Resistor Testing Methods and Experience

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Resistor Testing Methods and Experience [Copy link]

1. Detection of fixed resistors. A. Connect two test leads (regardless of positive and negative) to the two 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 nominal value of the measured resistor. Due to the nonlinear relationship of the ohm scale, its middle section is more finely divided, so the pointer indication value should be as close to the middle of the scale as possible, that is, within the 20% to 80% arc range of the full scale, so as to make the measurement more accurate. According to the different resistance error levels. The reading is allowed to have an error of ±5%, ±10% or ±20% between the nominal resistance value. 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 resistors with a resistance value of more than tens of kΩ, do not touch the test leads and the conductive part of the resistor with your hands; the resistor to be tested should be welded off from the circuit, and at least one end should be welded off 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 value when using it.

2? Detection of cement resistors. The method and precautions for detecting cement resistors are exactly the same as those for detecting ordinary fixed resistors.

3? Detection of fuse resistors. In the circuit, when the fuse resistor is melted and the circuit is opened, a judgment can be made based on experience: if the surface of the fuse resistor is found to be black or burnt, it can be concluded that it is overloaded and the current passing through it exceeds the rated value many times; if there is no trace on its surface and it is open, it means that the current flowing through is just equal to or slightly greater than its rated fuse value. To determine whether a fuse resistor without any traces on the surface is good or bad, you can use the multimeter R×1 gear to measure it. To ensure accurate measurement, one end of the fuse resistor should be welded off the circuit. If the measured resistance is infinite, it means that the fuse resistor has failed and is open-circuited. If the measured resistance is far from the nominal value, it means that the resistance has changed and should not be used again. In maintenance practice, it is found that a few fuse resistors are short-circuited in the circuit, and attention should be paid to this during detection.

4. Detection of potentiometer. When checking the potentiometer, first turn the handle to see if the 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 friction sound between the internal contact point of the potentiometer 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 position of the multimeter according to the resistance value of the potentiometer to be measured, and then perform the detection according to the following method.

A? Use the ohmmeter of the multimeter to measure the two ends of "1" and "2". The reading should be the nominal resistance of the potentiometer. If the pointer of the multimeter does not move or the resistance value is very different, it means that the potentiometer is damaged. B? Check whether the movable arm of the potentiometer is in good contact with the resistor. Use the ohmmeter of the multimeter to measure the two ends of "1" and "2" (or "2" and "3"), and rotate the shaft of the potentiometer counterclockwise to a position close to "off". At this time, 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 poor contact fault.

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 test (indoor temperature close to 25℃); touch the two pins of the PTC thermistor with two test leads to measure its actual resistance, and compare it with the nominal resistance. If the difference between the two is within ±2Ω, it is normal. If the actual resistance value is too different from the nominal resistance value, it means that its performance is poor or it is damaged. B? Heating test; on the basis of normal normal temperature test, the second step test - heating test can be carried out. Place a heat source (such as a soldering iron) close to the PTC thermistor to heat it, and 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. Be careful not to make the heat source too close to the PTC thermistor or directly contact the thermistor to prevent it from being burned.

6? Negative temperature coefficient thermistor (NTC) detection.

(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 ordinary fixed resistors, that is, selecting a suitable resistance block according to the nominal resistance value of the NTC thermistor can directly measure the actual value of Rt. However, since the NTC thermistor is very sensitive to temperature, the following points should be noted 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 done when the ambient temperature is close to 25°C to ensure the reliability of the test. B? The measured power must not exceed the specified value to avoid measurement errors caused by the thermal effect of the current. C? Pay attention to correct operation. During the test, do not pinch the thermistor body with your hands to prevent the body temperature from affecting the test.

(2) Estimating 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 use a thermometer to measure the average temperature t2 of the thermistor RT surface at this time and then calculate.

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, it means that the leakage current is large. If the measured resistance is very small, it means that the varistor is damaged and cannot be used.

8? Detection of photoresistor. A? Cover the light-transmitting window of the photoresistor with a piece of black paper. At this time, the pointer of the multimeter basically remains motionless, and the resistance value is close to infinity. The larger this 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 burned 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 significantly and the resistance value will be significantly reduced. The smaller this value, the better the performance of the photoresistor. If this value is very large or even infinite, it means that the internal open circuit of the photoresistor is damaged and it can no longer be used. C? Align the light-transmitting window of the photoresistor with the incident light, and shake a small black paper on the upper part of the light-shielding window of the photoresistor to make it receive light intermittently. At this time, the multimeter pointer should swing left and right with the shaking of the black paper. If the multimeter pointer 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.

yzstwz

Very detailed support

luowei01051145

Support, thumbs up!

catdogpanda

I need it, but it's expensive... :Q

vaejoly

Thank you, Lara!