12 to 5Vdc USB power Supply

Discussion in 'General Electronics Chat' started by axeman22, Jun 16, 2012.

  1. axeman22

    Thread Starter Active Member

    Jun 8, 2009
    Hi guys..

    with USB devices - say for example my HTC One X or the Samsung Galaxy S2 etc.. they just look for a micro USB to charge from etc..

    what's I'd like to understand it - the smarts, for charging the battery inside - are they done by the phone, or by the USB charger external.. ie if I could just hookup my simple old 0-30Vdc up to 5Amp bench power supply.. set it to 5Volts (accurately) and then connect that to the right pins on the USB cable.. is that at good as anything ..? ie from the perspective of the phone, looking back at the power supply - is it just looking for a dumb 5Vdc (clean, no ripple etc) ..??

    this then I guess leads on to the greater question - why can't I just take a 12V input, put it accross a 7805 (to-220 package) and then put the output into my phone.. what is wrong with that etc..?

    I have a 12V charger.. which just gives you a USB output - there is quite a bit of circuitry in there.. just wondering what else it would do on top of the 7805 etc..?

    I hope my question is not too confusing.. just trying to get my head around it a little.

  2. BMorse

    Senior Member

    Sep 26, 2009
    So basically some chargers would have some other circuitry to support some of these specs.....

    SO as long as you are supplying 5 volts on the proper connections on the USB port of the phone, it should be just fine, since the internal circuitry of your phone takes care of the charging of the battery and monitoring this charge.
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    I've heard some Apple products use some sort of "protection scheme" to keep you from doing exactly what you are (quite reasonably) attempting to do. So this is not always 100% effective, but should not cause any damage.

    The 7805 will still need the input and output caps to stay stable. And it has a nasty self-destruct mode if you unplug the 12V but the connected device is back-supplying the 5V output: it will die immediately and permanently. The fix for that is to add a reverse diode across the 7805. Anything like a 1N400x would do fine there, as would anything Radio Shack sells as a "rectifier" diode.