Current limiter-LM317 fail ! Why ?

Discussion in 'The Projects Forum' started by prashanthb, Aug 11, 2015.

  1. prashanthb

    Thread Starter Member

    May 17, 2015
    34
    2
    For the life of me I am not able to find out what mistake I am doing.
    I have built the same circuit as shown here :
    [​IMG]

    2015-08-11.jpg
    Sorry for bad quality of image ! Its from a web cam.
    Tx (26Vac)>> BR >> LM317 >> small indicator LED

    I have tried R values from 10E to 22K. The red LED keeps blowing up always and the LM317 heats up. I have tried many LM317 chips, they all behave the same. So I think the chips are ok but there is some dumb mistake I am doing in the connections. Please help.

    Thank you.
     
  2. absf

    Senior Member

    Dec 29, 2010
    1,492
    371
    I calculated 62.5Ω is for a constant current of 20mA for the LED. The nearest value to use is 68 ohm.

    I used 26V DC for the input and there was 22.5V drop across the LM317 which is 450mW of power dissipated on the 317. Not sure how you blew up the LED. Is your input voltage exceeded the 35V max voltage?

    lm317 const current.PNG
    Allen
     
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  3. prashanthb

    Thread Starter Member

    May 17, 2015
    34
    2
    AHHHHHH!

    Caught the mistake! Such a kiddish mistake. I was taking output from point no. 2 in your diagram. i.e BEFORE THE RESISTOR !

    Now the LED glows but 1/4W resistor bellows smoke. But that I understand.

    Thank you.
     
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  4. prashanthb

    Thread Starter Member

    May 17, 2015
    34
    2
    Thank you Allen for taking time out for simulating it.
     
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  5. snav

    Active Member

    Aug 1, 2011
    100
    10
    This has always come in handy for me. I don't know the author. Good luck.
     
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  6. absf

    Senior Member

    Dec 29, 2010
    1,492
    371
    Were you using Excel to formulate the table?
    So the value I calculated is correct!

    Allen
     
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