How to measure a diode with a multimeter

　　1. Selection of pointer tables and digital tables:

　　1. The reading accuracy of pointer tables is poor, but the process of pointer swing is more intuitive, and its swing speed amplitude can sometimes reflect the measured size more objectively (such as measuring TV data). The bus (SDL) jitters slightly 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 techniques (if no explanation is given, the pointer meter is used):

　　1. Measure speakers, headphones, and dynamic microphones: Use the R×1Ω gear, connect any test lead to one end, and touch the other end to the other end. Normally, it will make a crisp and loud "click" sound. 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. Measuring 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 during measurement. ①. Estimating the capacity of microwave-grade capacitors: It can be determined based on experience or with reference to standard capacitors of the same capacity, and based on 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. Estimating the picofarad capacitance size: Use the R×10kΩ scale, 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 sufficient. ③. Test whether the capacitor is leaking: For capacitors above 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 to continue testing for a while. At this time, the pointer does not move. It should return and 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, they can be charged in the R×1kΩ range. Then switch to the R×10kΩ range and 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: In actual circuits, the bias resistance of triodes or the peripheral resistance of diodes and voltage regulator tubes are generally relatively large, mostly above hundreds of thousands of ohms, so , 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, use the R×10Ω gear to measure the PN junction and it should have obvious forward and reverse characteristics (if the difference between the forward and reverse resistances is not obvious, you can use the R×1Ω gear to measure it). Generally, the forward resistance is in R When measuring in the ×10Ω range, the needle should indicate around 200Ω, and when measuring in the R×1Ω range, the needle should indicate around 30Ω (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 a certain PN junction and the forward resistance is too high, 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. Not working properly or unstable.

　　4. Measuring resistance: It is important to select the correct 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. Source: www.tede.cn

　　5. Measure the voltage regulator diode: The voltage regulator value of the voltage regulator diode we usually use is generally greater than 1.5V, and the resistance range below R×1k of the pointer meter uses the 1.5V in the meter. Battery-powered, in this way, measuring the Zener tube with a resistance range below R×1k is 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 value. resistance, but this resistance is still much 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, and then take another meter and put it in the R×10k position. At the voltage level V×10V or V×50V (according to the voltage regulator value), connect the red and black test leads to the black and red test leads of the watch just now. The voltage value measured at this time is basically this 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 need 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 direction as the diode. Electrically, the reverse resistance is infinite, and its forward resistance is about 10K. In order to further evaluate the characteristics of the tube, if necessary, the resistance level should be changed for multiple measurements. The method is: set the R×10Ω setting to measure the forward conduction resistance of the PN junction, which is about 200Ω; set the R×1Ω setting and measure The forward conduction resistance of the PN junction is about 30Ω. (The above data are measured by the 47-type meter. Other models of meters may be slightly different. You can test a few more good tubes to summarize and have an idea.) If the reading is too high If there are too many, it can be concluded that the characteristics of the tube are not good. You can also place the meter in R There may be some, and the needle will deflect slightly (generally not more than 1/3 of the full scale, depending on the pressure 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.