The universal meter is referred to as the multimeter, which is the most commonly used multi-purpose electrical measuring instrument for measuring and repairing measuring instruments, automatic devices and household appliances. For a variety of measured objects and under a variety of test conditions, how to correctly and ingeniously use a multimeter is directly related to the accuracy of the measurement results, and even affect the life and safety of the multimeter. This article summarizes the experience of using the multimeter's accumulated experience in the work and summarizes it as follows.
1. The multimeter has a digital multimeter and an analog multimeter (ie, an analog multimeter).
The digital multimeter has high accuracy and resolution, and has strong overload capacity, good anti-interference performance, multiple functions, small size, and light weight. It can also fundamentally eliminate the parallax when reading data. These advantages are all common simulations. The multimeter is incomparable. However, because the digital multimeter is measured and displayed in an intermittent manner, it is not convenient to observe the continuous change process of the measured quantity of electricity and its changing trend. For example, the digital multimeter checks the charging process of the capacitor, the resistance of the thermistor with temperature, and observes the characteristics of the resistance of the photoresistor with the illumination, which is not as convenient and intuitive as the analog multimeter. In short, we must reasonably select the type of the multimeter and the performance indicators of the table according to the measured object and test requirements.
2. Before using the multimeter, fully understand its technical performance and characteristics, must be familiar with the role of the various selector switches, knobs, buttons and jacks, find out the correspondence between the sub-shift position of the selector switch and the dial scale.
3. Pay attention to the test environment of the multimeter.
For the analog multimeter, the working environment temperature to ensure accuracy is usually specified as 23 °C ± 10 °C, if the temperature deviation is too large, will affect the sensitivity of the meter, and change the parameters of the components within the table, resulting in temperature error; ambient humidity is generally 40% ~ 60%, the limit range of 25% ~ 80%, too wet will reduce the dielectric strength of the multimeter, and promote the deterioration of the components within the table. Since this type of watch is a magnetoelectric meter, it should not be close to a larger magnetically permeable material when it is used. For example, it should not be placed directly on the workbench of a steel material or on a large-sized iron device. The vicinity of the watch must not be strong. Electromagnetic field.
For digital multimeters, pay close attention to the impact of environmental temperature and humidity on test performance and the multimeter itself. Since the display liquid crystal material is a crystalline substance between solid and liquid, it will liquefy when the temperature is too high, and curing may occur when the temperature is lower than 0°C. This phenomenon will reduce its use. life. Moreover, although the specified operating temperature range is generally 0°C to 40°C, the accuracy index is guaranteed under certain temperature and humidity conditions, and exceeding this range will cause additional errors. For example, the accuracy of the DT1000 four and a half multimeter is measured at 23°C ± 5°C and relative humidity below 75%, and the effective time is only set to one year.
4. The multimeter must not be subjected to strong mechanical vibration and drop impact under any circumstances, because severe impact will cause the case to rupture, the LCD of the digital multimeter will fail, and the magnet of the analog multimeter will demagnetize to cause a decrease in sensitivity. Damage to the movable part of the watch (moving circle, hairspring, shaft tip, etc.), and damage to the multimeter from direct sunlight, dusty air, and corrosive substances should be avoided as much as possible.
5. The surface of the analog multimeter is equipped with a mechanical zero adjustment knob or adjustment screw. If it is found that the hands are not pointing at the mechanical zero position (ie the zero point of the voltage scale and the infinite scale of the ohm scale), the finger or screwdriver must be used. Lightly and slowly turn the mechanical zero adjustment mechanism to zero the hands to eliminate the zero point error.
6. For a digital multimeter (such as DT-890A) with a zero adjustment knob in the capacitance file, you must adjust the zero before testing the capacitor and after changing the gear to zero the reading. Most multimeters do not have a zero adjustment knob because they have an auto-zero feature, so there is no need to consider the zero return problem. When testing a large-capacity charged capacitor, be sure to first let go of the stored charge so that excessive charges do not damage the multimeter through the internal circuit discharge.
7. Before using the ohmmeter, the analog multimeter should be adjusted to the zero point of the ohm scale by the ohmic zero adjustment knob. If the adjustment is valid but cannot be set to zero, the battery is old and the battery needs to be updated. The ohmic zero point should be readjusted every time the ohmic gear is changed, and the phenomenon of zero point migration should also be frequently checked during use.
8. In order to reduce the measurement error, the range must be reasonably selected. For the analog multimeter, the measurement error is small when the hands are deflected to the full-scale value of 2/3 to 3/4, and the closer the full-scale value is, the smaller the error is. The ohm range is different. It is better to place the hands as close to the center of the ohm scale as possible. The closer to the center, the more accurate.
For digital multimeters, the choice of gears is aimed at maximizing the number of significant digits of the measured data.
