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Digital measuring instruments have become mainstream due to their high sensitivity, accuracy, clear display, strong overload capability, portability, and ease of use.
Figure 1 below uses this type of digital multimeter as an example to briefly introduce its usage and precautions.
1. Insert the black probe into the COM port and the red probe into the VΩ port.
2. Rotate the function switch to V~ (AC), V- (DC), and select an appropriate range.
3. Touch the red probe to the positive terminal of the circuit being measured, and the black probe to the ground or negative terminal, connecting it in parallel with the circuit.
4. Read the number displayed on the LCD screen.
1. Turn off the circuit power.
2. Select the resistance setting (Ω).
3. Insert the black test probe into the COM input socket. Insert the red test probe into the Ω input socket.
4. Connect the probe tips across the two ends of the component or the section of the circuit you wish to measure.
5. Check the reading, confirming the measurement unit is ohms (Ω), kilo-ohms (kΩ), or mega-ohms (MΩ).
① Judging the condition of a diode: Set the dial to the 
position, insert the red probe into the rightmost hole, and the black probe into the second hole from the right. Connect the probe tips to the two terminals of the diode as shown below, then reverse the probes and measure again.
② Measurement results: If the results of the two measurements show: one displays “1” and the other shows a number close to zero, then this diode is a normal diode. If both measurements show the same result, the diode is damaged. The number displayed on the LCD indicates the forward voltage drop of the diode: around 0.6V for silicon diodes; around 0.2V for germanium diodes. Based on the characteristics of the diode, it can be determined that the red probe is connected to the positive terminal and the black probe to the negative terminal.
③ The most important characteristic of a diode is:
Unidirectional Conductivity
④ Short circuit test (to determine continuity):
Set the dial to the short circuit 
position, with the probes in the same position as above. Use the other ends of the probes to connect to the two points being tested. If these two points are indeed shorted, the multimeter’s buzzer will sound.
1. Disconnect the circuit;
2. Insert the black probe into the COM port, and the red probe into the mA or 20A port;
3. Rotate the function switch to A~ (AC), A- (DC), and select an appropriate range;
4. Disconnect the circuit being measured, and connect the digital multimeter in series with the circuit. The current flows into the red probe from one end and out through the black probe back into the circuit;
5. Turn on the circuit;
6. Read the number displayed on the LCD screen.
1. Short the two ends of the capacitor to discharge it, ensuring the safety of the digital multimeter.
2. Rotate the function switch to the capacitance (C) measurement setting, and select an appropriate range.
3. Insert the capacitor into the C-X socket of the multimeter.
4. Read the number displayed on the LCD screen.
Small Knowledge: Units of capacitance:
1F=1000mF=1000uF=1000nF=1000pF
1. If the voltage or current to be measured cannot be estimated in advance, first set it to the highest range and measure once,
then gradually reduce the range to an appropriate position based on the situation. After measurement, set the range switch to the highest voltage setting and turn off the power.
2. When at full scale, the meter only shows the number “1” at the highest position, while other positions disappear; at this point, a higher range should be selected.
3. When measuring voltage, the digital multimeter should be connected in parallel with the circuit being measured. When measuring current, it should be connected in series with the circuit being measured, and when measuring DC, polarity does not need to be considered.
4. When incorrectly using the AC voltage setting to measure DC voltage, or the DC voltage setting to measure AC voltage, the display will show “000”, or the numbers at the lower end will fluctuate.
5. It is prohibited to change the range when measuring high voltage (above 220V) or high current (above 0.5A) to prevent arcing and burning the switch contacts.
[How to Use a Multimeter]
A multimeter is an essential testing tool in electronic production. It has multiple functions for measuring current, voltage, and resistance. This article will introduce the structure of the multimeter and how to use it.
1. Observe and Understand the Structure of the Multimeter.
There are many types of multimeters with different appearances, but the basic structure and usage methods are the same. The structure and appearance of the commonly used multimeter are shown in the attached color page.
The front panel of the multimeter should have a meter head and a selection switch. There are also a zero adjustment knob for the ohm setting and probe sockets. Below is an introduction to the function of each part:
(1) Meter Head
The meter head of the multimeter is a sensitive ammeter. The dial on the meter head is printed with various symbols, scale lines, and values (as shown in Figure 3-4(B)). The symbols A, V, and Ω indicate that this meter can measure current, voltage, and resistance. The dial has multiple scale lines, with the rightmost one marked with “Ω” being the resistance scale, with the right end at zero and the left end at infinity, and the scale values are unevenly distributed. The symbol “-” or “DC” indicates direct current, while “~” or “AC” indicates alternating current, and “~” indicates a scale shared by both AC and DC. The numbers below the scale lines correspond to the different settings of the selection switch.
