The role of capacitors

灞波儿奔

The role of capacitors [Copy link]

The capacitance determination formula is: C=εS/4πkd, the definition formula is: C=Q/U, and it has a characteristic of blocking direct current and passing alternating current. This is also the understanding of most people on capacitance. Although they know what capacitance is, few people know what its specific role is. It was not until I started working and did several electronic research and development projects that I had a deeper understanding of the role of capacitance.

The role of capacitors

The main functions of capacitors include filtering, coupling, resonance, bypassing, timing, loading, integration, and differentiation. Let’s talk about how capacitors achieve these functions.

Filtering

The filter capacitor is an electrolytic capacitor, and the positive and negative poles cannot be reversed. It is called a filter capacitor, but what waves do they filter out?

As shown in the figure above, the waveform after passing through the rectifier bridge is as follows

The waveform after the filter capacitor is shown in the figure below. Its function is to stabilize the voltage similar to the sin waveform within a certain value. The larger the capacity of the capacitor, the smoother the waveform. It acts like a bucket, collecting the water flowing in the pipe first, and then flowing out through the bucket. In this way, the flow rate of the water flowing out will be relatively slow. This is the so-called filtering. If you feel that the waveform is not smooth enough, you can add a voltage regulator tube to stabilize the voltage, so that the waveform will be smoother. The filter capacitor is mostly used in the power supply part of the circuit.

Bypass, coupling

The functions of bypass capacitors and coupling capacitors are actually the same, both of which are to filter out high-frequency signals. The difference is that they are located in different positions in the circuit. The bypass capacitor filters out the high frequency of the input signal, while the coupling capacitor filters out the high frequency of the output signal. Although the filter capacitor also performs filtering, it is powerless for high-frequency signals. At this time, it is necessary to connect an appropriate bypass or coupling capacitor in parallel in the circuit to allow high-frequency AC to flow to the ground through this capacitor to ensure that there is no high-frequency signal in the subsequent circuit. As for how large the bypass capacitor should be, it is necessary to do a specific circuit and specific analysis.

Load Capacitance

I believe everyone is familiar with load capacitors. The capacitors C1 and C2 in parallel with the crystal oscillator X1 in the figure below are load capacitors. Changing the capacitance of the load capacitor can adjust the frequency of the crystal oscillator. The capacitance of the load capacitor is very small, usually only a few dozen pF (1pF=10^-12F). Commonly used capacitances are 22pF, 30pF, 50pF, and 100pF. The official MCU will generally give the capacitance of this load capacitor, so we don't need to go into it.

Some of the most commonly used functions of capacitors are listed. If there are any omissions, I will not hesitate to teach you some advanced functions of capacitors, such as integration and differentiation.

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When seeing so many capacitors in the circuit, I always wonder what they are used for. The main function of capacitors is filtering and something else.