## What’s a Capacitor?

Capacitor is a kind of circuit component with insulating medium between two metal plates. Its unit is Farad, the symbol is F. The capacitor uses the electric field between the two conductors to store energy, and the charge carried by the two conductors are equal, but the sign is reversed.

## Features of Capacitor

First, the capacitor can store the charge, with the role of blocking the DC When the capacitor of the two plates were connected to the DC power supply positive and negative, the positive and negative charge will gather in the capacitor on the two electrode plates, in the two A plate between the formation of voltage. As the charge on the capacitor stack increases, the voltage on the capacitor increases from small to equal to the DC supply voltage, the circuit will not flow, the charging process is stopped, which is the capacitor charge effect. If the DC power supply and the capacitor are disconnected, the charge is stored on the capacitor.

Second, AC power can be “through” the capacitor If the capacitor connected to the AC circuit, due to the size and direction of alternating current voltage changes, the capacitor will be alternately charged, discharge repeated, then the capacitor between the two plates still no charge through, but In the AC circuit has formed a direction and size are constantly changing the AC current, just as the capacitor can pass the same AC, which is the exchange can “pass” the truth of the capacitor.

Third, Capacitor capacitive capacitance of the AC has a special resistance characteristics, known as capacitive reactance. The larger the capacity of the capacitor, the higher the frequency of the current, the smaller the capacitive resistance, the more easily the AC current through the capacitor.

## What Does a Capacitor Do

The basic characteristics of the capacitor in the electronic circuit has been very widely used, it is in the filter circuit, tuning circuit, coupling circuit, bypass circuit, delay circuit, shaping circuit and other circuits are playing an important role. Here are two examples to illustrate the capacitor in the circuit some of the role. Here are two examples to show you how does a capacitor work.

Example 1: the reproduce two tubes of semiconductor radio circuit.

Circuit used a total of seven capacitors, the role of their circuit is described as follows:
C1 and L2 constitute a tuning circuit, by adjusting the size of the C1 capacity to achieve the purpose of selecting the radio.
C2 is a semi-adjustable capacitor. Amplified high-frequency signal can be L3 and C2 feedback to the tuning loop, so that high-frequency signal to enhance the radio to improve the sensitivity. Adjusting C2 can change the strength of feedback regeneration.

C3 bridging between L2 and VT1 emitter, it has a dual role: First, it is low capacity of the broadcast signal, L2 high-frequency signal can be successfully returned to the VT1 launch node to enlarge; Second, C3 also played a post-detection of the residual high-frequency signal bypass role.

C4 capacity is very small, only 100pF, it is the high-frequency signal capacity is small, and the low-frequency signal capacity of the larger, so the high-frequency signal can be added to the detector by C4 detection, the audio signal cannot pass C4, just L4 sent to VT2 to further enlarge.

C5 for the bypass capacitor, because of its high-frequency signal capacity is very small, you can leak from the L4 over the high-frequency signal bypass.

C6 has two roles: First, cut off the A, B between the two points of the DC channel to prevent A, B two points connected to destroy the VT1, VT2 static working state, so that the radio cannot work; Second, constitute the audio channel, the audio signal output from the VT1 collector is coupled to the base of the VT2 for the amplification. So you can put C6 as a straight-coupled coupling capacitor.

C7 of the larger capacity, it is low capacitance of the low capacitance. As the C7 parallel in the battery, when the battery with the old resistance increases, C7 can bypass the low-frequency signal to prevent the amplification of the signal through the battery resistance of the coupling effect of harmful low-frequency oscillation.

Example 2: Delay Circuit

The picture above is a single junction semiconductor tube composed of delay circuit. 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 the beginning S closed, the power through R3, RP to C charge. When the voltage on the C reaches a certain amplitude, VT1 conduction, C on the charge by the VT1, E, B1 and R2 discharge, triggering thyristor VT2 conduction, the relay K work, the contact will control the circuit work.

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