How to Test NPN Transistors

Written by michelle raphael
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How to Test NPN Transistors
NPN transistors are a commonly used type of transistors in circuits. (four transistors image by Eugene Tokarev from

You need to test NPN transistors to check if they have any damage. Heat produced by soldering and improper use of circuits can permanently incapacitate your transistors. If you use a malfunctioning NPN transistor for your operational duties, chances are these will disrupt the board and trigger other problems. You can replace your faulty component if this is the case. Replacing a malfunctioning NPN transistor is a lot easier than installing a whole new board. The NPN transistor test will show conduction.

Skill level:

Things you need

  • Analogue or digital multimeter

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  1. 1

    Prepare your multimeter for testing your NPN transistors by setting it on the "diode test" selection. Set the analogue units and ensure they are on a low resistance range.

  2. 2

    Get your NPN transistor and identity three terminals. The terminals are usually viewed as electronic valves. The small signal they send through their middle terminal controls the present flow through the others. The terminals also function as amplifiers and switches.

  3. 3

    Clip the red lead from your multimeter on the base terminal. Use the black lead to touch your emitter terminal, before moving to your collector terminal. The readings should be the same with your black lead on either terminal.

  4. 4

    Reverse your leads and clip the black to your base terminal. Touch your emitter and collector terminals separately using your red lead. Your readings should be the opposite of those in the previous step. If your original readings showed high resistance, then this test should come out low and vice versa.

  5. 5

    Test your collector and emitter resistance by touching the red lead to your collector, as well as the black lead to your emitter. Reverse your leads. The readings should then indicate high resistance for both of your tests.

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