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Odd LM317T question

Discussion in 'Electronic Design' started by Richie, Oct 17, 2003.

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  1. Richie

    Richie Guest

    I am trying to use a LM317T to drive some 3.8 volt LEDs. I built a
    circuit using a 1k resistor for the fixed and 10k pot for the
    adjustable. Using my trusty DMM i set the voltage to 3.8 volts and
    preceed to connect the LED. When I connected the LED the voltage drops
    to 2.8V and no matter how high I turn the pot the voltage of the LED
    stays at 2.8V. Does anyone know why it is doing this?
    Thanks
    Rich
     
  2. Paul Burke

    Paul Burke Guest

    Sounds like the LED is actually 2.8V and you are current limiting. Is
    there a resistor in series with the LED? Does the LED require more
    current than the LM317 can deliver?

    Paul Burke
     
  3. Robert Baer

    Robert Baer Guest

    LEDs should be driven by a *current* source.
    You may notice one or more things as you "turn up the pot":
    a) the LED gets brighter
    and/or
    b) there is more ripple at the LM317T output
    and/or
    c) the LM317T drops completely out of regulation
     
  4. Andre

    Andre Guest

    I'm assuming you have the LM317T connected round the right way and in
    constant current mode not constant voltage :)

    Check the current through the LEDs, your resistor should have some
    effect , the change may just not be large enough to have an effect on
    the voltage across the LEDs.
     
  5. Bill Sloman

    Bill Sloman Guest

    You were trying to drive some 3.8V LEDs from *what* supply voltage?

    If it was 5V, you would be out of luck. The LM317 needs at least 1.5V
    from input to output beofre it can work. This limits your output
    voltge to less than 3.5V, not enough to drive your LEDs.

    Your 1k fixed resistor is too high - the LM317 data sheet recommends
    120R - and this may be part of the problem.

    This - incidentally - happens to be the wrong way to drive LEDs. If
    you read the LED data sheet carefully, you will find that the "3.8V"
    is specified at a specific current (usually 20mA) as 3.8+/-0.4V or
    there abouts.

    You need to design your circut so that each LED gets its 20mA (or
    whatever) anywhere within the guaranteed voltage drop range around
    3.8V. If you are trying to drive several LEDs in parallel from a
    regulated voltage, most of the current will go through just one LED
    (the one with the lowest voltage drop) and the others probably won't
    light up - not what you want.

    You can configure an LM317 as a current regulator, but it won't work
    with a 3.8V LED and a 5.0V power rail. A low-drop-out linear regulator
    might work, but they can be picky about the output impedance they see,
    and the only handy data book doesn't show any of them configured as a
    regulated current sink, which is a bit worrying.

    You can use a current mirror to do the job. 1.2V of headroom is
    probably just enough. Horowitz and (Win) Hill's "The Art of
    Electronics" (ISBN 0-521-37095-7) will tell you all you need to know
    about current mirrors.
     
  6. Tim Shoppa

    Tim Shoppa Guest

    Two points:

    1. Do not drive LED's with regulated voltages. If you have to regulate
    them, just regulate the current. There's a circuit in the LM317 data sheet
    if you don't mind all the losses of linear current regulation. There
    are many switch-mode current-regulated LED drivers out there if you care
    more about efficiency.

    2. You need 2.5 or 3V of headroom over the output voltage when the LM317
    is used as a voltage regulator, as well as some bypass capacitors. What
    are you putting in to the LM317, 5V?

    Tim.
     
  7. Fred Bloggs

    Fred Bloggs Guest

    Yes- the LED clamps the ADJ terminal of the 317 to the LED voltage
    corresponding to Vref/1K=1.25mA ( less 280uA through the 10K pot at max
    setting). Remove the 10K pot altogether and replace the current setting
    resistor as shown below. If you want 10mA through the LED then
    R=1.25/0.01=125 ohm- use a 120 ohm- etc. The LM317 IN terminal must be
    at a minimum potential of 3.8+1.25+1.5=6.6V in this circuit.

    Please view in a fixed-width font such as Courier.



    LM317
    +------------+
    Vin >--+-----| IN OUT |------+
    | | | |
    | | | \
    | | ADJ | R
    | +------------+ \
    | | /
    === | |
    0.1U +-------------+
    | |
    | | LED |
    | + --- | 1.25V
    | Vled \ / ~~ | I= -----
    | - --- | R
    | | \|/
    +-----------+
    |
     
  8. happyhobit

    happyhobit Guest

    The LM317 has a 'minimum current to maintain regulation' requirement.
    (Typical 3.5 ma, max 10 ma) With a 1K resister you only have 1.25 ma. That's
    TOO LOW.

    Also see recommendations above re. current limit being better that volt
    Regulation for lighting LED's

    Jay
     
  9. Byron A Jeff

    Byron A Jeff Guest

    -I am trying to use a LM317T to drive some 3.8 volt LEDs.

    Um. Why?

    - I built a
    -circuit using a 1k resistor for the fixed and 10k pot for the
    -adjustable. Using my trusty DMM i set the voltage to 3.8 volts and
    -preceed to connect the LED. When I connected the LED the voltage drops
    -to 2.8V and no matter how high I turn the pot the voltage of the LED
    -stays at 2.8V. Does anyone know why it is doing this?

    Yes. Because you're mistaken about the role of voltage in driving an LED.

    the critical attribute for an led is current.

    Here's a nice thread from the PICLIST that talks about the role of current
    and voltage in driving an LED:

    http://tinyurl.com/rans

    What's your source voltage? Is it regulated? If it is then you don't need
    the LM317T to drive the LED, just a simple resistor whose value can be
    computed from the info given in the thread above. The voltage plays its
    role in fixing the value of the resistor in order to generate the correct
    current for the LED.

    Hope this helps,

    BAJ
     
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