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Adjustable Voltage Reg LM317, Should it be getting this hot ?


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Hi,


I've made the little circuit of resistors around an LM317 adjustable voltage regulator that is specified in its spec sheet (240R bet pin1 and pin2 and a 5k pot bet pin1 and gnd)...

When using a 12v lead acid battery as an input and regulating the output down to to just above 7.2 volts (running a portable DAT machine) the regulator will get too hot to touch - it is rated for 1.5A but during operation the DAT is only drawing from 400mA to just over 1A

I am just using a little clip-on TO220 heat sink with a little blob of compound

Question : is this heat sink enough ? is "too hot to touch" ok for the regulator ? or do I need a better heat sink or other solution ?

I am only leaving it on for a few minutes at the moment, would like to be sure I can leave it running at that heat ...

The operating temperature in the spec sheet says 0deg < T < 125deg

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Hi Nick,
Your little heatsink is too small.
The max internal temperature of the junction of the LM317 is 125 degrees C, the thermal resistance from its junction to its case is 4 degrees per Watt and you have it dissipating (12V-7.2V) x 1A= 4.8W, so if its case is at 106 degrees C then it is too hot. It lasts longer if is cooled properly.

The LM317 is smart. It shuts down if it gets too hot, at about 135 degrees C internally. Then it will cool and start working again. They say not to let it heat and cool like that too many times or it will fail with fatigue.

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And not all LM317's are created equally. The LM317T has thermal shut down.

No, they all have thermal shut down because they all use the same chip.
National Semi supplies the LM317 in 7 different packages. The T is the TO-220 package, the K is the TO-3 package and there are a few tiny surface-mount packages.
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