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BC547 Transistor as a switch

  1. Sep 20, 2016 #1
    Hello friends I'm new here and want some answers from u. plz help me to sort out the problem related to my circuit of high voltage protection. in this circuit i regulate the dc voltage using zener diode and then output of zener is given to the base of npn transistor. plz check the circuit diagram for detail.
    now coming to my problem that at second stage where i used transistor as a switch, actually transistor is not working as a switch. the led shown there must be ON/OFF but its nor happen here. when i changed pot value thn intensity of the LED changes only it not goes ON/OFF.
    can anyone help me regarding this? DSC_0071.JPG
     
  2. jcsd
  3. Sep 20, 2016 #2

    davenn

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    Hi there
    welcome to PF


    it would have been easier to read if you had posted the image the right way up
    here I will do it for you......

    upload_2016-9-20_19-28-47.png

    What is the zener type ... I cannot read the full part number
    there's no need for the first 10uF 63V when you already have a 4700uF there

    so what are you really trying to achieve in the second stage ?

    OK after a little figuring out .... the reason that the LED isn't switching off is because you have a continuous bias on the base of the transistor that the LED is in the collector cct.
    That bias voltage(current) is coming from the positive rail down through the 1k resistor and on to the base
    To make the BC547 a pure on/off switch then get rig of the second transistor and trimpot and other assoc components. Keep a single 1k resistor in the base and take that via a switch to the positive rail

    Dave
     
    Last edited: Sep 20, 2016
  4. Sep 20, 2016 #3

    CWatters

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    Do you want a simple on/off circuit or do you want to be able to change the brightness?
     
  5. Sep 20, 2016 #4

    davenn

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    OK revising your stage 2 circuit

    upload_2016-9-20_19-45-31.png

    just a quick circuit I had drawn for some one else in days gone bye, but it will give you the understanding of how to use the transistor as a switch
    You can still take the positive and negative rails from your PSU in your stage 1 circuit


    Dave
     
  6. Sep 20, 2016 #5

    davenn

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    an on /off switch by his comments
     
  7. Sep 20, 2016 #6
    Thanks Dave for ur reply..I'm using 1N4744 zener. and in the second stage i want a transistor to work as a switch only by using which i can operate relay. another modified diagram I/m attaching here. plz check it

    Circuit.JPG
     
  8. Sep 20, 2016 #7
    Yes i want simple ON/OFF circuit using transistor which may operate relay at the output side of transistor so i can cut off the mains from further circuit. as i ll put relay between mains and further circuit.
     
  9. Sep 20, 2016 #8
    Why u use switch here.? i don't want anything which will work manually. i want this circuit to use in remote place. Thanks for ur kind help.
     
  10. Sep 20, 2016 #9
    Yes simple ON/OFF circuit
     
  11. Sep 20, 2016 #10
    hei davenn I did as per ur circuit. but u can see in the simulation circuit runs for 3volt and also for 5volts. i checked it for 8volts also. if my DC voltage is varying from 8-14 volts as per AC mains changes from 180-250Volts this switch is continuously ON. but i want this to ON only After 7-9volts. den i can operate relay. otherwise it is not of any use.
     

    Attached Files:

  12. Sep 20, 2016 #11

    davenn

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    so what is the point of the trimpot ? it cannot be operated manually any easier than a physical switch

    what do you intend to use to turn the transistor on and off ?
     
  13. Sep 20, 2016 #12

    davenn

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    that isn't really clear
     
  14. Sep 20, 2016 #13
    I have further circuit which i need to operate. i'm designing this circuit for high voltage protection. here at the output of transistor i want to connect relay which then further operate to protect my circuit by cutting it OFF from main source.

    i'm using trimpot and fixing it to some value at which transistor ON. as voltage increases or decreases our resistance of trimpot is fixed and so current varries. which causes our transistor to ON or OFF as resistance is constant in this case. Getting my point?:sorry:
     
  15. Sep 20, 2016 #14
    switching circuit.JPG
     
  16. Sep 20, 2016 #15

    davenn

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    OK so let me see if I have this....
    you are wanting to monitor the mains voltage and then be able to switch off some other piece of gear that is on that mains voltage
    IF the mains voltage rises too high
    Is that correct ?


    D
     
  17. Sep 20, 2016 #16
    :biggrin:yes correct
     
  18. Sep 20, 2016 #17

    davenn

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    OK ... tis bed time here for me .... if some one else doesn't chime in with some circuitry, I will see what I can organise in the new day

    final Q for the nite .... what is the gear that needs to be switched ? ...
    what is it's recommended supply voltage and current ( or watts) ?


    Dave
     
  19. Sep 20, 2016 #18
    Okay.. 12Volts relay i'm using. answer it tomorrow. no problem. gd nyt.


    actually I have no of ques related to it.. can we discuss it ?
     
  20. Sep 20, 2016 #19

    jim hardy

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    One usually incorporates a little bit of positive feedback to give snap-action on/off transitions with a little bit of hysteresis so it doesn't 'chatter' right at the trip point..
     
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