transistor necessity

Discussion in 'General Electronics Chat' started by mxabeles, Jun 9, 2010.

  1. mxabeles

    Thread Starter Active Member

    Apr 25, 2009
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    hey there,
    when I have a circuit running that lights up various leds without using a tranistor the logic acts funny. Using transistors to drive the leds makes the funky errors disappear. Am I correct in thinking that this is because the base of transistor takes so little current that it doesn't "interrupt" or "confuse" the signal flow, which seemingly does happen if devices are powered by logic unaided by transistors?
    Thanks,
    M
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    To assess the difference in performance of the two circuits (one with and one without transistors) it would help us if you would please post the two schematics?

    hgmjr
     
  3. mxabeles

    Thread Starter Active Member

    Apr 25, 2009
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    and as an aside great website i was looking at but got off put by this:
    'the collector is a current source: it does not source a voltage'
    this is concerning npn transistor. Now i must say i am still not solid about voltage/current relationship (even after reading, there is no such thing as electrons haha). In my mind i thought current and voltage are forever linked, how can you have one without the other...ever?
    thanks 2,
    M
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    While you can't have current without a potential difference to move electrons (voltage), you can have a voltage and no current. Open a switch, and you can read a voltage across the contacts, but there will be no current.
     
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  5. mxabeles

    Thread Starter Active Member

    Apr 25, 2009
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    here is circuits in question:
    with transistor :see 8.59 pm screenshot attachment
    without transistor: see 9.00 pm screenshot attachment
    thanks
    m
     
  6. mxabeles

    Thread Starter Active Member

    Apr 25, 2009
    182
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    been there : thanks, thats interesting. Does that make the case that its not high voltage that burns out say an led but current? That, among other things, is still a sticking point. In other words, using series resistors although they drop voltage as a bi-product of limiting current, are truthfully used to limit current to device not voltage.
    thanks again,
    m
     
  7. hgmjr

    Moderator

    Jan 28, 2005
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    Your assessment of the advantage provided by the addition of a transistor is pretty accurate. One thing, from looking at your circuit that includes the transistor, you need to add a current limiting resistor between the output of the 555 and the base of the transistor. Otherwise you are clamping the output of the 555 timer at around 0.7 volts.

    hgmjr
     
  8. mxabeles

    Thread Starter Active Member

    Apr 25, 2009
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    right, actually tho i'm using 2803 array they got built in series resistor. The reason its not in the schematic is that eagle doesn't seem to have the uln2803a in its library. U know of any downloads of it...might have to learn how to make my own libraries after all :))
     
  9. beenthere

    Retired Moderator

    Apr 20, 2004
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    The PN junction has to have a certain voltage across it before electrons overcome the barrier, but after that, there is no "natural" limiting mechanism present to control current. As power is E X I, that power dissipated in the PN junction will heat the LED to destruction.
     
  10. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Why do people post "wrong" schematics just because the real part model is not in the library?

    Use an accurate drawing program to draw an accurate schematic not a pcb designing program that does it wrong.
     
  11. mxabeles

    Thread Starter Active Member

    Apr 25, 2009
    182
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    oh hey there audioguru!
     
  12. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    They are in the uln-udn.lbr - if you don't have it, you can download it from the CadsoftUSA site, or if it is not there any longer, I could upload the one I have for you.
     
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  13. mxabeles

    Thread Starter Active Member

    Apr 25, 2009
    182
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    got it. Thanks !
     
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