Capacitance detection method

In the absence of special instruments, the quality and quality of the capacitor can be tested and judged with the resistance file of a multimeter. For a fixed capacitor with a large capacity (above 1μF), the resistance profile (R×1000) of a multimeter can be used to measure the two electrodes of the capacitor. The needle should swing in the direction of small resistance, and then slowly swing back to around ∞. Then exchange the test rod and try again, see 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 the 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.
For a pressure gauge with a small capacity capacitor, when measuring with a multimeter, it is often difficult to see the needle swing. At this time, you can use an external DC voltage and the DC voltage range of a multimeter to measure. The method is shown in Figure 1, 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 hands. After swinging, the hands can gradually return to the zero position. If the pointer of the multimeter does not swing at the moment when 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 needs to be pointed out that the auxiliary DC voltage used for measuring capacitors 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 simple detection method mentioned above can only roughly judge the quality of the pressure gauge capacitor.
Method 1: Pointer type multimeter measurement.
1. Check the quality of the electrolytic capacitor with the resistance file of a multimeter
The two leads of an electrolytic capacitor are divided into positive and negative points. When checking its quality, for electrolytic capacitors with lower withstand voltage (6V or l0V), the resistance file should be placed in the R×100 or R×1K file. 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
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) when connecting positively and large leakage current when connecting reversely. The specific method is: touch the two leads of the capacitor with the red and black test leads, remember the magnitude of 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 Then, swap the red and black test leads before measuring the leakage current. Judgment is based on the small leakage current indication. The lead in contact with the black test lead is the positive terminal of the electrolytic capacitor. With this method, it is more difficult to distinguish the polarity of an electrolytic capacitor with a small 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 file of the multimeter to check whether there is any contact between the moving and fixed pieces. Use the 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 that 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
Using the multimeter resistance file can roughly distinguish the quality of capacitors above 5000PF (the ones below 5000PF can only judge whether the inside of the capacitor is broken). When checking, put the resistance range in the range value, and the two test probes 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 second, 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 capacity of the two capacitors according to the magnitude of the movement of the meter pointer.
2. 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, when measuring with a multimeter, it can only be qualitatively checked for leakage, internal short circuit or breakdown. When measuring, you can choose a multimeter R×10k block, and use two meter pens to connect the two pins of the capacitor arbitrarily, 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 Check whether the 10PF～0.01μF fixed capacitor is charged, and then judge whether it is good or bad. The multimeter selects R×1k block. The β value of the two transistors is above 100, and the penetration current should be 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, it is necessary to repeatedly exchange the contact points A and B of the tested capacitor pin to clearly see the movement of the multimeter pointer.
C For fixed capacitors 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 magnitude of the pointer swing to the right.
2 Testing 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, the capacitance between 1 and 47μF can be measured with the R×1k block, and the capacitance larger than 47μF can be measured with the R×100 block.
B Connect the red test lead of the multimeter to the negative pole and the black test lead to the positive pole. 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 Until it stops at a certain position. The resistance value 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 a few hundred kΩ, otherwise, it will not work properly. 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 to measure the leakage resistance arbitrarily, remember its size, and then exchange the test leads to measure a resistance value. The larger resistance in 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 capacitor
A Rotate the shaft gently with your hands, it should feel very smooth, and should not feel loose, sometimes tight or even stuck. When pushing the load shaft forward, backward, up, down, left, right and other directions, there should be no looseness of the rotating shaft. 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 Place the multimeter in the R×10k gear, connect the two test leads with one hand to the moving piece of the variable capacitor and the leading end of the fixed piece, and slowly rotate the shaft several times back and forth with the other hand. The pointers of the multimeter should be Do not move at the infinity position. 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 infinite 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 in the middle. The characteristic of the capacitor is mainly to block the direct current and pass the alternating current.
The size of the capacitor capacity is the size that can store electrical energy. The obstructive 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=1/2πf 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 Fara (F), and other units are: 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 represented by letters or numbers on the capacitor
Letter notation: 1m=1000 uF 1P2=1.2PF 1n=1000PF
Number notation: Generally, three digits are used to indicate the size of the capacity, the first two digits indicate the effective digits, and the third digit is the magnification.
Such as: 102 means 10×102PF=1000PF 224 means 22×104PF=0.22 uF
3, capacitance error table
Symbol F G J K L M
Allowable error ±1% ±2% ±5% ±10% ±15% ±20%
Such as: a ceramic capacitor of 104J means that the capacity is 0.1 uF, and the error is ±5%