LM317 Ripple/Power dissipation
Is 48 W dissipation too much?


Let's look at the thermal resistance between the chip and the tab on the back.



Figure 1. The TI LM317 datasheet shows the minimum thermal resistance between the junction and case is 15.9&deg;C/W for the KCS (TO220) version.

That means that with 48 W the temperature rise would be 48 &times; 15.9 = 763&deg;C above ambient if we were able to hold the tab at ambient temperature. Add in the thermal resistance of the chip to the heatsink and the heatsink to the air and you will increase the temperature further. 

Table 7, the Abolute Maximum Ratings (which you want to be operating well below) states, "[Max] Operating virtual junction temperature 150°C."

Yes, it's too much. Forget this design. I first used that chip > 40 years ago. Things have moved on.