Powering Arduino in a car.

I have searched on this a few times. Seems the best way is to buy a cigarette lighter style charger and disassemble it. I was wondering if something like this might work though: eBay DC-DC step down buck converter

I'm only interested in powering it while the car is running. I've got a few different functions being managed by an uno, and it will probably be switched to a pro mini once it's finished up.

I don't know much about dealing with a noisy supply, so if that part won't do the job, I'll buy a cigarette lighter to hack up. I just might be building a few more of these, so an off the shelf solution would be preferred.

Thanks in advance!

Most of the time automotive 12V supply is fairly well behaved, but sometimes things like alternators and regulators fail, or battery terminals corrode, chassis ground return wiring comes loose..., then it can be extremely nasty / noisy - so for robust operation its good to assume the worst case - upto 20V with lots of noise spikes (50V even). Generally a good automotive supply will have fuse, then RF choke / decoupling caps/ TVS diodes to remove most of the high frequency noise, then go to a converter or regulator with high input voltage range, possibly with a crow-bar circuit on the output to prevent overvoltage.

This is overkill most of the time, but it really depends on the value of what it protects - you'd like all this for a phone-charger as phones can cost a lot, but you would not care for a map-reading LED light!

I'd suggest fuse, RF choke, DC-DC converter as a reasonable compromise, that one tolerates 24V on the input which is pretty good. A small RF choke for a few amps isn't expensive, and inline fuseholder is simple. The RF choke will help reduce feeding noise back into the supply and being picked up by the radio too...

For 5V only a car USB phone-charger can be good. If you need a clean 5V for analog circuitry a linear regulator will be best though.