Several Measurement Methods and Operating Principles of Digital Multimeters
Measurement of voltage
1. Measurement of DC voltage, such as batteries, portable audio power supplies, etc. Firstly, insert the black probe into the "com" hole and the red probe into the "V Ω current voltage range". "V~" represents the AC voltage range, and "A" represents the current range. Then connect the probe to both ends of the power supply or battery; Maintain stable contact. The value can be directly read from the display screen. If it is displayed as "1.", it indicates that the range is too small, and then a large range needs to be added before measuring industrial appliances. If a "-" appears on the left side of the value, it indicates that the polarity of the probe is opposite to the actual power polarity, and the red probe is connected to the negative terminal.
2. Measurement of AC voltage. The probe socket is the same as measuring DC voltage, but the knob should be turned to the required range at the AC gear "V~". There is no positive or negative distinction in AC voltage, and the measurement method is the same as before. Whether measuring AC or DC voltage, attention should be paid to personal safety and do not touch the metal part of the probe with your hands casually.
Measurement of current
1. Measurement of direct current. First, insert the black probe into the "COM" hole. If measuring a current greater than 200mA, insert the red probe into the "10A" socket and turn the knob to the DC "10A" position; If measuring a current less than 200mA, insert the red probe into the "200mA" socket and turn the knob to the appropriate range within 200mA DC. After adjustment, it can be measured. Connect the multimeter in series into the circuit, maintain stability, and the reading will be ready. If it displays as "1.", then the range needs to be increased; If a "-" appears on the left side of the value, it indicates that current is flowing from the black probe into the multimeter.
2. Measurement of AC current. The measurement method is the same as 1, but the gear should be set to AC gear. After measuring the current, the red pen should be inserted back into the "V Ω" hole. If you forget this step and directly measure the voltage, your instrument will be scrapped.
Measurement of resistance
Insert the probe into the "COM" and "V Ω" holes, turn the knob to the desired range in "Ω", and connect the probe to the metal parts at both ends of the resistor. During measurement, you can touch the resistor with your hand, but do not touch both ends of the resistor at the same time, as this will affect the measurement accuracy - the human body is a conductor with a large but limited resistance. When reading, ensure good contact between the probe and the resistor; Attention to units: The unit is "Ω" in the "200" range, "K Ω" in the "2K" to "200K" range, and "M Ω" in the "2M" and above range.
Measurement of diodes
The digital multimeter can measure light-emitting diodes, rectifier diodes... When measuring, the probe position is the same as the voltage measurement, and the knob is turned to the "" position; Connect the red probe to the positive terminal of the diode and the black probe to the negative terminal, and the forward voltage drop of the diode will be displayed. The voltage drop of Schottky diodes is about 0.2V, while that of ordinary silicon rectifiers (1N4000, 1N5400 series, etc.) is about 0.7V, and that of light-emitting diodes is about 1.8-2.3V. If the probe is swapped and the display screen shows "1.", it is normal because the reverse resistance of the diode is very high, otherwise the diode would have been broken down.
Measurement of transistor
Insert the pen in the same position as above; Its principle is the same as that of a diode. Assuming that pin A is the base, connect the black probe to that pin and the red probe to the other two pins respectively; If both readings are 0 Around 7V, then connect the A pin with a red pen and the other two pins with a black pen. If both display "1", then the A pin is the base, otherwise it needs to be remeasured, and this transistor is a PNP transistor. So how to determine the collector and emitter? What should I do if a digital meter cannot use pointer swing to determine like a pointer meter? We can use the "hFE" gear to determine: first set the gear to "hFE", and you can see a row of small sockets next to the gear, which are used for measuring PNP and NPN tubes.
The tube type has been determined earlier. Insert the base into the corresponding "b" hole of the tube type, and insert the other two pins into the "c" and "e" holes respectively. At this time, the value can be read, that is, the β value; Fix the base again and swap the other two pins;
Compare two readings, and the position of the pin with the larger reading corresponds to the surface "c" and "e".
Tip: The above method can only directly measure small tubes such as the 9000 series. To measure large tubes, the wiring method can be used, which involves using small wires to lead out the three pins. This makes it much more convenient.
Measurement of MOSFET
There are domestically produced 3D01, 4D01, and Nissan's 3SK series for N-channel. Determination of C-pole (gate): Use the diode mode of the multimeter. If the positive and negative voltage drops between a certain pin and the other two pins are both greater than 2V, it will display "1", and this pin is the gate C. Then exchange the probes to measure the other two pins. In the case where the voltage drop is small, the black probe is connected to the D terminal (drain), and the red probe is connected to the S terminal (source).
