This topic contains 3 replies, has 2 voices, and was last updated by  mwpeters75 1 month, 3 weeks ago.

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  • #2708


    Hi all,

    By adding  a few components, the circuit simulates the lamps dimming in a theatre.

    The transistors form a darlington pair with very high input impedance. Therefore, the 100uF capacitor slowly discharges through the two resitors of 100k.

    Who can answer following questions ?

    What do you think the resistor of 10k contributes in the circuit ?

    Why is the lamp connected between the Emitter and the ground ?

    Do you think if the resistor of 47 Ohms could also be placed in series with the lamp, or is it better to place it at the Collector side ?

    Try the circuits out to answer these questions !


    Circuit 20

  • #2710


    Hi Frank,

    Thanks so much for posting this great learning moment!  Challenging questions for a non-engineer such as myself.

    Perhaps in a few days you can post the answers and explanations for further learning.

    Thank you for taking the time to make these great posts.  This is what the Lectron System is all about! MWP.

    • This reply was modified 2 months ago by  mwpeters75.
  • #3207


    Hi all,

    I hope you found some time to build the circuit and look for the answers to the questions.

    First of all, did you spot the error on purpose in the text of above circuit ?

    Careful readers should have noticed that the capacitor charges instead of discharges through the two 100K resistors. When you look at the diagram, you see that the switch is connected across the capacitor via a resistor of 5,6K. By pressing the switch, you’ll discharge the capacitor. After releasing it, the capacitor starts to charge. The negative pole of the capacitor will slowly build up a negative voltage through the two 100K resistors. This voltage is amplified in the first transistor and fed to the second transistor.

    The secret of the high impedance (seen at the negative pole of the capacitor) is the use of 2 emitter followers in series.

    The emitter of the first transistor is coupled to the base of the second transistor. The lamp is connected from ground to the emitter of the second transistor. This is the reason why the capacitor mainly charges through the two resistors of 100K.

    So, now for the questions :

    The 10K resistor adds to the high impedance of the circuit. Try to lower it’s value and see the results in the dimming speed !

    So, the lamp is connected between the emitter and the ground to make the transistor act as an emitter follower. Change it’s place to the collector side and watch the difference by observing the lamp. It will stay lit longer and suddenly get dark, due to the amplification difference between the two circuits.

    Aha, the 47 Ohm resistor ! Well, in fact it makes no difference for the lamp and it’s behavior. When we would put the resistor at the emitter side, the voltage will increase also at this side. The current flowing through the lamp remains the same. What will change is the base current, it will be shifted higher.

    Hope you enjoy this little circuit !

    • This reply was modified 1 month, 3 weeks ago by  mwpeters75.
    • This reply was modified 1 month, 3 weeks ago by  mwpeters75.
    • This reply was modified 1 month, 3 weeks ago by  mwpeters75. Reason: Changed font and size styling for easier reading
  • #3210


    Hi Frank,

    Excellent learning situation once again!  Thank you.  I have to say that I did not note the charge/discharge situation.  I just got a new Rigol 5000 series scope so perhaps now I can better participate in your teaching modules.  Thanks again for your great participation in this forum.  As we gain more registrants, these learning moments will be even more greatly appreciated and helpful.


    • This reply was modified 1 month, 3 weeks ago by  mwpeters75.

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