Turn on higher current LED with lower current BJT

Discussion in 'Analog & Mixed-Signal Design' started by spittips101, Oct 4, 2018.

  1. spittips101

    Thread Starter New Member

    Apr 9, 2018
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    I am trying to get a status LED to turn on using a circuit similar to the one below, but I can't quite figure out the right way to do it.
    The problem is my LED needs around 16mA to illuminate but my control BJT has a max collector current of 2mA, so I am trying to use the BJT to turn on some other circuit that can support my LED. Can anyone offer an idea of how to do this, preferably using a breakout board or some off-the-shelf solution.
    I would also prefer not to use a micro controller for this, but a more basic electrical circuit.
    [​IMG]

    -Taylor
     
  2. ebp

    Well-Known Member

    Feb 8, 2018
    2,243
    785
    What transistor are you using? 2 mA is an extraordinarily low rating and something that would only be expected of a very small transistor on an integrated circuit.

    If you are willing to consider different LEDs there are a few around that will produce reasonable brightness on 1 mA.
     
  3. crutschow

    Expert

    Mar 14, 2008
    20,251
    5,733
    Yes, 2mA is a very low maximum collector current for a BJT.

    But your schematic shows <2A.
     
  4. KeithWalker

    Member

    Jul 10, 2017
    113
    33
    Your diagram is confusing. You state that the maximum collector current for the BJT you are using is 2mA but the diagram indicates that is is drawing 2A. Which is correct?
    If R1 in your circuit is the load you are controlling, connect the led as shown: New-1.jpg
     
  5. spittips101

    Thread Starter New Member

    Apr 9, 2018
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    You're correct I made a mistake on that drawing, it should say 2mA. The transistor is an opto-isolator transistor controlled by a power good signal inside a power supply (Excelsys UX6). Here is a description from the designer manual:

    The open collector of an opto-isolator is available on J3 pin 7 and the emmiter on J3 pin 8. (opto-transistor ON= PowerGood).
    Maximum collector current is 2mA.
    Maximum Collector voltage is 30V.

    I agree that this is an extremely small current rating. That is why I am trying to come up with a creative way to use this to control another circuit.
    The LED I am using can not be changed, it is a specific panel mount LED. I would even be willing to build a small circuit on a proto board if no one knows of off-the-shelf boards that can work for this. Just looking for ideas. Thanks for your replies.
     
  6. KeithWalker

    Member

    Jul 10, 2017
    113
    33
    This will do what you want. Using standard values for the resistors, the collector current of the opto-isolator will be 1.6 mA and the current through the LED will be about 17 mA :
    New-2.jpg
     
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  7. crutschow

    Expert

    Mar 14, 2008
    20,251
    5,733
    Here's the LTspice simulation of a representative optoisolator circuit with an added 2N3906 PNP to drive the LED, similar to Keith's circuit above.
    The opto current is 1mA and the LED current is 15.6mA

    upload_2018-10-4_19-46-46.png
     
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  8. spittips101

    Thread Starter New Member

    Apr 9, 2018
    8
    0
    Keith and crutschow,

    Thank you so much! This definitely looks like it will work for me. Very much appreciated.

    Best regards,
    -Taylor
     
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