The detection of electronic components is a basic skill in home appliance maintenance. Many engineering maintenance and repair technologies in the security industry are actually derived from home appliance maintenance and repair technologies, or borrowed or homogeneous. THow to accurately and effectively detect the relevant parameters of components and judge whether the components are normal is not a one-size-fits-all matter. Different methods must be used according to different components to judge whether the components are normal or not. Especially for beginners, it is necessary to master the detection methods and experience of commonly used components. The following is an introduction to the detection experience and methods of commonly used electronic components for comparison.
Resistor
1 Detection of fixed resistors.
A Connect the two test leads (regardless of positive and negative) to the pins at both ends of the resistor to measure the actual resistance value. 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 relatively finely divided, so the pointer indication value should fall as far as possible to the middle position of the scale, that is, within the 20% to 80% radian range from the beginning of the full scale, so that the Measurement is more accurate. The error level varies according to the resistance. Errors of ±5%, ±10% or ±20% are allowed between the reading and the nominal resistance value, respectively. If it does not match and exceeds the error range, it means that the resistance value has changed.
B Note: When testing, especially when measuring resistances with a resistance value of tens of kΩ or more, do not touch the test leads and the conductive parts of the resistance; the resistance to be tested is welded from the circuit, and at least one head must be welded to avoid the circuit. Other components will affect the test and cause measurement errors; although the resistance value of the color ring resistor can be determined by the color ring mark, it is best to use a multimeter to test its actual resistance value 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 resistance.
3 Detection of blown resistors. In the circuit, when the fuse resistor fuses and opens the circuit, 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 the load is too heavy, and the current passing through it exceeds the rated value many times; If there is no trace on its surface and the circuit is open, it means that the current flowing is just equal to or slightly greater than its rated fusing value. For the judgment of the quality of the fuse resistor without any trace on the surface, it can be measured with the help of the multimeter R×1 block. To ensure the accuracy of the measurement, one end of the fuse resistor should be soldered off the circuit. If the measured resistance value is infinite, it means that the fuse resistor has failed to open circuit. If the measured resistance value is far from the nominal value, it means that the resistance value has changed and should not be used anymore. In the maintenance practice, it is found that there are also a few fuse resistors that are broken down and short-circuited in the circuit, and attention should also be paid when testing.
4 Potentiometer detection. 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 turned on and off, and listen to the friction between the contact point inside the potentiometer and the resistance body. If there is a "rustling" sound, it means the quality is not good. When testing with a multimeter, first select the appropriate resistance barrier of the multimeter according to the resistance value of the potentiometer to be tested, and then perform the detection according to the following method.
A Use the ohm stop of the multimeter to measure both ends of "1" and "2". The reading should be the nominal resistance value of the potentiometer. If the pointer of the multimeter does not move or the resistance value is very different, it indicates that the potentiometer is damaged.
B Check whether the contact between the movable arm of the potentiometer and the resistance sheet is good. Use the ohm gear of the multimeter to measure the two ends of "1", "2" (or "2", "3"), and turn the shaft of the potentiometer counterclockwise to the position close to "off", the smaller the resistance value at this time. the better. Then slowly rotate the shank clockwise, the resistance value should gradually increase, and the pointer in the meter head should move smoothly. When the shank is turned to the extreme position "3", the resistance value should be close to the nominal value of the potentiometer. For example, the pointer of the multimeter jumps during the rotation of the shaft handle of the potentiometer, indicating that the movable contact has a faulty contact.
5 Detection of positive temperature coefficient thermistor (PTC). When testing, use the multimeter R×1 block, which can be operated in two steps:
A Normal temperature detection (indoor temperature is close to 25°C); measure the actual resistance value of the two test pens contacting the two pins of the PTC thermistor, and compare it with the nominal resistance value, if the difference between the two is within ±2Ω, it is normal. If the actual resistance value is too different from the nominal resistance value, it means that its performance is poor or damaged.
B Heating detection; on the basis of normal temperature test, the second step of the test - heating detection can be carried out, a heat source (such as an electric soldering iron) is placed close to the PTC thermistor to heat it, and a multimeter is used to monitor whether its resistance value is It increases with the increase of temperature. If so, it means that the thermistor is normal. If the resistance value does not change, it means that its performance has deteriorated and cannot be used continuously. Be careful not to place the heat source too close to the PTC thermistor or contact the thermistor directly to prevent burning it.
