Describe the use and safety precautions for the various multimeter functions in detail.
Electric barrier is the most widely used
The electrical barrier (ohm barrier) is generally used to measure the resistance value of the resistance, measure the on-off status of the circuit, circuit components, and judge whether the component is good or bad by measuring whether the resistance value between the pins of the component is consistent with the normal data.
The pointer multimeter should be zero-adjusted before use. Short-circuit the two test leads. At this time, the pointer is biased to the right. Adjust the zero-adjustment potentiometer so that the pointer points to the zero scale on the right. If zero adjustment is not possible, use the Rx1 to Rx1k gear, indicating that the 1.5V battery is insufficient; use the Rx10k gear, indicating that the 9V or 12V laminated battery is insufficient.
1. Range selection
The ohm dial is marked with Ω, the right side is zero, and the value increases toward the left. The scales are arranged in a non-linear manner, and the middle section is more accurate. After changing the resistance range every time, zero adjustment must be carried out to ensure the measurement accuracy.
After recognizing the resistance value, select the appropriate range and adjust to zero. The principle of selecting the appropriate range is to make the pointer of the measured value close to the middle of the dial as much as possible. If the resistance is 10Ω, choose Rx1 gear, if the resistance is 220Ω, choose Rx10 gear, if the resistance is 4.7kΩ, you can choose Rx100 gear, if the resistance is 68kΩ, you can choose Rx1k or Rx10k gear. Put the test leads on the two pins of the resistance to be tested, read the value indicated by the pointer of the resistance dial (the uppermost scale line), and then multiply it by the multiplier of the file, which is the resistance value of the resistance.
If a digital multimeter is used, the closer the resistance value of the measured component is to the range, the more accurate the measurement result will be. For example, 150Ω resistance can be measured with Rx200 gear and Rx2k gear, but the 200 gear has more accurate digits, and the measurement result is naturally more accurate.
2. On-road measurement
Most of the maintenance needs to measure the resistance value directly on the circuit board first. Because the on-line measurement will be affected by other series and parallel components, the measurement results will be biased. Generally, it is necessary to adjust the pair of test leads twice, and take the one with the larger resistance value as the reference resistance value. If a 82kΩ resistor is measured (see Figure 2), the measurement result is smaller than the nominal value and the difference is not large, indicating that the resistor is normal. If the measured resistance value is greater than the actual resistance value, it may be that the resistance value has increased; if the measured resistance value is small, if there are no small resistance resistors or inductance components connected in parallel, in this case, the pins must be soldered or disassembled. The next component is measured again.
Explain in detail the use of various functions of the multimeter and precautions
Figure 2 82kΩ resistance measured on the road
Under normal circumstances, there is a possibility that the resistance value of the resistor will increase, and it may be open-circuited by a large current or other reasons, but it is rare for the resistance value to decrease.
Notice
On-road measurement components generally need to disconnect their pins, and discharge large-capacity electrolytic capacitors, high-voltage capacitors, high-voltage packs, picture tubes and other power storage components before measurement. There are many ways to discharge, such as directly short-circuiting the component pins, using 1kΩ or 10kΩ, 2W resistors to discharge, the author recommends that you can use a soldering iron to discharge several times, that is, connect the plug of the soldering iron to the discharge point.
When there is a large difference in the data of the on-circuit measurement, it is necessary to analyze the condition of the circuit where the components are located. When a semiconductor device is connected in parallel with it, it will affect the resistance value.
When measuring diodes and triodes with ohms in the way, the measured forward and reverse resistance of the PN junction is very small. When there are no small resistance components connected in parallel, the PN junction may be broken down and short-circuited. If both positive and negative resistance values are large, it means that the circuit is open.
It must be pointed out that because the volt-ampere characteristic of the diode is non-linear, when the resistance of the diode is measured with different resistances of the multimeter, different resistance values will be obtained, for example, the resistance value measured by R×100 gear is compared with R×10 gear. will be bigger.
The copper foil line on the PCB is very long. You can know whether there is an open circuit by measuring the resistance of this line. It is also possible to determine whether two points on the circuit belong to the same line. When the resistance value of the measured components is different, you can cut off the component pins, cut the copper skin to measure, or you can desolder one, two or all of the components to remove the measurement.
knowledge development
It should be noted that the pins of some components have been oxidized, and the solder joints of the circuit board are oxidized or covered by chemicals. You need to scrape the pins or solder joints, or you can use the tip of a test pen to press the solder joints tightly to scratch or scrape the circuit. Only when the solder mask layer of copper foil is green, can the test leads make good contact and ensure accurate measurement, otherwise there will be misjudgment phenomena such as open circuit, large resistance value, and suspected poor contact.
A circuit or component has a certain resistance value, and the ohmmeter measures its resistance value to judge whether it is good or bad. Of course, this kind of measurement only reflects the quality of the components from certain aspects and parameters of the components, which is not comprehensive. From another perspective, it is good to use a multimeter to measure, but the installation work may not be normal. For example, if the diode is installed, there may be phenomena such as reverse leakage, increased forward resistance, and poor rectification characteristics. The installation of the triode may have insufficient magnification, poor frequency characteristics, and soft breakdown.
For example, when you use a small resistance block to measure the forward resistance of a certain PN junction is too large, and then use the R×1k block to measure again, it may be normal again. In fact, the characteristics of this tube have deteriorated, and it can no longer work normally in the circuit. or job instability.
No matter what type of components to measure, it is very beneficial for measurement to be able to learn from materials, other people’s experience, or to disassemble components to understand the internal structure and working principle. Of course, it is also very beneficial for circuit analysis and maintenance. For example, you can find out the special switch on-off relationship from common multi-connector switches.
A good way to measure components is: when you are a beginner, or when it is difficult to judge whether it is good or bad, choose a good product and compare it with the current measurement part to make an accurate judgment.
