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This article teaches you how to use the resistance file of a multimeter to detect the quality and quality of capacitors without special instruments, and make judgments:
a. A fixed capacitor with a large capacity (above 1μF) can be used to measure the two electrodes of the capacitor with the resistance file (R&TImes;1000) of a multimeter. The needle should swing in the direction of small resistance, and then slowly swing back to near ∞. Then exchange the test rod and try again. Look at the swing of the hands. The larger the swing, the greater the capacitance of the capacitor.
If the test rod keeps touching the capacitor lead, the needle should be near ∞. Otherwise, it indicates that the capacitor has leakage. The smaller the resistance value, the greater the leakage and the poor quality of the capacitor; if the needle does not move at all during measurement , Which indicates that the capacitor has failed or is open; if the needle swings but cannot return to the starting point, it indicates that the capacitor has a large leakage and its quality is not good.
b. Pressure gauge For capacitors with small capacity, when measuring with a multimeter, it is often not seen that the needle swings. At this time, you can use an external DC voltage and the DC voltage file of the multimeter to measure, that is, adjust the multimeter to the corresponding DC voltage file. , The negative (black) test rod is connected to the negative pole of the DC power supply, the positive (red) test rod is connected to one end of the capacitor under test, and the other end is connected to the positive pole of the power supply.
When a capacitor with good performance is switched on, the needle of the multimeter should have a larger swing; the greater the capacity of the capacitor, the greater the swing of the needle. After swing, the needle can gradually return to the zero position. If the pointer of the multimeter does not swing at the moment the power is turned on, the capacitor is invalid or open; if the needle keeps indicating the power supply voltage without swinging, it indicates that the capacitor has been short-circuited; if the needle swings normally, it does not return to zero , Indicating that the capacitor has leakage phenomenon, the higher the indicated voltage value, the greater the leakage capacity. It should be pointed out that the auxiliary DC voltage used to measure the capacitor with small capacity cannot exceed the withstand voltage of the capacitor under test, so as to avoid the capacitor breakdown and damage due to the measurement. To accurately measure the capacitance of a capacitor, a capacitance bridge or Q meter is required. The above simple detection method can only roughly judge the quality of the pressure gauge capacitor.
Method 1: Measurement with an analog multimeter.
1. Use the resistance file of a multimeter to check the quality of the electrolytic capacitor
The two leads of an electrolytic capacitor are divided into positive and negative. When checking its quality, for electrolytic capacitors with lower withstand voltage (6V or l0V), the resistance should be placed in R&TImes;100 or R&TImes;1K. Connect the red test lead to the negative end of the capacitor and the black test lead to the positive end. At this time, the pointer of the multimeter will swing and then return to or near the 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. Use a multimeter to judge the positive and negative leads of the electrolytic capacitor
For some electrolytic capacitors with lower voltage resistance, if the positive and negative leads are not clearly marked, it can be judged based on the characteristics of low leakage current (large resistance value) during positive connection and large leakage current during reverse connection. The specific method is: touch the two leads of the capacitor with the red and black test pens, remember the leakage current (resistance value) (the resistance value indicated when the pointer swings back and stop), and then short the positive and negative leads of the capacitor Connect it again and measure the leakage current after swapping the red and black test leads. Judgment is based on the small leakage current display value. The lead in contact with the black test lead is the positive terminal of the electrolytic capacitor. This method is difficult to distinguish the polarity of electrolytic capacitors with low leakage current.
3. Check the variable capacitor with a multimeter
The variable capacitor has a set of fixed plates and a set of moving plates. Use the resistance of the multimeter to check whether there is any contact between the moving and fixed pieces. Use red and black test pens to connect the moving piece and the fixed piece, rotating the shaft handle, and the meter pointer does not move, indicating that there is no short circuit between the moving and fixed pieces ( If the pointer swings, it means there is a short circuit in the capacitor.
4. Use the resistance file of a multimeter to roughly identify the quality of capacitors above 5000PF
The resistance file of the multimeter can roughly distinguish the quality of capacitors above 5000PF (the ones below 5000PF can only judge whether the capacitor is broken). When checking, put the resistance range at the high range value, and the two test leads are in contact with the two ends of the capacitor respectively. At this time, the pointer swings quickly and then restores, and the reverse connection, the swing amplitude is greater than the first time, and then restores. Such a capacitor is good. The larger the capacity of the capacitor, the greater the movement of the meter pointer during measurement, and the longer it will take for the pointer to recover. We can compare the capacities of the two capacitors according to the magnitude of the movement of the meter pointer.
Capacitor testing methods and experience
1. Detection of fixed capacitors
A. Detect small capacitance below 10pF
Because the capacity of the fixed capacitor below 10PF is too small, it can only be qualitatively checked for leakage, internal short circuit or breakdown if it is measured with a multimeter. When measuring, you can choose a multimeter R&TImes; 10k block, and use two meter pens to connect the two pins of the capacitor randomly, and the resistance should be infinite. If the measured resistance value (the pointer swings to the right) is zero, it means that the capacitor is damaged by leakage or internal breakdown.
