12v DC to 5v DC, 0.6A with LM317

Hello,

Hoping this is the right forum for it (and I'm a bit of a beginner too):

I've got some solar power, 12v and would like to power a WiFI router outside.

The router is labelled as 5v, 1.2A but I checked it (using the mains connector) and it only takes 0.6A max.

So, I made a circuit based on an LM317:

- 1mF capacitor between 12v input & GND to smoothen the input (just in case as I was also using a car battery charger as test input)

- 220mF between output & GND (to smoothen output so the router won't complain)

- Adjust - GND via 639 Ohm (using a potentiometer)

- Adjust - Output via 229 Ohm (using a potentiometer)

This circuit results in a 6v output under load (but the idle voltage is higher, around 6.3v)

Interestingly the router won't start up below 6v actually, but ok 6v it is then.

The LM317 however gets really warm. I'm using a 38cm2 aluminium heat sink but it gets to 55° within 5min (under 0.55A = 6W for the whole circuit).

I also tried 2x LM317 in parallel but that won't get cooler either. They are designed for more than 0.6A, so why do they get so hot?