calculating heatsink for LM350At..need help!

Discussion in 'The Projects Forum' started by ayane, Oct 12, 2009.

  1. ayane

    Thread Starter Active Member

    May 3, 2009
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    i am looking for a heatsink. I am new to this henc i am not sure on how to calculate.

    Input supply ( from solar cells) : 15.6v, 1.33A ( max)

    ouput of the regulator is 12.85V.

    Power dissipation: ( 15.6-12.85)* load current ( approx 1.3) = 3.575w

    Everytime i run the lm350 it gets very hot. I had tested on the dc supply , the regulator switch to its thermal protection by limiting the current.

    jc- junction to case, cs- case to heatsink, sa- heatsink to ambient

    the junction temperature must not exceed 125 degrees.

    can i neglect Rcs?

    T(junction)- T(ambient) = Power loss ( Rjc + Rcs + Rsa)

    125- ( how do i determine?) = 3.575( 3+ Rsa)

    Rsa is to determine he minimum thermal resistance of the heatsink?.
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Too much math. Take a 5W or 10W power resistor, the ceramic kind that has an aluminum flange for mounting on a flat surface. Or better yet, one like this. Temporarily screw it to a 4" square of aluminum, about 1/8" thick or thicker. Hook the resistor to a power supply, adjust the voltage until the resistor is dissipating ~4W. E=sqrt(4/R).

    In the final application, can you mount the heatsink vertically or horizontally? Set the aluminum plate in the same orientation. Let it reach thermal equilibrium (might take a few minutes). Measure the temperature of resistor body and the plate near where the resistor is mounted.

    If the body of the resistor body is hotter than ~75 degC, you need a bigger heatsink, or it needs to be placed vertically instead of horizontally. If the temperature is < 50 degC, then you can make the plate smaller.

    If you have a heatsink in the junk box, you can use this test on that, too.
     
    Last edited: Oct 12, 2009
  3. bertus

    Administrator

    Apr 5, 2008
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  4. ayane

    Thread Starter Active Member

    May 3, 2009
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    thanks for the link..will try to understand the concept.
     
  5. ayane

    Thread Starter Active Member

    May 3, 2009
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  6. bertus

    Administrator

    Apr 5, 2008
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  7. ayane

    Thread Starter Active Member

    May 3, 2009
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    noted..had purchased some mica.. any negatives to screw the ic with a nut to the heatsink?.with the mica in between to provide insulation?..

    will be switching the heatsink once i completed my testing on my breadboard..thx..
     
  8. bertus

    Administrator

    Apr 5, 2008
    15,649
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    Hello,

    It can be that the adjust pin is connected to the housing.
    The regulator will give the wrong outpout when this is grounded.
    There are isolation rings to mount it isolated from the heatsink.

    [​IMG]

    Greetings,
    Bertus
     
  9. ayane

    Thread Starter Active Member

    May 3, 2009
    39
    0
    noted thanks
     
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