Commonly used resistor detection methods and experience

1. Detection of fixed resistors.

A. The actual resistance value can be measured by connecting the two test leads (without positive and negative) to the two 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 non-linear relationship of the ohmic scale, its middle section is finer. Therefore, 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% radians of the full scale, so that The measurement is more accurate. Depending on the level of resistance error. Errors of ±5%, ±10%, or ±20% are allowed between the reading and the nominal resistance, respectively. If it does not match, the error range is exceeded, indicating that the resistance value has changed.

B. Note: When testing, especially when measuring the resistance of tens of kΩ or more, do not touch the conductive part of the test leads and the resistor; the detected resistance is soldered from the circuit, at least one head should be soldered to avoid the circuit. The other components in the test have an influence on the test, causing measurement error; although the resistance of the color ring resistor can be determined by the color circle 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 resistors.

3. Detection of the fuse resistor.

In the circuit, when the fuse resistor is melted and disconnected, 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 its load is too heavy, and the current passing through it exceeds the rated value many times; If the surface is open without any trace, it means that the current flowing is just equal to or slightly larger than its rated blown value. The judgment of the fuse resistor with no trace on the surface can be measured by the multimeter R×1 block. To ensure accurate 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 open circuit. If the measured resistance value is far from the nominal value, it indicates that the resistance value is not suitable for reuse. In the maintenance practice, it is found that there are also a few blown resistors that are short-circuited in the circuit, and attention should 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 or off, and listen to the internal contact point of the potentiometer and the friction of the resistor body. Sound, if there is a "sand" sound, the quality is not good. When testing with a multimeter, first select the appropriate electrical blocking position of the multimeter according to the resistance of the potentiometer to be tested, and then perform the detection as follows. 

A. Uses the ohmmeter of the multimeter to measure the "1" and "2" ends. The reading should be the nominal resistance of the potentiometer. If the pointer of the multimeter does not move or the resistance value is different, it indicates that the potentiometer is damaged. 

B. Check that the movable arm of the potentiometer is in good contact with the resistor. Use the ohmmeter of the multimeter to measure both ends of "1", "2" (or "2", "3"), and turn the rotary shaft of the potentiometer counterclockwise to the position close to "off". The smaller the resistance value. The better. Rotate the shank slowly clockwise, the resistance value should be gradually increased, and the pointer in the meter should move smoothly. When the arbor is turned to the extreme position "3", the resistance should be close to the nominal value of the potentiometer. If the pointer of the multimeter has a jumping phenomenon during the rotation of the shaft handle of the potentiometer, it indicates that the movable contact has a fault of poor contact.

5. Detection of positive temperature coefficient thermistor (PTC).

When testing, use the multimeter R × 1 block, specifically can be divided into two steps: A normal temperature detection (indoor temperature is close to 25 ° C); two pens contact the two pins of the PTC thermistor to measure its actual resistance, and with the standard The relative resistance value is called, and the difference between the two is within ±2 Ω. If the actual resistance is too different from the nominal resistance, it indicates poor performance or damage. B Heating detection; on the basis of the normal temperature test, the second test can be carried out—heating detection, heating a heat source (such as a soldering iron) close to the PTC thermistor, and monitoring the resistance value with a multimeter. It increases with the increase of temperature. If so, the thermistor is normal. If the resistance value does not change, the performance is deteriorated and cannot be used. Be careful not to get the heat source too close to or directly in contact with the PTC thermistor to prevent it from being burnt.

6. Detection of negative temperature coefficient thermistor (NTC).

(1) Measuring 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 of the NTC thermistor can directly measure the Rt. Actual value. However, because the NTC thermistor is sensitive to temperature, the following points should be noted during the test: ARt is measured by the manufacturer at an ambient temperature of 25 ° C. Therefore, when measuring Rt with a multimeter, the ambient temperature should be close to 25 Perform at °C 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 pay attention to correct operation. When testing, do not hold the thermistor body with your hands to prevent the body temperature from affecting the test.

(2) Estimate the temperature coefficient αt firstly measure the resistance value Rt1 at room temperature t1, then use the electric iron as the heat source, close to the thermistor Rt, measure the resistance value RT2, and measure the thermistor RT with the thermometer. 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 positive and negative insulation resistance between the two pins of the varistor, 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-transmissive window of the photoresistor with a black piece of paper. At this time, the pointer of the multimeter is basically kept, and the resistance is close to infinity. The larger the value, the better the photoresistor performance. If this value is small or close to zero, the photoresistor has been burned through and can no longer be used.

B. Align a light source with the light-transmitting window of the photoresistor. At this time, the pointer of the multimeter should have a large amplitude swing, and the resistance value is significantly reduced. The smaller the value, the better the photoresistor performance. If the value is large or infinite, it indicates that the open circuit of the photoresistor is damaged and cannot be used any more.

C. Align the light-resistance window of the photoresistor with the incident light, and shake it with the small black paper on the upper part of the light-shielding window of the photoresistor to make it intermittently receive light. At this time, the pointer of the multimeter should swing left and right with the shaking of the black paper. If the multimeter pointer is always stopped at a certain position and does not oscillate with the paper, it indicates that the photosensitive material of the photoresist is damaged.