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Parallel connection of BJTs ,constant DC supply

Discussion in 'General Electronics Discussion' started by AshwinNambiar, Feb 1, 2015.

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  1. AshwinNambiar

    AshwinNambiar

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    Nov 11, 2014
    I'm new to electronics .I'm trying to simulate a BJT circuit on MULTISIM(BC635) .What i require is to provide an input DC voltage of 50V ,create a BJT circuit such that my voltage at the output is stepped down or rather the current at output > current at input.SO essentially if i want to build a circuit where i want my current to increase it will be in parallel .
    My questions are :
    1.How do i connect two BJT s in parallel i.e calculating the impedance values and providing bias?
    2.Is it possible to develop a design using BJT s where i get an exponentially decaying curve of an increasing current to a decreasing voltage.
    Also point out if i'm going wrong in my approach
    Thanks
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    In essence, you parallel bjt's by connecting their bases together, their collectors together, and by adding emitter resistors such that the voltage drop across them at full current is large compared to differences in vbe. Practically this might be around 1v. It is also important that the transistors are in thermal contact. Generally it is sufficient to place them in the same heat sink.

    I can't answer your other questions because your explanation of what you want is not clear to me. Perhaps you can tell us what you're trying to do rather than how you think it is done.
     
    Arouse1973 likes this.
  3. AshwinNambiar

    AshwinNambiar

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    Nov 11, 2014
    I'm trying to create an electronic circuit based on the property of
    ELECTRIC ARC (DC)which exhibits a decrease in voltage with an increasing current.The power is to be conserved,
    SO trying to design a circuit where i give a constant DC supply and enable the circuit design such that my output current is greater than the input ,thus lowering my voltage at output which can hence give me a characteristic as desired.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I'm still not completely sure I know what you're after but I think you should look at switch mode buck regulators. These will allow an output current greater than the input current (at a lower voltage) and aside from inevitable losses, exhibit the general behaviour of having the input power equal to the output power.
     
  5. BobK

    BobK

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    Jan 5, 2010
    I wonder if he is trying yet another free energy scheme?

    Bob
     
  6. AshwinNambiar

    AshwinNambiar

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    Nov 11, 2014
    What switch mode regulators i.e buck converter to be specific in this case does is give a lower output voltage and greater current compared to the input ,but isnt the output constant? how do i obtain a continuous decrease in the voltage with the current rise??I can keep varying the duty cycle and keep decrease the voltage further at the output but is it possible that an electronic circuit be designed where i can show decrease in voltage fora constant input not having to vary any of the parameter(for instance the duty cycle)?
     
  7. AshwinNambiar

    AshwinNambiar

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    Nov 11, 2014
    I'm not sure if i'am precise in explaining my requirement.to put it simple.I want to design an electronic load which gives me the exact V-I characteristics of a "DC ELECTRIC ARC" ,which is nothing but decreasing output voltage with increasing current at the output for a constant input ,power remaining constant .
    An electric arc has static VI characteristic with negative resistance slope.
    An analogous electronic design just to get this characteristic of the arc is what i'm trying to.Trying to make a simulated design on MULTISIM.
     
  8. AshwinNambiar

    AshwinNambiar

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    Nov 11, 2014
    one REALLY LAME design that i tried was:
    using BJT BUH100G(10A,700V),providiing Vcc =120v and DC supply 100volts ,i did a simple parallel connection of BJTs ,owing to the fact that for increased current parallel connection is done. Is there a way i can work through with this ?is there a possible design for an analogous electronic design of an electric arc charatceristic??
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,473
    2,819
    Jan 21, 2010
    Ah, so you want to design a load with negative resistance characteristics.

    Presumably you want something with a specific (negative) resistance? Or will any negative resistance be OK?

    One major issue is that any practical circuit will only exhibit a negative resistance over a certain range of voltages and currents, what are suitable end points?

    For simplicity, the circuit could operate only in the first quadrant (where voltage and current are both positive).

    What voltage do you want for zero current, and what current do you require for zero voltage?
     
  10. AshwinNambiar

    AshwinNambiar

    31
    0
    Nov 11, 2014
    This specific type of negative resistance slope is what i want.Exponentially decrease of output voltage and increase of output current for a constant input(Power is to be maintained constant) static V-I characteristics.This is a property of an "DC ELECTRIC ARC" .A simulated electronic design to obtain the same output is what i'm looking for
     

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