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Circuit not working

Discussion in 'General Electronics Discussion' started by kingofjong, Jun 1, 2018.

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  1. kingofjong

    kingofjong

    30
    1
    Aug 14, 2014
    Hi

    I have build a circuit but for some reason it is not working. The capacitor is suppose to discharge through the bjjt pnp transistor. Hence the transistor acts as a switch. But the capacitor isn't discharging when I apply current to the base of the transistor. Circuit diagram is bellow.

    upload_2018-6-1_6-44-8.png

    Thanks
     
    Last edited: Jun 1, 2018
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,411
    2,779
    Jan 21, 2010
    Your circuit is wrong in many ways.

    Are you working from an original circuit or did you come up with this yourself?
     
  3. kingofjong

    kingofjong

    30
    1
    Aug 14, 2014
    I came up with it my self. What is wrong with circuit?
     
  4. kingofjong

    kingofjong

    30
    1
    Aug 14, 2014
    Hi

    I figured out my mistake. So I don't need anymore help.

    Thanks
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I'd love to see what you came up with.

    Also, since you're staying the circuit yourself, there's a few conventions normally applied that make the circuit more readable.

    1. Power supply to one side
    2. Positive rail at the top, negative at the bottom
    3. Components horizontal or vertical
    4. Transistors generally drawn so the base connection is at the left or right side (not at the top). This should also result in the PNP emitter being at the top, and NPN emitter being at the bottom.
    These are generalisations, but following them will make your circuit more readable to other, and help you get your transistors connected right.

    I'll try to give you an example a little later.
     
  6. davenn

    davenn Moderator

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    Sep 5, 2009
    I would also like to see what you did to solve the problem

    and take good note on Steve's comments on drawing circuits :)
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,411
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    Jan 21, 2010
    OK, here's a more conventionally drawn version of your circuit:

    BadCircuit.png

    From this, it is clear that your circuit can't work. Pressing S1 will charge the capacitor, but the emitter of Q1 can never be more positive than the base, so the transistor cannot be turned on.

    Probably, the switch needs to be in the base circuit, pulling the base low.

    Let's see if I can draw something that might work...

    GoodCircuit.png

    In this circuit the capacitor charges up via R3 and R2, When the switch is pushed the capacitor discharges via Q1 and R2.

    The capacitor will be discharged down to about 0.6V. Using an NPN transistor and pulling the base to +ve would discharge the capacitor to a few tenths of a volt. You could also rearrange the circuit so the capacitor wasn't tied to the -ve rail to do this.

    And here's an option along those lines:

    GoodCircuit2a.png

    The solution you'd employ depends on the actual problem this circuit is intending to solve.
     
    Last edited: Jun 2, 2018
    davenn likes this.
  8. WHONOES

    WHONOES

    937
    238
    May 20, 2017
    In your solution, Q1 could really do with a pull up resistor otherwise when the switch is open, the base of Q1 is left floating and the device could be partially turned on.
     
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