![]() ![]() All other cases (like EB and BE both conduct, or BC and CB both not conducting, etc.) indicate the transistor is not good. And, if only the EB and CB junctions conduct, the transistor is still normal but the transistor type is PNP. Now, if only the BE and BC junctions conduct, the transistor is of NPN type and is working fine. If D2 is High, CB junction conducts, otherwise not. If D2 is High, EB junction conducts, otherwise not. If D3 is High, BC junction conducts, otherwise not. If D1 is High, BE junction conducts, otherwise not. The test sequence for a transistor would go like this. Three I/O pins of a microcontroller are required to implement the testing algorithm for a transistor. We can also identify the type (PNP or NPN) of the transistor by considering the direction of the current conduction. ![]() If both the junctions conduct in only one direction, the transistor is normal, otherwise it is faulty. This concept can be easily extended to test a transistor by realizing that a transistor consists of two PN junctions: one betwen the base and the emitter (BE junction), and the another between the base and the collector (BC junction). The circuit implementation of this logic is shown below. If it does in both the directions, it means the diode is short, and if it does in neither direction, it is open. Therefore, a good diode will conduct current in only one direction. A diode is a PN junction that allows the conduction of current only in one direction. The logic behind testing a diode is straightforward. A PIC16F688 microcontroller is used in this project that switches the bias voltage across the PN junctions of diode and transistors, and determines if a particular junction is normal, open or short. The testing algorithm is based on a simple fact that a working PN junction conducts current in only one direction. The purpose of this project is to demonstrate a simple way to construct a testing device for diodes and bipolar junction transitors (BJTs) using a microcontroller. ![]() Most of the digital multimeters these days have built-in features for testing diodes and sometimes transistors. ![]()
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