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Does a 1/4 watt resistor need any airflow if it is disapating 20mw ?

Discussion in 'Electronic Basics' started by SA Dev, Mar 25, 2005.

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  1. SA Dev

    SA Dev Guest


    I've made a really small LED light that I'm planning to use and I was
    thinking about shrink tube wrapping the resistor in with the leads on the
    LED. The LED is rated for 30ma continuous, but I'm only putting around 12ma
    through it. Would this be an issue for heat?


    SA Dev
  2. Lord Garth

    Lord Garth Guest

    If the voltage drop across the resistor multiplied by the current through it
    close to the 250 mW resistor maximum, it is way too much and there is no
    safety margin.
  3. John Fields

    John Fields Guest

  4. Hi John,
    The drop across the resistor is 1.8v and the current is 11.3ma, if I
    mutliply these I get 20.3mw, right? Ambient temp will be room temp ~75 deg


    SA Dev
  5. John Fields

    John Fields Guest

  6. Sounds about right and safe.

    A question not for you, exactly, but to anyone else would be "What is
    the C/W specification for a 1/4 watt resistor body in still,
    relatively dry air?" It would be hard to imagine it being much worse
    than the Rja=200 or so for a TO-92.

    You'd usually just multiply that Rja spec by the watts the resistor is
    dissipating to get the relative change in equilibrium temperature. If
    the value were 200, then the change would be about 4 Celsius or less
    than 8F. That would seem quite a safe change, over 75 F ambient.

    I'd suppose that you *could* wrap the resistor in thermal insulation
    and that if you really tried hard you might cause a problem. But the
    Rja would have to be in the area of 2500 or so before a 50 Celsius
    change would be seen. Hard to imagine that big a number for Rja
    unless you really worked at causing a problem.

  7. Hi,
    Thanks everyone, I appreciate the info!!

    SA Dev
  8. Rich Grise

    Rich Grise Guest

    If you're running it at ~20V or more, yes. Otherwise, no.

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