Summary analysis of commonly used electronic components detection methods

First, the detection method and experience of the resistor

1. Detection of fixed resistors. A. The actual resistance value can be measured by connecting the two test leads (not positive or negative) to the two ends of the resistor. In order to improve the measurement accuracy, the range should be selected according to the nominal value of the measured resistance. Due to the nonlinear relationship of the ohmic scale, its middle section is finer. Therefore, the pointer indication value should fall as far as possible to the middle position of the scale, that is, within the range of 20% to 80% radians of the full scale, so that The measurement is more accurate. Depending on the level of resistance error. Errors of ±5%, ±10%, or ±20% are allowed between the reading and the nominal resistance, respectively. If it does not match, if the error range is exceeded, the resistance value is changed. B. Note: When testing, especially when measuring the resistance of tens of kΩ or more, do not touch the conductive part of the test leads and the resistor; the detected resistance is soldered from the circuit, at least one head must be soldered to avoid the circuit. The other components in the test have an influence on the test, causing measurement error; although the resistance of the color ring resistor can be determined by the color circle mark, it is better to test the actual resistance value with a multimeter when using it.

2. Detection of cement resistance. The method and precautions for detecting cement resistance are exactly the same as those for detecting ordinary fixed resistors.

3. Detection of the fuse resistor. In the circuit, when the fuse resistor is melted and disconnected, it can be judged according to experience: if the surface of the fuse resistor is found to be black or burnt, it can be concluded that its load is too heavy, and the current passing through it exceeds the rated value many times; If the surface is open without any trace, it means that the current flowing is just equal to or slightly larger than its rated blown value. The judgment of the fuse resistor with no trace on the surface can be measured by the multimeter R×1 block. To ensure accurate measurement, one end of the fuse resistor should be soldered from the circuit. If the measured resistance is infinite, it means that the fuse resistor has failed open circuit. If the measured resistance value is far from the nominal value, it indicates that the resistance value is not suitable for reuse. In the maintenance practice, it is found that a small number of fuse resistors are broken through in the circuit, and attention should be paid to the detection.

4, the detection of potentiometers. When checking the potentiometer, first turn the handle to see if the rotation of the handle is smooth, whether the switch is flexible, whether the "click" sound is clear when the switch is on or off, and listen to the internal contact point of the potentiometer and the friction of the resistor body. The sound, if there is a "shasha" sound, indicates that the quality is not good. When testing with a multimeter, first select the appropriate electrical blocking position of the multimeter according to the resistance of the potentiometer to be tested, and then perform the detection as follows. A. Use the ohmmeter of the multimeter to measure the "1" and "2" ends. The reading should be the nominal resistance of the potentiometer. If the pointer of the multimeter does not move or the resistance value is different, it indicates that the potentiometer is damaged. B. Check whether the contact between the movable arm of the potentiometer and the resistor is good. Use the ohmmeter of the multimeter to measure both ends of "1", "2" (or "2", "3"), and turn the rotary shaft of the potentiometer counterclockwise to the position close to "off". The smaller the resistance value. The better. Then slowly rotate the shank clockwise, the resistance value should be gradually increased, and the pointer in the meter should move smoothly. When the shank is turned to the extreme position "3", the resistance should be close to the nominal value of the potentiometer. If the pointer of the multimeter has a jumping phenomenon during the rotation of the shaft handle of the potentiometer, it indicates that the movable contact has a fault of poor contact.

5. Detection of positive temperature coefficient thermistor (PTC). When testing, use the multimeter R × 1 block, specifically can be divided into two steps: A, normal temperature detection (indoor temperature is close to 25 ° C); two pens contact the two pins of the PTC thermistor to measure its actual resistance, and with The nominal resistance is relatively normal, and the difference between the two is within ±2 Ω. If the actual resistance is too different from the nominal resistance, it indicates poor performance or damage. B. Heating detection; on the basis of the normal temperature test, the second test can be carried out—heating detection, heating a heat source (such as a soldering iron)* near the PTC thermistor, and monitoring the resistance with a multimeter. Whether the value increases with the increase of temperature, if it is, the thermistor is normal, if the resistance value does not change, it indicates that its performance is deteriorated and cannot be used continuously. Be careful not to get the heat source and PTC thermistor* too close or directly in contact with the thermistor to prevent it from being burnt.

6. Detection of negative temperature coefficient thermistor (NTC).

(1) Measuring the nominal resistance value Rt The method of measuring the NTC thermistor with a multimeter is the same as the method of measuring the ordinary fixed resistance, that is, selecting the appropriate electrical barrier according to the nominal resistance of the NTC thermistor can directly measure the Rt. Actual value. However, because NTC thermistor is very sensitive to temperature, the following points should be noted during the test: A, Rt is measured by the manufacturer at an ambient temperature of 25 ° C, so when measuring Rt with a multimeter, it should also be at ambient temperature. Performed at approximately 25 ° C to ensure the reliability of the test. B. The measured power shall not exceed the specified value to avoid the measurement error caused by the current thermal effect. C, pay attention to correct operation. When testing, do not hold the thermistor body with your hands to prevent the body temperature from affecting the test.

