The function and principle of capacitor

In electronic circuits, capacitors are used to block DC through AC, and are also used to store and release charges to act as filters to smooth output pulsating signals.

Small-capacity capacitors commonly 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, which is filled with electrolyte, and leads out two electrodes as positive (+) and negative (-) poles. Unlike other capacitors, their polarity in the circuit cannot be wrong, while other capacitors are No polarity.

Connect the two electrodes of the capacitor to the positive and negative poles of the power supply. After a while, even if the power supply is disconnected, there will still be residual voltage between the two pins. You can observe it with a multimeter. We say that the capacitor stores the charge. A voltage is established between the plates of the capacitor and electric energy is accumulated. This process is called the charging of the capacitor. A charged capacitor has a certain voltage across it. The process of releasing the charge stored in the capacitor to the circuit is called the discharge of the capacitor.

How capacitors work

Capacitors work on the principle of storing electrical energy by storing charges on electrodes, and are usually used in conjunction with inductors to form LC oscillating circuits. The working principle of the capacitor is that the charge will move under the force in the electric field. When there is a medium between the conductors, the movement of the charge is hindered and the charge is accumulated on the conductor, resulting in the accumulation and storage of the charge.

Capacitors are one of the most widely used electronic components in electronic equipment, so they are widely used in DC blocking, coupling, bypassing, filtering, tuning circuits, energy conversion, control circuits, etc.

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working principle:

Capacitors are similar to batteries in that they also have two electrodes. Inside the capacitor, these two electrodes are connected to two metal plates separated by a dielectric. The dielectric can be air, paper, plastic, or any other substance that does not conduct electricity and prevents the two metal poles from touching each other. The metal plate on the capacitor connected to the negative terminal of the battery will absorb the electrons produced by the battery.

The metal plate on the capacitor connected to the positive terminal of the battery will release electrons to the battery. After charging is complete, the capacitor has the same voltage as the battery (if the battery voltage is 1.5 volts, the capacitor voltage is also 1.5 volts).

The role of capacitors

As one of the passive components of the capacitor, its role is nothing more than the following:

1. Applied to the power supply circuit to realize the functions of bypass, decoupling, filtering and energy storage. The categories are detailed below:

1) Bypass

The bypass capacitor is an energy storage device that provides energy to the local device, it can even out the output of the regulator and reduce the load demand. Like a small rechargeable battery, the bypass capacitor can be charged and discharged to the device. To minimize impedance, place bypass capacitors as close as possible to the power supply and ground pins of the load device. This is a good protection against ground potential rise and noise caused by excessive input values. Ground bounce is the voltage drop across the ground connection through a high current glitch.

2) Remove lotus root

Remove lotus root, also known as decoupling. From a circuit perspective, a distinction can always be made between the source being driven and the load being driven. If the load capacitance is relatively large, the driving circuit needs to charge and discharge the capacitance to complete the signal transition. When the rising edge is relatively steep, the current is relatively large, so the driving current will absorb a large power supply current. The inductance and resistance (especially the inductance on the chip pins, which will cause rebound), this current is actually a kind of noise compared to the normal situation, which will affect the normal operation of the front stage, which is the so-called "coupling" .

The decoupling capacitor acts as a "battery" to meet the change of the current of the drive circuit and avoid mutual coupling interference. Combining bypass and decoupling capacitors will make it easier to understand. The bypass capacitor is actually decoupling, but the bypass capacitor generally refers to the high-frequency bypass, that is, a low-impedance leakage prevention path for high-frequency switching noise. The high-frequency bypass capacitor is generally small, and is generally 0.1μF, 0.01μF, etc. according to the resonant frequency; while the capacity of the decoupling capacitor is generally larger, which may be 10μF or more, depending on the distribution parameters in the circuit and the change in drive current. to make sure.

Bypass is to take the interference in the input signal as the filtering object, and decoupling is to take the interference of the output signal as the filtering object to prevent the interference signal from returning to the power supply. This should be their essential difference.

