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Linear LED driver works too well?

Discussion in 'LEDs and Optoelectronics' started by eem2am, Jan 6, 2014.

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  1. eem2am

    eem2am

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    Aug 3, 2009
    Hello,
    This linear LED driver puts 50mA into each string, even though only one string is regulated....and even though the led strings are different lengths.

    Is it the emitter degeneration which is equalising the led current in each string?

    schem and LTspice sim attached
     

    Attached Files:

  2. duke37

    duke37

    5,361
    767
    Jan 9, 2011
    The emitter degeneration will determine the current.

    Should the transistors have base resistors to limit the base current if they are not working as true emitter followers ?
     
  3. eem2am

    eem2am

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    0
    Aug 3, 2009
    Yes I agree that above cct could do with a base resistor

    Anyway, this alternative circuit (below) uses a vbe to regulate the led current to 50mA
    Would you agree that this is not an accurate enough method to be used in an automotive led light?

    Incidentally The LEDs are power topleds by osram.

    So, the alternative circuit (below) is even cheaper....
    I tried changing transistors to see the effects of transistors with different parameters in the different strings......in the simulator its not much difference...the simulator appears "kind" to this circuit.

    The 2 Power BJT's in the actual circuit are BCX56-16
    http://www.nxp.com/products/automoti.../BCX56-16.html

    I really cannot see how accurate current regulation can be garnered with a BJT VBE......but this is happening inside cars today.

    Would you agree that the VBE of the BJT = BC847B could be anywhere between 500mV and 900mV depending on temperature and batch?
    http://www.nxp.com/documents/data_sheet/BC847_SER.pdf

    if someone presented this to you as a automotive rear led light regulator, would you agree that the led current accuracy is insufficient?

    LTspice sim and schematic of alternative attached

    here is the schem in jpeg (linreg_1)
    http://i44.tinypic.com/9hszt2.jpg
     

    Attached Files:

    Last edited: Jan 9, 2014
  4. duke37

    duke37

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    767
    Jan 9, 2011
    The two strings will not be exactly matched but will probably be close enough. The current will be 17mA.

    I do not see the purpose of D6 and you could perhaps do without R5.
    R4 could be higher than 10k.
     
    Last edited: Jan 7, 2014
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    As long as the bases are tied together the circuit will work. For better performance the transistors should be thermally coupled and be reasonably closely matched.

    This is essentially a current mirror, and for that to work, the bases must be joined together.

    When eem2am suggests a base resistor, I would hope he's suggesting a single resistor between the op-amp output and the coupled bases.

    Independent base resistors would allow the bases to be at different potentials.
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Is this another one of your "question lectures" eem2am?

    (Where you ask a question and then lecture us about the answer?)

    Just come out with the real question ("I don't understand current mirrors")

    And one of your links in post #3 is broken.
     
    Last edited: Jan 7, 2014
  7. eem2am

    eem2am

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    Aug 3, 2009
    ..the circuit wont work without D6.....without d6, Q3 turns on to quick and shorts out the power transistor bases, and they never turn on.
    As you know, q4 and q3 ensure that all the leds go out if the regulated string suffers an open led.
    The links in #3 work ok for me.
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Then you need to try the first one again.
     
  9. eem2am

    eem2am

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    Aug 3, 2009
    Actually. I have come up with a seemingly better version of the multi (2+) LED string regulator.
    In the following two schematics "LINREG 1 TL431REG" uses the TL431 as a negative feedback opamp to set the LED current.
    -Whereas LINREG 1 TL431REF uses the TL431 as a simple 2.5V zener diode.

    Which of these two circuits would you believe is best?, and is the negative feedback loop one in danger of oscillating?

    Schematics and LTspice simulations of the circuit with the TL431 used as a zener diode, and as an opamp with negative feedback attached .
    (please give me a shout if you wish to receive the .asy file and .sub file for the TL431)

    Here are schems in jpeg
    LINREG - TL431REF:
    http://i42.tinypic.com/17qtsi.jpg
    LINREG - TL431REG:
    http://i44.tinypic.com/33dzitx.jpg
     

    Attached Files:

    Last edited: Jan 9, 2014
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    You still haven't fixed your broken link...
     
  11. eem2am

    eem2am

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    Aug 3, 2009
    I went up and fixed it now.
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Nup, still broken. First link 404's
     
  13. eem2am

    eem2am

    414
    0
    Aug 3, 2009
    I just tried them and get them ok....I wonder if tinypic.com is blocked by some antivirus packages
     
  14. jpanhalt

    jpanhalt

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    4
    Nov 12, 2013
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