Introduction to the use of digital multimeters for measuring voltage/current/resistance

Digital multimeter, as the name suggests, its working principle is mainly based on detecting and analyzing signals, and then providing LCD display through the analog-to-digital converter.

Introduction to the use of digital multimeters for measuring voltage/current/resistance

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

Digital multimeters mainly include: DC voltage, DC current, AC voltage, AC current, resistance, circuit continuity test, diode test and other common measurement functions; with the development of electronic technology, multimeters have many application functions: data storage, data Transmission, oscilloscope, etc.: Some of them are developed in application areas: power meters, leakage current testers, insulation resistance testers for power testing, infrared thermometers used in the industrial and mining industry, noise meters for environmental testing, anemometers, etc. . Let's take the universal handheld digital multimeter UT58C from Unitech as an example to talk about the use of multimeter. 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, on-off buzzer, triode hFE, temperature measurement, and its unique large-screen display and input prompt The function also brings a lot of convenience to the user.

Use of digital multimeter

Introduction to the use of digital multimeters for measuring voltage/current/resistance

AC and DC voltage measurement:

Set the power switch to the ON position, and set the range switch to the appropriate range within the range of DCV (direct current) or ACV (alternating current) as required. Insert the red test lead into the V/Ω hole and the black test lead into the COM hole, and then insert the two test leads Connected to the measured point, the liquid crystal display will directly display the voltage of the measured point. When measuring the current voltage of instruments and meters, you should use a black meter pen to touch the low potential end of the measured voltage (such as the common ground terminal or the case of the signal generator) to reduce the measurement error.

Note: If you do not know the range of the measured voltage. It is recommended to shift the gear to gear (1000V), and then switch to a more suitable range according to the display reading to ensure the accuracy of the test results. When the user selects the range of the actual UT58C, the display will prompt you to select the corresponding input position of the test lead (AC and DC voltage, resistance, frequency are all used "COM", "Hz/V/Ω" two input terminals for test.

The test method of AC voltage V and DC voltage V is the same.

AC and DC current measurement:

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

After using the multimeter, unplug the red test lead from the current jack, and then plug it into the voltage jack.

Note: If the current range of this meter is less than 200mA, there is a safety device in the meter, so if the measured current is greater than the range. It may cause the fuse to blow. It can be replaced according to the instructions in the manual. When testing a current exceeding 200mA, the test leads need to be switched to "A".

Introduction to the use of digital multimeters for measuring voltage/current/resistance

Resistance, continuity buzzer test

The resistance of the UT58C multimeter can be tested to 20MΩ, and the connection method of the test leads is the same as that of the voltage test; ordinary multimeters do not support online testing. If you test a single resistance or conductor, you need to disconnect the measured resistance or conductor from the circuit; additionally delete the test It is necessary to make the user cut off the power supply of the tested circuit or conductor, and no live test is allowed to ensure the accuracy of the test result and the safety of the instrument. This is specially explained in the precautions for the use of the instrument.

The continuity buzzer test can be used to check the continuity of the circuit or switch, as shown in Figure 3 and Figure 4. Switch the range changeover switch to diode/peak gear, and use the two test leads to test the two ends of the switch according to the resistance test method. When the switch is open, the buzzer does not sound. When the switch is closed and the switch is turned on, the meter will beep to remind the user that the switch has been turned on. Note: The power supply of the device under test needs to be cut off during the continuity test. In order to avoid inaccurate test results or damage to the protection circuit of the meter.

Frequency test

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

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

Capacitance, transistor, temperature test

Capacitors, which are commonly used components in electronic circuits, and many users need to remove the capacitor under test from the circuit when testing. Because the pins of this type of capacitor are very short, the input terminal of a general multimeter cannot be effective. The design of UT58C solves this problem very well. This table is specially designed with an auxiliary adapter accessory (the entire UT58 series of Unitech are equipped with this accessory, and the user can use this adapter to Test capacitance, transistor, temperature, etc.) as shown in Figure 5. Switch the meter range switch to the appropriate range you need. Insert the adapter as shown in the figure, insert the two pins of the capacitor into the flat slot, and the display will show the corresponding measurement results.

Triode test

This delete test function can delete the hFE of PNP and NPN transistors: insert the e, b, and c pins of the transistor into the test hole, and the meter will display the test result.

Introduction to the use of digital multimeters for measuring voltage/current/resistance

Temperature test

What needs to be explained here is that the temperature test function that the ordinary digital multimeter does not have is based on the temperature characteristics of the K-type thermocouple. Most people can measure the range from -40℃～1000℃, but the K-plow thermocouples distributed randomly are generally low temperature, and the temperature sensing range is around -40qC～250℃, so if you want to test high temperature, you need another Choose a high-temperature thermocouple to ensure the accuracy of the test results.

Inductor and transformer testing methods and experience

1. Detection of color code inductors

Place the multimeter in the R×1 gear and connect the red and black test leads to any one of the color code inductors. At this time, the pointer should swing to the right. According to the measured resistance value, it can be distinguished in the following three situations:

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

2. Detection of mid-week transformers

Set the multimeter to the R×1 gear, and check the on-off conditions of each winding one by one according to the arrangement of the winding pins of the mid-circle transformer, and then judge whether it is normal.

B port test insulation performance

Place the multimeter in the R×10k block and do the following state tests:

(1) The resistance value between the primary winding and the secondary winding;

(2) The resistance value between the primary winding and the housing.