How to use a digital multimeter

　　Digital multimeters are relatively simple measuring instruments. In this article, the author will teach you how to use a digital multimeter correctly. Starting from the measurement methods of digital multimeters such as voltage, resistance, current, diodes, transistors, and MOS field effect transistors, you can better master the multimeter measurement methods.

　　1. Voltage measurement

　　1. Measurement of DC voltage, such as battery, Walkman power supply, etc. First, insert the black test lead into the "com" hole and the red test lead into the "V Ω". Select the knob to a range larger than the estimated value (Note: the values ​​on the dial are all the maximum range, "V-" indicates the DC voltage range, "V~" indicates the AC voltage range, and "A" is the current range), then connect the test lead to the power supply or both ends of the battery; keep the contact stable. The value can be read directly from the display. If it shows "1.", it means that the range is too small, so you need to increase the range before measuring industrial electrical appliances. If "-" appears on the left side of the value, it means that the polarity of the test lead is opposite to the actual power supply polarity, and the red test lead is connected to the negative pole.

　　2. Measurement of AC voltage. The probe jack is the same as the DC voltage measurement, but the knob should be turned to the AC range "V~". There is no positive or negative difference in AC voltage, and the measurement method is the same as before. Whether measuring AC or DC voltage, pay attention to personal safety and do not touch the metal part of the probe with your hands.

　　2. Current Measurement

　　1. Measurement of DC current. First insert the black test lead into the "COM" hole. If measuring a current greater than 200mA, insert the red test lead into the "10A" jack and turn the knob to the DC "10A" position; if measuring a current less than 200mA, insert the red test lead into the "200mA" jack and turn the knob to a suitable range within DC 200mA. After adjustment, you can measure. Connect the multimeter in series to the circuit, keep it stable, and then read the value. If it shows "1.", then increase the range; if "-" appears on the left side of the value, it means that the current flows from the black test lead into the multimeter.

　　Measurement of AC current. The measurement method is the same as 1, but the gear should be set to AC gear. After the current measurement is completed, the red pen should be inserted back into the "VΩ" hole. If you forget this step and measure the voltage directly, haha! Your meter or power supply will "go up to the sky in a wisp of green smoke" - scrapped!

　　3. Resistance Measurement

　　Insert the test lead into the "COM" and "VΩ" holes, turn the knob to the desired range in "Ω", and connect the test lead to the metal parts at both ends of the resistor. You can touch the resistor with your hands during measurement, but do not touch both ends of the resistor with your hands at the same time, which will affect the measurement accuracy - the human body is a conductor with a large but finite resistance. When reading, keep the test lead and the resistor in good contact; pay attention to the unit: the unit is "Ω" in the "200" range, the unit is "KΩ" in the "2K" to "200K" range, and the unit above "2M" is "MΩ".

　　4. Measurement of diode

　　Digital multimeter can measure light-emitting diodes, rectifier diodes, etc. When measuring, the position of the test pen is the same as that of voltage measurement. Turn the knob to the " " position; connect the red test pen to the positive pole of the diode and the black test pen to the negative pole. The forward voltage drop of the diode will be displayed. The voltage drop of Schottky diode is about 0.2V, the voltage drop of ordinary silicon rectifier tube (1N4000, 1N5400 series, etc.) is about 0.7V, and the light-emitting diode is about 1.8~2.3V. Replace the test pen, and the display shows "1." It is normal because the reverse resistance of the diode is large, otherwise the tube has been broken down.

　　5. Measurement of transistor

　　The probe insertion position is the same as above; its principle is the same as that of the diode. Assume that pin A is the base, connect the black probe to this pin, and the red probe to the other two pins respectively; if the two readings are both around 0.7V, then connect the red pen to pin A, and the black pen to the other two pins. If both display "1", pin A is the base, otherwise it needs to be re-measured, and this tube is a PNP tube. So how to judge the collector and emitter? The digital meter cannot use the pointer swing to judge like the pointer meter, so what should we do ? We can use the "hFE" gear to judge: first set the gear to the "hFE" gear, and you can see that there is a row of small jacks next to the gear, which are divided into PNP and NPN tube measurements. (Copyright http://www.diangon.com/) The tube type has been determined before. Insert the base into the corresponding tube type "b" hole, and the other two pins into the "c" and "e" holes respectively. Now you can read the value, that is, the β value; then fix the base and swap the other two pins; compare the two readings, the pin position with the larger reading corresponds to the surface "c" and "e".

　　Tips : The above method can only directly measure small tubes such as the 9000 series. If you want to measure large tubes, you can use the wiring method, that is, use a small wire to lead out the three tube pins. This is much more convenient.

　　6. Measurement of MOS Field Effect Transistor

　　The N-channel ones include the domestically produced 3D01, 4D01, and the Japanese 3SK series. Determination of the G pole (gate): Use the diode range of the multimeter. If the positive and negative voltage drops between a certain pin and the other two pins are both greater than 2V, that is, "1" is displayed, this pin is the gate G. Then exchange the test leads to measure the remaining two pins. In the case with the smaller voltage drop, the black test lead is connected to the D pole (drain) and the red test lead is connected to the S pole (source).

　　1. Voltage range:

　　During testing or manufacturing, it can be used to measure the voltage of each foot of the device, and compared with the normal voltage, it can be determined whether it is damaged. It can also be used to detect the voltage value of the voltage-stabilizing diode with a small voltage-stabilizing value. The principle is as shown in the figure: R is 1K, and the voltage at the power supply end depends on the nominal voltage-stabilizing value of the voltage-stabilizing tube, which is generally 3V higher than the nominal voltage, but not more than 15V. Then use a multimeter to detect the voltage value at both ends of the D tube, which is the actual voltage-stabilizing value of the D tube.

　　2. Current Range

　　Connect the meter in series to the circuit to measure and monitor the current. If the current deviates far from the normal value (based on experience or the original normal parameters), the circuit can be adjusted or repaired if necessary. You can also use the 20A range of the meter to measure the short-circuit current of the battery, that is, connect the two test leads directly to the two ends of the battery. Remember that the time must not exceed 1 second! Note: This method is only applicable to dry batteries, No. 5 and No. 7 rechargeable batteries, and beginners must be guided by personnel familiar with maintenance and must not operate on their own! The performance of the battery can be judged based on the short-circuit current. In the case of fully charged batteries of the same type, the larger the short-circuit current, the better.

　　3. Resistance range;

　　This is one of the methods that can be used to judge the quality of resistors, diodes, and transistors. For resistors, if the actual resistance value deviates too much from the nominal value, it is damaged. For diodes and triodes, if the resistance between any two pins is not very large (more than a few hundred K), it can be considered that the performance has degraded or it has been broken down. Note that this triode has no resistance. This method can also be used for integrated blocks. It should be noted that the measurement of integrated blocks can only be compared with the parameters when they are normal.

　　4. At present, the resistance of the test leads of ordinary multimeters is relatively large. Interested enthusiasts can make a pair of test leads by themselves. Method: prepare about one meter of high-quality speaker wire or multi-core copper wire, a pair of clips with insulating sleeves (red and black), and a pair of banana plugs for speaker wiring (red and black); solder one end of the wire to the clip, and connect the other end to the banana plug accordingly; a pair of excellent test leads is ready.