12v car battery to 5volt for Microcontroller, Help!

HoangTran83

Has anyone on here successfully connected a microcontroller to a car battery by stepping down the voltage?? If so, do you have the schematics??
I've tried searching this forum and google but come up empty handed. Please Help!!

SRLM

All you need are some regulators. Depending on how much current you're planning on using you can get by with one through four. More details on the application would be helpful.

HoangTran83

I'm trying to interface either a propeller or an SX to my cars to control various relays. Could i regulate the 12v to 5v using the 7805 voltage regulator and a voltage divider??

Joms

You wouldn't even need a voltage devider, just a 7805 and a few filter caps.· Because you are using this on a car the filter caps are VERY important and cars normally have a lot of noise on the DC.

John R.

Keep in mind that depending on the current, you may be generating a lot of heat, and even if you don't exceed the current capacity of the 7805, you still may want more than one to help keep the temps down. The amount of heat generated will roughly be:

(13.5 Volts [noparse][[/noparse]typ car voltage] - 5 Volts [noparse][[/noparse]regulator output]) x Current in Amps = Watts of Heat

or

7.7 Volts Dropped x Current in Amps = Watts of Heat


Basically, the excess voltage has to go "somewhere" and the somewhere is heat. Heat sinks on a the regulators can also help. What you need depends on how much energy you'll be using at 5 volts. If it's just a processor and a couple solid state relays, you're probably just fine with one regulator. If you start drawing some "working" current, it's time to be more thorough about the design.

The alternative would be a switching regulator (buck or boost/buck), but then there are usually a few other external components required. You may be able to find an "off the shelf" solution by looking at some of the Radio Control stuff. They use some of this for "battery eliminators" for the radios, and depending on the application are VERY concerned about wasted energy, less from heat than as it relates to battery life.

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John R.
Click here to see my Nomad Build Log

HoangTran83

Would it be better if i use diodes to drop some of the voltage before the 7805 or resistors?? Can i use several 7805 in series to lower the temperature? My circuits will drive several mosfets which will drive the 30A relays, will the 7805 source enough current??

$WMc%

HoangTran83

Take a look at the LM1084


________$WMc%____

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mctrivia

If you need 30a you are going to want to use a switching supply.

It is better to use a series resister with the input. Value should be chosen to drop 5v at max current.

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Post Edited (mctrivia) : 6/3/2009 2:12:12 AM GMT

Philldapill

I think he meant the 7805 regulators are supply the power for actuating the relays.

Mike Green

You would be better off running 12V coil voltage 30A contact rated relays directly off the 12V vehicle supply, switching the coil voltage with the MOSFETs. Your logic power supply wouldn't have to deal with any of the relay current, just the microcontroller and its logic peripherals.

dre

Would this link help?

http://www.seetron.com/an_vpwr1.htm

cheers, David

HoangTran83

That link has the exact circuit I was searching for. Thanks for the link, and thank you everyone for helping.