USB charging semantics

Discussion in 'General Electronics Chat' started by axeman22, Jun 29, 2012.

  1. axeman22

    Thread Starter Active Member

    Jun 8, 2009
    something that's on my electronic mind, bugging me.. I'm hoping you guys might be able to shed some light..

    USB - my favourite..

    USB charging actually. so the USB spec initially allows devices to pull 100mA - and do data transfer at the same time. Not enough power for most any device today - especially smart phones. so with a revision to the spec devices when they connect up look for les than 200Ohms between the data wires, if the case - for example short, then they will pull up to 5000mA - all good..

    so here;s what I want to do.. take a 12Vdc car battery - nice clean 12V .. put that into a 7905 reg (to-220), with required external caps as per the 7905 data sheet.. , short the two data wires - and then allow the phone to pull 500mA etc.. - sound simple sight - /? it does't work…..!!!!! phone goes all weird etc etc..

    can anyone tell me what is going on - what am I doing wrong..? my understanding is that the phone has internal charging circuits etc.. so just putting a nice clean 5Vdc into it should be all that is needed..?