitor for discharge, and connect the black probe of the multimeter to the positive pole of the electrolytic capacitor. The red probe is connected to the negative pole (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 larger 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 points to a certain point in the middle and no longer changes, 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 low-voltage capacitors. When the voltage resistance of the capacitor is high, although the measurement is normal during operation, leakage or breakdown may occur when high voltage is applied.

　　3. Precautions for using electrolytic capacitors

　　1. Since electrolytic capacitors have positive and negative polarities, they cannot be connected in reverse when used in circuits. In the power supply circuit, when outputting 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 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 both 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 side of the transformer can reach 22V. At this time, the 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 more than 250V, it is best to choose an electrolytic capacitor with a withstand voltage of more than 30V.