The role and working principle of capacitors

　　In electronic circuits, capacitors are used to block direct current while passing alternating current. They are also used to store and release charges 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 charges. And there is another feature. Generally, capacitors above 1μF are electrolytic capacitors, while capacitors below 1μF are mostly ceramic capacitors. Of course, there are others, such as monolithic capacitors, polyester capacitors, and small-capacity mica capacitors. Electrolytic capacitors have an aluminum shell filled with electrolytes, and two electrodes are led out as positive (+) and negative (-). 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 LED 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 a buzzing sound 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 flows only when the capacitor is charging. After the charging process is over, 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 AC and blocking DC" characteristics. So why can alternating current pass through capacitors? Let's first look at the characteristics of alternating current. Alternating current not only changes direction back and forth, but also changes in size according to a law. When a capacitor is connected to an AC power supply, the capacitor charges and discharges continuously, and charging and discharging currents consistent with the laws of alternating current will flow through the circuit.

　　The selection of capacitors involves many issues. The first is the voltage resistance. If the voltage applied to the two ends of a capacitor exceeds its rated voltage, the capacitor will be broken down and damaged. The voltage resistance of general electrolytic capacitors is divided into 6.3V, 10V, 16V, 25V, 50V, etc.