6 Detection of negative temperature coefficient thermistor (NTC).
(1) Measure 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 value of the NTC thermistor can directly measure the Rt actual value. However, because NTC thermistors are very sensitive to temperature, the following points should be paid attention to when testing: A Rt is measured by the manufacturer when the ambient temperature is 25 °C, so when measuring Rt with a multimeter, it should also be measured when the ambient temperature is close to 25 ℃ to ensure the reliability of the test. B The measurement power shall not exceed the specified value, so as to avoid measurement errors caused by the thermal effect of the current. C Pay attention to correct operation. When testing, do not pinch the thermistor body with your hands to prevent human body temperature from affecting the test.
(2) Estimate the temperature coefficient αt. First measure the resistance value Rt1 at room temperature t1, then use an electric soldering iron as a heat source, approach the thermistor Rt, measure the resistance value RT2, and use a thermometer to measure the thermistor RT at this time. The average temperature t2 of the surface is calculated again.
7 Detection of varistor. Use the R×1k block of the multimeter to measure the forward and reverse insulation resistance between the two pins of the varistor, both of 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-transmitting window of the photoresistor with a black piece of paper. At this time, the pointer of the multimeter remains basically unchanged, and the resistance value is close to infinity. The larger the value, the better the performance of the photoresistor. If this value is very small or close to zero, it means that the photoresistor has been burnt through and damaged and can no longer be used.
B. Aim a light source at the light-transmitting window of the photoresistor. At this time, the pointer of the multimeter should swing in a large amount, and the resistance value should be obviously reduced. The smaller the value, the better the performance of the photoresistor. If this value is very large or even infinite, it indicates that the internal open circuit of the photoresistor is damaged and cannot be used any longer.
C Align the light-transmitting window of the photoresistor with the incident light, and shake a small black piece of paper on the top of the light-shielding window of the photoresistor to make it receive light intermittently. At this time, the pointer of the multimeter should swing left and right with the shaking of the black paper. If the pointer of the multimeter always stops at a certain position and does not swing with the shaking of the paper, it means that the photosensitive material of the photoresistor has been damaged.
capacitor
1 Detection of fixed capacitors
A Detection of small capacitors 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 use the multimeter R×10k block, and use two test pens to connect the two pins of the capacitor at will, and the resistance value 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 10pF～0.01μF fixed capacitor is charged, and then judge whether it is good or bad. The multimeter selects R×1k block. The β values ​​of the two triodes are both above 100, and the penetrating current should be composed of 3DG6 and other silicon triodes 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 amplifying effect of the composite triode, the charging and discharging process of the capacitor under test is amplified, so that the pendulum of the multimeter pointer is increased, which is convenient for observation. It should be noted that during the test operation, especially when measuring small-capacity capacitors, it is necessary to repeatedly exchange the pins of the capacitor under test to contact points A and B, in order to clearly see the swing of the multimeter pointer.
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, appropriate ranges should be selected for different capacities. According to experience, in general, the capacitance between 1 and 47μF can be measured in R×1k block, and the capacitance larger than 47μF can be measured in 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 first contact, the multimeter pointer will deflect to the right by a large degree (for the same electrical block, 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. The actual use experience shows that the leakage resistance of electrolytic capacitors should generally be above several 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 needle does not move, it means that the capacity has disappeared or the internal circuit is broken; Can no longer be used.
C For electrolytic capacitors whose positive and negative signs are unknown, the above method of measuring leakage resistance can be used to identify them. 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 the larger resistance value in the two measurements is the forward connection method, 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 the multimeter to block the electricity, and use the method of forward and reverse charging to the electrolytic capacitor. According to the magnitude of the pointer swinging to the right, the capacity of the electrolytic capacitor can be estimated.
3 Detection of variable capacitors
A Gently rotate the shaft by hand, it should feel very smooth, it should not feel loose and tight or even stuck. When the carrier shaft is pushed forward, backward, up, down, left, right, etc., the rotating shaft should not be loose.
B Rotate the shaft with one hand and touch the outer edge of the moving plate with the other hand, you should not feel any looseness. A variable capacitor with poor contact between the rotating shaft and the moving plate cannot be used any longer.
C Put the multimeter in the R×10k block, connect the two test leads to the moving piece and the fixed piece of the variable capacitor with one hand, and slowly rotate the shaft with the other hand. Do not move at infinity. In the process of rotating the rotating shaft, if the pointer sometimes points to zero, it means that there is a short-circuit point between the moving piece and the fixed piece; if a certain angle is encountered, 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 plate and the stator.
