For automotive application: better to use external 5v supply than Arduino's?

I'm working on a project using a Mega in my car. I am wondering if it would be better to power the Arduino directly from the 12v car supply, and let the regulator on the Arduino do the work, or use something like this: http://www.alldatasheet.com/datasheet-pdf/pdf/38294/SANKEN/SI-8301L.html. (I salvaged a couple of the 8301L's from some old circuit boards).

The 8301L looks beefier that what is on the Mega, so my assumption is that it will do a better job of providing a regulated 5v supply. However, one thing I like about using the supply on the Arudino is that if I make a wrong connection, it (hopefully) can't supply enough current to fry something, whereas the 8301L probably could. Is that accurate?

Thoughts?

thank you! GB

First off have you read the disclaimers about automotive applications in the Amega data sheet. Second an automotive electrical environment is quite hostile. It is likely to go up to 18V so I would use a more robust regulator than the Arduino's.

one thing I like about using the supply on the Arudino is that if I make a wrong connection, it (hopefully) can't supply enough current to fry something, whereas the 8301L probably could. Is that accurate?

Not entirely accurate. You may not fry any high power devices you try connecting to the Arduino, but you can certainly fry components on the Arduino by trying to run high power devices on it.

Grumpy_Mike: I don't think I have seen that specific datasheet you're referring to. If it wouldn't be a bother, could you post a link to it?

The SI-8301L shows that it can handle an input voltage from 8 to 40v and output 5v up to 1a, so I'm thinking that I will incorporate one of those into my final design.

jraskell: yes, what I was trying to get across is, does the Arduino regulator supply enough current to fry itself, such as if I would inadvertantly short an output? I'm thinking that perhaps it cannot, whereas with a stronger external supply it would.

Grumpy_Mike: First off have you read the disclaimers about automotive applications in the Amega data sheet. Second an automotive electrical environment is quite hostile. It is likely to go up to 18V so I would use a more robust regulator than the Arduino's.

I agree with Mike. A car's charging system can be up to 14.4Vrms, 18Vpeak. I would use a 12V-20V to 9V regulator to power the arduino, then use best judgement (see: amp limits) on using the onboard 5V regulator.

Also, filter capacitors and diodes are your friend. AC voltage can still find it's way around a car (remember, an alternator is an AC output that gets rectified to DC), so you'll need to safeguard on that front as well.

I found the 444 page datasheet for the Arduino and when I did a search on "automotive" all I found was:

"Unless specifically provided otherwise, Atmel products are not suitable for, and shall not be used in, automotive applications."

it's all the way at the end. I couldn't find any reference as to why in the datasheet upon searching. If it's just because of the power fluctuation then hopefully providing a better source of 5v should be sufficient.

magnethead794: Any reason I should go from 12v-20v to 9v to the Arduino vs. using something like the 8301L to take me straight to 5v? (the link in my first post is to the 8301L datasheet if you want to check it out). It does call for a capacitor to be wired in very close to it.

If it's just because of the power fluctuation then hopefully providing a better source of 5v should be sufficient.

No it is because if it fails and someone gets killed or damaged as a result then you can't sue them because they told you not to use it. It is probably more about vibration exposure, they take it for granted that you are going to do a good job on the power supply.

At a first glance that regulator look alright although those are big caps it needs on the input and outputs.

GlitchBoy: magnethead794: Any reason I should go from 12v-20v to 9v to the Arduino vs. using something like the 8301L to take me straight to 5v? (the link in my first post is to the 8301L datasheet if you want to check it out). It does call for a capacitor to be wired in very close to it.

What I meant, is because arduino is not recommended for >12VDC, I would use a 9V regulator board that supports a 20 volt input. On our racecar, I've seen peak-peak values as high as 24 volts (~17 Vrms), but that's at 7000 RPM on a 100 amp charging system- something far from a street application...most street vehicles shouldn't have a higher peak-peak than 18 volts in normal conditions. That said, I would still suggest having diodes on both the positive and negative power leads going to the regulator and a filtering capacitor across the input leads, just for insurance.

All points well taken. Thanks for the input.

GlitchBoy: jraskell: yes, what I was trying to get across is, does the Arduino regulator supply enough current to fry itself, such as if I would inadvertently short an output? I'm thinking that perhaps it cannot, whereas with a stronger external supply it would.

None of the components on the Arduino are capable of protecting themselves from excessive current draw. Yes, the Arduino regulator will fry if you try drawing too much current. Yes, the AVR outputs can fry if you short them out. Plenty of people have accidentally done both of these things.

Another thing you should worry about is noise , so as another poster said, capacitors in parallel with the power supply will smooth things out.