How to use the resistance file of the multimeter
(1) Measure the resistance with a multimeter
The ohm range of a multimeter can measure the resistance of a conductor. The ohm level is represented by "Ω" and is divided into four levels: R×1, R×10, R×100 and R×1K. Some multimeters also have R×10k files. Use the multimeter ohm gear to measure the resistance, in addition to the requirements that should be done before use, you should also follow the following steps.
1. Set the selector switch to the R×100 gear, and short-circuit the two test pens to adjust the zero position adjustment knob of the ohm position, so that the needle points to the zero position on the right end of the resistance scale line. If the pointer cannot be adjusted to zero, it means that the battery voltage in the watch is insufficient, and the battery should be replaced.
2. Use two test pens to respectively touch the two pins of the resistance to be measured for measurement. Correctly read the value of the resistance pointed by the pointer, and then multiply by the magnification (R×100 gear should be multiplied by 100, R×1k gear should be multiplied by 1000…). is the resistance value of the resistor being measured.
3. In order to make the measurement more accurate, the pointer should be near the center of the scale line when measuring. If the declination angle of the pointer is small, the R×1k gear should be used, and if the pointer declination angle is large, the R×1O gear or R×1 gear should be changed. After each gear change, the ohmic zero adjustment knob should be adjusted again, and then measured again.
4. After the measurement, the test leads should be pulled out, and the selector switch should be set to the "OFF" gear or the maximum AC voltage gear. Put away the multimeter.
When measuring resistance, pay attention to:
1. The measured resistance should be removed from the circuit and then measured.
2. Do not touch the two test pens together for a long time.
3. Two hands cannot touch the metal rods of the two test leads or the two pins of the resistance under test at the same time. It is better to hold the two test leads with the right hand (as shown in Figure 3-8).
4. If the ohm gear is not used for a long time, the battery should be removed from the watch.
Skill training Measuring resistance with a multimeter
Objective To master the use of the ohm gear of the multimeter, and to practice measuring the resistance with the multimeter.
Equipment multimeter 10 resistors with different resistance value color rings
(1) Insert 10 resistors on the cardboard. According to the color circle on the resistor, write their nominal value.
(2) Adjust the multimeter as required, put it in the R×100 gear, and adjust the ohm gear zero adjustment knob to zero.
(3) Measure 10 resistances respectively. Write the measurement next to the resistance. When measuring, note that the reading should be multiplied by the magnification.
(4) If the declination angle of the pointer is too large or too small during measurement, it should be changed and then measured. After shifting, it should be zeroed again before it can be used.
(5) Check each other. How many of the 10 resistors did you measure correctly? Compare the measured value to the nominal value to understand the error of each resistor.
(6) Store the multimeter as required.
Detection methods and experience of resistors:
1. Detection of fixed resistors. A? The actual resistance value can be measured by connecting two test leads (regardless of positive and negative) to the pins at both 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 nonlinear relationship of the ohmic scale, its middle section is relatively finely divided, 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% to 80% radian from the beginning of the full scale, so that the Measurement is more accurate. The error level varies according to the resistance. Errors of ±5%, ±10% or ±20% are allowed between the reading and the nominal resistance value, respectively. 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 a resistance value of tens of kΩ or more, do not touch the test leads and the conductive parts of the resistance; the resistance to be tested is welded from the circuit, and at least one head must be welded to avoid the circuit. Other components in the tester will affect the test and cause 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 resistance. The method and precautions for detecting cement resistance are exactly the same as those for detecting ordinary fixed resistance.
3. Detection of fuse resistors. In the circuit, when the fuse resistor fuses and opens the circuit, 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 there is no trace on its surface and the circuit is open, it means that the current flowing is just equal to or slightly greater than its rated fusing value. For the judgment of the quality of the fuse resistor without any trace on the surface, it can be measured with the help of the multimeter R×1 block. To ensure the accuracy of the measurement, one end of the fuse resistor should be soldered off the circuit. If the measured resistance value is infinite, it means that the fuse resistor has failed to open circuit. If the measured resistance value is far from the nominal value, it means that the resistance value has changed and should not be used anymore. 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 also be paid when testing.
4. Detection of potentiometer. 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 and off, and listen to the friction between the contact point inside the potentiometer and the resistance body. Sound, if there is a "rustling" sound, it means the quality is not good. When testing with a multimeter, first select the appropriate resistance barrier of the multimeter according to the resistance value of the potentiometer to be tested, and then perform the detection according to the following method.
A. Use the ohm stop of the 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 is very different, it indicates that the potentiometer is damaged. B? Check whether the contact between the movable arm of the potentiometer and the resistance sheet is good. Use the ohm gear of the multimeter to measure the two ends of "1", "2" (or "2", "3"), and turn the shaft of the potentiometer counterclockwise to the position close to "off", the smaller the resistance value at this time. the better. Then slowly rotate the shank clockwise, the resistance value should gradually increase, and the pointer in the meter head should move smoothly. When the shank is turned to the extreme position "3", the resistance value should be close to the nominal value of the potentiometer. For example, the pointer of the multimeter jumps during the rotation of the shaft handle of the potentiometer, indicating that the movable contact has a faulty contact.
5. Detection of positive temperature coefficient thermistor (PTC). When testing, use the multimeter R×1 block, which can be divided into two steps: A? Normal temperature detection (indoor temperature is close to 25 ℃); Compared with the nominal resistance value, it is normal if the difference between the two is within ±2Ω. If the actual resistance value is too different 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 detection can be carried out, a heat source (such as an electric soldering iron) is heated close to the PTC thermistor, and its resistance value is monitored with a multimeter at the same time Whether it increases with the increase of temperature, if so, it means that the thermistor is normal, if there is no change in the resistance value, it means that its performance has deteriorated and cannot be used continuously. Be careful not to place the heat source too close to the PTC thermistor or contact the thermistor directly to prevent burning it.
6. Detection of negative temperature coefficient thermistor (NTC).
(1), measure the nominal resistance value Rt
The method of measuring NTC thermistor with a multimeter is the same as the method of measuring ordinary fixed resistance, that is, selecting an appropriate electrical barrier according to the nominal resistance value of the NTC thermistor can directly measure the actual value of Rt. However, because NTC thermistors are 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 measured at the ambient temperature. It is carried out when it is close to 25 °C to ensure the reliability of the test. B? The measurement power shall not exceed the specified value, so as to avoid the measurement error caused by the thermal effect of the current. C? Pay attention to correct operation. When testing, do not pinch the thermistor body with your hands to prevent human body temperature from affecting the test.
(2), estimated 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 on the surface of the thermistor RT at this time before calculating.
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 of 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-transmitting window of the photoresistor with a black piece of paper. At this time, the pointer of the multimeter remains basically unchanged, and the resistance value is close to infinity. The larger the 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 burnt 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 have a relatively large swing, and the resistance value will be significantly reduced. The smaller the value, the better the performance of the photoresistor. If this value is very large or even infinite, it indicates that the internal open circuit of the photoresistor is damaged and cannot be used any longer. C? Align the light-transmitting window of the photoresistor with the incident light, and use a small black piece of paper to shake the upper part of the light-shielding window of the photoresistor 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 shaking of the paper, it means that the photosensitive material of the photoresistor has been damaged.