The meter head also has a mechanical zero adjustment knob to calibrate the pointer to the zero position on the left end.
(2) Selection Switch
The selection switch of the multimeter is a multi-position rotary switch used to select measurement items and ranges (as shown in Figure 3-4(B)). Common measurement items for the multimeter include: “mA”; DC current, “V”: DC voltage, “V”: AC voltage, “Ω”: resistance. Each measurement item is divided into several different ranges for selection.
(3) Probes and Probe Sockets
Probes come in red and black. When using, the red probe should be inserted into the socket marked with a “+” sign, and the black probe should be inserted into the socket marked with a “-” sign.
2. How to Use the Multimeter
(1) Before using the multimeter, you should:
1. Place the multimeter horizontally.
2. Check whether the pointer is at the zero position on the dial. If it is misaligned, gently turn the mechanical zero adjustment knob with a small screwdriver to make the pointer point to zero.
3. Insert the probes into the probe sockets as required above.
4. Rotate the selection switch to the corresponding item and range. You can then use it.
(2) After using the multimeter, you should:
1. Remove the probes.
2. Rotate the selection switch to the “OFF” position; if there is no “OFF” position, rotate it to the maximum AC voltage range, such as the “1000V” range.
How to Use a Digital Multimeter [Illustration]
[Efficient Usage Tips for Digital Multimeters]
1. Determine whether a circuit or device is live
The AC voltage setting on a digital multimeter is very sensitive and can detect even small induced voltages. Based on this characteristic, it can be used as a voltage tester. The method is as follows: Set the multimeter to AC20V, hold the black probe in the air, and touch the red probe to the wire or device being tested. If the display shows a value between several volts and a dozen volts (different multimeters may show different values), the wire or device is live. If it shows zero or a very small value, the wire or device is not live.
2. Distinguish between live wire and neutral wire
The first method: You can use the above method to determine: the wire with the larger displayed number is the live wire, and the one with the smaller displayed number is the neutral wire. This method requires contact with the wire or device being measured.
The second method: No contact with the wire or device is needed. Set the multimeter to AC2V, hold the black probe in the air, and gently slide the red probe along the wire. If the display shows several volts, that wire is live. If it only shows a few tenths of a volt or even smaller, that wire is neutral. This method is safe and convenient as it does not require direct contact with the wire.
3. Locating cable breakpoints
When there is a breakpoint in a cable, the traditional method is to use the multimeter’s resistance setting to find the breakpoint segment by segment, which not only wastes time but also greatly damages the cable’s insulation. Using the induction characteristics of the digital multimeter, the breakpoint can be quickly located. First, use the resistance setting to determine which wire in the cable has an open circuit, then connect one end of the broken wire to a 220V AC power source. Next, set the multimeter to AC2V, hold the black probe in the air, and gently slide the red probe along the wire. If the display shows several volts or a few tenths of a volt (depending on the type of cable), and the display suddenly drops significantly at a certain point, remember that position: generally, the breakpoint is within 10-20cm of that position.
This method can also be used to find the open circuit point of faulty heating wires in electric blankets.
4. Measuring the frequency of UPS power supply
For UPS power supplies, the stability of the output voltage is an important parameter, and the output frequency is also crucial. However, the frequency setting of the digital multimeter cannot be used directly for measurement because it can only handle low voltages of a few volts. Instead, connect a 220V/6V or 220V/4V step-down transformer to the output of the UPS to lower the voltage without changing the frequency of the power supply, then connect the frequency setting to the output of the transformer to measure the UPS frequency.
The usage of multimeters
Multimeters, also known as multifunctional meters, tri-meters, or universal meters, are portable measuring instruments with multiple functions and ranges. They can generally measure AC and DC voltages, DC currents, and resistances, and some can also measure AC currents, inductance, capacitance, and the DC amplification factor of transistors, etc.
1. Analog Multimeter
There are many types of analog multimeters, but the usage methods are basically the same. Here, we will take the MF30 as an example to introduce the usage methods and precautions for the multimeter:
① The test leads should be intact and well insulated.
② Observe whether the pointer is pointing to the zero position for voltage and current; if not, adjust the mechanical zero adjustment knob to make it point to zero.
③ Select the position and range of the switch according to the type and size of the parameter being measured, and try to make the pointer deflect to about 2/3 of the full scale. If the range of the measurement is unknown, start from the maximum range and gradually decrease to an appropriate range.
