Capacitor Basic: How do Capacitors Work?

21 July 2020

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Capacitors store electrical energy by storing charges on electrodes, and are usually used together with inductors to form an LC oscillator circuit. The working principle of the capacitor is that the electric charge will move under the force in the electric field. When there is a medium between the conductors, the electric charge will be prevented from moving and the electric charge will accumulate on the conductor, resulting in the accumulation of electric charge.

Catalog

Ⅰ Introduction

The working principle of the capacitor is to store electrical energy by storing charge on the electrode, and it is usually used together with the inductor to form an LC oscillation circuit. The working principle of the capacitor is that the charge will move under the force of the electric field. When there is a medium between the conductors, it will hinder the movement of the charge and cause the charge to accumulate on the conductor, resulting in the accumulation of charge. Capacitors are one of the electronic components used in a large number of electronic equipment, so they are widely used in DC blocking, coupling, bypass, filtering, tuning loop, energy conversion, control circuit, etc.

In a sense, capacitors are a bit like batteries. Although the two work in very different ways, they can both store electrical energy. If you have learned the working principle of the battery, then you should know that the battery has two electrodes. Inside the battery, a chemical reaction causes one electrode to generate electrons, and the other electrode absorbs electrons. A capacitor is much simpler, and it cannot produce electrons-it just stores them.

Capacitors, resistors, and inductors are also called the three major passive components, and their annual output has reached about 2 trillion units worldwide. The most widely used capacitors are ceramic capacitors. At the same time, various types of capacitors such as film capacitors with excellent insulation and stability, electrolytic capacitors are known for their large capacity, etc., are also used by people with their respective advantages and characteristics.

Ⅱ How do capacitors work?

Like a battery, a capacitor also has 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 material that does not conduct electricity and prevents the two metal poles from coming into contact with each other. Using two pieces of aluminum foil and a piece of paper, you can easily make a capacitor. Although the capacitor you made is not ideal in terms of storage capacity, it does work.

 The basic structure of a capacitor

The basic structure of a capacitor is two electrodes (metal plates) facing each other. Applying a DC voltage (V) to the two electrodes, the electrons instantly gather on one of the electrodes, the electrode is negatively charged, and the other electrode is in a state of insufficient electrons, which is positively charged. This state still exists after removing the DC voltage. That is, electric charge (Q) is accumulated between the two electrodes. A dielectric (ceramic, plastic film, etc.) is inserted between the electrodes. The polarization of the dielectric increases the accumulated charge. The index indicating how much charge is stored in a capacitor is called capacitance (C).

The capacitor in the electronic circuit is shown in the figure:

Let's see what happens when we connect the capacitor and battery together:

The metal plate on the capacitor connected to the negative electrode of the battery will absorb the electrons generated by the battery; The metal plate on the capacitor connected to the positive electrode of the battery will release electrons to the battery.

In the circuit, the movement of the charge forms a current. Due to the repulsive effect of the isoelectric charge, the current is the largest at the beginning of the charge movement, and then gradually decreases; and the charge of the capacitor is the smallest at the beginning of the charge movement, which is zero. The charge capacity gradually increases, and the voltage between the two metal plates gradually increases. When it increases to be equal to the power supply voltage, the charging is completed and the current decreases to zero.

After charging, the capacitor and the battery have the same voltage (if the battery voltage is 1.5 volts, the capacitor voltage is also 1.5 volts). Small capacitors have lower capacity, but large capacitors can hold a lot of charges. For example, a soda can-sized capacitor can hold enough charge to light a flashlight bulb for a few minutes. When you see the lightning in the sky, you see a huge capacitor, one of which is the dark cloud in the sky, the other is the earth. Lightning is the charge release phenomenon between the two "poles" of the dark cloud and the earth. Obviously, such a huge capacitor can hold a lot of charges!

Next, let us assume that you connect the capacitor to the circuit as follows:

You have a battery, a light bulb, and a capacitor. If the capacitor is very large, then you will see that after connecting the battery, current flows from the battery to the capacitor to charge it, and the bulb will be lit. The bulb will gradually dim, and finally, once the capacitor reaches its capacity, the bulb will immediately go out. You can then remove the battery and replace it with a piece of wire. Current will flow from one pole of the capacitor to the other. At this point, the bulb will replay brightly, but soon the bulb gradually dims. Finally, the capacitor is discharged (the number of electrons on the two poles of the capacitor is equal), and the bulb goes out again.

In the circuit, the movement of the charge forms a current. Due to the attraction of the opposite charge, the current is the largest at the beginning of the discharge process and then gradually decreases; the charge capacity of the capacitor is the largest at the beginning of the discharge process and gradually decreases afterwards. When the power is reduced to zero, the discharge is completed, and the current is reduced to zero.

After the capacitor is charged, no current flows in the circuit, so the capacitor can play a role in blocking DC. In the DC circuit, it can be regarded as an open circuit.

