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Real Quick LED Driver Question

Discussion in 'Electronic Design' started by Tom, May 11, 2007.

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  1. Tom

    Tom Guest

    I am trying to drive an LED (2.1Vf, 20mA) from a digital signal
    (3.3V). From what I have been able to understand, the logic level
    output I am using does not provide enough current to drive an LED
    brightly. I have another power souse (7.4V) on board to drive the
    LEDs, but I do not know what transistor to use. There are so many
    different ones. What would be my best option? Can someone point me
    out to a model number?


    Thanks,
    Tom
     
  2. DJ Delorie

    DJ Delorie Guest

    Pretty much anything will do, if it's just for blinkenlights (if it's
    a fiberoptic laser LED, that's a different issue). You need either
    an NPN bipolar or N-channel MOSFET.

    On my furnace board, I used ON Semi's DTC144EET1G, mostly because I
    wanted a SOT-523 (i.e. really small) package. This is an NPN bipolar
    with a built-in base resistor, since you need some sort of resistor
    when driving bipolar transistors. Then you have to do the math - how
    much Ice current do you want, divide by worst-case Hfe (gain) to get
    Ibe, determine voltage difference between worst case Voh of your
    driver and highest Vbe, select base resistor.

    For N-MOSFETs it's a little easier, as no base resistor is needed and
    the turn-on voltage drop across the MOSFET is very little; you can
    almost pick the resistor as if the resistor/led were connected right
    to the voltage rails. You just have to pick any N-MOSFET with a Vgs
    turn-on voltage sufficiently below your minimum Voh, and a Ids current
    capability high enough for your LED.

    To recap:

    1. Select a physical size, like TO-92 or SOT-23.

    2. Choose between bipolar or MOSFET.

    3. Current capability: For bipolar, select for Ice. For MOSFET,
    select for Ids. Choose at least 2x your expected LED current.

    4. Pick the cheapest one that's still listed ;-)
     
  3. John Fields

    John Fields Guest

     
  4. My personal favorite NPN is MPSA06 (TO92), and for higher power PNP I like
    MJE170 (TO225). Mostly because I have several hundred of the one and a
    couple thousand of the other, among my surplus stock. I design many of my
    projects around these and other components I have in bulk, unless I have to
    compromise too much on specs.

    For driving 8 LEDs (or even relays) it's hard to beat the 18 pin ULN2803
    octal darlington driver.

    Paul
     
  5. Meat Plow

    Meat Plow Guest

    Wasn't quick enough.
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  6. feebo

    feebo Guest

    any GP NPN signal transistor will do really - 2N4124, BC107, BC184,
    ZTX300 anything really - provided you drive it in common emitter mode
    (i.e the LED & resistor to the collector, emitter to ground and drive
    the base with your 3.3v signal via a resistor). Base resistor needs to
    be about 1K5 and your LED resistor will be about 270R. Should light up
    nicely.
     
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