How to determine Transistor Terminals

In this post, I will show you how to determine transistor terminals or pinouts that is; Base (b), Collector (c), and Emitter (e) of an unknown Bipolar Junction Transistor (BJT). Transistors are of different pin configurations and this is refers to as transistor case or package type. Some common packages for both NPN and PNP include TO18 and TO39, TO92A, TO92B, TO92C, TO218 and TO220, etc. with different pins configurations.
Before we can determine the correct terminals or pinouts (bce), we must first of all be sure if the transistor is an NPN type or PNP. Therefore, I assume you know how to test and identify NPN and PNP transistors already. But however, if you can not figure out whether the transistor is an NPN or PNP BJT, read this article; how to test NPN and PNP transistor to learn how to.

Alright! Once you are sure of the transistor’s type that is either NPN or PNP, you can now identify the pin configurations by following the steps below

how to determine transistor terminals


Using multimeter (Digital or Analog), set it in this form: Black Probe’s plug inserted into the “COM” socket and the Red Probe’s plug into the “VΩmA” socket of the multimeter.


Turn your meter into continuity or diode tester range (if your meter has the range). If the meter has not the continuity or diode tester range, set it to resistance tester range at 2000Ω (2KΩ).


If the transistor is an NPN type, the Base (b) pin which is the common junction (P-Type) is the pin which you placed your Red Probe on when testing the transistor. Meanwhile, if the transistor is PNP type, the pin which the Black Probe was placed on when testing the transistor is the Base (b).


Next, test to determine the Collector (c) and the Emitter (e) pin. Note that for every ideal BJT, the Emitter (e) pin will have slightly higher resistance value from the Base (b) while the Collector (c) will have lower resistance value from the Base (b). Therefore, connecting the probes Base (b) to Collector (c) = lower value whereas Base (b) to Emitter (e) = higher value.
That’s all! You have successfuly determined the Base, Collector, and the Emitter (bce) pins of the unknown bipolar junction transistor. Don’t forget to share this short but interesting post!

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