Multimeters, also known as multiplex meters, multimeters, triple meters, multimeters, etc., are indispensable measuring instruments in power electronics and other departments. Generally, the main purpose is to measure voltage, current and resistance. Multimeters are divided into pointer multimeters and digital multimeters according to the display mode. It is a multi-function and multi-range measuring instrument. Generally, a multimeter can measure DC current, DC voltage, AC current, AC voltage, resistance and audio level, etc., and some can also measure AC current, capacitance, inductance and semiconductor. Some parameters (like β) etc.
Measurement techniques (pointer table if not stated):
1. Measuring speakers, earphones, and dynamic microphones: use R×1Ω gear, connect one end of any test lead, and touch the other end with the other test lead. Under normal circumstances, a crisp and loud "da" sound will be emitted. If there is no sound, the coil is broken. If the sound is small and sharp, there is a problem of rubbing the coil and it cannot be used.
2. Capacitance measurement: Use the resistance gear, select the appropriate range according to the capacitance capacity, and pay attention to the positive electrode of the capacitor for the black test lead of the electrolytic capacitor during measurement. ①. Estimate the size of the microwave-class capacitor capacity: it can be determined by experience or by referring to the standard capacitor of the same capacity, according to the maximum amplitude of the pointer swing. The reference capacitors do not have to have the same withstand voltage value, as long as the capacity is the same. For example, estimating a 100μF/250V capacitor can be referenced by a 100μF/25V capacitor. As long as the maximum amplitude of their pointer swings is the same, it can be concluded that the capacity is the same. ②. Estimate the capacitance of the picofarad-level capacitor: use the R×10kΩ file, but only the capacitance above 1000pF can be measured. For 1000pF or slightly larger capacitors, as long as the needle swings slightly, it can be considered that the capacity is sufficient. 3. Measure whether the capacitor is leaking: For capacitors above 1,000 microfarads, you can use the R×10Ω gear to quickly charge it first, and initially estimate the capacitance, then change to the R×1kΩ gear and continue to measure for a while. Should return, but should stop at or very close to ∞, otherwise there will be leakage. For some timing or oscillating capacitors below tens of microfarads (such as the oscillating capacitors of color TV switching power supplies), their leakage characteristics are very demanding, as long as there is a slight leakage, they cannot be used. Then use the R×10kΩ gear to continue the measurement, and the needle should stop at ∞ instead of returning.
3. Test the quality of diodes, triodes, and Zener tubes on the road: because in actual circuits, the bias resistors of transistors or diodes, and the peripheral resistance of Zener tubes are generally relatively large, mostly above hundreds of thousands of ohms, so , we can use the R×10Ω or R×1Ω gear 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 should have obvious forward and reverse characteristics (if the difference between the forward and reverse resistance is not obvious, you can use the R×1Ω gear to measure). Generally, the forward resistance is at R The needle should indicate about 200Ω when measuring in the ×10Ω gear, and around 30Ω when measuring in the R×1Ω gear (there may be slight differences depending on the phenotype). If the forward resistance value of the measurement result is too large or the reverse resistance value is too small, it means that there is a problem with the PN junction, and there is a problem with the tube. This method is particularly effective for repairs, where bad tubes can be found very quickly, and even tubes that are not completely broken but have deteriorated characteristics can be detected. For example, when you measure the forward resistance of a PN junction with a small resistance value, if you solder it down and test it again with the commonly used R×1kΩ file, it may be normal. In fact, the characteristics of this tube have deteriorated. Not working properly or unstable anymore.
4. Resistance measurement: It is important to choose a good range. When the pointer indicates 1/3 to 2/3 of the full range, the measurement accuracy is the highest and the reading is the most accurate. It should be noted that when using the R×10k resistance gear to measure the large resistance value of the megohm level, do not pinch your fingers at both ends of the resistance, so that the resistance of the human body will make the measurement result small.
5. Measure the Zener diode: the voltage regulator value of the Zener diode we usually use is generally greater than 1.5V, and the resistance file below R×1k of the pointer meter is powered by the 1.5V battery in the meter. The resistance range below R×1k is the same as measuring the diode, which has complete unidirectional conductivity. However, the R×10k gear 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 regulation value less than 9V or 15V, the reverse resistance value will not be ∞, but a certain value. resistance, but this resistance is still much higher than the forward resistance of the Zener tube. In this way, we can preliminarily estimate the quality of the Zener tube. However, a good voltage regulator must have an accurate voltage regulation value. How to estimate this voltage regulation value under amateur conditions? It's not difficult, just find another pointer watch. The method is: first place a watch in the R×10k gear, and the black and red test pens are connected 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 another watch is placed on the On the voltage range V×10V or V×50V (according to the voltage regulation 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 regulator value of the Zener tube. Saying "basically" is because the bias current of the first watch to the voltage regulator tube is slightly smaller than the bias current in normal use, so the measured voltage regulation value will be slightly larger, but the difference is basically the same . This method can only estimate the voltage regulator tube whose voltage regulation value is less than the voltage of the pointer meter's high voltage battery. If the voltage regulation value of the Zener tube is too high, it can only be measured by means of an external power supply (in this way, when we choose a pointer meter, it is more suitable to choose a high-voltage battery with a voltage of 15V than 9V).
6. Measure the triode: usually we use the R×1kΩ file, whether it is an NPN tube or a PNP tube, whether it is a low-power, medium-power, or high-power tube, the be and cb junctions of the test should be exactly the same as the diode. Electricity, the reverse resistance is infinite, and its forward resistance is about 10K. In order to further estimate the quality of the tube characteristics, if necessary, the resistance gear should be changed for multiple measurements. The method is: set the R×10Ω gear to measure the forward conduction resistance of the PN junction at about 200Ω; set the R×1Ω gear to measure The forward conduction resistance of the PN junction is about 30Ω. (The above is the measured data of the 47-type meter, and other models are slightly different. You can test a few more good tubes to summarize, so that you can know what you have in mind.) If the reading is too large Too many and it can be concluded that the characteristics of the tube are not good. You can also place the meter in R×10kΩ and test again. The tube with low withstand voltage (basically the withstand voltage of the triode is above 30V), the reverse resistance of its cb junction should also be ∞, but the reverse resistance of its be junction 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 tube) or ce (for PNP tube) with R×10kΩ, the needle may deflect slightly, but this does not mean that the tube is bad. However, when measuring the resistance between ce or ec with the gear below R×1kΩ, the indication of the meter should be infinite, otherwise there is a problem with the tube. It should be noted that the above measurements are for silicon tubes and not applicable to germanium tubes. But now germanium tubes are also rare. In addition, the so-called "reverse" refers to the PN junction, and the direction of the NPN tube and the PNP tube is actually different.
