Can't you check the quality of capacitors without special equipment?

In the absence of special instrumentation, the quality and quality of the capacitor can be measured with the multimeter's resistance profile and judged. Large capacity (1μF or more) of the fixed capacitor can be used to measure the resistance of the multimeter (R × 1000) capacitor electrodes, the hands should be to the direction of the resistance of the small swing, and then slowly swing back to near ∞. Then exchange the test bar and try again to see the swing of the hands. The larger the swing, the greater the capacitance of the capacitor. If the test stick always touches the capacitor lead, the hands should be in the vicinity of ∞. Otherwise, it indicates that the capacitor has a leakage phenomenon. The smaller the resistance, the greater the leakage, the poorer the quality of the capacitor; if the watch is not moving at all It indicates that the capacitor has failed or broken; if the hands are oscillating but cannot return to the starting point, it indicates that the capacitor leakage is large and its quality is not good.

Pressure gauges For capacitors with smaller capacitances, the measurement is often not possible with a multimeter. At this time, it is possible to measure with an external DC voltage and the DC voltage range of the multimeter. The method is shown in Fig. 1, that is, the multimeter is adjusted. To the corresponding DC voltage range, the negative (black) test bar is connected to the negative pole of the DC power supply, the positive (red) test bar is connected to one end of the tested capacitor, and the other end is connected to the positive pole of the power supply.

When a capacitor with good performance is connected to the power supply, the hands of the multimeter must have a large swing; the larger the capacity of the capacitor, the larger the swing of the hands of the watch. After the swing, the hands can gradually return to zero. If the capacitor is turned on at the instant of power-on, the pointer of the multimeter will not swing, which means that the capacitor is in failure or open circuit; if the hands indicate the power supply voltage without swinging, it indicates that the capacitor has been punctured and short-circuited; if the hands are swinging normally, they will not return to zero position. This shows that the capacitor has a leakage phenomenon, and the higher the indicated voltage value, the greater the leakage current. It should be pointed out that the auxiliary DC voltage used by the capacitor with small measurement capacity cannot exceed the withstand voltage of the tested capacitor, so as to avoid damage to the capacitor due to measurement. To accurately measure the capacitance of a capacitor, a capacitive bridge or Q-meter must be used. The above simple detection method can only roughly judge the quality of the pressure gauge capacitor.

Method one, pointer multimeter measurement

1, check the quality of the electrolytic capacitor with a multimeter resistance file

The two leads of the electrolytic capacitor have positive and negative points. When checking the quality of the electrolytic capacitor (6V or l0V), the resistance file should be placed in the R×100 or R×1K file. The red marker is connected to the negative terminal of the capacitor, and the black marker to the positive terminal. At this time, the pointer of the multimeter will swing and then return to the zero or zero position. Such an electrolytic capacitor is good. The larger the capacity of the electrolytic capacitor, the longer the charging time, and the slower the pointer swings.

2, using a multimeter to determine the positive and negative leads of the electrolytic capacitor

Some electrolytic capacitors with lower withstand voltages can be judged according to the characteristics of leakage current at the time of positive connection and negative lead when the positive and negative leads are unclear. The specific method is: contact the two leads of the capacitor with the red and black meter pens, and remember the magnitude of the leakage current (resistance value) (the indicated resistance when the pointer swings back and stops), and then short the positive and negative leads of the capacitor. After that, turn the red and black pens back and measure the leakage current. Judging from the indication of a small leakage current as a criterion, the lead that comes into contact with the black meter pen is the positive terminal of the electrolytic capacitor. This method is more difficult to distinguish the polarity of an electrolytic capacitor whose leakage current is small.

3, check the variable capacitor with a multimeter

Variable capacitors have a set of stators and a set of rotors. Use a multimeter resistor file to check the movement of the multimeter and whether there is any contact between the stators. Use the red and black meter pens to move the plate and the stator respectively. Rotate the shaft handle and the pointer of the meter does not move. It indicates that there is no short circuit between the movable and fixed plates. If the pointer swings, it indicates where the capacitor has a short circuit.

4, using a multimeter resistor file to roughly identify the capacity of more than 5000PF capacitance

With a multimeter resistor file can roughly identify the quality of capacitors above 5000PF (5000PF below can only determine whether the capacitor is broken down). During the inspection, the resistance range is placed on the high end of the measuring range. The two pens are in contact with the two ends of the capacitor respectively. Then, the pointer rapidly swings and then recovers. In the reverse connection, the amplitude of the swing is greater than that of the first time, and then it is restored. Such a capacitor is good. The greater the capacity of the capacitor, the greater the swing of the meter pointer during the measurement, the longer the pointer recovery time, we can compare the size of the two capacitors based on the size of the meter pointer swing.

