1. Detection of fixed resistors.
A. Connect the two pen pens (regardless of positive and negative) with 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 magnitude of the nominal value of the resistance to be measured. Due to the nonlinear relationship of the ohmic scale, the middle section of the index is finer, so the pointer indication value should fall as far as possible to the middle of the scale, that is, the range of 20%-80% of the radian from the beginning of the full scale, so as to make the measurement more accurate. It varies according to the resistance error level. Errors of ±5%, ±10%, or ±20% are allowed between the reading and the nominal resistance, 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 the resistance of more than tens of kΩ, do not touch the conductive part of the pen and the resistor; The resistor to be detected is soldered down from the circuit, and at least one head should be soldered open, so as to avoid other components in the circuit from affecting the test and causing measurement errors; Although the resistance value of the color ring resistance can be determined by the color ring 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 resistance.
3. Detection of fuse resistor.
In the circuit, when the fuse resistor is blown and the circuit is opened, it can be judged according to experience: if the surface of the fuse resistor is found to be black or scorched, it can be concluded that its load is too heavy, and the current passing through it exceeds the rated value many times; If it is open without any traces on its surface, it indicates that the current flowing is just equal to or slightly greater than its rated fuse value. For the judgment of the quality of the fuse resistor without any traces on the surface, it can be measured with the help of the multimeter R×1 gear, in order to ensure the accuracy of the 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 to open the circuit, if the measured resistance value is far from the nominal value, it indicates that the resistance value has changed, and it is not suitable for use again. 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 to when testing.
4. Detection of potentiometer.
When checking the potentiometer, first turn the rotary handle to see whether the rotating handle rotates smoothly, whether the switch is flexible, whether the "click" sound is crisp when the switch is on and off, and listen to the sound of friction between the internal contact point and the resistor of the potentiometer, if there is a "rustle" sound, it means that the quality is not good. When testing with a multimeter, first select the appropriate resistance position of the multimeter according to the size of the resistance of the potentiometer to be measured, and then it can be detected by the following method.
A. Use the ohmic block of the multimeter to measure the two ends of "1" and "2", and its 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 has been damaged.
B. Check whether the contact between the movable arm of the potentiometer and the resistor plate is good. Use the ohmic file of the multimeter to measure the ends of "1", "2" (or "2", "3"), and rotate the rotating shaft of the potentiometer in a counterclockwise direction to a position close to "off", the smaller the resistance value, the better. Then slowly rotate the shaft handle clockwise, the resistance value should gradually increase, and the pointer in the meter head should move smoothly. When the shaft handle is rotated to the extreme position "3", the resistance value should be close to the nominal value of the potentiometer. If the pointer of the multimeter jumps during the rotation of the shaft handle of the potentiometer, it means that the movable contact has a fault of poor contact.
5. Detection of positive temperature coefficient thermistor (PTC).
When detecting, use the multimeter R × 1 gear, which can be operated in two steps:
A. Normal temperature detection (indoor temperature close to 25°C); The actual resistance value of the two pins of the PTC thermistor is measured by the two pins of the two watch pens contacting the PTC thermistor, and compared with the nominal resistance value, the difference between the two is normal within ±2Ω. If the actual resistance value is too different from the nominal resistance, it is not performing well or is damaged.
B. Heating detection; On the basis of normal temperature test, you can carry out the second step test - heating detection, heat a heat source (such as an electric soldering iron) close to the PTC thermistor to heat it, and at the same time use a multimeter to monitor whether its resistance value increases with the increase of temperature, if so, the thermistor is normal, if the resistance value does not change, it means that its performance has deteriorated and can not be used continuously. Be careful not to bring the heat source too close to the PTC thermistor or directly contact the thermistor to prevent it from burning.
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, the actual value of Rt can be directly measured by selecting the appropriate resistance according to the nominal resistance value of the NTC thermistor. However, because NTC thermistors are very sensitive to temperature, the following points should be paid attention to during 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 carried out when the ambient temperature is close to 25 °C to ensure the reliability of the test.
B. The measured power shall not exceed the specified value, so as to avoid the measurement error caused by the current thermal effect.
C. Pay attention to correct operation. When testing, do not pinch the thermistor body with your hands to prevent the body temperature from affecting the test.
(2) Estimate the temperature coefficient αt First, the resistance value Rt1 is measured at room temperature t1, and then the electric soldering iron is used as a heat source, close to the thermistor Rt, and the resistance value RT2 is measured, and the average temperature t2 of the thermistor RT surface is measured with a thermometer and then calculated.
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 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. Use a piece of black paper to cover the light transmission window of the photoresistor, and the pointer of the multimeter is basically unchanged at this time, and the resistance value is close to infinity. The higher the value, the better the photoresistor performance. If this value is small or close to zero, the photoresistor has burned through and can no longer be used.
B. Aim a light source at the light transmission window of the photoresistor, at this time, the pointer of the multimeter should have a large swing, and the resistance value should be significantly reduced. If this value is large or even infinite, it means that the internal open circuit of the photoresistor is damaged and can no longer be used.
C. Shake the photoresistor light transmission window to the upper part of the photoresistor's shading window with a small black piece of paper to make it receive light intermittently, and the multimeter pointer should swing left and right with the shaking of the black paper.
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