Right or wrong?

Discussion in 'The Projects Forum' started by MikeK, Apr 22, 2016.

  1. MikeK

    Thread Starter Member

    Apr 1, 2016
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    A page in the 'IC Timer Cookbook'. On item 'B' is an NPN the correct transistor? The diode seems to be keeping current from getting to the base. If this was a PNP setup, I would understand it. A 2N6055 is NPN.
     
  2. #12

    Expert

    Nov 30, 2010
    16,298
    6,811
    It might be a 2N3055, one of the most used transistors ever, but we can't tell because we don't have the book. If you post a drawing of the circuit you might get some good answers.
     
  3. MikeK

    Thread Starter Member

    Apr 1, 2016
    33
    0
    I'll try again, I think I messed up the first time.

    upload_2016-4-22_18-16-40.png
     
  4. #12

    Expert

    Nov 30, 2010
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    The 680 ohm resistor is supplying the base current so this Darlington transistor is normally on. The timer is in charge of shutting it off by dumping the base current through the diode.
     
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  5. Papabravo

    Expert

    Feb 24, 2006
    10,145
    1,791
    The reason it might look strange is that the transistor is a Darlington, but the schematic symbol does not show that fact. A Darlington transistor has a higher Vbe than an ordinary transistor. With the base at 0.7V the Darlington transistor will be OFF.
     
  6. MikeK

    Thread Starter Member

    Apr 1, 2016
    33
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    Thanks, #12, that makes sense.
     
  7. dannyf

    Well-Known Member

    Sep 13, 2015
    1,819
    362
    Npn is the right type there.

    Also, the diode is a must in that circuit. Alternatively, you can tie the base to pin 3 through a resistor and get rid of the diode.
     
  8. MikeK

    Thread Starter Member

    Apr 1, 2016
    33
    0
    Yeah, in the 'A' the 'out' was high, so when I looked at the 'B' I presumed it was high again. But no, it was low. No fair changing these things without telling someone.
     
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