Method two, capacitor testing methods and experience

1 Fixed capacitor detection

A detection of small capacitors below 10pF

Because the capacity of the fixed capacitor below 10PF is too small, use the multimeter to carry on the measurement, can only qualitatively inspect whether there is leakage, internal short circuit or breakdown phenomenon. When measuring, use the multimeter R×10k block, and connect the two pins of the capacitor to the two pins of the capacitor. The resistance value should be infinite. If the resistance is measured (the pointer swings to the right) is zero, it indicates that the capacitor is damaged by leakage or internal breakdown.

B Check whether the 10PF to 0 01μF fixed capacitor has a charging phenomenon and judge whether it is good or bad. The multimeter uses R×1k block. Both triodes have a beta value of more than 100 and a small penetration current. Can use 3DG6 and other types of silicon triodes to form a composite pipe. The red and black pens 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 capacitance is amplified, so that the amplitude of the pointer of the multimeter is increased, thereby facilitating observation. It should be noted that when the test is performed, especially when measuring a small-capacity capacitor, it is necessary to repeatedly exchange the two points of the measured capacitor pin to contact A and B so as to clearly see the swing of the multimeter pointer.

C For a fixed capacitance of 0 01μF or more, the multimeter's R×10k block can be used to directly test whether the capacitor is charged and whether there is an internal short circuit or leakage, and the capacity of the capacitor can be estimated based on the amplitude of the pointer swinging to the right.

2 Electrolytic capacitor detection

A Because the capacitance of the electrolytic capacitor is much larger than the fixed capacitance, the appropriate range should be chosen for different capacities when measuring. According to experience, under normal circumstances, the capacitance between 1 ~ 47μF can be measured with R × 1k block, and the capacitance greater than 47μF can be measured with R × 100 block.

B Connect the red meter of the multimeter to the negative pole and the black lead to the positive pole. At the instant of the moment of contact, the pointer of the multimeter will be deflected to the right by a greater degree of deflection (for the same electric resistance, the larger the capacity, the greater the swing amplitude), and then gradually turn to the left Until stopped in a certain position. The resistance at this time is the positive 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 more than several hundred kΩ, otherwise, it will not work properly. In the test, if there is no charging in the forward and reverse directions, ie, the hands are not moving, the capacity disappears or the internal disconnection occurs. If the measured resistance is small or zero, the capacitor has a large leakage current or has been damaged by breakdown. Can not be used again. C For electrolytic capacitors with unknown positive and negative electrode marks, the above-mentioned method of measuring the leakage resistance can be used to determine. That is, first measure the leakage resistance, remember its size, and then exchange the test leads to measure a resistance. In the two measurements, the one with the largest resistance is the positive connection. That is, the black pen is connected to the positive electrode, and the red pen is connected to the negative electrode. D. Use a multimeter to block electricity. Use the method of charging the electrolytic capacitor forward and backward. According to the magnitude of the pointer's swing to the right, the capacity of the electrolytic capacitor can be estimated.

3 Variable capacitor detection

A Rotate the shaft gently with your hand, it should feel very smooth, and you should not feel tight or even stuck during loosening. When the axial direction of the loading shaft is pushed forward, rearward, upward, downward, leftward and rightward, the shaft should not be loosened. B Rotate the shaft with one hand. The other hand gently touches the outer edge of the rotor pack and should not feel any loosening. The variable capacitor with poor contact between the shaft and the rotor cannot be used anymore. C Put the multimeter into R×10k block, one hand connects the two table pens to the leading end of the moving piece of the variable capacitor and the stator, and the other hand slowly rotates the shaft several times. The pointer of the multimeter should be Infinite position does not move. In the process of rotating the shaft, if the pointer sometimes points to zero, it means that there is a short circuit point between the rotor and the stator; if you touch a certain angle, the meter reading is not infinite but there is a certain resistance, indicating that the variable capacitor is moving There is leakage between the chip and the stator.

1, capacitance in the circuit is generally used "C" plus digital representation (such as C13 represents the number 13 capacitor). The capacitor is composed of two pieces of metal film and the middle is separated by insulating material. The characteristics of the capacitor are mainly DC-to-DC communication.

The size of the capacitance is the size of the energy that can be stored, and the impedance of the capacitor to the AC signal is called capacitive reactance. It is related to the frequency and capacitance of the AC signal.

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

2. Identification method: The identification method of the capacitor and the identification method of the resistor are basically the same, and there are 3 types of methods: the straight label method, the color label method, and the number standard method. The basic unit of capacitance is F (F), and there are other units: millifarad (mF), microfarad (uF), nanofarad (nF), and picofarad (pF). Among them: 1 Farah = 103 millifa = 106 microfa = 109 nanofa = 1012 picofarads

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

The capacity of a small-capacity capacitor is represented by a letter or a numeral on the capacitor.

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

Digital representation: Generally, three digits are used to indicate the size of the capacity, the first two digits are valid digits, and the third digit is the multiplier.

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%

Such as: a ceramic capacitor 104J that capacity is 0. 1 uF, the error is ± 5%