Correct use of multimeter

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1. Selection of pointer tables and digital tables: 　　1. The reading accuracy of the pointer meter is poor, but the process of pointer swing is relatively intuitive, and its swing speed can sometimes reflect the measured size more objectively (such as measuring the slight jitter of the TV data bus (SDL) when transmitting data) ； The digital meter reading is intuitive, but the process of digital changes looks messy and not easy to watch. 　　2. There are generally two batteries in an analog watch, one with a low voltage of 1.5V and one with a high voltage of 9V or 15V. The black test lead is the positive terminal relative to the red test lead. Digital meters commonly use a 6V or 9V battery. In the resistance mode, the output current of the test pen of the pointer meter is much larger than that of the digital meter. Using the R×1Ω gear can make the speaker make a loud "click" sound, and using the R×10kΩ gear can even light up the light-emitting diode (LED). 　　3. In the voltage range, the internal resistance of the pointer meter is smaller than that of the digital meter, and the measurement accuracy is relatively poor. In some high-voltage and micro-current situations, it is even impossible to measure accurately because the internal resistance will affect the circuit under test (for example, when measuring the acceleration stage voltage of a TV picture tube, the measured value will be much lower than the actual value). The internal resistance of the voltage range of the digital meter is very large, at least in the megohm level, and has little impact on the circuit under test. However, the extremely high output impedance makes it susceptible to the influence of induced voltage, and the measured data may be false in some situations with strong electromagnetic interference. 　　4. In short, pointer meters are suitable for measuring analog circuits with relatively large currents and high voltages, such as televisions and audio amplifiers. Digital meters are suitable for measuring digital circuits with low voltage and small current, such as BP machines, mobile phones, etc. It is not absolute. Pointer tables and digital tables can be selected according to the situation. 　　2. Measurement skills (if no explanation is given, it means that a pointer meter is used): 　　1. Test speakers, headphones, and dynamic microphones: Use the R×1Ω setting, connect any test lead to one end, and touch the other test lead to the other end. Normally, a clear and loud "click" sound will be emitted. If there is no sound, the coil is broken. If the sound is small and sharp, there is a friction ring problem and it cannot be used. 　　2. Measure capacitance: Use the resistance setting, select the appropriate range according to the capacitance, and note that the black test lead of the electrolytic capacitor must be connected to the positive electrode of the capacitor when measuring. ①, Estimate the size of the microwave-grade capacitor: It can be determined based on experience or with reference to a standard capacitor of the same capacity and the maximum amplitude of the pointer swing. The reference capacitors do not have to have the same voltage resistance, as long as they have the same capacity. For example, when estimating a 100μF/250V capacitor, a 100μF/25V capacitor can be used as a reference. As long as the maximum amplitude of their pointer swings is the same, it can be concluded that the capacities are the same. ②, estimate picofarad capacitance size: Use the R×10kΩ range, but you can only measure capacitances above 1000pF. For a capacitor of 1000pF or slightly larger, as long as the watch needle swings slightly, the capacity is considered to be sufficient. ③, Test whether the capacitor is leaking: For capacitors of more than 1,000 microfarads, you can first use the R×10Ω gear to quickly charge it and initially estimate the capacitance capacity, and then change to the R×1kΩ gear and continue measuring for a while. At this time, the pointer should not return, but should stop at or very close to ∞, otherwise there will be leakage. For some timing or oscillation capacitors below tens of microfarads (such as the oscillation capacitor of a color TV switching power supply), the requirements for their leakage characteristics are very high. As long as there is a slight leakage, they cannot be used. In this case, you can charge the capacitor in the R×1kΩ range and then switch to the R×10kΩ range to continue measuring. Similarly, the needle should stop at ∞ and should not return. 　　3. Test the quality of diodes, transistors and voltage regulator tubes on the road: Because in actual circuits, the bias resistance of the transistor or the peripheral resistance of the diode and voltage regulator tube are generally relatively large, mostly above hundreds of thousands of ohms. In this way, we can use the R×10Ω or R×1Ω range of the multimeter to measure the quality of the PN junction on the road. When measuring on the road, the PN junction should have obvious forward and reverse characteristics when measured in the R×10Ω range (if the difference in forward and reverse resistance is not obvious, you can use the R×1Ω range to measure). Generally, the forward resistance should indicate around 200Ω when measured in the R×10Ω range, and the needle should indicate around 30Ω when measured in the R×1Ω range (there may be slight differences depending on different phenotypes). If the measurement result is that the forward resistance is too large or the reverse resistance is too small, it means there is a problem with the PN junction and the tube. This method is particularly effective for repairs. It can find bad pipes very quickly, and can even detect pipes that are not completely broken but have deteriorated characteristics. For example, when you use a small resistance setting to measure the forward resistance of a certain PN junction and it is too large, if you solder it down and use the commonly used R×1kΩ setting to measure again, it may still be normal. In fact, the characteristics of this tube have deteriorated and it cannot work normally or is unstable. 　　4. Measure resistance: The important thing is to choose the right range. When the pointer indicates 1/3 to 2/3 of the full scale, the measurement accuracy is the highest and the reading is the most accurate. It should be noted that when using the R×10k resistor range to measure a large megohm resistance resistor, do not pinch your fingers at both ends of the resistor, as this will cause the measurement result to be smaller due to human body resistance. 　　