usb power in car

[lvlaveryk]

i am trying to install usb ports in my dash to be used as power supply for multiple devices.** going to tap into remote power supply for radio (12v) want to avoid lighter adapters.* i'm pretty sure i need to reduce voltage to 5v and amps to 1A. i am a beginner with this type of project, and would like to know how to reduce the voltage and amps...dont want to wreck devices. thanks in advance

 

[Blueteeth]

A voltage regulator is what you need.

Standard part numbers are 7805 for 5v, 7809 for 9v etc..etc..

However, these are linear regulators, which drop the voltage by absorbing the difference. with 12v in and 5v out (for a 5v regulator) thats 7v drop across it. If its carrying 1amp, thats 7x1 = 7watts to disappate as heat. So you would definately need a heatsink on the regulator! (TO-220 packages can do a maximum of 2 watts without one).

That would be the easiest, you could then just add panel mount USB sockets. Since USB is rated at 500mA, no single device will draw more than that, so I would suggest using one regulator per USB socket. (you could have two but thats 1000mA = 1amp max, the very limit of the regulator). Very cheap and cheerful, requiring only the regulator, a diode, and a couple of capacitors per socket.

There are also regulators rated for higher current, but you would still require a heatsink.

For a more expensive, but more efficient system, buy a premade switched mode regulator. These can convert the 12V to 5V (or adjustable) with great efficiency, negating the need for bulky heatsinks. I wouldn't recommend building one form scratch, ebay has plenty of premade modules which will work just fine.

A word of warning about automotive electric systems...they are noisy! The voltage can vary greatly, theres many large voltage spikes from the ignition system, and lots of frequencies mixed in with the 12V line from the battery. Even with a regulator, they can't always supress these transients, and so any USB device you plug in (purely for power of course) may misbehave or even be detroyed. USB power from a PC is generally well filtered and fused (using a resetable fuse).

I'm sure theres some posts in the forum about using automotive power, and the methods/circuits to use them to power other things whilst cleaning up the voltage.

.
Blueteeth

 

[3v0]

Given that they have been putting computers and other electronics in cars for a few decades there are purpose built regulators on the market.

If you go to Mouser Electronics - Electronic Component Distributor and search for "regulator automotive" you will find over 100 of them up to 1A.

3v0

 

[Noggin]

I've been using an LM2937 for 5v power in cars now and haven't had any problems yet. It is very simple to use. You can get a datasheet from Digi-Key Corporation - USA Home Page