Electronic components detection method for household induction cooker failure maintenance
Electronic components detection method
Component detection It is a basic skill of home appliance maintenance. How to accurately and effectively detect the relevant parameters of components and judge whether the components are normal is not a one-size-fits-all thing. Different methods must be used according to different components to judge the normality of components. 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.
I. Detection methods and experience of resistors:
1. Detection of fixed resistors.
A. The actual resistance value can be measured by connecting two test leads (regardless of positive and negative) to the pins at both 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 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 range of 20% to 80% radian from the beginning of the full scale, so that the Measurement is more accurate. The error level varies according to the resistance. An error of ±5%, ±10% or ±20% is 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 above tens of kΩ Solder a head to prevent other components in the circuit from affecting the test and causing 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 when using it. value.
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 It is caused by many times exceeding the rated value; if there is no trace on its surface and an open circuit, it indicates 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. In order 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. Detection of potentiometer. When checking the potentiometer, first turn the handle to see if the handle rotates smoothly and the switch is flexible.
When the switch is on and off, whether the “click” sound is clear, and listen to the potentiometer The sound of friction between the contact point inside the device and the resistor body, if there is a “rustling” sound, it means that 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 block of the multimeter to measure the two ends of “1” and “2”, the reading should be the nominal resistance value of the potentiometer, such as 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 range 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”, At this time, the smaller the resistance value, 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 shaft 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 gear, which can be divided into two steps:
A normal temperature detection (indoor temperature is close to 25 ℃); The actual resistance value, and
compared with the nominal resistance value, the difference between the two is within ±2Ω 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 test; on the basis of normal temperature test, the second step test can be carried out – heating test, place a heat source (such as an electric
soldering iron) close to PTC thermistor heats it, and at the same time, use a multimeter to monitor whether its resistance value 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, select the appropriate electric resistance according to the nominal resistance value of the NTC thermistor. Blocking can directly measure the actual value of Rt. However, because NTC thermistors are very sensitive to temperature, the following points should be paid attention to when testing: ARt is measured by the manufacturer when the ambient temperature is 25°C, so when measuring Rt with a multimeter, the ambient temperature should also be close to 25°C. ℃ to ensure the reliability of the test. B
The measurement power should not exceed the specified value to avoid measurement error caused by the current heating effect. CPay 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), estimated temperature coefficient αt First measure the resistance value Rt1 at room temperature t1, then use an electric soldering iron as a heat source, close to the thermistor Rt, measure the resistance value RT2, and use The thermometer measures the average temperature t2 on the surface of the thermistor RT at this time and then calculates.
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 a lot, and the resistance value is 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 use a small black piece of paper to shake the upper part of the light-shielding window of the photoresistor to make it receive light intermittently. At this time, the pointer of the multimeter should follow the black paper. Shaking and swinging from side to side. 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.
Second, the detection method and experience of capacitors
1. Detection of fixed capacitors
A . Detection of small capacitors below 10pF Because the capacity of fixed capacitors below 10pF is too small, use a multimeter to measure, only qualitatively check whether there is leakage, internal short circuit or breakdown. When measuring, a multimeter R× 10k block can be used, and two test pens are used 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～001μF fixed capacitor is charged, and then judge whether it is good or bad. Use the R×1k block for the multimeter. The β values of the two triodes are both above 100, and the penetration current is less, and the 3DG6 and other silicon triodes 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 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 001μF, 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 it can be determined according to the magnitude of the pointer swinging to the right Estimate the capacity of the capacitor.
2. Detection of electrolytic capacitors
A. Because the capacity of electrolytic capacitors is much larger than that of general fixed capacitors, the measurement should be carried out for different Select the appropriate range for the capacity. According to experience, under normal circumstances, the capacitance between 1 ~ 47μF can be measured with R×1k block, and the capacitance greater than 47μF can be measured with 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 first contact, the multimeter pointer will deflect to the right by a large degree of deflection (for the same electrical barrier, the greater the capacity, the greater the swing). large), then gradually turn 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 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 a 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 There is a stuck phenomenon. 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 the other
The hand rotates the shaft slowly a few times back and forth, and the pointer of the multimeter should not move at the infinity position. 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.
III. Testing methods and experience of inductors and transformers
1. For the testing of color-coded inductors, set the multimeter to R×1 block, red , Each black test lead is connected to any terminal of the color-coded inductor,
At this time, the pointer should swing to the right. According to the measured resistance value, it can be identified in the following three cases:
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 to wind the inductor coil and the number of turns. As long as the resistance value can be measured, it can be considered to be Color code inductors are normal.
2. Detection of mid-cycle transformer
A. Set the multimeter to R×1 gear, and follow the pin arrangement of each winding of the mid-cycle transformer , check the on-off condition of each winding one by one, and then judge whether it is normal.
B. To test the 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 situations:
(1) The resistance value is infinite: normal;
(2) The resistance value is zero: there is a short circuit
(3) The resistance value is less than infinity, but greater than zero: there is a leakage fault.
3. Power Transformer Inspection
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 gear 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 pointers should all point to infinity and do not move. Otherwise, the transformer insulation performance is poor.
C. Detection of coil on and off. Put the multimeter in the R×1 gear, during the test, if the resistance value of a certain winding is infinite, it means that this winding has an open circuit fault.
D. Identify the 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. Detection of no-load current.
(a) Direct measurement method. Open all the secondary windings, put the multimeter in the AC current block (500mA, and connect the primary winding in series. When the plug of the primary winding is inserted into the 220V AC mains, the multimeter indicates the no-load current value. This value should not be More 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, it means that the transformer has a short-circuit fault.
(b ) indirect measurement method. Connect a 10/5W resistor in series with the primary winding of the transformer, and the secondary is still all unloaded. Turn the multimeter to the AC voltage block. After powering 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 mains, and use a multimeter to connect the AC voltage. Measure the no-load voltage value of each winding in turn (U21, U22, U23, U24) should meet the required value, the allowable error range is generally: high voltage winding ≤± 10%, low voltage winding ≤± 5%, with center The voltage difference between the two symmetrical windings of the tap should be ≤±2%.
G. Generally, the allowable temperature rise of low-power power transformers is 40℃～50℃. The temperature rise can also be improved.
H. Detection and identification of the same name end of each winding. When using a power transformer, sometimes in order to obtain the required secondary voltage, two or more secondary windings can be connected 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 cannot be mistaken. Otherwise, the transformer will not work normally.
I. Synthesis of short-circuit faults of power transformers Detection and judgment.
The main symptoms after the short-circuit fault of the power transformer are severe heating and abnormal output voltage of the secondary winding. Usually, the more short-circuit points between turns inside the coil, the greater the short-circuit current, and the transformer The more serious the heating is.The 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 serious, the transformer will quickly heat up within tens of seconds after no-load power-on, and the iron core will feel hot when you touch it with your hand. At this time, it can be concluded that there is a short-circuit point in the transformer without measuring the no-load current.
