Multimeter: Various Measurement Techniques for Different Objects
1. Testing loudspeakers, headphones, and dynamic microphones: Use the R×1Ω range. Connect one probe to one end and touch the other end with the other probe. Normally, a crisp and loud "da" sound will be heard. If no sound is heard, it indicates a broken coil. If the sound is small and shrill, it indicates a coil rubbing issue and cannot be used.
2. Measuring Capacitance: Use the resistance setting, select an appropriate range based on the capacitance value, and note that for electrolytic capacitors, the black probe should be connected to the positive terminal of the capacitor during measurement. ① Estimating the capacitance of microwave-level capacitors: This can be done based on experience or by referencing a standard capacitor of the same capacity, judging by the maximum amplitude of the pointer swing. The reference capacitor does not need to have the same voltage rating, as long as the capacities are the same. For example, to estimate the capacitance of a 100μF/250V capacitor, one can use a 100μF/25V capacitor as a reference, as long as their pointer swings with the same maximum amplitude, it can be concluded that the capacities are the same. ② Estimating the capacitance of picofarad-level capacitors: Use the R×10kΩ setting, but it can only measure capacitors above 1000pF. For capacitors of 1000pF or slightly larger, as long as the pointer swings slightly, it can be considered that the capacitance is sufficient. ③ Testing whether a capacitor is leaking: For capacitors above 1000μF, first use the R×10Ω setting to quickly charge them and make a preliminary estimate of the capacitance. Then switch to the R×1kΩ setting to continue measuring for a while. At this point, the pointer should not return to its original position but should stop at or very close to ∞. Otherwise, there is a leakage phenomenon. For some timing or oscillating capacitors (such as the oscillating capacitor in a color TV's switching power supply) with capacitance below tens of microfarads, the leakage characteristics are very critical. As long as there is any leakage, they cannot be used. In this case, after charging with the R×1kΩ setting, switch to the R×10kΩ setting to continue measuring. Similarly, the pointer should stop at ∞ and not return to its original position.
3. Testing the quality of diodes, triodes, and zener diodes in-circuit: In practical circuits, the bias resistors of triodes or the peripheral resistors of diodes and zener diodes are generally large, mostly in the hundreds or thousands of ohms. Therefore, we can use the R×10Ω or R×1Ω range of a multimeter to test the quality of PN junctions in-circuit. When measuring in-circuit, using the R×10Ω range to test the PN junction should exhibit clear forward and reverse characteristics (if the difference between forward and reverse resistance is not too significant, you can switch to the R×1Ω range for measurement). Generally, the forward resistance should indicate around 200Ω when measured in the R×10Ω range, and around 30Ω when measured in the R×1Ω range (there may be slight variations depending on different meter types). If the measured forward resistance is too high or the reverse resistance is too low, it indicates that there is a problem with the PN junction, and thus the transistor is faulty. This method is particularly effective for maintenance, as it can quickly identify faulty transistors, and even detect transistors that have not completely failed but have deteriorated characteristics. For example, if you measure the forward resistance of a PN junction using a low resistance range and find it to be too high, if you solder it off and measure it again using the commonly used R×1kΩ range, it may still appear normal. However, in fact, the characteristics of this transistor have deteriorated, making it unable to work properly or stably.
