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LEDs and Thermal Resistance

Discussion in 'LEDs and Optoelectronics' started by 96300NAZ, Apr 20, 2013.

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  1. 96300NAZ


    Apr 20, 2013
    I am trying to determine the size of a heat sink necessary for my project. I have been racking my brain over there to the point of complete confusion.

    For experimental purposes, I'll explain the known.

    • Sealed Headlight enclosure, including bulb connection. (Motorcycle)

    • Forward voltage 21v
    • Current Draw 700mA
    • Power Dissipation 14.7w
    • Listed Thermal Resistance 7.5*C/W
    • Max Recommended Junction Temperature: 135*C
    • Ambient Temperature: 25*C

    When I do the calculations, (Tj - Ta)/Pd = 7.5*C. This is what is listed on the spec sheet. This would tell me at an ambient temp of 25*C the LED system's increase based on the listed thermal resistance is equal to the maximum recommended junction temperature. This means no heat sink required??? Somewhere, something is wrong or maybe I'm not understanding this stuff right.

    I don't want the temperature inside the headlight to be over 45*C. I'm guessing at the temperature, but I would assume this temperature is less than that of a 55w Halogen bulb I'm trying to replace and should be safe not to melt any of the plastic case components.

    So where am I going wrong?

    Thank you for your help,

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