When the two electrode plates of the capacitor are respectively connected with the positive and negative electrodes of the DC power supply, the positive and negative charges will gather on the two electrode plates of the capacitor to form a voltage between the two electrode plates. With the increase of charges on the two plates of the capacitor, the voltage on the capacitor gradually increases from small to equal to the DC power supply voltage, so there will be no current flowing in the circuit and the charging process will stop. This is the charging function of the capacitor. If the DC power supply is disconnected from the capacitor, the capacitor will be charged at this time, and the amount of charge stored in it can be obtained by the following formula, namely

Q=C U

Where: q- the amount of charge stored on the capacitor (c);

C- capacitor capacity (f);

U- voltage across the capacitor (v).

As can be seen from the above formula, when the voltage across the capacitor is constant, the larger the capacity of the capacitor, the greater the amount of charge it stores. It can be seen that the capacitance of a capacitor is a parameter to measure the capacity of a capacitor to store charge.

After the charge is stored in the capacitor, because the two plates of the capacitor are separated by an insulating medium, although there is a voltage across the capacitor, the charge cannot pass between the electrodes, so the capacitor has the function of isolating DC.

If the two electrodes of a capacitor storing charge are connected by wires, the positive and negative charges on the capacitor plate will be neutralized by the wires at the moment of connection, which is the discharge function of the capacitor. The process of capacitor discharge is a process of energy release, which will do work in the discharge circuit and convert electric energy into other forms of energy.

When a capacitor is used in an electronic circuit, if the voltage on the electronic circuit is higher than the voltage across the capacitor, the capacitor will be charged until the voltage established on the capacitor is equal to the voltage of the circuit; If the voltage on the electronic circuit is lower than the voltage across the capacitor, the capacitor discharges.

(2) Alternating current can "pass" through the capacitor.

If the capacitor is connected to the AC circuit, the capacitor will be charged and discharged alternately because the magnitude and direction of the AC voltage are constantly changing. At this time, there is still no charge passing between the two plates of the capacitor, but alternating current with changing direction and size is formed in the alternating current circuit, just as the capacitor can pass through alternating current, which is why alternating current can "pass through" the capacitor.

(3) Capacitance reactance of capacitor

Capacitors have special resistance characteristics to alternating current, which is called capacitive reactance. Capacitance reactance can be calculated by the following formula, namely

Where: xc- capacitive reactance (0);

F- frequency (Hz);

C-capacitance of capacitor (f).

It is not difficult to see from the above formula that the larger the capacity of the capacitor, the higher the frequency of the current, the smaller the capacitive reactance, and the easier it is for AC current to pass through the capacitor.

2. The role of capacitor in the circuit

The basic characteristics of capacitors are widely used in electronic circuits, and play an important role in filter circuits, tuning circuits, coupling circuits, fiber channels, delay circuits, shaping circuits and other circuits. Here are two examples to illustrate some functions of capacitors in circuits.

[Example 1]: Recycle the dual transistor radio.

Figure 4-3 shows the regeneration circuit of two transistor electron tubes. Seven capacitors are used in the circuit, and their functions in the circuit are as follows:

C 1 and L 1 form a tuning loop. By adjusting the capacity of C 1, the purpose of selecting radio stations can be achieved.

C2 is a semi-adjustable capacitor. The amplified high-frequency signal can be fed back to the tuning loop through L3 and C2, thus enhancing the high-frequency signal and improving the sensitivity of the radio. Adjusting Cz can change the intensity of feedback regeneration.

C3 is connected between L2 and the emitter of VT 1, which has dual functions: first, it has low capacitive reactance for broadcast signals, so that the high-frequency signal of L2 can be successfully added to the emitter junction of VT 1 for amplification; Secondly, C3 also plays the role of bypassing the residual high-frequency signal after detection.

The capacity of C4 is very small, only 100pf, which has small capacitive reactance for high-frequency signals and large capacitive reactance for low-frequency signals. Therefore, high-frequency signals can be added to the detector for detection through C4, while audio signals cannot pass through C4, but are sent to VT2 through L4 for further amplification.

C5 is a bypass capacitor. Because of its small capacitive reactance to high-frequency signals, it can bypass the high-frequency signals leaked by L4.

C6 has two functions: one is to block the DC path between point A and point B, and prevent the connection between point A and point B from damaging the static working state of VT 1 and VT2, so that the radio station cannot work normally; The second is to form an audio channel, which combines and amplifies the audio signal output by the collector of VT 1 with the base of VT2. Therefore, C6 can be called a right-angle capacitor.

C7 has a large capacitance and a small capacitive reactance to low-frequency signals. Because C7 is connected in parallel with the battery, when the daily internal resistance of the battery increases, C7 can bypass the low-frequency signal and prevent the signals of each amplification stage from generating harmful low-frequency oscillation through the pomegranate effect of the internal resistance of the battery.

[Example 2]: Delay Circuit

The picture shows a delay circuit composed of a single junction semiconductor tube. It uses the charging and discharging characteristics of capacitor to realize the purpose of delay control time. The length of the delay time is determined by R3, RP and C. When the switch S is closed, the power supply charges C through R3 and RP. When the voltage on C reaches a certain amplitude, VT 1 is turned on, and the charge on C is discharged through terminals VT 1, E and B 1, which triggers the thyristor VT2 to be turned on, and the relay K is energized, and its contacts will control the controlled circuit to work.