Identification, classification and measurement of capacitors

1. Unit: Farad (F)

1F=10 ³ mF=10 ⁶ uF=10 ⁹ nF=10 ¹² pF

Symbol : " C , TC, MC, EC"


Domestic symbol International symbol

The white one is the negative pole. If there is no positive or negative pole mark, it is a non-polar capacitor

2. Types of capacitors:

There are two main types according to the structure: one is fixed capacitor and the other is variable capacitor. According to the dielectric, there are mainly organic dielectric capacitors, inorganic dielectric capacitors, electrolytic capacitors, etc. According to the material, there are ceramic capacitors, mica capacitors for high frequencies; polyester capacitors, used for medium and low frequencies; metal film capacitors, used for low frequencies; electrolytic capacitors are fixed capacitors, generally larger in size, used in low-frequency filter circuits, they have positive and negative poles and cannot be connected in reverse when used, otherwise leakage or explosion will occur.

1. SMD capacitors :

Symbol : SMD capacitors "CB, BC, CM, MC, CD"; row capacitors "CN, CP"


SMD capacitors are divided into single SMD capacitors and row capacitors

Single SMD capacitor row capacitor 2. Electrolytic capacitors : Symbol : SMD capacitors "C, TC, CT, BC, EC, CE" Polarized capacitors: The "long" pin is the negative pole, and the "short" pin is the positive pole. The pin with the colored band on the capacitor is the negative pole. 3. Non-polar capacitors : III. Basic parameters of capacitors: 1. Withstand voltage and capacity Withstand voltage : The highest voltage that a capacitor can withstand when it works continuously in a circuit. Capacitance : The ability of a capacitor to store charge is called capacity. The larger the capacity, the more charge it can store, and vice versa. Example: A: Capacitor mark: 25V, 1300uF, indicating a withstand voltage of 25V and a capacity of 1300uf B : Capacitor mark: 16V, 2200uF, indicating a withstand voltage of 16V and a capacity of 2200uF C: Non-polar capacitor mark: 100, indicating a capacity of 100pF D: Non-polar capacitor mark: 0.01, indicating a capacity of 0.01uF 2. Capacitive reactance: The special blocking effect of capacitors on alternating current is capacitive reactance. The frequency is inversely proportional to the capacitive reactance. The higher the frequency, the smaller the capacitive reactance. Therefore, capacitors have the characteristics of passing high frequencies and blocking low frequencies. When the frequency is constant, the capacity is inversely proportional to the capacitive reactance. The larger the capacity, the smaller the capacitive reactance, and the smaller the capacity, the larger the capacitive reactance. When the frequency is 0, the capacitive reactance of the DC capacitor is infinite.

4. Capacitor nominal method:

The first method of capacitor nominal is Direct marking method : If the nominal value is an integer and has no unit, it is read as "pF"; if the nominal value is a decimal and has no unit, it is read as "uF"; if the nominal value is a three-digit number and has no unit, the first and second digits are the effective digits "AB", and the third digit is the multiple "10 ^C "; imported capacitors have "47uFD", which is "47uF"; the capacitor nominal value is "3R3", "R" is the decimal point, indicating "3.3pF"; the nominal value is "0.47k, 2.2J", indicating "0.47uF, 2.2uF", and "k, J" are error values; the second method is Color marking method , which is the same as the color marking method of resistors. The third method Special nominal value : "109J, 219k, 379k", etc., with "*10 ^-1 " with 9.

5. Characteristics of capacitors :


pass high frequencies and block low frequencies; pass AC and block DC (refer to capacitive reactance)

6. Functions of capacitors:

filtering, coupling, energy storage

1. Filter capacitor:

connected in parallel between the positive and negative poles of the circuit, using the characteristics of capacitors to pass AC and isolate DC, to filter out the AC current in the circuit. Polarized capacitors usually have the negative pole grounded.

2. Coupling capacitor:

connected between the signal source and the signal processing circuit or the two-pole amplifier to isolate DC power, allowing AC or pulsating signals to pass through, so that the DC operating points of adjacent amplifiers do not affect each other.

3. Decoupling capacitor:

connected in parallel between the positive and negative poles of the circuit to prevent parasitic oscillation caused by the positive feedback path formed by the power supply.

4. Bypass capacitor:

connected in parallel between the two ends of the resistor to set up an energy path for the AC in the AC and DC signals to avoid the voltage drop of the AC component when passing through the resistor.

5. Bootstrap capacitor:

use the energy storage to increase the potential of a certain point in the circuit, so that its potential value is higher than the power supply voltage that supplies power to that point.

6. Frequency stabilization capacitor:

used to stabilize the oscillation frequency in the oscillation circuit.

7. Timing capacitor:

connected in series with resistor R in the RC timing circuit to jointly determine the length of time.

8. Soft start capacitor:

usually connected to the base of the power switch tube to prevent the switch tube from being damaged by the surge current or voltage added to the base of the switch tube when the power is turned on.


VII. Measurement of capacitors and judgment of quality

1. Capacitor measurement

Set the multimeter to the buzzer diode mode, put the test leads on the two pins, and you should see that the value is increasing continuously. When it reaches infinity, reverse the two test leads, and the value should change rapidly from negative to infinity. This process is the charging and discharging process of the capacitor.

2. Good or bad judgment

Electrolytic capacitors After damage, the appearance shows bulging, leakage, deformation, etc. When measured with a multimeter, there is no discharge process or the discharge process is very short, and the jump change is relatively slow or even cannot jump to infinity, which indicates that the capacitor is leaking or has poor performance; if the multimeter reading is always zero, it means that the capacitor is short-circuited. For SMD capacitors , it is difficult to judge the quality by measuring on the motherboard. You can only take it off and measure it. When measuring, the two capacitors should be infinite. The color of the chip capacitor with leakage will be slightly darker than the surrounding capacitors; a bad capacitor will cause the computer to enter the system blue screen, freeze, and freeze when running a large program. Leakage will cause the computer to restart . VI. Principles of capacitor replacement 1. The positive and negative poles cannot be connected in reverse. 2. The withstand voltage value must be greater than or equal to the original value. 3. The capacity can differ by +/-20% from the original value. 4. Chip capacitors can be replaced as long as the color and size are the same. 5. The frequency stabilizing capacitors on the two pins of the crystal oscillator must be replaced with the original value (original position) (you can find a capacitor in the same position on another motherboard to replace it).