BJT comparator and Open Drain Comparator

Discussion in 'General Electronics Chat' started by artmaster547, Aug 16, 2017.

  1. artmaster547

    Thread Starter Member

    Jan 6, 2016
    388
    7
    Hi All
    I am in the process of designing a comparator circuit I want the output of this comparator to then go into a simple BJT latch which keeps an LED on (i.e. if an error occurs, the LED remains on). I have attached screen shots of the simulations I carried out the issue I am having is that I have been able to get the latch work on it's on and the same for the comparator however when connecting the two up I am having an issue the latch functionality ceases to exist I am quite puzzled as to how I can fix it and any assistance would be greatly appreciated.

    Kind Regards

    Art
     
  2. jayanthd

    Active Member

    Jul 4, 2015
    689
    64
    Use SCR. CD106M and a 2N2222A. 2N2222A will be used to turn off the SCR.
     
  3. ericgibbs

    Moderator

    Jan 29, 2010
    5,723
    1,066
    hi 547,
    If you check Q1 and Q2 Bases you will see they are biassed so that they are never switched Off.
    When used in isolation, the latch, I guess your Vsource for the test pulse has zero ohms output resistance.???
    E
     
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  4. ericgibbs

    Moderator

    Jan 29, 2010
    5,723
    1,066
    The OP is asking why his circuit is not working, he is NOT asking for a different circuit.
     
    artmaster547 likes this.
  5. artmaster547

    Thread Starter Member

    Jan 6, 2016
    388
    7
    Thank you so much I really appreciate the explanation, can you suggest whether this circuit could be improved or what I could use for the same effect?

    Kind Regards

    Art
     
  6. artmaster547

    Thread Starter Member

    Jan 6, 2016
    388
    7
    Sorry I mean can I ammed
    the circuit so it behaves in the way I intent or can you recommend any other typologies that I could use?
     
  7. ericgibbs

    Moderator

    Jan 29, 2010
    5,723
    1,066
    hi 547,
    This is one option.
    E
     
  8. artmaster547

    Thread Starter Member

    Jan 6, 2016
    388
    7
    Hi thank you, is there a way for the LED to have zero current flowing through it when there is no signal from the comparator I think atm the LED I am thinking of using the following:
    http://uk.farnell.com/kingbright/kp...ategoryId=&searchView=table&iscrfnonsku=false
    So I think with the simulation when the pulse is zero at the start it comes out to 13mA flowing so the LED will be slightly ON
     
  9. artmaster547

    Thread Starter Member

    Jan 6, 2016
    388
    7
    If I change R4 to say 10Ohms would that be suitable or would that much current damage the BJT's?
     
  10. artmaster547

    Thread Starter Member

    Jan 6, 2016
    388
    7
    I can to the following BJT's if that works out better: BC817
     
  11. artmaster547

    Thread Starter Member

    Jan 6, 2016
    388
    7
    *BC817-40
     
  12. ericgibbs

    Moderator

    Jan 29, 2010
    5,723
    1,066
    hi,
    To isolate your LED driver I would add a final transistor, you can drop that intermediate transistor.
    E
     
  13. artmaster547

    Thread Starter Member

    Jan 6, 2016
    388
    7
    Brilliant thank you so much, really appreciate the assistance learning a lot from creating designs and your input is really helping out with the learning process
     
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