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How to judge the quality, type and pin discrimination of the triode?
Transistors are fundamental and essential components in analog circuit design. Most of us have learned about their working principles, and terms like "high level on, low level off," "common amplifier circuit," and "emitter follower" are commonly encountered in our studies. However, in real-world applications, just knowing these definitions isn't enough to solve practical problems. For example, questions like "Is this transistor good?" or "How can I quickly determine the polarity of a transistor?" aren’t usually covered in textbooks, yet they are crucial skills for every electronics engineer or hobbyist. In this article, we’ll share three essential methods that you must master from a practical perspective.
First, determining the quality of a transistor.
When debugging circuits, it's often necessary to check if a transistor is functioning properly. One common method is using a multimeter. Set the multimeter to the resistance mode and measure between each pair of the transistor’s three pins, performing a total of six measurements. If two of them show a measurable resistance, the transistor is likely good. If not, it may be faulty.
Second, identifying the type (NPN or PNP) and pin configuration of a transistor.
Set the multimeter to the resistance mode. Touch the black probe to one pin and the red probe to the other two. If resistance is measured twice, the black probe is touching the base, and the transistor is NPN type. To distinguish between the emitter and collector, use the diode mode. If no resistance is found in any of the three configurations, try using the red probe as the common terminal and repeat the steps. This helps identify the base and the type of transistor. For PNP transistors, the same method applies, but the roles of the probes are reversed.
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.
An infrared thermometer is convenient and easy to use, allowing direct handheld measurement. However, its accuracy depends on the material, surface texture, and color of the component being measured. On the other hand, a thermocouple works by generating a small electromotive force when two different metals are at different temperatures. This signal is then converted into a temperature reading. Thermocouples are widely used due to their durability, wide range, high precision, and ease of signal transmission. However, installation and environmental factors can affect their performance.
In practice, I recommend using an infrared thermometer during debugging and switching to a thermocouple for long-term aging tests.
Lastly, determining the operating state of a transistor.
Set the multimeter to the voltage mode and test according to the table provided. This allows you to assess whether the transistor is in active, saturation, or cutoff mode. Visual aids such as the images below will help clarify the process.