5. Measure the Zener diode: The voltage stabilization value of the voltage regulator tubes we usually use is generally greater than 1.5V, and the resistance range below R×1k of the pointer meter is powered by the 1.5V battery in the meter. In this way, measuring the voltage regulator tube with the resistance range below R×1k is just like measuring a diode, with complete one-way conductivity. However, the R×10k range of the pointer meter is powered by a 9V or 15V battery. When using R×10k to measure a voltage regulator tube with a voltage regulator value less than 9V or 15V, the reverse resistance will not be ∞, but will have a certain resistance, but this resistance still needs to be * * Higher than the forward resistance of the voltage regulator tube. In this way, we can initially estimate the quality of the voltage regulator tube. However, a good voltage regulator tube must have an accurate voltage regulator value. How to estimate this voltage regulator value under amateur conditions? It's not difficult, just find an analog watch. The method is: First put a meter in the R×10k position, and connect its black and red test leads to the cathode and anode of the voltage regulator tube respectively. At this time, the actual working state of the voltage regulator tube is simulated. Then take another meter and place it in the voltage range V×10V or V×50V (according to the voltage stabilization value). Connect the red and black test leads to the black and red test leads of the meter respectively. The voltage value measured at this time is basically the voltage stabilization value of the voltage regulator tube. I say "basically" because the bias current of the voltage regulator tube of the first meter is slightly smaller than the bias current during normal use, so the measured voltage regulator value will be slightly larger, but the difference is basically not big. This method can only estimate the voltage regulator tube whose voltage regulator value is smaller than the voltage of the high-voltage battery of the pointer meter. If the voltage stabilization value of the voltage regulator tube is too high, it can only be measured with an external power supply (from this point of view, when we choose a pointer meter, it is more suitable to use a high-voltage battery with a voltage of 15V than a 9V one). 　　6. Test the transistor: Usually we have to use the R×1kΩ range. Whether it is an NPN tube or a PNP tube, whether it is a low-power, medium-power, or high-power tube, when measuring its be junction and cb junction, it should show the same unidirectional conductivity as the diode. The reverse resistance is infinite, and its forward resistance is about 10K. In order to further evaluate the quality of the tube characteristics, if necessary, the resistance level should be changed for multiple measurements. The method is:: Set the R×10Ω setting and measure the forward conduction resistance of the PN junction, which is about 200Ω. ； Set the R×1Ω setting to measure the forward conduction resistance of the PN junction, which is about 30Ω. (The above data are measured by the 47-type meter. Other types of meters may be slightly different. You can test a few good tubes and summarize them to get an idea.) If the reading is too high, it can be concluded that the characteristics of the tube are not good. You can also place the meter at R×10kΩ and measure again. For tubes with lower voltage resistance (basically the voltage resistance of triodes is above 30V), the reverse resistance of the cb junction should also be ∞, but the reverse resistance of the be junction may be a little, and the needle of the meter will deflect slightly (generally not more than 1/3 of the full scale, which varies according to the voltage resistance of the tube). Similarly, when measuring the resistance between ec (for NPN tubes) or ce (for PNP tubes) using the R×10kΩ scale, the needle of the meter may deflect slightly, but this does not mean that the tube is bad. However, when measuring the resistance between ce or ec with R×1kΩ or lower, the meter indication should be infinite, otherwise there is something wrong with the tube. It should be noted that the above measurements are for silicon tubes and are not applicable to germanium tubes. But now germanium tubes are rare. In addition, the so-called "reverse direction" is for PN junctions, and the directions for NPN tubes and PNP tubes are actually different. 　　Most of the common transistors nowadays are plastic-packed. How to accurately determine which of the three pins of the transistor is b, c, or e? The B pole of the triode is easy to detect, but how to determine which one is C and which one is E? Here are three recommended methods: first method: For a pointer meter with a jack for measuring the hFE of a triode, first measure the b pole, then insert the triode into the jack at will (of course the b pole can be inserted accurately), measure the hFE value, and then turn the tube upside down and measure it again. If the measured hFE value is relatively large, the insertion position of each pin is correct. Second method: This method can be used for meters without hFE measurement jacks, or if the tube is too large to be inserted into the jack.: For NPN tubes, first measure the b pole (it is easy to detect whether the tube is NPN or PNP and its b pin, right? ), place the meter in the R Pick up the tube, lick pole B with the tip of your tongue, and see that the pointer on the meter head should deflect to a certain extent. If you connect the test leads correctly, the pointer deflection will be larger. If you connect it incorrectly, the pointer deflection will be smaller. The difference is obvious. From this, the c and e poles of the tube can be determined. For PNP tubes, connect the black test lead to the assumed e pole (do not touch the pen tip or pin with your hand), and connect the red test lead to the assumed c pole. At the same time, pinch the tip of the test lead and the pin with your fingers, and then lick the b pole with the tip of your tongue. If the test leads are connected correctly, the meter pointer will deflect larger. Of course, the test leads need to be exchanged and measured twice during measurement, and the final judgment can be made only after comparing the readings. This method is suitable for triodes of all shapes and is convenient and practical. According to the deflection amplitude of the watch hand, the amplification ability of the tube can also be estimated. Of course, this is based on experience. Third method: After first determining the NPN or PNP type of the tube and its b-pole, place the meter in the R×10kΩ range. For NPN tubes, when the black test lead is connected to the e-pole and the red test lead is connected to the c-pole, the meter needle may deflect to a certain extent. For PNP tubes, when the black test lead is connected to the c-pole and the red test lead is connected to the e-pole, the meter needle may have a certain deflection, and vice versa. From this, the c and e poles of the triode can also be determined. However, for high pressure pipes, this method is not applicable. 　　For common imported high-power plastic tubes, the c pole is basically in the middle (I have never seen one with b in the middle). Some b's of medium and small power tubes are most likely to be in the middle. For example, the b pole of the commonly used 9014 triode and its other series of triodes, 2SC1815, 2N5401, 2N5551 and other triodes are in the middle. Of course, they also have the c-pole in the middle. Therefore, when repairing and replacing transistors, especially these low-power transistors, do not just install them as they are, you must test them first.