Detection of fixed resistors

The actual resistance value can be measured by connecting the two test pens (both positive and negative) with the pins at both ends of the resistor respectively. In order to improve the measurement accuracy, the range should be selected according to the nominal value of the measured resistance. Due to the non-linear relationship of the ohm block scale, the middle section of the scale is finer. Therefore, the pointer indication value should fall to the middle section of the scale as much as possible, I .e. within the range of 20% ~ 80% radian of the start of the full scale, so as to make the measurement more accurate. Depending on the resistance error level. An error of ± 5%, ± 10% or ± 20% is allowed between the reading and the nominal resistance. 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 resistance values above tens of kΩ, do not touch the conductive parts of the stylus and resistor with your hands; The detected resistor should be welded from the circuit, and at least one head should be welded 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, but in use, it is best to use a multimeter to test its actual resistance.

Detection of fuse resistor 

In the circuit, when the fuse resistor is fused and opened, 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 its surface is open without any trace, it indicates that the current flowing through it is just equal to or slightly greater than its rated fuse value. For the judgment of the fuse resistor without any trace on the surface, it can be measured with the help of a multimeter R× 1 block. In order to ensure accurate measurement, one end of the fuse resistor should be welded from the circuit. If the measured resistance value is infinite, it indicates that the fuse resistor has failed to open circuit. 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 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 be paid to the detection.

Detection of potentiometer

When checking the potentiometer, first turn the handle to see if the handle rotates smoothly, if the switch is flexible, if the "click" sound is clear when the switch is on and off, and listen to the sound of friction between the contact point inside the potentiometer and the resistor body. If there is a "rustling" sound, the quality is not good. When testing with a multimeter, first select the appropriate resistance gear of the multimeter according to the resistance value of the measured potentiometer, and then perform the test according to the following methods.

A. Use the ohm block of a 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 differs a lot, it indicates that the potentiometer is damaged. 

B. Whether the contact between the movable arm of the potentiometer and the resistor is good. Use the ohm gear of the multimeter to measure both ends of "1" and "2" (or "2" and "3"), and turn the rotating 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 shaft clockwise, the resistance value shall gradually increase, and the pointer in the meter head shall move steadily. When the mandrel is turned to the extreme position "3", the resistance should be close to the nominal value of the potentiometer. For example, the pointer of the multimeter has a beating phenomenon during the rotation of the shaft handle of the potentiometer, indicating that the movable contact has a poor contact fault.

Detection of Positive Temperature Coefficient Thermistor (PTC)

When testing, use multimeter R× 1 gear, the specific operation can be divided into two steps:

A. Normal temperature detection (indoor temperature is close to 25 ℃); The actual resistance value of the two pins of the PTC thermistor contacted by the two test pens is measured and compared with the nominal resistance value. The difference between the two is normal within ± 2Ω. If the actual resistance value is too large 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 test-heating test can be carried out. A heat source (such as electric soldering iron) is placed close to the PTC thermistor to heat it. At the same time, a multimeter is used 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, its performance is deteriorated and cannot be used continuously. Be careful not to make the heat source and PTC thermistor too close or direct contact with the thermistor to prevent it from being burned.

Detection of Negative Temperature Coefficient Thermistor (NTC)

1) Measure the nominal resistance value Rt: The method of measuring NTC thermistor with multimeter is the same as that of measuring ordinary fixed resistance, that is, the actual value of Rt can be directly measured according to the nominal resistance value of NTC thermistor. However, because NTC thermistor 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 measuring Rt with a multimeter, it should also be carried out when the ambient temperature is close to 25 ℃ to ensure the credibility of the test. The measuring power shall not exceed the specified value to avoid measurement error caused by current thermal effect. C. Pay attention to correct operation. During the test, 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, close to the thermistor Rt, measure the resistance value RT2, and at the same time measure the average temperature t2 on the surface of the thermistor RT at this time with a thermometer, and then calculate.

Detection of varistor

Measure the positive and reverse insulation resistance between the two pins of the varistor with the R× 1k block of the multimeter, which are both infinite, otherwise, the leakage current is large. If the measured resistance is very small, the varistor is damaged and cannot be used.

Photosensitive resistance detection

A, with a piece of black paper to the light transmission window of the photosensitive resistor cover, at this time the pointer of the multimeter remains basically motionless, the resistance value is close to infinity. The larger the value, the better the performance of the photosensitive resistor. If this value is very small or close to zero, indicating that the photosensitive resistor has been burned through damage, can no longer continue to use.

B. Aim a light source at the light transmission window of the photosensitive resistor. At this time, the pointer of the multimeter should swing by a large margin, and the resistance value is obviously reduced. The smaller the value, the better the performance of the photosensitive resistor. If this value is large or even infinite, it indicates that the internal open circuit of the photosensitive resistor is damaged and can no longer be used. 

C. Align the light transmission window of the photosensitive resistor with the incident light, and use a small piece of black paper to shake the upper part of the light shielding window of the photosensitive resistor 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 paper shaking, it means that the photosensitive material of the photosensitive resistor has been damaged.