Transistor circuit help

Discussion in 'The Projects Forum' started by 8-bit, Jan 26, 2014.

  1. 8-bit

    Thread Starter New Member

    Jan 7, 2014
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    I've just started dipping my toes into electronics and got hold of a few bits and pieces. I thought I would mess around with the transistors I have to see them in action. I setup a simple circuit with the collector attached to the positive rail, the emitter connected to a LED then onto the negative rail. I just wanted to see how the LED will only light when I run a current through the base of the transistor. The problem is the LED lights WITHOUT a current running into the base. What have I done wrong? :confused:
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Can you post a schematic of the circuit you made?
    That way we can see if you did something wrong.

    Bertus
     
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  3. 8-bit

    Thread Starter New Member

    Jan 7, 2014
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    Thanks, will do. :)
     
  4. LvW

    Active Member

    Jun 13, 2013
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    8-bit, you are using a positive AND a negative supply voltage, correct?
    In this case, the negative voltage at the emitter and zero voltage at the base (I assume) cause together a positive B-E voltage which opens the transistor.
    In this case, of course there is a base current.
     
  5. 8-bit

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    Jan 7, 2014
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  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    To make that circuit schematic work you need two modifications:

    first, turn the LED over so that the cathode (not the anode) is connected to 0V.

    second, the bottom lead of your transistor must be the emitter (usually drawn with an arrow to indicate current direction.) If you have the resistor connected to the collector, you will have to turn the transistor over by interchanging the collector and emitter.
     
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  7. 8-bit

    Thread Starter New Member

    Jan 7, 2014
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    Thanks Mike. Luckily the LED was the right way around but the transistor may not be. I'll post an image of my circuit so you can see maybe what the problem is.
     
  8. 8-bit

    Thread Starter New Member

    Jan 7, 2014
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    Here's the image. Hope you can make it out okay?

    [​IMG]
     
  9. 8-bit

    Thread Starter New Member

    Jan 7, 2014
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    I realize now there is no need for the red connecting wire at the bottom of the image! :)
     
  10. MikeML

    AAC Fanatic!

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    Or the black one? If the LED is lit all the time, try shorting the base to the emitter. If the LED is still lit, the transistor is shorted. (Based on the assumption that it the pin order is EBC)
     
  11. 8-bit

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    Jan 7, 2014
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    The black connects from the battery's input that just out of picture.
     
  12. bertus

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    Apr 5, 2008
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    Hello,

    Your transistor is shorted by the strips inside the breadboard.

    [​IMG]

    Bertus
     
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  13. 8-bit

    Thread Starter New Member

    Jan 7, 2014
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    The black one goes to the other terminal of the battery. :)
     
  14. 8-bit

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    Jan 7, 2014
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    Ah! So thats the problem? But surely it wouldn't short without a current to its base? I assumed it would act as an open switch as it stands in the image.
     
  15. bertus

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    Hello,

    The metal strip inside the breadboard is shorting the C-E of the transistor, so it can not switch.
    Putting the emittor and the resistor in an other row will solve the problem.

    Bertus
     
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  16. 8-bit

    Thread Starter New Member

    Jan 7, 2014
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    Thanks Bertus, will alter it and see what happens. :)
     
  17. 8-bit

    Thread Starter New Member

    Jan 7, 2014
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    I did not think that it would be able to short as no current can flow between the collector and emitter until the transistor is 'turned on' by a base current.(?)
     
  18. 8-bit

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    Jan 7, 2014
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    Well, altered the mistake you pointed out and it works fine! Thanks :)
     
  19. bertus

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    Apr 5, 2008
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    Hello,

    Did you understand what I said abou moving the wires?

    [​IMG]

    Bertus
     
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  20. 8-bit

    Thread Starter New Member

    Jan 7, 2014
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    Yes thank you.

    But it seems something odd is happening. When I apply a current to the base of the transistor the led lights up but I can't see how it does as the collector isn't connected to the positive rail(??)

    I assumed the base current allowed a current to flow from collector to emitter but I've noticed the collector isn't actually connected to a the positive rail at the moment. :s
     
    Last edited: Jan 26, 2014
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