SCR gate trigger problem

Discussion in 'The Projects Forum' started by MEA, Feb 22, 2013.

  1. MEA

    Thread Starter New Member

    Feb 18, 2013
    4
    0
    Hello, I am having problems providing a gate signal to turn on the SCR. The transistor AND gate seems to be okay. I have attached the cct I am using. Thanks.
     
  2. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
    40
    Is your gate just connected to the 12 volt supply?

    The output of the 741 is not guaranteed to get all the way down to 0 volts to turn the transistors completely off.

    When your two transistors are ON, you basically connect you 12 volt supply to ground.
     
  3. MEA

    Thread Starter New Member

    Feb 18, 2013
    4
    0
    The transistors are switching on/off. The problem I am having is where to put the SCR gate lead. Would a logic gate be a better solution?
     
  4. #12

    Expert

    Nov 30, 2010
    16,278
    6,788
    Use a 220 ohm resistor to connect the gate lead between the two transistors, ground the Kathode, expect conduction when the upper transistor is on and the lower transistor is off.
     
  5. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
    40
    When exactly are you trying to trigger the signal? You can explain in terms of the opamp outputs.
     
  6. MEA

    Thread Starter New Member

    Feb 18, 2013
    4
    0
    I am trying to trigger the SCR gate when both transistors are switched on. One op-amp is set to output at 0 degrees Celcuis, the other is to o/p when rain is detected. The combined o/p (AND) is to gate an SCR circuit.
     
  7. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
    40
    To create an AND gate you need to put a resistor between the 12volt supply and the collector of the upper transistor, start around 10k. The upper collector than gives you a NAND output. Adding one more transistor resistor pair will invert the output to a logical AND. For this pair feed the NAND output into the base of the next transistor and the point t between the collector and resistor will be the AND output.

    As #12 suggests, add a resistor between the output and the gate of the SCR and connect the SCR as instructed.
     
  8. MEA

    Thread Starter New Member

    Feb 18, 2013
    4
    0
    Thankyou for the help StayatHomeElectronics and #12. The circuit works fine now!
     
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