SparkFun Forums

[bagy]

Hello,
I know this has been discused many times, but I just wanted to be sure.

I'd like to build an integrated car charger (I'll place it right behind the cigarette lighter and use it's power source). Since I can only get 12V and I need 5V, I figured it'd be best to use a linear regulator (in this case, a 7805 should do).

My phone's wallcharger says it's 5V/1A, but since 1A is the maximum output of a 7805 (and it would produce a lot of heat) I figured I should lower the current somehow. Will the lowering of the current help to cool down the IC?

Thanks!

[omagasohe]

A charging usb port is simple and complex at the same time. http://www.ladyada.net/make/mintyboost/ has a lot of really good info, specifically resistor values for the D+/D- lines.

A linear regulator is fine. a heat sink is recommended if your going to be using it for a long time. though for a few dollars on ebay you can get a 1A charger and modify it. fairly simple.

[Philba]

The problem with a linear going from 12v to 5 is that you are make a lot of heat (7v * current). At about 140 mA you will be dissipating 1 Watt which is enough to get your regulator finger scorching hot - you'll want a heat sink making your solution bulky. At the USB 2.0 limit of 500 mA we are talking bout 3.5W. So, a lot depends on what your max curent draw is going to be.

A buck regulator will be much more efficient, smaller (no heat sink) and cooler.

[trophosphere]

Here is a switching regulator that is similar to the 7805 form factor and pin out.

http://www.murata-ps.com/data/power/oki-78sr.pdf

[macegr]

You can get tiny switching USB 5V regulators that plug into a cig socket already. Just splice on some wires, add some heatshrink, drill a hole where you want the plug, and glue it in place as is. Already shaped like a panel-mount socket.

Examples:
http://www.dealextreme.com/p/designer-s ... rger-72932
http://www.dealextreme.com/p/car-cigare ... -12v-65855