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LM338T got hot at 2.5A...

Dai Vu96212
Prodigy 50 points
Other Parts Discussed in Thread: LM338

Hi.  the LM338T (TO-220) is rated for up to 5A.  I had configured the regulator circuit to +5V output (with +9V input) and attached a 2 ohm 50W resistor load to test.

At 2.5A it got very hot then thermal shutdown occurred after about 30 seconds.  I then added a 12.5TR (thick) heat sink.  It now lasted up to 35 seconds before it got really hot.  Shouldn't the LM338T stay "relatively" warm at half its rated current?  what can I do to keep it cooler?  thanks.

  • 0
    TI__Mastermind 22610 points

    LM338 5A Adjustable Regulator

    " ... Shouldn't the LM338T stay "relatively" warm at half its rated current? ..."

    The heat does not come from the current alone. The heat is from the power dissipated in the package.

       Pdiss = ((Vin - Vout) x Iout)

       Pdiss = ((9V- 5V) x 2.5A) = 10 Watts

    With that much disspation in the TO-220 (NDE) package, yes, it is going to get very warm.

    The LM338T in the TO-220 (NDE) package has a θjc rating of 4°C/W.

    If you want to keep the internal temperature (Tj) less than, for instance, ≤ 85°C (a modest 185°F) then you will need an external heatsink that has a thermal resistance of ...

       θsa ≤ ((Tj - Ta) / Pdiss ) - θjc

       θsa ≤ ((85°C - 25°C)/10W ) - 4°C/W 

       θsa ≤ 2°C/W

    and that presumes that the TO-220 tab to heat-sink thermal interface, θcs, is near perfect.

    " ... what can I do to keep it cooler? ..."

    There only a few variables involved and you probably will not be able to change any single one enough to make a significant difference.

       Lowering input voltage

       Add a fan to your heat sink

       Use a water cooled heat-sink

       Use multiple LM338T's to share the current and the dissipation.

    The best option is to not use a linear regulator for this application. Most any buck SMPS can run cool to the touch under your described conditions.