Transistors are fundamental and essential components in analog circuit design. Most of us are familiar with their basic working principles, concepts like "high level on, low level off," "common amplifier circuits," and "emitter followers" are commonly encountered during our studies. However, in real-world applications, just knowing these theoretical definitions isn't enough. Questions such as "Is this transistor good?" or "How to quickly determine the polarity of a transistor" aren’t often covered in textbooks, yet they are crucial for every electronics engineer or hobbyist to master. In this article, we’ll walk you through three practical methods that are essential for real-world troubleshooting and testing.
First, how to determine the quality of a transistor. When debugging a circuit, it's common to need to check if a transistor is functioning properly. One simple method involves using a multimeter set to the resistance mode. You test all six combinations of the three pins. If two of the measurements show a consistent resistance value, the transistor is likely good. If not, it may be damaged.
Second, identifying the type (NPN or PNP) and pin configuration of a transistor. Start by setting the multimeter to the resistance mode. Touch the black probe to one pin, and then touch the red probe to the other two. If you get a reading twice, the black probe is on the base, and the transistor is NPN. To distinguish between the emitter and collector, use the diode mode. If no readings are found, try switching the probes and repeat the process—this helps identify PNP transistors.
Third, measuring the temperature rise of key power components. This includes checking the surface temperature of critical parts like MOSFETs, transformers, output rectifier diodes, and capacitors. Common tools include infrared thermometers and thermocouples. Infrared thermometers are easy to use and allow quick, non-contact measurements, but they can be affected by surface material and color. Thermocouples, on the other hand, offer high accuracy and are more suitable for long-term monitoring. They work based on the thermoelectric effect, converting temperature differences into electrical signals. While they are reliable, their installation and calibration can be more complex.
During debugging, I recommend using an infrared thermometer for fast checks, while thermocouples are better suited for production aging tests.
Lastly, how to assess the working state of a transistor. Set your multimeter to the voltage mode and refer to the table provided to measure the voltages at each pin. This will help determine if the transistor is operating correctly in its circuit. The images below illustrate typical voltage readings for different configurations.
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