The charging process of the capacitor is the process of storing charge. When the capacitor is connected to the DC power supply, the charge on the metal plate connected to the positive electrode of the power supply will run toward the metal plate connected to the negative electrode of the power supply under the action of the electric field force. So that the metal plate connected to the positive pole of the power supply loses its charge and is positively charged. The metal plate connected to the negative pole of the power supply gets negatively charged (the charges of the two metal plates are equal and the signs are opposite), and the capacitor begins to charge.

The discharge process is the process of the capacitor releasing the stored charge. When the charged capacitor is located in a closed path without power, the charge on the negatively charged metal plate will be directed to the positively charged metal under the action of the electric field force. The plate runs away, so that the positive and negative charges are neutralized, and the capacitor begins to discharge.

 The charge accumulated in the capacitor

The role of the capacitor can be visually described by a water tower connected to the water pipe. The water tower can be used to "storage" water pressure-when the water supplied by the water pump of the water supply system exceeds the amount of water required by the town, the excess water will be stored in the water tower. Then, when the water demand is high, excess water will flow out of the water tower to maintain the water pressure. Capacitors store electrons in the same way.

Ⅲ Capacity Unit- F

The unit of electric capacity is F. A capacitor with a capacity of 1F can store 1 coulomb of electricity at a voltage of 1 volt. 1 Coulomb is 6.25e18 (6.25*10^18, or 62.5 billion) electrons. 1 ampere represents the flow rate of electrons flowing through 1 coulomb of electrons per second. Therefore, a capacitor with a capacity of 1F can store 1 ampere-second of electrons at a voltage of 1 volt.

The 1F capacitor is usually quite large. Depending on the voltage tolerance of the capacitor, it may be as large as a can of tuna or a 1 liter soda bottle. Therefore, the capacitors you see are usually measured in microfarads (parts per million).

To understand how big the 1 method is, it may be calculated like this:

A typical alkaline AA battery stores about 2.8 amp-hours of electricity. This means that an AA battery can produce a current of 2.8 amps for 1 hour at 1.5 volts (about 4.2 watt-hours, that is, an AA battery can keep a 4 watt light bulb continuously lit for a little more than an hour).

For the convenience of calculation, we simply counted the voltage of the AA battery as 1 volt. To store the energy of one AA battery in the capacitor, a capacitor with a capacity of 3,600*2.8=10,080F is needed, because 1 amp-hour is equivalent to 3600 amp-seconds.

If the capacity of 1 method needs to be stored with a tuna-sized capacitor, then the size of an AA battery is nothing compared to the size of a 10,080 method capacitor! Obviously, unless a capacitor has a high withstand voltage, it is not practical to use a capacitor to store a large amount of energy.

Ⅳ The main application of capacitors

1.  Filtering

The capacitor connected between the positive and negative poles of the DC output of the power module can filter out unnecessary AC components in the DC module, which can make the DC power smoother.

2.  Decoupling

The capacitor connected between the positive and negative poles of the power supply of the amplifier circuit can prevent the parasitic oscillation caused by the positive feedback formed by the internal resistance of the power supply.

3.  Bypassing

In the circuit of AC and DC signals, connect the capacitor in parallel with the two ends of the resistor or jump to the common potential from a certain point of the circuit. You can set a path for the AC signal or pulse signal to avoid AC signal components. The voltage drop attenuation due to the resistance.

4.  Coupling

In the AC signal processing circuit, it is used to connect the signal source and the signal processing circuit or as the inter-stage connection of the two amplifiers. It is used to cut off the DC, so AC signal or the pulse signal will pass. And the DC working points of the front and rear amplifier circuits do not affect each other.

5.  Tuning

A capacitor is connected at both ends of the oscillating coil of the resonant circuit, which plays the role of selecting the oscillating frequency.

6.  Compensation

Auxiliary capacitors connected in parallel with the main capacitor of the resonance circuit. Adjusting this capacitor can expand the frequency range of the oscillation signal.

7.  Neutralization

Capacitors connected in parallel between the base and emitter of the triode amplifier to form a negative feedback network to suppress self-oscillation caused by the capacitance between the triodes.

8.  Frequency stabilization

The capacitor plays a role in stabilizing the oscillation frequency in the oscillation circuit.

9.  Timing

The capacitor in series with the resistor R in the RC time constant circuit can determine the charge and discharge time.

10.  Acceleration

Connecting to the oscillator feedback circuit to accelerate the positive feedback process and increase the amplitude of the oscillation signal.

11.  Start-up

Connected in series with the auxiliary winding of the single-phase motor to provide the starting phase-shifted AC voltage for the motor, and disconnected from the auxiliary winding after the motor runs normally.

12.  Operation

Connected in series with the secondary winding of a single-phase motor to provide phase-shifted alternating current for the secondary winding of the motor. When the motor is running normally, it is connected in series with the auxiliary winding.

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