Capacitor detection method and experience

1, the detection of fixed capacitors

A detects small capacitors below 10pF

Since the fixed capacitor capacity below 10 PF is too small, the measurement with a multimeter can only be qualitatively checked for leakage, internal short circuit or breakdown. When measuring, you can use the multimeter R × 10k block, use two meter pens to connect the two pins of the capacitor, the resistance should be infinity. If the measured resistance value (the pointer swings to the right) is zero, the capacitor leakage is damaged or internal breakdown.

B Check if the 10PF~0 01μF fixed capacitor has a charging phenomenon, and then judge whether it is good or bad. The multimeter uses R × 1k block. The beta values ​​of the two triodes are all above 100 and the penetration current is small. A composite tube of 3DG6 and other types of silicon triodes can be selected. The red and black test leads of the multimeter are respectively connected to the emitter e and the collector c of the composite tube. Due to the amplification effect of the composite triode, the charging and discharging process of the measured capacitor is amplified, so that the multimeter pointer swing amplitude is increased, thereby facilitating observation. It should be noted that when testing *, especially when measuring a small capacity capacitor, it is necessary to repeatedly exchange the measured capacitor pins to contact A and B points to clearly see the swing of the multimeter pointer.

C For a fixed capacitance of 0 01μF or more, use the R × 10k block of the multimeter to directly test whether the capacitor has a charging process and whether there is an internal short circuit or leakage, and estimate the capacity of the capacitor according to the amplitude of the pointer swinging to the right.

2. Detection of electrolytic capacitors

A Because the capacity of electrolytic capacitors is much larger than that of general fixed capacitors, the appropriate range should be selected for different capacities when measuring. According to experience, in general, the capacitance between 1 ~ 47μF can be measured by R × 1k block, and the capacitance larger than 47μF can be measured by R × 100 block.

B Connect the multimeter red pen to the negative pole and the black pen to the positive pole. At the moment of contact, the multimeter pointer deflects to the right with a large degree of skewness (for the same electrical block, the larger the capacity, the larger the swing), then gradually turns to the left. Until stopped at a certain location. The resistance at this time is the forward leakage resistance of the electrolytic capacitor, which is slightly larger than the reverse leakage resistance. Practical experience shows that the leakage resistance of electrolytic capacitors should generally be several hundred kΩ or more, otherwise, it will not work properly. In the test, if there is no charging in the forward and reverse directions, that is, the hands are not moving, the capacity disappears or the internal circuit is broken; if the measured resistance is small or zero, the leakage of the capacitor is large or the breakdown is broken. Can no longer be used.

C For electrolytic capacitors whose positive and negative signs are unknown, the above method for measuring the leakage resistance can be used for discrimination. That is, first measure the leakage resistance arbitrarily, remember its size, and then exchange the test leads to measure a resistance value. The one with a large resistance value in the two measurements is the forward connection method, that is, the black meter is connected to the positive pole, and the red meter is connected to the negative pole. D Using a multimeter electric block, using the method of positive and negative charging of the electrolytic capacitor, the capacity of the electrolytic capacitor can be estimated according to the magnitude of the swing of the pointer to the right.

3. Detection of variable capacitors

A Slightly rotate the shaft by hand, it should feel very smooth, and should not feel sometimes loose or even stuck. When the axial direction of the load is pushed forward, backward, up, down, left, right, etc., the shaft should not be loose.

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

C Place the multimeter in the R×10k block, one hand connects the two test leads to the movable piece of the variable capacitor and the leading end of the fixed piece, and the other hand will slowly rotate the rotating shaft several times back and forth, and the multimeter pointer should be Do not move at infinity. In the process of rotating the shaft, if the pointer sometimes points to zero, it means there is a short circuit between the moving piece and the fixed piece; if it touches an angle, the multimeter reading is not infinite but a certain resistance value appears, indicating that the variable capacitor is moving. There is leakage between the film and the stator.

1) Capacitance is generally indicated by a "C" plus a number in the circuit (for example, C13 represents a capacitor numbered 13). The capacitor is an element consisting of two metal films abutting and separated by an insulating material. The characteristics of the capacitor are mainly DC-blocking AC.

The size of the capacitor capacity is the size of the energy that can be stored. The blocking effect of the capacitor on the AC signal is called capacitive reactance, which is related to the frequency and capacitance of the AC signal.

Capacitive reactance XC=1/2Ï€f c (f indicates the frequency of the AC signal, C indicates the capacitance). The types of capacitors commonly used in telephones are electrolytic capacitors, ceramic capacitors, chip capacitors, monolithic capacitors, tantalum capacitors, and polyester capacitors. .

2) Identification method: The identification method of the capacitance is basically the same as the identification method of the resistance, and there are three types: the direct label method, the color scale method and the number standard method. The basic unit of the capacitance is pulled (F), and other units are: millifarad (mF), microfarad (uF), nanofarad (nF), and picofarad (pF). Where: 1 Farad = 103 millifarads = 106 microfarads = 109 nanofarads = 1012 picofarads

The capacity of a large capacity capacitor is directly indicated on the capacitor, such as 10 uF/16V.

Capacitance values ​​of small capacitances are represented by letters or numbers on the capacitors.

Letter notation: 1m=1000 uF 1P2=1.2PF 1n=1000PF

Digital notation: Generally speaking, the three-digit number indicates the capacity. The first two digits represent the significant digits and the third digit is the magnification.

For example: 102 means 10×102PF=1000PF 224 means 22×104PF=0.22 uF

3) Capacitance capacity error table

Symbol FGJKLM

Allowable error ±1% ±2% ±5% ±10% ±15% ±20%

For example, a ceramic capacitor has a capacitance of 104 J, and the capacity is 0.1 μF, and the error is ±5%.