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Class problem help (Converter U-I)

Discussion in 'Electronics Homework Help' started by Razvan, Jun 5, 2017.

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  1. Razvan

    Razvan

    3
    0
    Jun 5, 2017
    Hello everyone, I have this problem where i need to propose the supply/input voltage and the output current (that I want to obtain) and then to calculate and choose the correct components so my circuit can work.
    asd.png

    So, I choose the supply to 5V and the output that i want to obtain to 20mA. From my calculations, I got R = 250ohms, and i choose it 270ohms (standard). But now, my teacher wants to calculate the values for the NPN transistor and choose one properly (I choosed bc107a, but i dont think it's good, because i didn't make any calculations). I also tried to do my best with the schematic in orcad.
    asd2.png

    Can i get a little help?

     
  2. Alec_t

    Alec_t

    2,927
    797
    Jul 7, 2015
    Welcome to EP!
    If 20mA flows through R1, how much current would you expect to flow through R2 and what would be the expected voltage on the transistor collector?
    Are all the transistor voltages and currents within the limits specified in the transistor's datasheet?
     
  3. Cannonball

    Cannonball

    191
    53
    May 6, 2017
    Hello Razvan,
    Just a word of encourgement. Alec-t has given you a very good hint. Good Luck.
     
  4. Razvan

    Razvan

    3
    0
    Jun 5, 2017
    That's the problem. I dont't have a particular transistor imposed, i need to choose one from the 'catalog' that checks my circuit... And about the R2 (witch i think is RL) I don't know how to choose (or calculate) it. I'm so confused.
     
  5. Razvan

    Razvan

    3
    0
    Jun 5, 2017
    I got it now, so: i choosed my Vi=5V and the output current (Ie) that i wanted to obtain (≈20mA), because this circuit it's an U-I Converter. AND i got the Ie to 18.52mA, which is very ok and i'm happy.

    Screenshot_1.png

    Now i have calculate the VCE on the transistor (with formulas and stuff) and choose the correct transistor from the catalog (i put BC107a because it's the only transistor i know :banghead:). Also, do you know what is the role of the diode in the whole circuit?
     
  6. Alec_t

    Alec_t

    2,927
    797
    Jul 7, 2015
    It does nothing useful if V1 stays at 5V. It would prevent the transistor base-emitter junction being driven beyond its reverse voltage rating if V1 were negative.
     
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