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How to adjust the LM317T voltage regulator?

Discussion in 'Electronic Basics' started by [email protected], Mar 7, 2006.

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  1. Guest

    I got the LM317T from Radio Shack and I am trying to wire up an
    external battery pack to run an mp4 player which has an internal 3.7v
    battery.

    Can someone explain to me (in plain english, I'm new with this stuff)
    what I need and how to adjust this thing?
    I want to run this mp4 player on a rechargable 9.6v battery that I
    have.

    Thanks in advance.
     
  2. Figaro

    Figaro Guest

    |I got the LM317T from Radio Shack and I am trying to wire up an
    | external battery pack to run an mp4 player which has an internal 3.7v
    | battery.
    |
    | Can someone explain to me (in plain english, I'm new with this stuff)
    | what I need and how to adjust this thing?
    | I want to run this mp4 player on a rechargable 9.6v battery that I
    | have.
    |
    | Thanks in advance.
    |


    Try the 1st circuit on

    http://ourworld.compuserve.com/homepages/Bill_Bowden/page12.htm

    You'll need 192 ohms for R2
     
  3. Guest

    Ok well I'm a total "noob" at all this but I think that diagram makes
    sense. I'll see what I can make of it. Thanks.
     
  4. Byron A Jeff

    Byron A Jeff Guest

    Be aware that you'll burn up much of your power in the LM317. So don't
    expect long battery life with this method.
    In plain English: The output pin regulates to a voltage that is 1.25V
    higher than the voltage on the ADJ pin. The two resistors form a voltage
    divider to set the ADJ pin voltage. You can use the formula on this page

    http://www.circuit-innovations.co.uk/LM317.html

    To compute the values of the resistor.
    It won't last long. You really need a switching regulator to get any
    real usable battery life.

    BAJ
     
  5. Guest

    What more can you tell me about a switching regulator? What would it
    take to use one of those instead of the LM317T?
     
  6. Bill Bowden

    Bill Bowden Guest

    Switching regulators make a lot of noise. You might get better sound
    with the LM317. The battery life will be a about 1/2 as much compared
    to a switching regulator since the input is about twice the output.

    Basically, a switching regulator charges a capacitor very quickly to
    the desired output voltage and then waits for the capacitor voltage to
    fall a small amount and then blast it again with a pulse of current to
    bring the voltage back up. It sort of switches on and off as necessary
    to keep the output fairly constant. But it won't be as smooth as the
    linear LM317 and so needs more filtering on the output to get rid of
    the noise and small variations in output voltage.

    -Bill
     
  7. ehsjr

    ehsjr Guest

    Here's a "picture".


    +-----------------------> Gnd To MP3 (-)
    |
    ------ | ---------
    +---|470ohm|---+---|-BATTERY+|---+
    | ------ --------- |
    | |
    | ------ |
    | | o | |
    | |______| |
    | /______/| |
    | | || |
    | |LM317 |/ |
    | ------ |
    | | | | |
    +----------+ | +---------------+
    | |
    | |
    | ------ |
    +---|240ohm|--+------------------------> 3.7V To MP3 +
    ------

    With the 240 and 470 resistors shown, you'll be within
    ..003 volts of target (3.7) per the computation. You do
    not need any capacitors if you keep the LM317 relatively
    close to the battery.

    Ed
     
  8. Guest

    Thanks a lot. So far that is working perfectly. I will post a link to
    my project once I have the site up, for anyone who is interested.
     
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