In car use of the AVR

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I'm busy designing a small data logger based on the AVR for my car. I have only one question, do have to use filters in the power supply (and if yes what type) or can I use a simple 78xx?

All electric components run on smoke. If you let the smoke out, they won't work anymore.

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It is wise to filter the power supply. Depending on what protection you have a 7805 may/may not survive.

Occasionally you can have what is termed a 'load dump' from the car's alternator which puts around 40V onto the 12V. When this occurs, your 7805 will probably not like it. There are regulators specifically for automotive use - see National Semiconductor. However, with some protection, we can have a happy 7805 reg. I would suggest a polyswitch in series (these are a thermal fuse) and a 24V MOV to gnd. The polyswitch is to protect the MOV and everything else in case of a surge - the polyswitch will limit the current and reset when things go back to normal. The MOV will soak up the surge. You must be careful with the rating of the polyswitch - since these are thermal devices, you don't want them going open circuit when the interior of the car gets to 70degreesC, so you will need a higher rated polyswitch than you might first anticipate.

Remember to put the correct capacitors on the 7805 and watch the power dissipation - the ambient temperature in cars can get hot, so you don't want to exceed the max temperature of the 7805.

A 7mm MOV should be sufficient for currents < 1A.

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There are SAE standards for transients in cars/light-truck and big trucks. They're pretty severe, requiring beefy inductor-chokes ahead of the power supply.

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Look at the RBO-40. I've used them before and they work well.

A rock solid combination is a couple of inductors for both +12V and GND, then the RBO-40, then a couple of ferrite beads (optional) followed by an AN7705 regulator. That regulator is specifically designed to handle large source voltage fluctuations.

I used a pair of 680uH inductors -> RBO-40 -> ferrites -> AN7705 with several good caps in a project a couple of years ago. The power line is smooth as can be.

Don't forget to protect your inputs as well. Personally I like a TVS like the Littelfuse SP723 first, then a resistor divider (for signals less than 5V) front-ending an opamp section. The opamp output goes to your ADC inputs on the AVR.

The last thing you want is some surge not only frying your board, but also starting a fire ...

Dean 94TT
"Life is just one damn thing after another" Elbert Hubbard (1856 - 1915)