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Temperature

Discussion in 'General Electronics Discussion' started by jfk, May 4, 2016.

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  1. jfk

    jfk

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    Mar 28, 2016
    I want to adjust an LED, based on board temperature. As the temperature hits a threshold, I'd like to dim the LED a bit to control heat.

    The temperature measurement ICs that I see raise an output as the temp goes up, so driving a FET from this output is backwards.

    Any suggestions? Drive a PNP high side? Invert and use an NPN? (Simple schematic even nicer but not essential)

    Thanks very much!
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    if the heat is generated by the LED, then a switch mode constant current source may be a solution.

    As the LED temperature rises its forward voltage drops and a constant current results in a reduction in power.

    if you already have a constant current driver, then paralleling a thermistor with one of the legs of a voltage divider from your reference voltage will result in a reduction of current with temperature. The actual reduction will depend on a lot of factors.
     
  3. jfk

    jfk

    13
    1
    Mar 28, 2016
    This will be just use a resistor for current limiting, as it need to be cheap and very low part count. I was thinking a temp IC that has a thermal trigger point that just switches a FET.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    well you could use a series PTC thermistor. Some are designed for relatively high current and may have the range of resistances needed. However I believe most of these respond to self heating rather than external temperature and may "latch" due to this.
     
  5. AnalogKid

    AnalogKid

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  6. Herschel Peeler

    Herschel Peeler

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    Feb 21, 2016
    It would be helpful if we knew what your present circuit looked like.
     
  7. jfk

    jfk

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    Mar 28, 2016
    I'm still thinking about the best way to proceed, so nothing yet.
     
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