Capacitance detection method

In the absence of special instruments, the quality of capacitors can be tested and judged using the resistance range of a multimeter. Fixed capacitors with large capacity (above 1μF) can use the resistance range (R×1000) of a multimeter to measure the two electrodes of the capacitor. The needle should swing in the direction of the smaller resistance value, and then slowly swing back to near ∞. Then exchange the test rod and try again to see the swing of the needle. The larger the swing, the larger the capacitance of the capacitor. If the test rod keeps touching the capacitor lead, the needle should point to near ∞, otherwise, it indicates that the capacitor has leakage. The smaller the resistance value, the greater the leakage, and the capacitor is of poor quality; if the needle does not move at all during measurement, it indicates that the capacitor has failed or is broken; if the needle swings but cannot return to the starting point, it indicates that the capacitor has a large leakage and its quality is poor.

For capacitors with smaller capacity, the needle of the pressure gauge often cannot be seen to swing when measured with a multimeter. At this time, an external DC voltage and the DC voltage range of the multimeter can be used for measurement. The method is shown in Figure 1, that is, the multimeter is adjusted to the corresponding DC voltage range, 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 to be measured, and the other end is connected to the positive pole of the power supply.

At the moment when a capacitor with good performance is connected to the power supply, the needle of the multimeter should have a large swing; the larger the capacity of the capacitor, the larger the swing of its needle, and after swinging, the needle can gradually return to zero. If the pointer of the multimeter does not swing at the moment when the capacitor is connected to the power supply, it means that the capacitor has failed or is open; if the needle always indicates the power supply voltage without swinging, it means that the capacitor has been broken down and short-circuited; if the needle swings normally but does not return to zero, it means that the capacitor has leakage, and the higher the voltage value indicated, the greater the leakage. It should be pointed out that the auxiliary DC voltage used to measure capacitors with small capacity cannot exceed the withstand voltage of the capacitor being measured, so as to avoid capacitor breakdown and damage due to measurement. To accurately measure the capacity of the 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: Pointer multimeter measurement.

1. Use the multimeter resistance range to check the quality of the electrolytic

capacitor. The two leads of the electrolytic capacitor are positive and negative. When checking its quality, for electrolytic capacitors with lower withstand voltage (6V or l0V), the resistance range should be set to R×100 or R×1K, connect the red test pen to the negative end of the capacitor, and the black test pen to the positive end. At this time, the multimeter pointer will swing and then return to zero or near zero. 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 determine the positive and negative leads of electrolytic capacitors For

some electrolytic capacitors with low withstand voltage, if the positive and negative lead marks are unclear, you can judge based on its characteristics that the leakage current is small (the resistance value is large) when it is positively connected and the leakage current is large when it is reversely connected. The specific method is: use the red and black test pens to touch the two leads of the capacitor, remember the size of the leakage current (resistance value) (the resistance value indicated when the pointer swings back and stops), then short-circuit the positive and negative leads of the capacitor, swap the red and black test pens, and then measure the leakage current. The judgment is based on the indication of the small leakage current. The lead that contacts the black test pen is the positive end of the electrolytic capacitor. This method is more difficult to distinguish the polarity of electrolytic capacitors with small leakage current.

3. Use a multimeter to check variable capacitors

Variable capacitors have a set of fixed plates and a set of moving plates. Use the resistance range 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 and fixed pieces respectively, rotate the shaft handle, and the meter pointer does not move, indicating that there is no short circuit (touching piece) 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 range of the multimeter to roughly identify the quality of capacitors with a capacity of more than 5000PF.

Use the resistance range of the multimeter to roughly identify the quality of capacitors above 5000PF (those below 5000PF can only determine whether the capacitor is broken down). When checking, set the resistance range to the high value of the range, and the two test pens touch the two ends of the capacitor respectively. At this time, the pointer swings quickly and then recovers. Reverse connection, the swing amplitude is larger than the first time, and then recovers. Such a capacitor is good. The larger the capacity of the capacitor, the larger the swing of the meter pointer during measurement, and the longer the time it takes for the pointer to recover. We can compare the size of the capacity of the two capacitors based on the swing of the meter pointer.

2. Capacitor detection method and experience

1. Fixed capacitor detection

A. Detection of small capacitors 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 by measuring with a multimeter. When measuring, you can use the multimeter R×10k block, and use two test pens to connect the two pins of the capacitor at will, 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 leaking or damaged or internally broken down.

