Cant get transistor into saturation using an rc timer element

Discussion in 'The Projects Forum' started by Mrdouble, Mar 9, 2016.

  1. Mrdouble

    Thread Starter Member

    Aug 13, 2012
    82
    0
    image.jpeg
    The circuit above energizes the relay after a sec or so. It works great except that I can drive the base any harder because of the resistor. I know I could reduce the resistor but then I won't have to timing element. The Vce is amost 2 volts and thus cooking my little 3904. I know saturation depends almost entirely on collector resistance which is the coil at 120 ohms. I'm sure there's a simple solution but boy am I stumped.
    Thanks in advance
    Micheal
     
    Last edited: Mar 9, 2016
  2. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,539
    1,251
    Round numbers, the desired collector current is 100 mA so the base current should be 10 mA. But if the transistor is on the edge of satiration then it is running at a gain of around 100 so you need only 1 mA. 12 V / 1 mA = 12 K. You have 31 K so you don't have nearly enough base current. 10K resistor, 1500 uF cap.

    ak
     
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  3. Ramussons

    Active Member

    May 3, 2013
    557
    92
    Use a 2N2222 or similar to make a Darlington with the 3904
     
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  4. crutschow

    Expert

    Mar 14, 2008
    13,014
    3,234
    You could also use an N-MOSFET, which would allow a much higher resistor and small capacitor value for the same delay.
     
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  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    Use an IRF510, IRF540 mosfet, or Tip121 Darlington pair and zener diode to increase charge voltage range.
     
    Last edited: Mar 10, 2016
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  6. Mrdouble

    Thread Starter Member

    Aug 13, 2012
    82
    0
    Thank you all, I just decided to throw another 470uf cap in parallel and reduce the resistance. Not EXACLY saturated but good enough.

    Spike
     
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