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BJT as switch...

Discussion in 'General Electronics Chat' started by koolguy, Sep 8, 2013.

  1. ronv

    ronv Well-Known Member Most Helpful Member

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    I thought he wanted it dimmable as well, but I'm not sure anymore. Really should start over.:banghead:
     
  2. audioguru

    audioguru Well-Known Member Most Helpful Member

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    His post #59 shows the common-emitter transistor driven from a 50kHz square-wave so it is dimmed a little with PWM.
    But the base current is too low for the transistor to saturate.
     
  3. ronv

    ronv Well-Known Member Most Helpful Member

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    I guess I was stuck all the way back to #1 with the pot in the base.
     
  4. dave

    Dave New Member

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  5. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Maybe he used a pot on the base because he didn't read the datasheet for the transistor, where it says the base current must be 1/10th the collector current for fairly good saturation at high currents.
     
  6. koolguy

    koolguy Active Member

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    OK, after using common emitter will current also increase?
     
  7. koolguy

    koolguy Active Member

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    As i am using PIc16F877a so, without base resistance it will damage it..
     
  8. koolguy

    koolguy Active Member

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    OK, I have noticed this in data sheet: that mean i have to raise the base current as pic16f is not giving sufficient current so, so using another transistor at base like bc547 will be fine??
    VCE (sat) Collector-Emitter Saturation Voltage IC=500mA, IB=50mA 0.7 V

    I will do this now pls telll will this be fine?
    PIC16f output to 5K base resistance of BC547 and its emitter to Vcc of 5V and collector to 10K pott. meter at base of BC337 and emitter to gnd and collector with 55Ohm 2watt series resistance to led...
     
    Last edited: Sep 19, 2013
  9. koolguy

    koolguy Active Member

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    Anyway the problem is solved it was nothing on load the voltage level was getting low so, i changed it to 12 for red led working very bright only bc337 is used giving good amount of current to led.

    I have doubt using in // resistance of same value will double the power??
     
  10. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Do not operate the BC337 transistor at its maximum collector current because it works poorly above 200mA.

    Why is the emitter of the BC547 at 5V? A BC547 can be an emitter-follower to drive the base of the BC337 with its emitter.
    or a PNP common-emitter transistor can drive the base of the BC337 with its collector through a resistor.

    You did not post a new schematic so I do not know what you are talking about.

    Ohm's Law says that two resistors with the same value in parallel double the current and double the power if the voltage remains the same.
    But the voltage will probably be reduced when the current is higher.
     
  11. koolguy

    koolguy Active Member

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    why???
     
  12. audioguru

    audioguru Well-Known Member Most Helpful Member

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    A BC337 current gain.png graph in the datasheet shows the current gain.
     
  13. koolguy

    koolguy Active Member

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    why common collector in reversed biased switching?
     
  14. audioguru

    audioguru Well-Known Member Most Helpful Member

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    What is "reverse biased switching"?

    You want the BC337 to turn on and turn off but its base needs about 50mA of current for its collector to saturate with a load current of 500mA.
    Your signal does not provide 50mA, it has only about 20mA or less Driver circuits for LEDs.png .

    Then a common-collector (emitter-follower) NPN transistor can be used to provide 50mA when its base uses only 5mA or less. Its collector MUST BE POSITIVE or connected to the collector of the BC337 to make a darlington pair. When the input signal to the base goes positive then the emitter follows and also goes positive but at a diode voltage drop (0.7V) lower.

    A PNP common-emitter transistor and a series resistor can also be used to drive the BC337. But it inverts the function.

    I do not know why you do not show a clear and tidy schematic like this:

    EDIT: Hee, hee. the forum's new software does tricks when you insert a schematic!
     
  15. tvtech

    tvtech Well-Known Member Most Helpful Member

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    Hi AG

    I like the BC327 and BC337 trannies. Great family especially the BC327-40 and BC337-40. Highest gain versions.

    My little project loves them. Max current at any time is around 140mA that they are carrying/switching and the Voltage drop over Collector-Emitter at the same time is around 0.2V Max. Never get hot and never moan.

    As you said in a previous post...these little guys are great up to 200Ma where they start to battle.

    Kind Regards,
    tvtech
     
  16. koolguy

    koolguy Active Member

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    Hi, here is the result...
     

    Attached Files:

  17. audioguru

    audioguru Well-Known Member Most Helpful Member

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    It is obvious that each LED has a different brightness. Some LEDs look like they are turned off and other LEDs look like they are ready to burn out.
    Just like we told you!
     
  18. koolguy

    koolguy Active Member

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    the viewing angle problem, anyway how to make equal?
     
  19. audioguru

    audioguru Well-Known Member Most Helpful Member

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    LEDs in parallel MUST all be measured then only the few with IDENTCAL forward voltages can be connected in parallel.
    LEDs in series all operate with the same current and will look the same brightness. But each series string might have different brightness.
    Chinese flashlight companies pay somebody almost nothing to measure and sort LEDs into groups that have IDENTICAL forward voltages.
     

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