Explanation of professional testing organization: how to test the capacitance of electronic components

Date：2021-04-29 15:09:00Views：1333

Capacitance (or capacitance) refers to the charge storage under a given potential difference; Marked as C, the international unit is farad (f). Generally speaking, the charge will move under force in the electric field. When there is a medium between the conductors, it hinders the charge movement and makes the charge accumulate on the conductor; The most common example of cumulative charge storage is two parallel metal plates. It is also commonly known as capacitor.

Capacitor identification

The capacity of two arbitrarily shaped conductors separated by a medium to hold charge under a certain voltage is called the capacitance between the two conductors. The symbol of capacitor in the circuit is（）, the letter symbol is C, and the unit is method (f). In addition, there are millimethod (MF), micro method (UF), nanomethod (NF), skin method (PF), 1F = 103mf = 106uf = 109nf = 1012pf. Capacitor is an important component in electrical equipment, which has a very important application in electronic technology and electrical technology. The capacitor can contain charge, making the capacitor charged is called charging, there is an electric field between the two poles of the charged capacitor, the loss of charge of the charged capacitor is called discharge, and there is no charge between the two poles of the discharged capacitor. The capacitors commonly used in the workshop are chip ceramic capacitors and tantalum electrolytic capacitors. The capacitance value of chip ceramic capacitors is marked on the surface of the capacitor, which is the same as that of chip resistors, and there is no difference between positive and negative poles. Reading the capacitance value of capacitors is the same as that of resistors. Give a few examples. For example: 330 = 33x100 = 33pf, 104 = 10x104 = 100000pf = 100nF = 0.1uF.

Function of capacitor

The function of capacitor is to isolate direct AC, which is also its characteristic. There are bypass filtering, signal coupling, etc. Therefore, it is applied to AC coupling, DC isolation, filtering, AC or pulse bypass, RC timing, LC resonance frequency selection, power decoupling, bootstrap, compensation and other circuits.

Detection method of capacitor quality

1. Detection of fixed capacitor

A. Detect small capacitance below 10PF

Because the capacity of fixed capacitors below 10PF is too small, it can only be qualitatively checked for leakage, internal short circuit or breakdown when measured with a multimeter. Multimeter R can be selected for measurement × 10K gear, use two probes to connect the two pins of the capacitor arbitrarily, and the resistance value shall be infinite. If the measured resistance value (the pointer swings to the right) is zero, it indicates that the capacitor is damaged by leakage or internal breakdown.

B. Test 10PF ～ 0.01 μ F check whether the fixed capacitor is charged, and then judge whether it is good or bad. R is selected for multimeter × 1K gear. Two triodes β The values are above 100, and the penetration current should be small. 3DG6 and other models of silicon triode can be selected to form a composite tube. The red and black probes of the multimeter are connected with the emitter E and collector C of the composite tube respectively. Due to the amplification of the composite triode, the charging and discharging process of the measured capacitor is amplified, so that the swing range of the multimeter pointer is increased, which is convenient for observation. It should be noted that during the test operation, especially when measuring the capacitance of small capacity, it is necessary to repeatedly exchange the measured capacitance pin to contact two points a and B, so as to clearly see the swing of the multimeter pointer.

C. For 0.01 μ For fixed capacitance above F, R of multimeter can be used × 10K gear directly tests whether the capacitor has charging process and internal short circuit or leakage, and the capacity of the capacitor can be estimated according to the amplitude of the pointer swinging to the right.

2. Detection of electrolytic capacitor

A. Because the capacity of electrolytic capacitor is much larger than that of general fixed capacitor, appropriate measuring range shall be selected for different capacities during measurement. According to experience, generally, 1 ~ 47 μ Capacitance between F, R can be used × 1K gear measurement, greater than 47 μ The capacitance of F can be used as R × 100 gear measurement.

B. Connect the red lead of the multimeter to the negative electrode and the black lead to the positive electrode. At the moment of contact, the multimeter pointer deflects to the right by a large amplitude (for the same electrical barrier, the greater the capacity, the greater the swing), and then gradually turn to the left until it stops at a certain position. The resistance at this time is the forward leakage resistance of the electrolytic capacitor, which is slightly greater than the reverse leakage resistance. The practical application experience shows that the leakage resistance of electrolytic capacitor should generally be more than several hundred K Ω, otherwise it will not work normally. In the test, if there is no charging phenomenon in the forward and reverse directions, that is, the gauge needle does not move, it indicates that the capacity disappears or the internal circuit is open; If the measured resistance value is very small or zero, it indicates that the capacitor has large leakage or has been broken down and damaged and can not be used again.

C. For electrolytic capacitors with unclear positive and negative marks, the above method of measuring leakage resistance can be used to distinguish. That is, measure the leakage resistance arbitrarily, remember its size, and then exchange the probes to measure a resistance value. The one with the largest resistance in the two measurements is the positive connection method, that is, the black probe is connected to the positive electrode and the red probe is connected to the negative electrode.

D. The capacity of the electrolytic capacitor can be estimated and measured by using the resistance block of the multimeter and the method of charging the electrolytic capacitor in the forward and reverse directions. According to the swing range of the pointer to the right.

3. Detection of variable capacitor

A. Rotate the rotating shaft gently by hand. It should feel very smooth. It should not feel sometimes loose, sometimes tight, or even stuck. When the rotating shaft is pushed forward, backward, up, down, left, right and other directions, the rotating shaft shall not be loose.

B. Rotate the rotating shaft with one hand and gently touch the outer edge of the moving plate group with the other hand. There should be no looseness. The variable capacitor with poor contact between the rotating shaft and the moving plate can no longer be used.

C. Place the multimeter in R × At 10K gear, connect the two probes to the leading out ends of the moving piece and the fixed piece of the variable capacitor with one hand, and slowly rotate the rotating shaft several times with the other hand. The pointer of the multimeter shall not move at infinity. In the process of rotating the rotating shaft, if the pointer sometimes points to zero, there is a short-circuit point between the moving plate and the fixed plate; If a certain angle is encountered, the reading of the multimeter is not infinite, but a certain resistance value appears, indicating that there is leakage between the moving piece and the fixed piece of the variable capacitor.