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you have a choice of linear passive methods or switching. The passive is easiest, but will generate heat. The switching is more efficient.
Whichever method you select, protecting the input is most important. There can be big spikes on the 12 V line when you start your car.
Put a pi filter on the input with shunt capacitors of large value and a big inductor. This will remove most of the spikes. Then put a zener at 15 V across the input. Use a power type, not the 400 mW types.
You can also get MOV type suppressors for this voltage range.
If you want your power supply to work during motor starting, you need to use a low dropout regulator instead of a standard as LM7805.
In very cold weather the battery could drop down to 6V.
Use an automotive regulator from Infineon, St or National instead.
The are better protected against battery reversal, line transient and load dumps.
Read more in https://www.st.com/stonline/books/pdf/docs/1691.pdf
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