The LM338 is a linear regulator. About 60% of the power in a 12 to 5 volt linear regulator gets burnt up as heat. To deliver 50W to the load 65W is burnt as heat.
Also, this design doesn't have a disable. It never turns off. Unless you have a relay controlling power to the circuit, your battery is toast! Running off your car's battery without the eingine running would drain it really quickly.
I linear regulator is very stable and free of power line noise, but it is just not worth the waste.
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