Cell phone battery charger motorcycle

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ElectronicsDevil

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Hi Friends,
I am a newbie to electronics and really this electronics is getting interesting and interesting.
I want to build a portable battery charger for my cell phone.
Can anybody suggest how should i proceed, Because bike battery is having great voltage fluctuations. If you have a schematic ready then please please give it to me.
Rating of battery charger: 5V, 0.7A
Battery of motorcycle : 12V

Added after 2 minutes:

I also have participated in electronics project in my school....
 

You may consider switching dc-dc step-down voltage regulator and the LM2575-5 seems to be a good candidate for the job ..
**broken link removed**
It'll compensate for fluctuations in the bike's battery voltage and, what is also important, it limits the output current to max. 1A ..

Rgds,
IanP
 

but the max allowed current is 700mA and it Should be kept at 350mA..
Can you suggest how to suppress this current to 350 mA.
 

You haven’t mentioned 350mA before, I’m not a clairvoyant ..

In that case you can consider a constant current circuit based on the LM317 (mounted on a heatsink) ..
Regardless (almost) of the input voltage will output the current of 350mA .. see attached picture .. and example at:
http://users.telenet.be/davshomepage/current-source.htm

R ≈ 3.6Ω

Rgds,
IanP
 

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