B. Detect whether the 10PF~0 01μF fixed capacitor has charging phenomenon, and then judge whether it is good or bad. The multimeter uses the R×1k block. The β values ​​of the two transistors are both above 100, and the penetration current should be small. 3DG6 and other models of silicon transistors can be used to form a composite tube. The red and black test pens of the multimeter are connected to the emitter e and collector c of the composite tube respectively. Due to the amplification effect of the composite triode, the charging and discharging process of the measured capacitor is amplified, so that the swing amplitude of the multimeter pointer is increased, which is convenient for observation. It should be noted that: when testing, especially when measuring capacitors with smaller capacity, the pins of the measured capacitor should be repeatedly switched to contact points A and B, so that the swing of the multimeter pointer can be clearly seen.

C For fixed capacitors above 0.01μF, the R×10k block of the multimeter can be used to directly test whether the capacitor has a charging process and whether there is an 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 capacitors

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

B. Connect the red probe of the multimeter to the negative pole and the black probe to the positive pole. At the moment of contact, the pointer of the multimeter deflects to the right with a large deviation (for the same resistance block, the larger the capacity, the larger the swing), and then gradually turns 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 the electrolytic capacitor should generally be above several hundred kΩ, otherwise it will not work properly. In the test, if there is no charging phenomenon in both the forward and reverse directions, that is, the needle does not move, it means that the capacity disappears 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 and damaged and cannot be used again. C. For electrolytic capacitors with unclear positive and negative pole markings, the above-mentioned method of measuring leakage resistance can be used to distinguish them. That is, first measure the leakage resistance at random, remember its size, and then exchange the probes to measure another resistance value. The one with the larger resistance value in the two measurements is the forward connection method, that is, the black probe is connected to the positive pole and the red probe is connected to the negative pole. D Use the resistance block of the multimeter to charge the electrolytic capacitor in the forward and reverse directions. The capacity of the electrolytic capacitor can be estimated based on the amplitude of the pointer swinging to the right.

3 Detection of variable capacitors

A Gently rotate the shaft by hand, it should feel very smooth, and should not feel sometimes loose, sometimes tight, or even stuck. When the carrier shaft is pushed forward, backward, up, down, left, right, etc., the shaft should not be loose. B Rotate the shaft with one hand and touch the outer edge of the moving plate group with the other hand. You should not feel any looseness. Variable capacitors with poor contact between the shaft and the moving plate cannot be used any more. C Set the multimeter to the R×10k position, connect the two test leads to the moving plate and the fixed plate of the variable capacitor with one hand, and slowly rotate the shaft back and forth with the other hand. The pointer of the multimeter should be at the infinite position and 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 moving plate and the fixed plate; if it hits a certain angle, the multimeter reading is not infinite but a certain resistance value, indicating that there is leakage between the moving plate and the fixed plate of the variable capacitor.

1. Capacitors are generally represented by "C" plus numbers in circuits (such as C13 for capacitor numbered 13). Capacitors are components composed of two metal films close together, separated by insulating materials. The main characteristic of capacitors is

to block direct current and pass alternating current. The size of the capacitance is the size of the electrical 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=1/2πf c (f represents the frequency of the AC signal, C represents the capacitance) Common types of capacitors used in telephones include electrolytic capacitors, ceramic capacitors, chip capacitors, monolithic capacitors, tantalum capacitors and polyester capacitors.

2. Identification method: The identification method of capacitors is basically the same as that of resistors, which is divided into three types: direct marking method, color marking method and digital marking method. The basic unit of capacitance is expressed in farad (F), and other units include: millifarad (mF), microfarad (uF), nanofarad (nF), and 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 value of a small-capacity capacitor is represented by letters or numbers on the capacitor
Letter representation: 1m=1000 uF 1P2=1.2PF 1n=1000PF
Digital representation: Generally, three digits are used to represent the capacitance size, the first two digits represent the significant digits, and the third digit is the multiple.
For example: 102 represents 10×102PF=1000PF 224 represents 22×104PF=0.22 uF

3. Capacitor capacity error table

Symbol FGJKLM
Allowable error ±1% ±2% ±5% ±10% ±15% ±20%
For example: a ceramic chip capacitor is 104J, which means the capacity is 0. 1 uF and the error is ±5%