Capacitance, also known as "capacitance", refers to the amount of free charge stored under a given potential difference, denoted as C, and the international unit is farad (F). Generally speaking, electric charges move under force in an electric field. When there is a medium between conductors, the movement of electric charges is hindered and the electric charges accumulate on the conductors, resulting in accumulation and storage of electric charges. The amount of stored electric charges is called capacitance.

 

 

Capacitance refers to the ability to hold charge. Any electrostatic field is composed of many capacitors. When there is an electrostatic field, there is a capacitance. The capacitance is described by an electrostatic field. It is generally believed that an isolated conductor forms a capacitance with infinity, and the conductor grounding is equivalent to being connected to infinity and connected to the earth as a whole.

Capacitance (or capacitance) is a physical quantity that expresses the ability of a capacitor to hold charge. In terms of physics, a capacitor is a static charge storage medium, and the charge may exist forever. This is its characteristic, and it has a wide range of uses. It is an indispensable electronic component in the electronics and power fields. Mainly used in power supply filtering, signal filtering, signal coupling, resonance, filtering, compensation, charging and discharging, energy storage, DC blocking and other circuits.

The role of capacitance:

1) Bypass: The bypass capacitor is an energy storage device that provides energy for local devices. It can make the output of the voltage stabilizer uniform and reduce the load demand. Just like a small rechargeable battery, the bypass capacitor can be charged and discharged to the device. In order to minimize the impedance, the bypass capacitor should be as close as possible to the power supply pin and ground pin of the load device. This can well prevent the ground potential rise and noise caused by the input value being too large. The ground potential is the voltage drop at the ground connection when a large current glitch passes through it.

2) Decoupling: Decoupling, also known as decoupling. From the circuit point of view, it can always be divided into the driving source and the driven load. If the load capacitance is relatively large, the drive circuit must charge and discharge the capacitance to complete the signal jump. When the rising edge is relatively steep, the current is relatively large, so that the drive current will absorb a large power supply current. The inductance and resistance (especially the inductance on the chip pins) will rebound. This current is actually a kind of noise compared to normal conditions, which will affect the normal operation of the previous stage. This is the so-called "coupling".

3) Filtering: In theory (that is, assuming that the capacitor is a pure capacitor), the larger the capacitor, the smaller the impedance, and the higher the passing frequency. But in fact, most of the capacitors over 1μF are electrolytic capacitors, which have a large inductance component, so the impedance will increase when the frequency is high. Sometimes you will see a large electrolytic capacitor with a small capacitor in parallel. At this time, the large capacitor filters low frequencies and the small capacitor filters high frequencies. The function of the capacitor is to pass the AC to block the DC, and pass the high frequency to block the low frequency. The larger the capacitance, the easier it is for high frequencies to pass. Specifically used in filtering, a large capacitor (1000μF) filters low frequencies, and a small capacitor (20pF) filters high frequencies. Some netizens have vividly compared the filter capacitor to a "pond." Since the voltage at both ends of the capacitor does not change suddenly, it can be seen that the higher the signal frequency, the greater the attenuation. It can be said that the capacitor is like a pond and will not change the amount of water due to the addition or evaporation of a few drops of water. It converts changes in voltage into changes in current. The higher the frequency, the greater the peak current, thus buffering the voltage. Filtering is the process of charging and discharging.

4) Energy storage: The energy storage capacitor collects the charge through the rectifier and transfers the stored energy to the output terminal of the power supply through the lead of the converter. Aluminum electrolytic capacitors with a voltage rating of 40-450VDC and a capacitance value of 220-150 000μF are more commonly used. According to different power requirements, the devices sometimes adopt the form of series, parallel, or a combination thereof. For power supplies with a power exceeding 10KW, a larger tank-shaped screw terminal capacitor is usually used.

 

Use a digital multimeter to detect capacitors, you can do as follows.

1. Direct detection with capacitance file

Some digital multimeters have the function of measuring capacitance, and their ranges are divided into five ranges: 2000p, 20n, 200n, 2μ and 20μ. During measurement, the two pins of the discharged capacitor can be directly inserted into the Cx jack on the meter board, and the display data can be read after selecting the appropriate range.

2. Detect with resistance file

Practice has proved that the charging process of the capacitor can also be observed with a digital multimeter, which is actually a discrete digital quantity that reflects the change of the charging voltage. Assuming that the measurement rate of the digital multimeter is n times/second, in the process of observing the charging of the capacitor, n independent and successively increasing readings can be seen every second. According to this display feature of the digital multimeter, the quality of the capacitor can be detected and the size of the capacitance can be estimated. The following describes the method of using the resistance file of a digital multimeter to detect capacitors, which is of practical value for meters without a capacitance file. This method is suitable for measuring large-capacity capacitors from 0.1μF to several thousand microfarads.

Three, use the voltage file to detect

Using a digital multimeter to detect capacitors with DC voltage is actually an indirect measurement method. This method can measure small-capacity capacitors from 220pF to 1μF, and can accurately measure the size of the capacitor's leakage current.