How to test the quality of capacitors

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 can be used to measure with the DC voltage range of the multimeter. The method is shown in Figure 1, that is, adjust the multimeter to the corresponding DC voltage range, connect the negative (black) test rod to the negative pole of the DC power supply, connect the positive (red) test rod to one end of the capacitor to be measured, and the other end to the positive pole of the power supply.

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. After swinging, the needle can gradually return to zero. If the pointer of the multimeter does not swing when the capacitor is powered on, 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 short-circuited; if the needle swings normally but does not return to zero, it means that the capacitor has leakage, and the higher the indicated voltage value, 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 breakdown and damage to the capacitor 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: Measurement with a pointer multimeter.

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

The two leads of the electrolytic capacitor are divided into positive and negative. When checking its quality, for electrolytic capacitors with lower withstand voltage (6V or 10V), the resistance range should be set to R×100 or R×1K, and the red test lead should be connected to the negative end of the capacitor and the black test lead 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 the electrolytic capacitor

For some electrolytic capacitors with low withstand voltage, if the positive and negative lead marks are unclear, they can be judged based on the characteristics that the leakage current is small (the resistance value is large) when it is connected in the positive direction, and the leakage current is large when it is connected in the reverse direction. 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. 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 range of the multimeter to check whether there is any contact between the moving and fixed plates. Use the red and black test leads to connect the moving and fixed plates respectively, rotate the shaft handle, and if the meter pointer does not move, it means there is no short circuit (touching) between the moving and fixed plates; if the pointer swings, it means there is a short circuit in the capacitor.

4. Use the resistance range of a multimeter to roughly identify the quality of capacitors with a capacity of more than 5000PF

The resistance range of a multimeter can be used to roughly identify the quality of capacitors above 5000PF (those below 5000PF can only be used to determine whether the capacitor is broken down). When checking, set the resistance range to the high range value, and the two test leads are in contact with the two ends of the capacitor. 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 capacity of the two capacitors based on the swing of the meter pointer.

2. Capacitor detection methods and experience

1. Detection of fixed capacitors

A Detects small capacitance below 10pF

Because the capacity of fixed capacitors below 10PF is too small, using a multimeter to measure can only qualitatively check whether there is leakage, internal short circuit or breakdown. When measuring, you can choose the multimeter R×10k block, use two test leads to connect the two pins of the capacitor at will, and the resistance should be infinite. If the measured resistance (the pointer swings to the right) is zero, it means that the capacitor is leaking or damaged or has internal breakdown.

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 types of silicon transistors can be used to form a composite tube. The red and black test leads 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 transistor, 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, it is necessary to repeatedly switch the pins of the measured capacitor to contact points A and B, so that the swing of the multimeter pointer can be clearly seen.

For fixed capacitors above 0.01μF, the R×10k range of the multimeter can be used to directly test whether the capacitor is in the charging process 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. 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, the capacitance between 1 and 47μF can be measured with the R×1k range, and the capacitance greater than 47μF can be measured with the R×100 range.

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's swing to the right.

[page]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 anymore. C Set the multimeter to the R×10k gear, 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 touches 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 a number in a circuit (e.g. C13 represents capacitor numbered 13). Capacitors are components composed of two metal films placed close together and separated by insulating material. The main characteristics of capacitors are to block direct current and pass alternating current.

The size of the capacitor indicates the amount of electrical energy that can be stored. The resistance of the capacitor to 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 can be 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 capacitor is represented by letters or numbers on the capacitor.

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

Digital representation: Generally, three digits are used to represent the capacity, the first two digits represent the significant digits, and the third digit is the multiplier.

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

3. Capacitance error table

Symbols FGJKLM

Permissible error: ±1% ±2% ±5% ±10% ±15% ±20%

For example, a ceramic capacitor of 104J means the capacity is 0.1 uF and the error is ±5%.