9. If it is impossible to estimate the approximate range of the measured voltage or current in advance, you must first set the range selection switch to the highest level of the corresponding measurement type, and then change the value according to the size of the measurement display (indication) value. For digital multimeters with automatic conversion range (such as DT-860, DT-910, etc.), this concern can be avoided and overload can be reliably avoided. However, the measurement process of this kind of watch is long. Even if the measured quantity of electricity is very small, it must follow the rules of procedure and gradually decrease from the highest range until it is appropriate. In contrast, the DT-960T digital multimeter is automatically shifted from the lowest range to the highest range.
10. When measuring higher voltages, if you do not know the type of voltage in advance, you can place the range selection switch at the highest DC voltage level for the analog multimeter. When the test is performed, the deflector is a DC voltage indicating that there is no deflector. For AC voltage.
11. Analog multimeter to determine the performance of the transistor, generally should use R × 100Ω or R × 1kΩ file, but not appropriate to use R × 1Ω and R × 10kΩ file. Because R×1Ω file is not convenient to observe the leakage current of the pipe; R×10kΩ file is equipped with high-voltage battery inside (MF24 type, 500 type is 9V; MF10 type, MF12 type and MF30 type are 15V; MF5 type, MF121 type For 22.5V), it will inevitably lead to some high-voltage breakdown of the tube with a lower voltage, resulting in erroneous test results, or even damage the tube under test.
Due to the high internal resistance of the digital multimeter's ohm file, the test current that can be supplied is extremely weak (eg, 20kΩ: 75μA for the DT-830 and 60μA for the DT-840D). This is not enough to overcome the PN junction when judging semiconductor components. The dead-zone voltage, therefore measured resistance is much higher than the analog multimeter, and there is no linear proportional relationship between the readings of the two tables, so the structure does not form the basis for judging the performance of the tube, it should be changed to the diode test file Conduct the test.
12 digital multimeter in the Ohm file, diode test file and buzzer file position, the red meter and the table in contact with high potential with positive electricity, black table pen because of the virtual ground in the table and negatively charged, which is obviously analog and analog The polarity of the pens on the multimeter's ohmmeter is completely reversed, and care must be taken when testing polar components or related circuits.
13. When using ohms to detect circuit components or circuit systems, the power supply to the device or system under test must be disconnected first. If the object to be measured contains a capacitor with a large amount of charge, it must also be discharged in an appropriate manner. The measurement can only be performed on the premise that the measured part has no power factor. Otherwise, the multimeter can easily be damaged, especially the analog multimeter.
14. When measuring the current of a low internal resistance circuit (including a network with a low internal resistance power supply and a network with low load resistance), a large current range should be selected as much as possible; the voltage of the high internal resistance circuit (or power supply) should be measured When analog multimeter should try to choose a higher voltage range, digital multimeter due to its higher internal resistance and easier to meet the test requirements.
15. Do not use ohms to detect the internal resistance of various batteries, nor can you directly measure the internal resistance of a high-sensitivity meter. The former can easily damage the multimeter, which often causes the gauge under test to break the hands, and may even burn out the moving coil.
16. For digital multimeters, when the measured current is large (for example, greater than 200mA), use a special socket for large current (such as 10A or 20A, etc.) on the electric surface board to plug in the test leads, but most of the current of the meter is high. Overcurrent protection measures are not set in the range and must be guarded against overload. In addition, long-term metering may not be used as a long-range ammeter in the load line, and the measurement time should not exceed 15 seconds.
17. Ordinary multimeter AC measurement file is only suitable for measuring sine wave voltage or current RMS, it can not directly measure the sawtooth wave, triangle wave, square wave and other non-sinusoidal electricity. Even the sine wave power, its frequency parameter and waveform distortion degree must accord with the technical condition of the multimeter, otherwise, the measurement error will increase obviously. Non-sinusoidal voltages or currents can typically be measured with an electrodynamic, electromagnetic meter, or an rms digital multimeter such as the DT-980.
18. In the process of measuring voltage and current, it is better not to change the gear of the selector switch, especially in the case of higher voltage and higher current, the selector switch can easily generate an arc during the switching process and burn the touch of the switch. Points and damage the internal components and lines.
19. When the fuse in the watch is blown, it should be replaced according to the specifications specified in the manual, and should not be expanded or reduced at will.
20. For analog multimeters, in order to reduce the parallax of the read data, the eyes must be facing the hands. For a dial equipped with a mirror, the line of sight should be adjusted to coincide with the needle in the mirror and the parallax is minimized. The multimeter must also be placed horizontally and the maximum tilt angle must not exceed 10 degrees.
21. After the measurement is completed, the selector switch should be placed in the highest voltage level or neutral position (such as the "OFF" position of the M64 type, the "*" position of the 500 type) to prevent accidental burning of the watch during the next use. Since it has short-circuited the meter in the watch, it has better damping and protection functions. It not only can prevent the meter from being burned out, but also can withstand the harm caused by vibration and bumps on the meter during the carrying and transportation of the meter. . For a non-neutral multimeter, you can make a special line with plugs at both ends, insert the two plugs into the “+, -†jacks of the meter, and place the selector switch at the lowest level of DC current. This is also effective. .