I. What is a capacitor?
Capacitors play an important role in circuits for tuning, bypassing, coupling, and filtering. They are used in the tuning circuits of transistor radios, as well as in the coupling and bypass circuits of color televisions.
With the rapid development of electronic information technology, digital electronic products are being updated at an increasingly faster pace. The production and sales of consumer electronics products, mainly flat-panel TVs (LCD and PDP), laptops, and digital cameras, have continued to grow, driving the growth of the capacitor industry.
The simplest capacitor consists of two plates and an insulating dielectric (including air) in between. When current flows through it, the plates become charged, creating a voltage (potential difference). However, due to the insulating material, the capacitor as a whole is not conductive. This is under the condition that the capacitor's critical voltage (breakdown voltage) is not exceeded. We know that all materials are relatively insulators. When the voltage across a material increases to a certain level, the material can conduct electricity; this voltage is called the breakdown voltage. Capacitors are no exception; once a capacitor breaks down, it is no longer an insulator. However, in high school, such a voltage is not encountered in circuits, so they operate below the breakdown voltage and can be considered insulators.
II. The role of capacitors in circuits
The main functions of a capacitor in a circuit are as follows:
1. It serves a coupling function.
It refers to the capacitor in the coupling circuit. When used in AC amplifiers and other capacitor-coupled circuits, it is called a coupling capacitor. The coupling capacitor connects the front and rear stages and plays the role of blocking DC and passing AC.
2. It serves as a filter.
The capacitors used in filtering circuits are called filter capacitors. They are used in power filters and various filtering circuits. Filter capacitors can effectively filter out harmful signals within a certain frequency band.
3. It serves a decoupling function.
The capacitor used in a decoupling circuit is called a decoupling capacitor. It is used in the DC power supply circuit of a multi-stage amplifier to eliminate harmful low-frequency cross-connections between different stages of the amplifier.
4. It serves as a bypass.
The capacitor used in a bypass circuit is called a bypass capacitor. If it is necessary to remove a signal of a certain frequency band from the signal in a circuit, a capacitor is often used to bypass it. Depending on the frequency of the signal being removed, there are full-frequency domain (all AC signals) bypass capacitor circuits and high-frequency bypass capacitor circuits.
5. It serves a timing function.
A capacitor used in a timing circuit is called a timing capacitor. In circuits where timing is controlled by charging and discharging the capacitor, the capacitor plays a role in controlling the time constant.
6. It serves as a derivative and integral calculus tool.
Capacitors used in differentiating circuits are called differentiating capacitors. Differentiating capacitor circuits are often used in trigger circuits to obtain peaked trigger signals from various types of signals, primarily rectangular pulse signals. Capacitors used in integrating circuits are called integrating capacitors. For example, in the synchronization separation circuit of a television field scanning system, an integrating capacitor circuit is used to extract the field synchronization signal from the composite field synchronization signal.
7. It serves a compensatory function.
Capacitors used in compensation circuits are called compensation capacitors, such as those used for reactive power compensation in power grids. They utilize the principle that the current in a capacitor is exactly opposite to the inductive current. As long as the number of capacitors connected to the line matches the inductive component of the load, the resulting capacitive current can eliminate or reduce the reactive power absorbed by the load from the power grid. This reduces the burden on electrical lines and transformer equipment, and also improves their utilization rate.
8. It serves as a frequency divider.
The capacitor used in a crossover circuit is called a crossover capacitor. In a speaker crossover circuit, the high-frequency speaker operates in the high-frequency range through the crossover capacitor, the mid-frequency speaker operates in the mid-frequency range, and the low-frequency speaker operates in the low-frequency range.
