Transistor Not Working as Expected

Discussion in 'General Electronics Chat' started by shodai, Apr 4, 2016.

  1. shodai

    Thread Starter New Member

    Apr 4, 2016
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    So I have been experimenting with transistors, trying to learn. However, I have run into a problem that I just do not get. All of the schematics that I look at have really high resistance for the base of the transistor (usually 10k) but when I use that it seems like nothing passes through. I change it to 1k and everything appears to work properly, with one pretty significant issue. It seems like the circuit work even when nothing is connected to the collector. So I am confused on both of these issues and was wondering if anyone has any ideas as to what I might be doing wrong?
     
  2. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
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    Welcome to forum.

    Your question is not defined well enough. Post the schematic you are talking about. Be more specific.
     
  3. shodai

    Thread Starter New Member

    Apr 4, 2016
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    0
    Sorry. Let me give the most simple example that I have tried.
     
  4. dl324

    Distinguished Member

    Mar 30, 2015
    3,249
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    One rule of thumb is to assume a beta of 10 when driving a transistor in saturation mode. To get 10mA collector current, you need 1mA of base current. With 100K, you're only getting 0.05mA. Try using 5K; 10K will also likely work. It depends on the beta of the transistor being used.
     
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  5. wayneh

    Expert

    Sep 9, 2010
    12,145
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    And then in many cases, beta is greater than ten and if minimizing the base current is important, you experiment. You use the largest ohms resistor that still works well. If you don't care about current and just want to be 100% sure it works, use the rule of thumb.
     
  6. DGElder

    Member

    Apr 3, 2016
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    "It seems like the circuit work even when nothing is connected to the collector. So I am confused on both of these issues and was wondering if anyone has any ideas as to what I might be doing wrong?"

    Define "works" precisely when you say "seems like the circuit work even when nothing is connected to the collector." What are your measured values?
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,024
    3,236
    A transistor "works" by the base current controlling the collector current.
    If you have nothing connected to the collector than the transistor can't be working.
     
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