How to test a bipolar transistor

This post refers to how to test a bipolar junction transistor (BJT) with an ohmmeter.

A bipolar transistor has two PN junctions; a PN junction basically behaves as a simple diode. Bipolar transistors come in two types, NPN and PNP, based on how the base of the transistor is doped.

A simple method to see if the transistor is good ( presuming you have just an ohmmeter ), is to use the ohmmeter to test each of the two junctions.

Note: You can use any type of ohmmeter ( analogical or digital )

Set your ohmmeter on the X1 domain, and:

Step1: Connect the red probe to the base(B) of the transistor and the black probe to the emitter(E)

Now we test the PN junction between the base and emitter ( BE ).

Red probe to base and black to emitter.

Step2: Connect the red probe to the base(B) of the transistor and the black probe to the collector(E)
Now we test the PN junction between the base and collector ( BC ).

Red probe to base black to collector.

If you get readings ( higher then 0Ω ) in both cases, then the junctions are good, and because the red probe was connected to the base, means that the transistor is NPN type. ( You should NOT get any readings ( Ω ) with the black probe connected to the base. )

Step3: If you didn’t get readings at Step1 and 2, then move the black probe to base(B), and repeat Step1 and 2

If you get readings, as in steps 1 and 2, but with the black probe connected to the base, and no readings at all with the red probe connected to the base, that means your transistor is also good, but is of PNP type.

Summary:

  • Red probe connected to base, and readings with the black probe connected to both emitter and collector ( one at the time ) => transistor is GOOD and is NPN.
  • Black probe connected to base, and readings with the red probe connected to both emitter and collector ( one at the time ) => transistor is GOOD and is PNP.
  • Both cases above, but no readings ( Ω )  or short-circuit ( 0Ω ), means the transistor is fried.

Comments

  1. written by: Maurizio on October 26, 2013 at 2:55 pm - Reply

    Hi, I noticed that the BC resistance is always slightly lower than that of BE. Is that difference significanta and diagnostic to identify C and E in the case of unknown-wired transistor?

  2. written by: mihai on October 26, 2013 at 8:04 pm - Reply

    Hi,

    I don’t think so, usually the pins (B,C, and E) are identified by the transistor case and pin orientations.

    Here you can find a lot of cases styles and pin orientations http://www.beavisaudio.com/techpages/Transistor-Pinouts/

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