(2) Estimate the temperature coefficient αt first measure the resistance value Rt1 at room temperature t1, then use the electric iron as the heat source, * near the thermistor Rt, measure the resistance value RT2, and measure the thermistor with the thermometer The average temperature t2 of the RT surface is calculated again.

7, the detection of varistor. Use the R × 1k block of the multimeter to measure the positive and negative insulation resistance between the two pins of the varistor, which are infinite. Otherwise, the leakage current is large. If the measured resistance is small, the varistor is damaged and cannot be used.

8. Detection of photoresistor. A. Cover the light-transmissive window of the photoresistor with a black piece of paper. At this time, the pointer of the multimeter is basically kept, and the resistance is close to infinity. The larger the value, the better the photoresistor performance. If the value is small or close to zero, the photoresistor has been burned through and can no longer be used. B. Align a light source with the light-transmitting window of the photoresistor. At this time, the pointer of the multimeter should have a large amplitude swing, and the resistance value is significantly reduced. The smaller the value, the better the photoresistor performance. If the value is large or infinite, it indicates that the open circuit of the photoresistor is damaged and cannot be used any more. C. Align the light-resistance window of the photoresistor with the incident light, and shake it with the small black paper on the upper part of the light-shielding window of the photoresistor to make it intermittently receive light. At this time, the pointer of the multimeter should swing left and right with the shaking of the black paper. If the multimeter pointer is always stopped at a certain position and does not oscillate with the paper, it indicates that the photosensitive material of the photoresistor has been damaged.

Second, the detection method and experience of capacitors

1. Detection of fixed capacitors A. Detecting small capacitors below 10pF. The capacity of fixed capacitors below 10pF is too small. Use a multimeter to measure, and only qualitatively check for leakage, internal short circuit or breakdown. When measuring, you can use the multimeter R × 10k block, use two meter pens to connect the two pins of the capacitor, the resistance should be infinity. If the measured resistance value (the pointer swings to the right) is zero, the capacitor leakage is damaged or internal breakdown. B. Detect whether the 10PF~0, 01μF fixed capacitor has a charging phenomenon, and then judge whether it is good or bad. The multimeter uses R × 1k block. The beta values ​​of the two triodes are all above 100 and the penetration current is small. A composite tube of 3DG6 and other types of silicon triodes can be selected. The red and black test leads 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 capacitor is amplified, so that the multimeter pointer swing amplitude is increased, thereby facilitating observation. It should be noted that during the test operation, especially when measuring the capacitance of a small capacity, it is necessary to repeatedly exchange the two points of the measured capacitor pins to contact A and B to clearly see the swing of the multimeter pointer. C. For fixed capacitors above 0 and 01μF, the Rx10k 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, the appropriate range should be selected for different capacities during measurement. According to experience, in general, the capacitance between 1 ~ 47μF can be measured by R × 1k block, and the capacitance larger than 47μF can be measured by R × 100 block. B. Connect the multimeter red pen to the negative pole and the black pen to the positive pole. At the moment of contact, the multimeter pointer deflects to the right with a large degree of skewness (for the same electrical block, the larger the capacity, the larger the swing), then gradually to the left. Swing 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 several hundred kΩ or more, otherwise, it will not work properly. In the test, if there is no charging in the forward and reverse directions, that is, the hands are not moving, the capacity disappears or the internal circuit is broken; if the measured resistance is small or zero, the leakage of the capacitor is large or the breakdown is broken. Can no longer be used. C. For the electrolytic capacitor whose positive and negative signs are unknown, the above method for measuring the leakage resistance can be used for discrimination. That is, first measure the leakage resistance arbitrarily, remember its size, and then exchange the test leads to measure a resistance value. The one with a large resistance value in the two measurements is the forward connection method, that is, the black meter is connected to the positive pole, and the red meter is connected to the negative pole. D. Using a multimeter electric block, using the method of positive and negative charging of the electrolytic capacitor, the capacity of the electrolytic capacitor can be estimated according to the magnitude of the swinging direction of the pointer to the right.

3, the detection of variable capacitor A, gently rotate the shaft by hand, should feel very smooth, should not feel sometimes loose or even stuck. When the axial direction of the load is pushed forward, backward, up, down, left, right, etc., the shaft should not be loose. B. Rotate the shaft with one hand and gently touch the outer edge of the group with the other hand. There should be no looseness. The variable capacitor with poor contact between the rotating shaft and the moving piece can no longer be used. C. Place the multimeter in the R×10k block. Connect one of the two test leads to the movable piece of the variable capacitor and the lead end of the fixed piece with one hand. The other hand will slowly rotate the rotating shaft several times, and the multimeter pointers are Should not move at infinity. In the process of rotating the shaft, if the pointer sometimes points to zero, it means there is a short circuit between the moving piece and the fixed piece; if it touches an angle, the multimeter reading is not infinite but a certain resistance value appears, indicating that the variable capacitor is moving. There is leakage between the film and the stator.

