Principles and application knowledge of capacitors
Electrical measurement and Ohm's law

Principles and application knowledge of capacitors

A capacitor is an electronic component with special properties. It can first “store” electrons, and then release them into the circuit to form an electric current when needed.

A capacitor (see Figure 1) is an electronic component that can store electrons and release them when needed. (Note: A capacitor is not a battery.) Figure 2a shows a simple capacitor consisting of two conductive plates and an insulating dielectric between them (such as a layer of paper sandwiched between two layers of aluminum foil). Figure 2b shows a parallel plate capacitor with oil-impregnated paper as the dielectric. In the fully discharged or neutral state, each plate has the same number of free electrons, so there is no potential difference between the plates. In the charged state, there is a potential difference between the plates. Figure 3 shows the circuit symbol of the capacitor.

Principles and application knowledge of capacitors
Figure 1 a) Wet electrolytic capacitor; b) Capacitor
Principles and application knowledge of capacitors
Figure 2 a) The structure of a simple plate capacitor; b) an oil-immersed capacitor
Principles and application knowledge of capacitors
Figure 3 Circuit symbol of capacitor

①The working principle of the capacitor
Figure 4 shows the charging process of the capacitor. There is a plate capacitor, a battery and a resistive load in the circuit. The initial state of the capacitor is the discharged state (there are equal amounts of free electrons on each plate). Now close the switch, as shown in Figure 4. At the moment the switch is closed, electrons flow from the plate of the capacitor connected to the positive (+) of the battery through the external circuit to the plate connected to the negative (-) of the battery. The plate connected to the positive electrode of the battery becomes “positive” (the number of electrons decreases), and the plate connected to the negative electrode of the battery becomes negative (the number of electrons increases). No electrons can pass through the dielectric layer between the plates, but an electric field is established between the positive and negative plates of the capacitor. When the switch is turned off, electrons stop flowing. Or, although the switch is still closed, but the capacitor has been charged to the power supply voltage, electrons will stop flowing. In Figure 4, the capacitor is charged from 0V to 12V. When the capacitor is disconnected from the power supply, the voltage can still be measured between its positive and negative plates. Now that the capacitor is in a charged state, the time it will remain charged after the circuit is disconnected depends on the construction and type of the capacitor. After a period of time, the plates will gradually “bleed out” the electrons through the dielectric. At this time, although the capacitor is not connected to the circuit, it is still discharged.

Principles and application knowledge of capacitors
Figure 4 The charging process of the capacitor

The amount of charge that a capacitor can store is called capacitance, and the unit of measurement is farad (F). The capacitance of most capacitors used in circuits is far less than 1 farad, and the usual capacitance range is from microfarad (μF) to picofarad (pF). The capacitor will be repeatedly charged and discharged during use. Because the capacitor can still store and hold charge after being removed from the circuit, it may cause danger if it is not handled properly.

Capacitors can be divided into two types, namely non-polarized capacitors and polarized capacitors. Non-polarized capacitors can be used in AC or DC circuits. Polarized capacitors must be consistent with the polarity of the charging source when the circuit is connected: positive (+) to positive (+), and negative (-) to negative (-). The polarity is usually marked on the capacitor case. Polarized capacitors can only be used in DC circuits.

The main factors that affect the capacitance are the area of ​​the plate, the number of plates, the distance between the plates and the dielectric constant of the dielectric. Can be expressed as
In the formula, C is the capacitance (pF); 0.225 is the constant; K is the dielectric constant of the insulating layer between the conductor plates; A is the area of ​​a plate (in²); d is the distance between the plates (in); N is The number of plates.

Principles and application knowledge of capacitors
Figure 5 Capacitor after charging

In Figure 5, a capacitor has been charged to Vc=12V and is connected to an open switch. Figure 6 shows the discharge process of the capacitor. In the picture, the power supply is removed and the switch is closed. At this time, the electrons stored in the capacitor will flow through the load to form a current. The negative plate loses electrons and the positive plate gains electrons. After the capacitor is discharged to zero volts (Vc=0V), the current stops flowing through the circuit. At this time, the potential difference between the capacitor plates disappears, and the number of free electrons in each plate is equal.

Principles and application knowledge of capacitors
Figure 6 Capacitor discharge

Do not touch the electrodes of the capacitor before it is discharged. }

example 1
Calculate the capacitance of a parallel plate capacitor, the plate area is 0.5in², and the thickness of the mica sheet between the plates is 0.001in (the dielectric constant of mica is 5.5).
The dielectric material in the capacitor has two properties. The dielectric constant in Example 1 indicates the ability of the capacitor to establish an electric field. The greater the dielectric constant, the greater the capacitance. The second dielectric property is dielectric strength. Dielectric strength is the ability of an insulating material to withstand the voltage between the plates without being short-circuited by breakdown. For example, the dielectric strength of air is about 80V/mil (Note: 1mil = 0.001in), and the dielectric strength of mica is about 1700V/mil. Some tables will list both the dielectric constant and the dielectric strength of the dielectric.

②Application of capacitor
Capacitors have been widely used in many electrical and electronic fields, including:
1) Power factor compensation. Adjust the active power and apparent power in the power distribution system.
2) The starting of some AC motors. Generate starting torque for split-phase motors.
3) Phase shift. It is used to start SCR (thyristor rectifier) ​​and TRIACS (triac thyristor switching device).
4) Power supply filter. Filter ripples in the power supply to reduce noise.
5) Timing. Control the triggering and closing time of security systems (such as alarms).