This paper expounds the use skills of digital multimeters, emphasizes the precautions for using digital multimeters, introduces the repair methods and repair skills of digital multimeters, and puts forward the reasons and countermeasures for the damage of digital multimeters.
Key words: digital multimeter; use method; repair method; repair skills
1 How to use a digital multimeter
1.1 Measurement of diodes. The digital multimeter can measure light-emitting diodes. When measuring rectifier diodes, the position of the test leads is the same as that for voltage measurement; connect the red test lead to the positive electrode of the diode and the black test lead to the negative electrode, and the forward voltage drop of the diode will be displayed at this time.
1.2 Measurement of triode. Assume that pin A is the base, connect it with a black pen, and connect the red pen with the other two pins; if the two readings are about 0.7V, then connect the red pen to pin A, and the black pen Touch the other two pins, if both display "1", then the A pin is the base, otherwise it needs to be re-measured, and this tube is a PNP tube.
1.3 Measurement of MOS FET. N-channel has domestic 3D01, 4D01, Nissan 3SK series. Determination of G pole: use the diode file of the multimeter.
1.4 Measurement of voltage. ①Measurement 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 "VΩ". Select the knob to a range larger than the estimated value, then connect the test leads to the power supply or both ends of the battery; keep the contact stable. The value can be read directly from the display screen. If it is displayed as "1.", it means that the range is too small, so it is necessary to increase the range before measuring. ②Measurement of AC voltage. The test pen jack is the same as the DC voltage measurement, but the knob should be turned to the required range at the AC gear "V~".
1.5 Measurement of current. ①Measurement of DC current. Insert the black test lead into the "COM" hole first. If measuring the current greater than 200mA, insert the red test lead into the "10A" jack and turn the knob to the DC "10A" level; if measuring the current less than 200mA, insert the red test lead into the "200mA" jack, turn the knob to a suitable range within 200mA of DC. ② measurement of alternating current. When measuring, the gear should be switched to the AC gear. After the current measurement, the red pen should be inserted back into the "VΩ" hole.
1.6 Measurement of resistance. Insert the test leads into the "COM" and "VΩ" holes, turn the knob to the desired range in "Ω", and connect the test leads to the metal parts at both ends of the resistor. You can touch the resistance with your hands during measurement.
2 Precautions for using a digital multimeter
2.1 The digital multimeter should be calibrated regularly. When calibrating, a digital meter of the same type or with higher precision should be used, and the sequence of calibrating the DC gear first, then the AC gear, and finally the capacitance gear should be carried out.
2.2 Pay attention to the correct selection of the measuring range and the red test lead jack.
2.3 When changing the range, the test leads should be disconnected from the measured point.
2.4 When measuring current, avoid overload.
2.5 When not measuring, please turn off the power.
2.6 It is not allowed to measure voltage with resistance gear and current gear.
2.7 When measuring the precision small resistance below 1OΩ (200Ω range), first short-circuit the two meter rods, measure the meter rod wire resistance (about 0.2Ω), and then subtract this value during the measurement.
2.8 Although there are relatively complete protection circuits inside the digital multimeter, we should try to avoid misoperation when using it.
2.9 To restart the power supply after the digital multimeter is automatically powered off, press the power switch twice continuously.
3 digital multimeter repair method
3.1 Open circuit method. Disconnect the suspicious part from the whole machine or unit circuit. If the fault disappears, it means that the fault is in the disconnected circuit. This method is mainly suitable for short circuits in the circuit.
3.2 Measuring element method. When the fault has been narrowed down to a place or a few components, it can be measured online or offline. If necessary, replace with good components. If the fault disappears, the components are broken.
3.3 Interference method. Using the induced voltage of the human body as an interference signal to observe the changes of the liquid crystal display, it is often used to check whether the input circuit and the display part are in good condition.
3.4 Feeling method. By means of the senses to directly judge the cause of the failure, through the visual inspection, it can be found such as disconnection, de-soldering, short-circuiting, broken fuse tube, burnt components, mechanical damage, and copper foil on the printed circuit. ; You can touch the temperature rise of batteries, resistors, transistors, and integrated blocks, and you can refer to the circuit diagram to find out the reason for the abnormal temperature rise.
3.5 Voltage measurement method. Measure whether the working voltage of each key point is normal, and the fault point can be found quickly. Such as measuring the working voltage of the A/D converter, the reference voltage, etc.
3.6 Short circuit method. The short-circuit method is generally used in the method of checking the A/D converter mentioned above, and this method is used more when repairing weak and micro-electrical instruments.
4 Digital Multimeter Repair Tips
4.1 If the measurement data is unstable, and the value always increases cumulatively, the input terminal of the A/D converter is short-circuited, and the displayed data is not zero, which is generally caused by the poor performance of the 0.1μF reference capacitor.
4.2 If there is a problem with individual files, it means that the A/D converter and the power supply are working normally. Because DC voltage and resistance files share a set of voltage divider resistors; AC and DC current share a shunt; AC voltage and AC current share a set of AC/DC converters; others such as Cx, HFE, F, etc. are composed of independent different converters .
4.3 If all gears fail to work, focus on checking the power supply circuit and A/D converter circuit. When checking the power supply part, you can remove the laminated battery, press the power switch, connect the positive test lead to the negative power supply of the meter under test, connect the negative test lead to the positive power supply, and turn the switch to the diode measurement gear. If the voltage is large, it means that the power supply part is good. If the deviation is large, it means that there is a problem with the power supply part.
5 Digital multimeter damage causes and countermeasures
5.1 The damage of the digital multimeter is caused by the measured voltage and current exceeding the range. When measuring DC voltage, the measured voltage exceeds the measurement range, which is also likely to cause circuit failure in the meter. When measuring the current, if the actual current value exceeds the range, it will generally only cause the fuse in the multimeter to blow, and no other damage will be caused.
5.2 The damage of the digital multimeter is caused by the wrong selection of the 1000V DC voltage range. If it exceeds 1000V, it is very likely to cause damage to the multimeter. If the measured voltage exceeds the range, the method of reducing the resistance of the resistor can be used to measure it.
5.3 The damage of the digital multimeter is caused by the wrong measurement gear in most cases. For example, when measuring AC mains, the measurement gear is selected to be placed on the electric block. In this case, once the pen contacts the mains, the internal components of the multimeter can be damaged instantly.
6 Conclusion
At present, there are many manufacturers of digital multimeters, and the quality is also good and bad. It is not easy to find the quality problems of double-sided copper-clad laminates during repair. Due to improper use of digital square meters, it is easy to cause damage to the components in the meter during actual detection, resulting in failure. Therefore, it is necessary to correctly master the use and repair skills of the digital multimeter.
