Resistance is one of the most basic components in electronic circuits. Testing resistors is the basic skill for mastering and learning electronic technology. The following describes the testing methods and experience of common resistors.
1. Fixed resistor
Test Methods:
The actual resistance value can be measured by connecting the two test leads (not positive or negative) to the two ends of the resistor. In order to improve the measurement accuracy; range should be selected according to the nominal value of the sense resistor size.
Test experience:
(1) Due to the nonlinear relationship of the electrical barrier scale; its middle section is more delicately distributed, so the pointer indication value should fall as far as possible to the middle position of the scale, that is, within the range of 20%-80% radians of the beginning of the full scale. To make the measurement more accurate. Depending on the resistance error level, an error of 5%, ±10% or ±20% of the soil is allowed between the reading and the nominal resistance. If it does not match, if the error range is exceeded, the resistance value is changed.
(2) When testing, especially when measuring the resistance of resistance above tens of k ohms, do not touch the conductive part of the test leads and the resistor. The detected resistance is soldered from the circuit, at least one end must be welded to avoid the circuit. The other components have an influence on the test, resulting in measurement error. Although the resistance of the color ring resistor can be determined by the color circle mark, it is better to use a multimeter to measure the actual resistance value when using it.
For the detection of cement resistance, since it is usually also a fixed resistor, the method of detecting the cement resistance is exactly the same as detecting the ordinary fixed resistor.
2. Fuse resistor
Test Methods:
(1) 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 it is open without any trace on its surface, it means that the current flowing is just equal to or slightly larger than its rated melting value.
(2) For the judgment of the fuse resistor without any trace on the surface, it can be measured by the multimeter Rxl 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.
Test experience
In practice, there are also a small number of blown resistors that are broken down or shorted in the circuit.
3. Potentiometer
Test Methods:
(1) When checking the potentiometer, first turn the handle, try to check whether the rotation of the handle is smooth, and whether the switch is flexible. When the switch is turned on or off, the "click" sound is crisp, and the sound of the internal contact point of the potentiometer and the friction of the resistor body is heard. If there is a "sand" sound, the quality is not good.
(2) 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.
Use the multimeter's electrical blocking to measure the "1" and "3" 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. Check if the movable arm of the potentiometer is in good contact with the resistor. Use the electric resistance of the multimeter to measure both ends of "1" and "2", and turn the rotary shaft 2 of the potentiometer counterclockwise to a position close to "off". At this time, the smaller the resistance value, the better. Then slowly rotate the shank clockwise, the resistance value should be gradually increased, and the pointer in the meter should move smoothly. When the shank is turned to the extreme position "3", the resistance should be close to the nominal value of the potentiometer (similar to when measuring "2", "3").
Test experience:
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.
4. Positive temperature coefficient thermistor (PTC)
Test Methods:
Use the multimeter Rxl block, specifically can be divided into two steps: First, the normal temperature detection (indoor temperature is close to 25 ° C), the two pins touch the two pins of the PTC thermistor to measure its actual resistance, and the nominal resistance is relative to the nominal resistance Compared, the difference between the two is within _±2Ω. If the actual resistance is too different from the nominal resistance, it indicates poor performance or damage. The second is the heating test. On the basis of the normal temperature test, the second step test can be carried out. The heating test is performed, and a heat source (such as a soldering iron) is heated near the PTC thermistor, and the resistance value is monitored by a multimeter. Whether it increases with the increase of temperature, if it is, the thermistor is normal, if the resistance value does not change, it indicates that its performance is deteriorated and cannot be used continuously.
Test experience:
Do not place the heat source too close to or directly in contact with the PTC thermistor to prevent it from being burnt.
5. Negative temperature coefficient thermistor (NTC)
Test Methods:
(1) The nominal resistance value Rt is measured. The method of measuring the NTC thermistor with a multimeter is the same as the method of measuring the ordinary fixed resistor, that is, selecting the appropriate electrical block according to the nominal resistance of the NTC thermistor, the actual value of Rt can be directly measured.
(2) Estimate the temperature coefficient. First, the resistance value Rtl is 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 average temperature t2 of the surface of the thermistor RT is measured by a thermometer.
Test experience:
Because the NTC thermistor is sensitive to temperature, the following points should be noted when testing:
(1) Rt is measured by the manufacturer at an ambient temperature of 25 ° C. Therefore, when measuring Rt with a multimeter, it should also be carried out when the ambient temperature is close to 25 ° C to ensure the reliability of the test.
(2) The measured power shall not exceed the specified value to avoid the measurement error caused by the current thermal effect. When testing, do not hold the thermistor body with your hands to prevent the body temperature from affecting the test.
6. Varistor
Test Methods:
Use the Rxlk block of the multimeter to measure the positive and negative insulation resistance between the two pins of the varistor, which should be infinite.
Test experience:
If the measured resistance is not infinite, it indicates leakage current. If the measured resistance is small, the varistor is damaged; it cannot be used.
7. Photoresistance
Test Methods:
(1) The black light film is used to cover the light-transmitting window of the photoresistor. At this time, the pointer of the multimeter is basically kept, and the resistance is close to infinity.
(2) 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.
(3) Align the light-receiving 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 photoresistor has been damaged.
Test experience:
For method (1), the larger the test value, the better the photo resistance performance. If the value is small or close to zero, the photoresistor has been burned through and can no longer be used. For method (2), the smaller the value, the better the photoresistor performance. If this value is large, it indicates that the open circuit of the photoresistor is damaged and cannot be used again.
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