Inductor Transformer
1 Detection of the color-coded inductor Put the multimeter in the R×1 block, connect the red and black test pens to any terminal of the color-coded inductor, and the pointer should swing to the right at this time. According to the measured resistance value, it can be identified in the following three situations:
A The resistance value of the tested color-coded inductor is zero, and there is a short-circuit fault inside.
B The DC resistance value of the color-coded inductor to be tested is directly related to the diameter of the enameled wire used for winding the inductor coil and the number of winding turns. As long as the resistance value can be measured, the color-coded inductor to be tested can be considered normal. .
2 Detection of mid-cycle transformers
A Set the multimeter to the R×1 gear, check the on-off status of each winding one by one according to the pin arrangement of each winding of the mid-cycle transformer, and then judge whether it is normal.
B Test insulation performance Put the multimeter in the R×10k block, and do the following state tests:
(1) The resistance value between the primary winding and the secondary winding;
(2) The resistance value between the primary winding and the casing;
(3) The resistance value between the secondary winding and the casing.
The above test results are divided into three cases:
(1) The resistance value is infinite: normal;
(2) The resistance value is zero: there is a short-circuit fault;
(3) The resistance value is less than infinity, but greater than zero: there is a leakage fault.
3 Detection of power transformers
A Check whether there is any obvious abnormality by observing the appearance of the transformer. Such as whether the coil lead is broken, desoldered, whether the insulating material has scorch marks, whether the iron core tightening screw is loose, whether the silicon steel sheet is rusted, whether the winding coil is exposed, etc.
B Insulation test. Use the multimeter R×10k block to measure the resistance values ​​between the core and the primary, the primary and each secondary, the core and each secondary, the electrostatic shielding layer and the secondary, and the secondary windings. The pointer of the multimeter should point to infinity. verb: move. Otherwise, the transformer insulation performance is poor.
C coil on/off detection. Put the multimeter in the R×1 block. During the test, if the resistance value of a winding is infinite, it means that the winding has an open circuit fault.
D Distinguish primary and secondary coils. The primary and secondary pins of the power transformer are generally drawn from both sides, and the primary winding is mostly marked with 220V, and the secondary winding is marked with a rated voltage value, such as 15V, 24V, 35V, etc. Recognize based on these marks.
E No-load current detection.
(a) Direct measurement method. Open all the secondary windings, put the multimeter in the AC current block of 500mA, and connect it to the primary winding. When the plug of the primary winding is inserted into the 220V AC mains, the no-load current value indicated by the multimeter is. This value should not be greater than 10% to 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, the transformer has a short-circuit fault.
(b) Indirect measurement method. A 10/5W resistor is connected in series with the primary winding of the transformer, and the secondary is still fully unloaded. Set the multimeter to AC voltage. After power-on, use two test pens to measure the voltage drop U across the resistor R, and then use Ohm's law to calculate the no-load current I empty, that is, I empty = U/R.
F Detection of no-load voltage. Connect the primary of the power transformer to 220V commercial power, and use a multimeter to connect the AC voltage to measure the no-load voltage value of each winding (U21, U22, U23, U24) in turn, which should meet the required value, and the allowable error range is generally: high-voltage windings≤±10 %, the low-voltage winding is ≤±5%, and the voltage difference between the two sets of symmetrical windings with center taps should be ≤±2%.
G General low-power power transformer. The allowable temperature rise is 40°C to 50°C. If the insulating material used is of good quality, the allowable temperature rise can also be increased.
H Detection and identification of the end of the same name of each winding. When using a power transformer, sometimes in order to obtain the required secondary voltage, two or more secondary windings can be used in series. When the power transformer is used in series, the ends of the same name of each winding participating in the series must be connected correctly, and no mistakes can be made. Otherwise, the transformer will not work properly.
I Comprehensive detection and judgment of short-circuit faults of power transformers. The main symptoms after the short-circuit fault of the power transformer are severe heating and abnormal output voltage of the secondary winding. Generally, the more short-circuit points between turns inside the coil, the greater the short-circuit current, and the more serious the transformer heats up. A simple way to detect and judge whether the power transformer has a short-circuit fault is to measure the no-load current (the test method has been introduced earlier). For transformers with short-circuit faults, the no-load current value will be much greater than 10% of the full-load current. When the short circuit is severe, the transformer is energized at no loadIt will quickly heat up within a few seconds, and it will feel hot when you touch the iron core with your hands. At this time, it can be concluded that there is a short-circuit point in the transformer without measuring the no-load current.