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delay circuit

Discussion in 'General Electronics Discussion' started by behi2011, Oct 9, 2011.

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  1. behi2011

    behi2011

    2
    0
    Oct 9, 2011
    i want a circuit for herein application:
    [​IMG]

    when i turn on the SW ,the relay activated after seconds
    but because of low impedance of relay(160 ohms), this circuit did not work
    my question is: how i can solve this problem?
    is there any high impedance 12V relay that can i use it
     
  2. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    A good method is to use a much higher resistance to give a longer time constant and then a transistor (FET) to switch the relay. Relays with built in transistors are available but are expensive.

    A few seconds is possible but to go much longer brings capacitor leakage as a significant factor.

    Duke
     
  3. behi2011

    behi2011

    2
    0
    Oct 9, 2011
    can you design a circuit
     
    Last edited: Oct 9, 2011
  4. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    Sketch attached. The time constant is R*C but the time will depend on the voltage at which the fet turns on.
     

    Attached Files:

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