④ Before measuring resistance, zero the corresponding ohm setting (i.e., touch the two probes together and rotate the zero adjustment knob until it shows 0Ω). Zeroing should be done each time the ohm setting is changed. If the pointer cannot reach zero after adjusting the zero knob, the battery voltage may be low and needs to be replaced. When measuring, separate the resistance from the circuit and do not operate under voltage.
⑤ When measuring DC quantities, pay attention to polarity and connection: for DC current, the current flows in from the “+” terminal and out from the “-” terminal; for DC voltage, the red probe should be connected to the high potential and the black probe to the low potential.
⑥ When reading values, read from the corresponding scale and pay attention to the range. If the measured value is voltage or current, read the full scale; if it is resistance, read the value from the scale and multiply it by the scale factor.
⑦ During measurement, do not touch the metal parts of the probes to ensure safety and accuracy.
⑧ Do not change the switch position under voltage.
⑨ Do not use the multimeter to measure the internal resistance of sensitive meters such as microammeters and galvanometers directly.
⑩ When measuring transistor parameters, use the low voltage high range setting (R×100Ω or R×1kΩ). Note that “-” is the positive terminal of the internal power supply and “+” is the negative terminal.
⑪ After measurement, rotate the switch to the highest AC voltage setting, and if there is an “OFF” position, rotate it to “OFF”.
2. Digital Multimeter
Compared to analog multimeters, digital multimeters have many advantages: high sensitivity and accuracy, intuitive display, comprehensive functions, stable performance, compactness, and portability, with features like polarity selection, overload protection, and over-range display. There are also many models of digital multimeters; here, we will take the DT890 as an example to introduce the usage methods and precautions. Figure 1-13 shows the panel of the DT890 digital multimeter.
Before operation, turn the power switch to the “ON” position. If the display shows “LOBAT” or “BATT”, it indicates that the battery voltage of the digital multimeter is low and needs to be replaced; otherwise, it cannot be used further.
① Measuring AC and DC Voltage
a. Insert the black probe into the COM socket and the red probe into the V/Ω socket.
b. Set the function selection switch to the appropriate range for DCV (DC) or ACV (AC) (if the range of the voltage to be measured is unknown, start from the highest range and gradually decrease to the appropriate range), and connect the probes to the two ends of the circuit being measured. The display will show the measured voltage value and the polarity of the red probe (if the display shows only “1”, it indicates over-range; switch to a higher range).
c. The triangle next to the test probe socket indicates that the DC voltage should not exceed 1000V and the AC voltage should not exceed 700V.
② Measuring AC and DC Current
a. Insert the black probe into the COM socket; if the measured current is ≤200mA, insert the red probe into the A socket; if the measured current is between 200mA-10A, insert the red probe into the 10A socket.
b. Set the function selection switch to the appropriate range for DCA (DC) or ACA (AC), and connect the probes in series with the circuit being measured. The display will show the magnitude of the current as well as the polarity of the red probe.
③ Measuring Resistance
a. Insert the black probe into the COM socket and the red probe into the V/Ω socket (the polarity of the red probe is “+”, which is different from the analog multimeter).
b. Set the function selection switch to the appropriate range for OHM, and connect the probes to the resistor being measured; the display will show the resistance value.
④ Measuring Diodes
a. Insert the black probe into the COM socket and the red probe into the V/Ω socket.
b. Set the function selection switch to the “Diode” position and connect the probes to the two ends of the diode being measured; the display will show the forward voltage drop of the diode in millivolts. If the diode is reverse-biased, it will show “1”.
c. If both directions show “1”, it indicates the diode is open; if both directions show “0”, it indicates the diode is shorted. Both situations indicate the diode is damaged and cannot be used.
d. This range can also be used for audible continuity testing, where the internal buzzer sounds when the resistance of the circuit being tested is below 70Ω, indicating the circuit is conductive.
⑤ Testing the hFE of Transistors
a. Set the function selection switch to the hFE position.
b. Confirm whether the transistor is a PNP or NPN type, and insert the E, B, and C legs into the corresponding sockets. The display will show the approximate value of the transistor’s amplification factor hFE (test conditions are IB=10μA, UCE=2.8V).
⑥ Measuring Capacitance
a. Set the function selection switch to the appropriate range for CAP, and adjust the capacitor zero adjustment to make the display show 0.
b. Insert the capacitor to be measured into the “Cx” test socket; the display will show its capacitance value.