Third, inductors, transformer testing methods and experience

1. The detection of the color code inductor places the multimeter in the R×1 block, and the red and black test leads are connected to any of the lead terminals of the color code inductor. At this time, the pointer should swing to the right. According to the measured resistance value, the following three cases can be specifically identified: A. The measured color code inductor resistance value is zero, and there is a short circuit fault inside. B. The magnitude of the DC resistance value of the measured color code inductor is directly related to the enamelled wire diameter and the number of winding turns used for winding the inductor coil. As long as the resistance value can be measured, the measured color code inductor is considered normal. of.

2, the detection of the transformer in the middle of the week A, the multimeter is dialed to R × 1 block, according to the arrangement of the windings of the windings of the transformer in the middle of the transformer, check the on and off conditions of each winding one by one, and then determine whether it is normal. B. Detecting insulation performance Place the multimeter in R × 10k block and do the following status tests: (1) the resistance between the primary winding and the secondary winding; (2) the resistance between the primary winding and the outer casing; 3) The resistance value between the secondary winding and the outer casing. There are three cases in the above test results: (1) the resistance is infinite: normal; (2) the resistance is zero: there is a short circuit fault; (3) the resistance is less than infinity, but greater than zero: there is leakage fault.

3. Detection of the power transformer A. Observe the apparent appearance of the transformer by observing the appearance of the transformer. If the coil lead is broken, de-soldering, whether the insulating material has burnt marks, whether the iron-tightening screw is loose, whether the silicon steel sheet is rusted, whether the winding coil is exposed or the like. B. Insulation test. Use the multimeter R×10k block to measure the resistance between the core and the primary, the primary and the secondary, the core and each secondary, the electrostatic shielding layer and the secondary and secondary windings. The multimeter pointer should refer to the infinity position. move. Otherwise, the transformer insulation performance is poor. C, the detection of the coil on and off. Place the multimeter in the R×1 gear. During the test, if the resistance value of a winding is infinite, the winding has a faulty fault. D. Determine the primary and secondary coils. The primary and secondary pins of the power transformer are generally led out from both sides, and the primary winding is marked with 220V, and the secondary winding is labeled with rated voltage, such as 15V, 24V, 35V. Then identify them based on these markers. E. Detection of no-load current.

(a) Direct measurement method. Open all the secondary windings and place the multimeter in the AC current block (500mA, stringed into the primary winding. When the primary winding plug is inserted into 220V AC mains, the multimeter indicates the no-load current value. This value should not be It is greater than 10%-20% of the full load current of the transformer. Generally, the normal no-load current of the power transformer of common electronic equipment should be about 100mA. If it exceeds too much, it means that the transformer has short-circuit fault. (b) Indirect measurement method. In the primary winding, a 10/5W resistor is connected in series, and the secondary is still completely empty. Turn the multimeter to the AC voltage block. After power-on, measure the voltage drop U across the resistor R with two test leads, and then calculate with Ohm's law. No-load current I is empty, that is, I empty = U / R. F, no-load voltage detection. Connect the primary of the power transformer to 220V mains, and use the multimeter AC voltage to sequentially measure the no-load voltage value of each winding (U21, U22, U23, U24) should meet the required value, the allowable error range is generally: high voltage winding ≤ ± 10%, low voltage winding ≤ ± 5%, the voltage difference between two sets of symmetrical windings with center tap should be ≤ ± 2%. General small work The power transformer allows the temperature rise to be 40 ° C ~ 50 ° C. If the quality of the insulating material used is good, the temperature rise can be increased. H, detection and identification of the same name of each winding. When using the power transformer, sometimes in order to get the required number of times The voltage of the stage can be used in series with two or more secondary windings. When the power transformer is used in series, the same name of each winding participating in the series must be correctly connected, and no mistake can be made. Otherwise, the transformer can not work normally. Comprehensive detection and identification of short-circuit faults of power transformers. The main symptoms after short-circuit faults in power transformers are severe heat generation and abnormal output voltage of the secondary winding. Generally, the more short-circuit points between turns in the coil, the larger the short-circuit current, and the transformer The more severe the heat is. The simple way to detect whether the power transformer has a short-circuit fault is to measure the no-load current (tested earlier in the test method). For a transformer with a short-circuit fault, the no-load current value will be much greater than 10% of the full-load current. When the short circuit is severe, the transformer will heat up quickly within a few seconds after the no-load power is applied. Touching the iron core will have a hot feeling. At this time, it is not necessary to measure the no-load current to conclude that the transformer has a short-circuit point.