B. Detect whether the fixed capacitor of 10PF～0 01μF is charged, and then judge whether it is good or bad. The multimeter selects R×1k gear. The β value of the two transistors is above 100, and the penetration current is small. 3DG6 and other types of silicon transistors can be used to form a composite tube. The red and black test leads of the multimeter are respectively connected to the emitter e and collector c of the composite tube. Due to the amplification effect of the composite triode, the charge and discharge process of the capacitor under test is amplified, and the amplitude of the pointer of the multimeter is enlarged, which is convenient for observation. It should be noted that when testing, especially when testing smaller capacity capacitors, you have to repeatedly switch the contacts A and B of the tested capacitor pin to clearly see the movement of the multimeter pointer.
C. For fixed capacitance above 0.01μF, the R×10k block of a multimeter can be used to directly test whether the capacitor has charging process and whether there is internal short circuit or leakage, and the capacity of the capacitor can be estimated according to the amplitude of the pointer swing to the right.
2. Detection of electrolytic capacitors
A. Because the capacity of electrolytic capacitors is much larger than that of general fixed capacitors, suitable ranges should be selected for different capacities when measuring. According to experience, in general, capacitance between 1 and 47μF can be measured with R×1k block, and capacitance larger than 47μF can be measured with R×100 block.
B. Connect the red test lead of the multimeter to the negative electrode and the black test lead to the positive electrode. At the moment of contact, the pointer of the multimeter will deflect to the right to a greater degree (for the same electrical barrier, the greater the capacity, the greater the swing), and then gradually turn to the left Turn around until it stops at a certain position. 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 more than several hundred kΩ, otherwise, it will not work normally. In the test, if there is no charging phenomenon in the forward and reverse directions, that is, the hand does not move, it means that the capacity has disappeared or the internal circuit is broken; if the measured resistance value is very small or zero, it means that the capacitor has a large leakage or has been broken down. Can no longer be used.
C. For electrolytic capacitors with unknown positive and negative signs, the above method of measuring leakage resistance can be used to distinguish. That is, first measure the leakage resistance randomly, remember its size, and then exchange the test leads to measure a resistance value. The larger resistance of the two measurements is the positive connection, that is, the black test lead is connected to the positive electrode, and the red test lead is connected to the negative electrode.
D. Use a multimeter to block electricity, and use the method of forward and reverse charging the electrolytic capacitor. According to the magnitude of the pointer swing to the right, the capacity of the electrolytic capacitor can be estimated.
3. Detection of variable capacitors
A. Rotate the shaft gently with your hands, it should feel very smooth, and should not feel tight, sometimes tight or even stuck. When pushing the load shaft forward, backward, up, down, left, right, etc., the rotating shaft should not be loose.
B. Rotate the shaft with one hand and gently touch the outer edge of the film set with the other hand. You should not feel any looseness. The variable capacitor with poor contact between the rotating shaft and the moving plate can no longer be used.
C. Put the multimeter in the R×10k gear, connect the two test leads to the moving piece and the lead end of the fixed piece of the variable capacitor with one hand, and slowly rotate the shaft several times with the other hand. The pointers of the multimeter are all Should not move at infinity. In the process of rotating the shaft, if the pointer sometimes points to zero, it indicates that there is a short-circuit point between the moving piece and the fixed piece; if it encounters a certain angle, the multimeter reading is not infinity but a certain resistance value, indicating that the variable capacitor is moving There is a leakage phenomenon between the film and the stator.
1. Capacitors are generally represented by "C" plus a number in the circuit (for example, C13 represents the capacitor numbered 13). A capacitor is a component composed of two metal films close together and separated by an insulating material. The characteristic of the capacitor is mainly to block DC and pass AC.
The size of the capacitance means the amount of energy that can be stored. The obstructive effect of the capacitance on the AC signal is called capacitive reactance, which is related to the frequency and capacitance of the AC signal.
Capacitive reactance XC=1/2πf c (f represents the frequency of the AC signal, C represents the capacitance 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, etc. .
2. Identification method: The identification method of capacitance is basically the same as that of resistance, which can be divided into three types: direct labeling method, color labeling method and numerical labeling method. The basic unit of capacitance is expressed in Faraday (F), and other units include: millifarad (mF), microfarad (uF), nanofarad (nF), picofarad (pF). Among them: 1 farad=103 millifarad=106 microfarad=109 nanofarad=1012 picofarad
The capacitance value of a large capacity capacitor is directly marked on the capacitor, such as 10 uF/16V
The capacitance of a small capacity capacitor is indicated by letters or numbers on the capacitor
Letter notation: 1m=1000 uF 1P2=1.2PF 1n=1000PF
Digital representation: generally use three digits to indicate the size of the capacity, the first two digits indicate the effective digits, and the third digit is the magnification.
For example: 102 means 10×102PF=1000PF 224 means 22×104PF=0.22 uF
3. Capacitor capacity error table
Symbol F G J K L M
Allowable error ±1% ±2% ±5% ±10% ±15% ±20%
For example: a ceramic capacitor of 104J means that the capacity is 0.1 uF and the error is ±5%