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Testing BJT Transistor

Discussion in 'General Electronics Discussion' started by fscii, Dec 6, 2012.

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  1. fscii


    Nov 13, 2012
    I have a project I'm working on in the "Make Electronics" book where you take a tiny signal and amplify it via transistors into a louder sound. It was working great then got quiet. I switched out the transistor for a new one (same exact model) and it works loudly so obviously my transistor went bad.

    However, I wanted to learn to test it on the meter and see what the deal was and I found this video:

    Its GREAT and helped me understand whats going on behind the scenes. However, it left me in a gray area because the faulty one tested very close to the operational one. The transistor in question is NPN 2N2222

    I tested them using the diode function of my meter and got this:

    Working Unit
    B-C .6v
    B-E .6v

    Broken one
    B-C .6v
    B-E .74v

    When trying to reverse bias it on the meter I got the expected "OL" errors too.

    So whats going on here? Why can't I "prove" on my meter that this thing doesn't work, when I can very easily put it in the circuit and see (hear) that its cooked?

    Any suggestions on other tests I can/should be doing with it on the meter? Please be detailed as I'm noob :)
    Last edited: Dec 6, 2012
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    There's more to a functioning transistor than just the B-C and B-E diode voltage drops that you are measuring. Using this simple test you can only sort out very bad transistors.
    For the amplification the most important parameter is the current gain of the transistor which requires a three-wire setup to measure. You inject a base current and measure the collector current from Ic/Ib you get the gain of the transistor.
    Some multimeters have a built-in simple transistor tester.
  3. fscii


    Nov 13, 2012
    Awwww I figured there was an easy way. I have a circle of holes in my meter to put transistors into but I think it only does hfe testing which I don't really understand.

    My good one gives 191 hfe, bad one gives 196

    If this isn't the built in transistor testor you are talking about, what is? Here is my multi-meter:

    Also I'm not sure what you meant by "lc/lb" ? I do understand injecting a base current to measure collector output tho, I'll give that a shot but prefer to understand how to use my meter to test.
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    That's not much of a difference. (191 vs 196) and is easily explained by variation between devices.

    Can you replicate the "one amplifies more than the other" experiment?
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