Question on testing transistor 3904

Discussion in 'General Electronics Chat' started by Rattle49, Sep 29, 2015.

  1. Rattle49

    Thread Starter New Member

    Sep 22, 2015
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    Hello Everyone,

    I am testing transistor 3904 with a digital multimeter. When I put it on diode function the test result was perfect. reads 0.66V and 0.63V. But I also want to be able to use resistance function to test. While my results are around 7 or 8 mega ohms. I thought it should be somewhere around 500 ohms. I am just so confused. Please help........
     
  2. OBW0549

    Well-Known Member

    Mar 2, 2015
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    The reason you're not getting the results you expect is that your DMM uses a very low voltage to measure resistance, and this voltage is not enough to forward bias semiconductor junctions and get a meaningful reading. One of my DMMs applies only 400 mV and the other one, only 200 mV. Yours is probably similar.
     
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  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
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    If your DMM has a diode test mode as one of its Ohmmeter ranges, then it applies sufficient voltage across the device being measured to forward bias a Si junction. Mine even causes a red LED (Vf=~2V) to glow dimly...
     
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  4. Rattle49

    Thread Starter New Member

    Sep 22, 2015
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    Ah that makes sense. Thank you so much
     
  5. dannyf

    Well-Known Member

    Sep 13, 2015
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    Why did you think so?

    The "resistance" of a p-n junction is a function of the current through it, or alternatively, the voltage across it. Without knowing either of that, it could be anything from 100s of Mohms down to a few ohm.
     
  6. dl324

    Distinguished Member

    Mar 30, 2015
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    Aside from performing a diode test (dead or alive), a multimeter isn't well suited to test any other transistor parameters.
     
  7. vk6zgo

    Active Member

    Jul 21, 2012
    677
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    It won't give you definitive results like a proper tester,but you can check your NPN BJT for gain with a multimeter.

    On "diode test" position,place the red lead on the collector,the black on the emitter.
    Now spit on your finger & place it between the base & collector.
    The meter will show collector current as a drop in resistance.

    This will also work for PNP with the red & black leads transposed.

    I mainly use this as a "go/no-go" test,or to determine which lead is the collector on unknown transistors.
    A reversed transistor will show lower gain than a correctly connected one.
     
  8. Rattle49

    Thread Starter New Member

    Sep 22, 2015
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    I read it from a book saying that if you test your transistor with resistance function, "the DMM should read a low forward resistance, typically less than 1kΩ". the figure in the book just shows a DMM with 1 lead on emitter and the other on base.
     
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