Introduction to the use of digital multimeter, how to measure voltage/current with digital multimeter

　　As the name suggests, the working principle of a digital multimeter is mainly based on digital circuits to detect and analyze signals, and then provide LCD display through an analog-to-digital converter.

　　Although the digital multimeter is a more complex electronic measuring tool, if you operate it correctly according to the instructions, pay attention to the general precautions, and maintain a good use environment, the service life and accuracy of the instrument will be greatly improved and stabilized. The digital multimeter uses a liquid crystal display as a reading device, which has the characteristics of high measurement accuracy and safe and reliable use. It has a wide variety of models and is very easy to measure.

　　Digital multimeters mainly have common measurement functions such as DC voltage, DC current, AC voltage, AC current, resistance, circuit continuity test, diode test, etc. With the development of electronic technology, multimeters have many application functions: data storage, data transmission, oscilloscope, etc. Some of them are developing into professional application fields: power meter, leakage current tester, insulation resistance tester for power testing, infrared thermometer used in the industrial and mining industries, noise meter, anemometer for environmental testing, etc. The following takes the UT58C handheld ordinary digital multimeter of Uni-T Company as an example to talk about the use of multimeters. UT58C is a 3l/2-digit digital multimeter, but in addition to the voltage, current, and resistance test functions, it also has capacitance, frequency, diode, continuity buzzer, transistor hFE, and temperature measurement. Its unique large-screen display and input prompt function also bring a lot of convenience to users.

　　Use of Digital Multimeter

　　Measurement of AC and DC voltage:

　　Put the power switch in the ON position, and adjust the range switch to the appropriate range within the DCV (direct current) or ACV (alternating current) range as needed. Insert the red test lead into the V/Ω hole, the black test lead into the COM hole, and then connect the two test leads to the measured point. The voltage of the measured point will be directly displayed on the LCD. When measuring the current voltage of the instrument, the black test lead should be used to touch the low potential end of the measured voltage (such as the common ground terminal or housing of the signal generator) to reduce the measurement error.

　　Note: If you do not know the range of the voltage to be measured, it is recommended to pull the gear to the highest gear (1000V), and then switch to a more appropriate range according to the display reading to ensure the accuracy of the test results. In fact, when the user selects the range of UT58C, the display screen will prompt you to select the corresponding test pen input position (AC and DC voltage, resistance, and frequency are all tested using the two input terminals of "COM" and "Hz/V/Ω").

　　The testing method of AC voltage V is the same as that of DC voltage V.

　　Measurement of AC and DC current:

　　Set the range switch to the appropriate range within the DCA (direct current) or ACA (alternating current) range, insert the red test lead into the A hole (≤200 mA) or 10 A hole (>200 mA), insert the black test lead into the COM hole, and connect the multimeter in series in the circuit to be tested through the two test leads. When measuring DC current, the digital multimeter can automatically convert or display the polarity.

　　After using the multimeter, remove the red test lead from the current jack and insert it into the voltage jack.

　　Note: This meter has a fuse inside for current ranges below 200mA, so if the current being measured is greater than the range, the fuse may blow. You can replace it according to the instructions. When testing currents exceeding 200mA, you need to switch the test lead to "A".

　　Resistance, continuity and buzzer test

　　The maximum resistance that the UT58C multimeter can test is 20MΩ. The connection method of the test leads is the same as that of the voltage test. Ordinary multimeters do not support online testing. If a single resistor or conductor is tested, the resistor or conductor to be tested needs to be disconnected from the circuit. In addition, when testing, the operator needs to cut off the power supply of the circuit or conductor to be tested. It is not allowed to test with power on to ensure the accuracy of the test results and the safety of the instrument. This is specially explained in the precautions for use of the instrument.

　　The on-off buzzer test can be used to check the on-off status of the circuit or switch, see Figure 3 and Figure 4. Switch the range conversion switch to the diode/peak range, and use the two test leads to detect the two ends of the switch according to the resistance test method. When the switch is in the open circuit state, the buzzer does not sound. When the switch is closed, the switch is connected, and the instrument will emit a buzzer to remind the user that the switch has been connected. Note: The power supply of the device under test needs to be cut off during the on-off test. To avoid inaccurate test results or damage to the instrument protection circuit.

　　Switch the range conversion switch to the Hz2K/20K position and select the range according to the test frequency.

　　Note: The frequency measurement function of general instruments is only suitable for testing low-voltage frequencies (such as audio signals), and the test input sensitivity is at the mV level; if the voltage of the measured frequency signal is too high, in addition to inaccurate test results, it will also cause damage to the instrument protection circuit: the manual generally prompts "maximum input sensitivity ≤30vrms", which means that the voltage range of the tested frequency signal cannot exceed the effective value of 30V.

　　Capacitor, transistor, temperature test

　　Capacitors are commonly used components in electronic circuits, and many users need to remove the capacitors to be tested from the circuit during testing. Since the pins of such capacitors are very short, the input end of a general multimeter cannot effectively test them. The design of UT58C solves this problem very well. This meter is specially designed with an auxiliary adapter (the entire UT58 series of products of Uni-T is equipped with this accessory, and users can use this adapter to test capacitors, transistors, temperature, etc.) See Figure 5. Switch the instrument range conversion switch to the appropriate range you need. Insert the adapter as shown in the figure, and insert the two pins of the capacitor into the flat slot accordingly, and the display will show the corresponding measurement results.

　　Transistor test

　　This test function can measure the hFE of PNP and NPN transistors: insert the e, b, and c pins of the transistor into the test holes accordingly, and the meter will display the test results.

　　Temperature test

　　It should be noted here that the temperature test function of ordinary digital multimeters is based on the temperature characteristics of K-type thermocouples. The measured range can be from -40℃ to 1000℃, but the K-type thermocouples that are randomly distributed are generally low-temperature, and the temperature sensing range is about -40℃ to 250℃. Therefore, if users want to test high temperatures, they need to purchase high-temperature thermocouples to ensure the accuracy of the test results.

　　Inductor and transformer testing methods and experience

　　1. Detection of color-coded inductors

　　Set the multimeter to R&TImes;1, and connect the red and black test leads to any lead-out terminal of the color-coded inductor. The pointer should swing to the right. According to the measured resistance value, it can be identified in the following three situations:

　　The resistance value of the color-coded inductor under test is zero, and there is a short-circuit fault inside it. The DC resistance value of the color-coded inductor under test at port B is directly related to the diameter of the enameled wire used to wind the inductor coil and the number of windings. As long as the resistance value can be measured, the color-coded inductor under test can be considered normal.

　　2. Detection of intermediate frequency transformer

　　Set the multimeter to R&TImes;1, and check the on-off status of each winding one by one according to the arrangement rules of the winding pins of the intermediate-circuit transformer to determine whether it is normal.

　　B port insulation performance test

　　Set the multimeter to R&TImes;10k and do the following state tests:

　　(1) Resistance between primary winding and secondary winding;

　　(2) Resistance value between primary winding and casing.