LM317t heatsink

Discussion in 'General Electronics Chat' started by stoopkid, Mar 12, 2011.

  1. stoopkid

    Thread Starter Member

    Mar 3, 2011
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    1
    My LM317t gets too hot to touch. Hot enough to burn maybe. How do I figure out what thermal resistance I need for it? I want to keep it compact, but like I said it gets awfully hot. I don't want to over estimate and waste space and I really don't want to underestimate and have it be too hot. Because of how hot it is, I'm worried even if I think I'm using going overkill, It may not be enough. So I'd like an actual figure and I have no clue how to do that.

    It's converting 5v to 1.8v. And it's doing about 200ma in, I think. Actually I have no clue. The actual lights are only doing a couple miliamps, when I test the input of the regulator it all goes dim and tells me 200ma.
     
    Last edited: Mar 12, 2011
  2. Audioguru

    New Member

    Dec 20, 2007
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    1.8V? Lights?
    Are you using an LM317 with its output set to 1.8V to drive some 1.8V red LEDs?
    You don't doo dat!

    LEDs are powered with current, not voltage. LEDs set their own voltage.
    If the voltage is high enough then a series current-limiting resistor sets the current.

    LEDs are never connected in parallel like light bulbs unless you measure and sort them to all have exactly the same voltage (they are all different).

    LEDs are frequently connected in series because then they all have the same current then a single current limiting resistor is needed for this string of LEDs.
     
  3. stoopkid

    Thread Starter Member

    Mar 3, 2011
    136
    1
    Actually, the 1.8v is going into a transformer, which is sorting that stuff out for me. I just need to supply it 1.8(actually I've decided on 1.5v).
     
  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Try a Buck converter
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    The output of an LM317 is DC. A transformer uses AC, not DC. LEDs operate on DC.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You need to post a detailed schematic of your circuit, and the typical forward voltage @ current of your LEDs.

    I suspect that you have a number of LEDs in parallel with no current limiting resistors. This won't work.
     
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