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BJT as switch...

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

  1. koolguy

    koolguy Active Member

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    Or should i use transformer rectifier ?
     
  2. alec_t

    alec_t Well-Known Member Most Helpful Member

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    A potential problem is that an SMPS probably has a much higher output current capability than your little battery, so with your apparent inability to grasp the basics of driving LEDs safely the SMPS could fry a lot of LEDs in the blink of an eye if things aren't wired properly.
     
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  3. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Nobody makes a 3V white LED. Some will be 2.8V and others will be 3.2V. If you use a 9V supply and connect three 3.2V LEDs in series and in series with a current-limiting resistor then THEY WILL NOT LIGHT! So you connect only two "3V" Leds in series and have a very hot current-limiting resistor.
     
  4. dave

    Dave New Member

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  5. Ian Rogers

    Ian Rogers Super Moderator Most Helpful Member

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    When did the game plan change???

    I'm loosing the will to live..... Do you know the reason they call it a transistor!!!!

    Transfer resistor.... You need to design the BJT to your circuit... If you fully turn on the transistor then there should be nearly 7.2v across your resistor... BUT if you work out your current gain you can partially switch on the BJT to give a smaller voltage across your resistor so the BJT dissipates some of the power...

    At the moment ALL of the power is lost in the resistor... Using the BJT share the power loss between them..

    Look at these two designs..
    ritesh1.png ritesh2.png
     
  6. koolguy

    koolguy Active Member

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    OK, using low value Resistance will BJT to diss less power rather than Resistance.
    I am using 10k variable at base but i have found changing its value is not changing current passing from BJT, why at 0 it off only few 1 or 2mA changes why?
    can you explain further more how to BJT has raised it voltage drop so high across it?


    i was testing with 9Vbattery and i have to used SMPS for long time.
     
  7. Electroenthusiast

    Electroenthusiast Member

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    hello Ritesh, i asked you a question. You didn't answer it.
     
  8. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Are you still using a BC337 PNP transistor? What is driving its base?
    Its datasheet shows that some of them saturate poorly (VCE=0.7V) when the collector current is 500mA and the base current is 50mA.
     
  9. Ian Rogers

    Ian Rogers Super Moderator Most Helpful Member

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    He has a 20k pot!!
     
  10. audioguru

    audioguru Well-Known Member Most Helpful Member

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    If the base current is 50mA then part of the 20k pot will have 8V across it (VBE is about 1V) and with 50mA then its value is only 8V/50mA= 160 ohms.
    Then the portion of the pot is only 160/20k= 0.8% which is nothing. The tiny part of the pot will smoke when it tries to dissipate 8V x 50mA= 0.4W.
     
  11. koolguy

    koolguy Active Member

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    I am using 5V vcc at base with 20K....
     
  12. audioguru

    audioguru Well-Known Member Most Helpful Member

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    With a 20k base resistor at ground, the base current is (5V - 0.7V)/20k= 0.215mA. Then the maximum collector current can be 0.215mA x 20= 4.3mA, or maybe 0.215mA x 50= 10.75mA. Very low currents.

    With the transistor turned off when its base resistor is +5V then you cannot use a 9V supply for the emitter.
     
  13. ronv

    ronv Well-Known Member Most Helpful Member

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    Dark beer or light beer? I think I only have light beer....... In small bottles...... No glasses.......

    R,
    Try this one.
     

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  14. koolguy

    koolguy Active Member

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    [Error: String\ is\ too\ long\ (699, limit 300)]

    What is this 20and 50 here?
    The 5v is given by Data sheet maximum 5v can be used..
     
  15. audioguru

    audioguru Well-Known Member Most Helpful Member

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    5V is the maximum allowed reverse-bias emitter to base voltage for most transistors. Exceeding this voltage causes the emitter-base junction to breakdown like a zener diode which damages the transistor.

    But the base to emitter is almost always forward-biased in most circuits and 5V is NEVER used.
     
  16. Ian Rogers

    Ian Rogers Super Moderator Most Helpful Member

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    The pot is in series..... (I hope there is a 1k in there as well) So he shouldn't see 5v base to emitter... But then we are talking about Ritesh...
     
  17. Little Ghostman

    Little Ghostman Well-Known Member Most Helpful Member

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    sorry for a hijack but can i ask a question please? why is 5v never used at the base? what about a pic pin?? or am i not understanding hmmmm i think i am missing the meaning of base to emiter rather than base?? sorry for the question
     
  18. Little Ghostman

    Little Ghostman Well-Known Member Most Helpful Member

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    sorry one other thing i wanted to say, it would never have occured to me to work out what % of the pot was being used. its seeing info like this and real working out that makes threads like this real gem's ok i admit the OP drives a few mad but beginners like me take notes of this stuff ;) is real good learning stuff thanks guy's
     
  19. tvtech

    tvtech Well-Known Member Most Helpful Member

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    Hi Lg

    Base Emitter Voltage for Silicon Transistors is around 0.6 V for it to start turning on or switch on completely.

    The 5V mentioned earlier is the Max the same junction can handle without breaking down.

    Regards,
    tvtech
     
  20. Little Ghostman

    Little Ghostman Well-Known Member Most Helpful Member

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    thank you, i looked up the datasheet and now it makes sense, sorry for the questions but i take notes and hate having wrong notes!
     
  21. Little Ghostman

    Little Ghostman Well-Known Member Most Helpful Member

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    really sorry Ritesh for hijacking, is Q1 and Q2 in the Emitter follower configuration? just checking if i am learning any of this stuff :D

    LG
     

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