(1) Capacitors can store charges and have the function of blocking DC

When the two electrode plates of the capacitor are connected to the positive and negative poles of the DC power supply respectively, the positive and negative charges It will accumulate on the two electrode plates of the capacitor, forming a voltage between the two plates. As the charge on the two plates of the capacitor continues to increase, the voltage on the capacitor gradually increases from small to small, until it is equal to the DC power supply voltage, no current will flow in the circuit, and the charging process stops. This is the charging of the capacitor. effect. If the DC power supply is disconnected from the capacitor, charge is stored on the capacitor. The amount of charge stored can be calculated by the following formula, namely

Q=C·U

In the formula: Q——the amount of charge stored on the capacitor (c);

c——the capacity of the capacitor (F);

U——the voltage across the capacitor (V) .

It can be seen from the above formula that when the voltage across the capacitor is constant, the greater 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 that measures the ability of the capacitor to store charge.

After the charge is stored on the capacitor, since the two plates of the capacitor are separated by an insulating medium, although there is voltage at both ends of the capacitor, the charge cannot pass between the electrodes, so the capacitor has the function of blocking DC.

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

When using a capacitor in an electronic circuit, if the voltage on the electronic circuit is higher than the voltage across the capacitor, the capacitor will charge until the voltage established on the capacitor is equal to the voltage of the circuit; if the voltage on the electronic circuit is The voltage is lower than the voltage across the capacitor, and the capacitor discharges.

(2) Alternating current can "pass" a capacitor

If a capacitor is connected to an AC circuit, the capacitor will charge and discharge alternately due to the constant changes in the magnitude and direction of the AC voltage. At this time, there is still no charge passing between the two plates of the capacitor, but in the AC circuit, an alternating current with changing direction and magnitude is formed, just like the capacitor can pass alternating current, this means that alternating current can "pass" The principle of capacitor.

(3) Capacitive reactance of capacitor

Capacitor has special resistance characteristics to alternating current, which is called capacitive reactance. Capacitive reactance can be calculated from the following formula, namely

where: Xc——capacitive reactance (0);

f——frequency (Hz);

C ——The capacity of the 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 its capacitive reactance, and the easier it is for AC current to pass through the capacitor.

2. The role of capacitors in circuits

The basic characteristics of capacitors have been widely used in electronic circuits. They are used in filter circuits, tuning circuits, coupling circuits, and bypasses. Circuits, delay circuits, shaping circuits and other circuits all play an important role. Two examples are used below to illustrate some of the functions of capacitors in circuits.

[Example 1]: Rifle regenerates a two-tube semiconductor radio

The circuit of Rifu regenerating a two-tube semiconductor radio is shown in Figure 4-3. Seven capacitors are used in the circuit. Their functions in the circuit are described below:

C1 and L1 form a tuning loop. By adjusting the capacity of C1, the purpose of selecting a radio station is achieved.

C2 is a semi-adjustable capacitor. The amplified high-frequency signal can be fed back to the tuning loop via L3 and C2 to strengthen the high-frequency signal and improve the sensitivity of the radio. Adjusting Cz can change the strength of feedback regeneration.

C3 is connected between L2 and VT1 emitter. It has dual functions: First, it has low capacitive impedance to the broadcast signal, which allows the high-frequency signal in L2 to be smoothly added to VT1. Amplification is performed on the transmitting junction; secondly, C3 also plays the role of bypassing the residual high-frequency signal after detection.

The capacity of C4 is very small, only 1OOpF. Its capacitive reactance to high-frequency signals is small, but its capacitive reactance to low-frequency signals is large, so high-frequency signals can be added to the detector through C4. During detection, the audio signal cannot pass through C4, but is sent to VT2 via L4 for further amplification.

C5 is a bypass capacitor. Since its capacitive reactance to high-frequency signals is very small, it can bypass the high-frequency signals leaking from L4.

C6 has two functions: first, it cuts off the DC path between points A and B to prevent the connection of points A and B from destroying the static working state of VT1 and VT2, making the radio unable to work normally; The second is to form an audio channel, and combine the audio signal output by the collector of VT1 to the base of VT2 for amplification. So C6 can be called a DC blocking capacitor.

C7 has a large capacitance, and its capacitive reactance to low-frequency signals is small. Since C7 is connected in parallel to the battery, when the internal resistance of the battery increases, C7 can bypass the low-frequency signal to prevent the signals of each amplification stage from generating harmful low-frequency oscillations through the interaction of the internal resistance of the battery.

[Example 2]: Delay circuit

The picture shows a delay circuit composed of single-junction semiconductor tubes. It uses the charge and discharge characteristics of the capacitor to achieve the purpose of delay control time. The length of the delay time is determined by R3, RP and C. When switch S is closed, the power supply charges C through R3 and RP. When the voltage on C reaches a certain amplitude, VT1 is turned on, and the charge on C is discharged through VT1, E, B1 terminals and t, triggering the thyristor VT2 to turn on, and the relay K is powered to work, and its contacts will control the controlled circuit Work.