3) Filtering

Theoretically (that is, assuming that the capacitor is a pure capacitor), the larger the capacitor, the smaller the impedance and the higher the passing frequency. But in fact, most of the capacitors exceeding 1μF are electrolytic capacitors, which have a large inductance component, so the impedance will increase when the frequency is high. Sometimes it is seen that there is an electrolytic capacitor with a large capacitance connected in parallel with a small capacitor. At this time, the large capacitor is connected to the low frequency, and the small capacitor is connected to the high frequency. The function of the capacitor is to pass high resistance and low frequency, and pass high frequency and block low frequency. The larger the capacitor, the easier it is for low frequencies to pass, and the larger the capacitor, the easier it is for high frequencies to pass. Specifically used in filtering, the large capacitor (1000μF) filters the low frequency, and the small capacitor (20pF) filters the high frequency. Some netizens have vividly compared the filter capacitor to a "pond". Since the voltage across the capacitor will not change abruptly, it can be seen that the higher the signal frequency, the greater the attenuation. It can be vividly said that the capacitor is like a pond, and the water volume will not change due to the addition or evaporation of a few drops of water. It converts changes in voltage into changes in current, and the higher the frequency, the greater the peak current, which buffers the voltage. Filtering is the process of charging and discharging.

4) Energy storage

The energy storage capacitor collects the charge through the rectifier and transfers the stored energy to the output of the power supply through the inverter leads. Aluminum electrolytic capacitors (such as B43504 or B43505 from EPCOS) with a voltage rating of 40 to 450 VDC and a capacitance of 220 to 150 000 μF are commonly used. Depending on the power supply requirements, the devices are sometimes used in series, parallel, or a combination thereof. For power supplies with power levels exceeding 10KW, bulky can-shaped screw terminal capacitors are usually used.

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The main purpose of capacitors:

1. Capacitors are used to store electricity for high-speed discharge. This is what the flash uses. Large lasers also use this technique for very bright instantaneous flashes.

2. Capacitors can also eliminate pulsation. If the line carrying the DC voltage contains pulsations or spikes, a bulk capacitor can smooth out the voltage by absorbing the peaks and filling in the valleys.

3. Capacitors can block DC. If you connect a smaller capacitor to the battery, after the capacitor is fully charged (the charging process can be completed in an instant when the capacitor has a small capacity), there will be no more current flowing between the two poles of the battery. However, any alternating current (AC) signal can flow through the capacitor unimpeded. The reason for this is that as the AC current fluctuates, the capacitor is continuously charged and discharged as if the AC current was flowing.

4. Capacitors are used together with inductors to form oscillators.

To take a real-life example, we see that after the commercially available rectifier power supply is unplugged, the light-emitting diodes on it will continue to light up for a while, and then gradually go out, because the capacitors inside store the electric energy in advance and then release it. Of course, this capacitor was originally used for filtering. As for the capacitor filter, I wonder if you have any experience of listening to the Walkman with the rectified power supply. Generally, the low-quality power supply uses a small-capacity filter capacitor for cost savings, resulting in a buzzing sound in the headphones. At this time, you can connect a large-capacity electrolytic capacitor (1000μF, pay attention to the positive pole to the positive pole) at both ends of the power supply, which can generally improve the effect. Audiophiles must use capacitors of at least 10,000 microfarads for filtering when making HiFi audio. The larger the filter capacitor, the closer the output voltage waveform is to DC, and the energy storage function of the large capacitor makes the circuit when a sudden large signal arrives. There's enough energy to convert into a powerful audio output. At this time, the role of the large capacitor is a bit like a reservoir, which makes the original turbulent water flow smoothly output, and can ensure the supply of large amounts of water downstream.

In the electronic circuit, current flows only during the charging process of the capacitor. After the charging process is over, the capacitor cannot pass direct current, and it plays a role of "blocking direct current" in the circuit. In the circuit, capacitors are often used for coupling, bypassing, filtering, etc., all of which use its characteristics of "passing AC and blocking DC". So why can alternating current pass through the capacitor? Let's first look at the characteristics of alternating current. Alternating current not only reciprocates in direction, but its magnitude also changes regularly. When the capacitor is connected to the AC power supply, the capacitor is continuously charged and discharged, and the charging current and discharging current that are consistent with the changing law of the AC current will flow in the circuit.

The selection of capacitors involves many issues. The first is the problem of pressure resistance. If the voltage across a capacitor exceeds its rated voltage, the capacitor will be damaged by breakdown. Generally, the withstand voltage of electrolytic capacitors is 6.3V, 10V, 16V, 25V, 50V, etc.