Multimeter: Introduction to Different Techniques for Measuring Different Objects
1. Testing Speakers, Headphones, and Dynamic Microphones: Use the R×1Ω range. Connect one probe to one terminal and tap the other probe on the opposite terminal. A normal component should produce a clear and loud "click" sound. If there's no sound, the coil is broken. A weak and high-pitched sound indicates a rubbing coil issue, rendering the component unusable.
2. Testing Capacitors: Use the resistance range and select an appropriate scale based on the capacitor's capacitance. Note that for electrolytic capacitors, connect the black probe to the positive terminal during measurement.
① Estimating the Capacitance of Microwave-level Capacitors: Rely on experience or compare with a standard capacitor of the same capacitance. Judge the capacitance by the maximum swing amplitude of the multimeter needle. The reference capacitor need not have the same voltage rating, only the same capacitance. For example, to estimate a 100μF/250V capacitor, use a 100μF/25V capacitor as a reference. If their needle swings match, their capacitances are deemed identical.
② Estimating the Capacitance of Picofarad-level Capacitors: Use the R×10kΩ range, which can only measure capacitors above 1000pF. For a 1000pF or slightly larger capacitor, a slight needle movement indicates sufficient capacitance.
③ Testing Capacitor Leakage: For capacitors above 1000μF, quickly charge them using the R×10Ω range to estimate capacitance initially. Then switch to the R×1kΩ range. The needle should remain at or very close to ∞; otherwise, leakage exists. For timing or oscillation capacitors under (e.g., oscillation capacitors in color TV switching power supplies), which require high leakage resistance, switch to the R×10kΩ range after charging with R×1kΩ. The needle should stay at ∞ without returning.
3. Testing Diodes, Transistors, and Zener Diodes in Circuit: In practical circuits, the bias resistors of transistors or the surrounding resistors of diodes and Zener diodes are generally large (hundreds to thousands of ohms). Thus, use the R×10Ω or R×1Ω range to measure the PN junction in-circuit. A good PN junction should exhibit distinct forward and reverse characteristics. Typically, the forward resistance should read around 200Ω on the R×10Ω range and around 30Ω on the R×1Ω range (values may vary slightly by multimeter model). Excessive forward resistance or low reverse resistance indicates a faulty PN junction and a defective component. This method efficiently identifies faulty components during repairs, even detecting partially damaged ones with degraded characteristics. For instance, if a PN junction shows high forward resistance on a low-resistance range but tests normal off-circuit with the R×1kΩ range, its characteristics have deteriorated, rendering it unreliable.
