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I. Functions of a digital multimeter A digital multimeter generally has functions such as resistance measurement, voltage measurement, current measurement, detection of continuity with sound indication, measurement of the forward voltage drop of diodes, and measurement of the amplification factor and performance of transistors. Some digital multimeters come with additional functions such as capacitance measurement, frequency measurement, temperature measurement, and data retention. 1. Input jacks: The multimeter has four input jacks; (1) the black test lead is always inserted into the “COM” jack ； (2) When measuring voltage, resistance, diodes, and checking on/off status, insert the red test lead into the “V/Ω” socket ； (3) When measuring AC/DC currents below 200mA, insert the red test lead into the “mA” socket ； (4) When measuring AC or DC currents of 200mA or higher, insert the red test lead into socket “A”. 2. Function and range selection switch: The ranges for AC and DC voltage settings are generally 200mV, 2V, 20V, 200V, and 1000V, totaling 5 ranges. The ranges for AC and DC current settings are 200μA, 2mA, 20mA, 200mA, and 10A, totaling 5 ranges. The resistance range of the ohmmeter includes 7 settings: 200Ω, 2kΩ, 20kΩ, 200kΩ, 2MΩ, and 20MΩ. The lowest setting is used to determine whether a circuit is closed or open; a beeping sound is emitted when the resistance is below a certain value (usually around 100Ω). 3. β socket: Used to measure the β value of transistors; be sure to note whether the transistor type is NPN or PNP. II. Proper use of multimeters: If digital multimeters are not used correctly, it can easily lead to damage of the components inside the meter during testing, resulting in malfunctions, or it may cause damage to the instruments and equipment being tested. 1. Before using a multimeter for measurements, be sure to check that the measurement setting is correct first. In most cases, damage to a digital multimeter is caused by an incorrect measurement setting; for example, when measuring alternating current from the power supply, if the multimeter is set to the resistance measurement mode, the internal components of the device can be damaged instantly once the test leads make contact with the power supply. 2. Do not get the test leads mixed up. Using the wrong socket can result in no signal being detected, leading to incorrect readings. For example, after measuring voltage and then attempting to measure current, failing to switch the setting and use the appropriate socket may cause a short circuit. Of course, it generally only causes the fuse inside the multimeter to blow, without causing any other damage, but we still need to be cautious. 3. Pay attention to the measurement range of the signal being used. If the approximate range of the signal being measured is unknown, start by setting the measurement scale to the highest setting; after measuring its value, switch to another setting to obtain a more accurate figure. Damage to a digital multimeter occurs when the voltage or current being measured exceeds its range. For example, measuring mains electricity on the 20V AC setting can easily damage the AC amplification circuit of the digital multimeter, resulting in the loss of its AC measurement capability. When measuring DC voltage, if the measured voltage exceeds the measurement range, it can also easily cause faults in the circuit inside the meter. 4. When measuring resistance, care must be taken not to do so while the device is powered on. First, it causes inaccurate measurements; additionally, it may damage the multimeter. 5. It is important to note that current measurement requires connection in series within the circuit, while resistance or voltage measurement requires connection in parallel within the circuit. 6. At full scale, the instrument displays only the digit “1” in the highest position, with all other digits disappearing; in this case, a higher scale should be selected. 7. When testing the instrument, do not rotate the function switch; especially under high voltage and high current conditions, it is strictly prohibited to change the range while the device is powered on. Change the range when the voltage is above 220V or the current is above 0.5A, to prevent arcing and the burning out of the switch contacts. These are some materials I have collected as well as some summaries from my daily work in this area; I welcome everyone’s feedback and suggestions.