Resistors, or resistors for short, are one of the most basic components in electronic circuits. Testing resistors is the basic skill for mastering and learning electronic technologies! The following describes common resistor testing methods and experiences.
1. Fixed resistor test method: the two table pens (unnecessarily plus or minus) are connected to the two ends of the resistor 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 measured resistance.
Test experience:
(1) Due to the non-linear relationship of the electric barrier scale; its distribution in the middle section is finer, so the pointer indication value should fall as far as possible in the middle position of the scale, that is, within the range of 20% to 80% of the arc from the start of full scale. To make the measurement more accurate. According to different resistance error levels, errors of 5%, ±10% or ±20% of soil are allowed between the reading and the nominal resistance, respectively. If it does not match, the error range is exceeded, indicating that the resistance has changed.
(2) When testing, especially when measuring a resistance value of several tens of kilo-ohms or more, do not touch the conductive parts of the test leads and the resistors. The detected resistance is soldered off from the circuit. At least one end must be soldered to avoid the circuit. The other components affect the test and cause measurement errors. Although the resistance value of the color ring resistor can be determined by the color circle mark, it is best to use a multimeter to measure the actual resistance value. For the detection of the cement resistance, since it is usually a fixed resistance, the method of detecting the cement resistance is the same as that of the ordinary fixed resistance.
2. Fuse resistor test method:
(1) In the circuit, when the fuse resistor melts and disconnects the circuit, judgment can be made based on experience; if it is found that the surface of the fuse resistor is black or burnt, it can be concluded that the load is too heavy, and the current passing through it exceeds the rated value by many times As a result; if the surface is open without any traces, it indicates that the current flowing is exactly equal to or slightly larger than its rated melting value.
(2) For the judgement of the quality of the fuse resistor without any trace on the surface, it can be measured by means of the multimeter Rxl block. In order to ensure the accuracy of measurement, one end of the fuse resistor should be soldered off from the circuit. If the measured resistance is infinite, it indicates 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 re-use. Test experience; In practice, there are also a few blown resistors that are broken or shorted in the circuit.
Potentiometer test method:
(1) When checking the potentiometer, first turn the handle and try to see if the handle rotates smoothly and the switch is flexible. When the switch is on and off, the sound of “click†is crisp, and the sound of friction between the internal contact point of the potentiometer and the resistor body is heard. If there is a “rustle†sound, the quality is not good.
(2) When testing with a multimeter, first select the proper barrier for the multimeter according to the resistance of the potentiometer under test, and then perform the test as described below. Use the multimeter's electrical resistance to measure the "1", "3" at both ends, its reading should be the potentiometer's nominal resistance. If the multimeter's pointer is not moving or the resistance value is much different, it indicates that the potentiometer is damaged. Check whether the contact between the movable arm and the resistor chip of the potentiometer is good. Measure the “1†and “2†at both ends with a multimeter and turn the potentiometer's shaft 2 counterclockwise to a position close to “off.†The smaller the resistance, the better. Rotate the shaft shank again 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 (similar to when the "2" and "3" ends are measured). Test experience: if the indicator of the multimeter has a beating phenomenon during the rotation of the shaft handle of the potentiometer, it indicates that the movable contact has a fault of poor contact.
4. Positive temperature coefficient thermistor (PTC)
Test method: Use a multimeter Rx1 block, which can be divided into two steps: First, the room temperature detection (indoor temperature close to 25 °C), the two table pen touch the PTC thermistor of the two pins to measure the actual resistance, and with the nominal Relative resistance, the difference between the two within _± 2Ω is normal. If the actual resistance value differs significantly from the nominal resistance value, it indicates that the performance is poor or has been damaged. The second is heating detection. On the basis of normal temperature testing, the second step of testing can be performed. Heating detection is performed. A heat source (such as electric iron) is placed close to the PTC thermistor to heat it, and its resistance is monitored with a multimeter. Whether it increases with increasing temperature. If yes, it indicates that the thermistor is normal. If the resistance does not change, it indicates that the performance is deteriorating and cannot be used.
Test experience: Do not place the heat source close to or directly contact the thermistor with the PTC thermistor to prevent it from scorching.
5. Negative temperature coefficient thermistor (NTC)
Test Methods:
(1) Measure the nominal resistance value Rt. The method of measuring the NTC thermistor with a multimeter is the same as measuring the ordinary fixed resistor. That is, according to the nominal resistance of the NTC thermistor, the proper resistance can be selected and the actual value of Rt can be directly measured.
(2) Estimate the temperature coefficient. The resistance value Rtl is first measured at room temperature T1, and then the electric iron is used as a heat source, and the resistance value RT2 is measured near the thermistor Rt. At the same time, the thermometer is used to measure the average temperature t2 of the surface of the thermistor RT at the same time.
Test experience:
Since the NTC thermistor is sensitive to temperature, the following points should be noted during the test:
(1) Rt is measured by the manufacturer when the ambient temperature is 25°C. Therefore, when using a multimeter to measure Rt, it should also be performed when the ambient temperature approaches 25°C to ensure the reliability of the test.
(2) The measured power must not exceed the specified value, so as to avoid measurement errors due to current heating effect. Do not hold the thermistor body by hand during the test to prevent the body temperature from affecting the test.
6. Varistor test method: Measure the positive and negative insulation resistances between the two terminals of the varistor with the Rxlk block of the multimeter, all of which should be infinite.
Test experience:
If the measured resistance is not infinity, there is a leakage current. If the measured resistance is small, the varistor is damaged; it cannot be used.
7. Photosensitive resistance test method:
(1) Use a black paper to cover the light-transmitting window of the photoresistor. At this time, the pointer of the multimeter will basically remain unchanged, and the resistance value will approach infinity.
(2) A light source is aimed at the light-transmitting window of the photoresistor. At this time, the pointer of the multimeter should have a large swing, and the resistance value is obviously reduced.
(3) Align the light-transmitting window of the photoresistor with the incident light, and use a small black paper to shake the upper part of the light-shielding window of the photoresistor so that it will receive light intermittently. At this time, the pointer of the multimeter should swing around with the shaking of the black paper. If the pointer of the multimeter is always stopped at a certain position, it will not oscillate with the shaking of the paper, which means that the photosensitive material of the photoresistor has been damaged. Test experience: For method (1), the larger the test value, the better the performance of the photoresistor. If this value is very small or close to zero, it indicates that the photoresistor has burned through and damaged, and can no longer be used. For method (2), the smaller the value, the better the performance of the photoresistor. If this value is large, it indicates that the open circuit of the photoresistor is damaged and can no longer be used.
(This article is translated from the world of electronic engineering: http://Test_and_measurement/2011/1011/article_3683.html)
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