1. Fixed resistor detection
A connect the two gauges (both positive and negative) with the pins at both ends of the resistance respectively 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 resistance to be measured. Due to the nonlinear relationship of the oomdial scale, its middle section is relatively thin, so the pointer should be as far as possible to fall into the middle of the scale, that is, within the range of 20% ~ 80% radian at the beginning of the full scale, in order to make the measurement accurate. Depending on the resistance error level. Tolerance of ± 5%, ± 10%, or ± 20%, respectively, between reading and nominal resistance is allowed. If it does not match, it indicates that the resistance value has changed. B Note: when testing, especially when measuring the resistance value of tens of K ω above, do not touch the pen and the conductive part of the resistance; The resistance to be tested is welded down from the circuit, at least one head must be welded, so as to avoid the influence of other components in the circuit on the test, resulting in measurement error; Although the resistance value of the color ring resistance can be determined by the color ring mark, it is best to use a multimeter to test the actual resistance value.
2. Test of cement resistance. The method and precautions for testing cement resistance are exactly the same as those for testing common fixed resistance.
3. Fuse resistor detection. In the circuit, when the fuse resistor fuses open circuit, we can make judgment according to experience: if the fuse resistor surface is found black or scorched, we can conclude that it is caused by the overload, the current through it exceeds the rated value many times; If its surface is open without any marks, it indicates that the current flowing is equal to or slightly greater than its rated fuse value. To judge whether the fusible resistor without any marks on its surface is good or bad, it can be measured by using a multimeter R×1 block. To ensure accurate measurement, one end of the fusible resistor should be welded down from the circuit. If the measured resistance value is infinite, it indicates that the fuse breaker has failed to open. If the measured resistance value is far from the nominal value, it indicates that the resistance value is changed and should not be used again. In the practice of maintenance, it is found that a few fusible resistors are broken down and short-circuited in the circuit, which should be paid attention to in the detection.
4. Potentiometer detection. When checking potentiometer, want to turn turn handle above all, see whether turn turn handle is smooth, switch is flexible, when switch is connected "click" sound whether clear, listen to the sound of friction of point of contact inside potentiometer and resistance body, if have "rustle" sound, explain quality is bad. When testing with a multimeter, choose the appropriate resistance of the multimeter according to the size of the resistance value of the potentiometer to be measured first, and then test according to the following method.
A use the ohm block of the multimeter to measure "1" and "2" at both ends, and the reading should be the nominal resistance value of the positioner. If the pointer of the multimeter does not move or the resistance value differs A lot, it indicates that the potentiometer has been damaged. B Check whether the movable arm of potentiometer is in good contact with the resistor piece. With the multimeter ohm file measurement "1", "2" (or "2", "3") at both ends, the potentiometer rotating axis by counterclockwise to close to the "off" position, then the smaller the resistance value, the better. Then rotate the shaft handle slowly clockwise, the resistance value should gradually increase, the pointer in the head should move smoothly. When the shaft shaft rotates to the extreme position "3", the resistance value should be close to the nominal value of the potentiometer. If the pointer of multimeter is in potentiometer shaft handle turns in the process have jumble phenomenon, the fault that explains active contact has bad contact.
5. Positive temperature coefficient thermistor (PTC) detection. Multimeter R×1 is used for detection, which can be divided into two steps: A Normal temperature detection (indoor temperature is close to 25℃); The two pins of the PTC thermistor are measured out and the actual resistance is compared with the nominal resistance. The difference between the two is normal within ±2 ω. If the difference between the actual resistance value and the nominal resistance value is too large, it indicates that the performance is poor or damaged. B heating detection; Can be carried in the normal temperature test, on the basis of normal, the second step test - heating test, a heat source (such as soldering iron) close to the PTC thermistor for the heating, at the same time using a multimeter to monitor whether the resistance value increases with the rise of temperature and, if yes, explain thermistor is normal, if there were no changes in resistance, illustrate its performance becomes bad, can't continue to use. Do not make the heat source and PTC thermistor too close or direct contact thermistor, in order to prevent its hot damage.
6. Negative temperature coefficient thermistor (NTC) detection
(1) Measurement of nominal resistance value Rt The method of measuring NTC thermistor with a multimeter is the same as that of measuring ordinary fixed resistance, that is, according to the nominal resistance value of NTC thermistor to choose the appropriate resistance can directly measure the actual value of Rt. But because NTC thermite is 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℃, so when using A multimeter to measure Rt, it should also be carried out when the ambient temperature is close to 25℃, 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 Ensure correct operations. During the test, do not pinch the thermistor body by hand to prevent human body temperature from affecting the test.
(2) To estimate the temperature coefficient αt, the resistance value Rt1 was measured at room temperature T1, and then the electric soldering iron was used as the heat source, close to the thermistor Rt, and the resistance value RT2 was measured. At the same time, the average temperature T2 on the surface of the thermistor Rt was measured with a thermometer and then calculated.
7. Detection of varistor. Measure the positive and reverse insulation resistances between the two pins of the varistor with the R× 1K block of the multimeter. The positive and reverse insulation resistances are both large. Otherwise, the leakage current is large. If the resistance measured is too small, the varistor is damaged and cannot be used.
8. Photosensitive resistance detection. A covers the transmittance window of the photosensitive resistor with A black paper. At this time, the pointer of the multimeter is basically kept stationary and the resistance value is close to great. The higher the value, the better the performance of the photoresistor. If this value is very small or close to zero, it indicates that the photoresistor has been burned through and damaged and can no longer be used. B Aligns a light source with the transmittance window of the photoresistor. In this case, the pointer of the multimeter should swing greatly and the resistance value decreases significantly. The lower the value, the better the performance of the photoresistor. If this value is very large or even very large, it indicates that the internal open circuit of the photoresistor is damaged and can no longer be used. C Align the light transmission window of the photosensitive resistor with the incident light, and shake the upper part of the shading window of the photosensitive resistor with a small black paper to make it intermittently exposed to light. At this time, the pointer of the multimeter should swing from side to side with the shaking of the black paper. If the multimeter pointer stays in a certain position and does not move with the paper, the photosensitive material of the photosensitive resistor is damaged.
