Charging your Smartphone from USB without fear



From time to time many of us are facing the same problem when trying to charge your smartphone or tablet from the USB port - it just not charging. Recently I have bought USB car charger to power my Samsung Galaxy Tab 10.1 on the road just to discover it doesn't work. Even though the Galaxy actually sees the charger connected is just marking it as unrecognized power source. I was trying to power up my Motorola Droid 3 phone - it won't work either. The reason is simple - the Galaxy Tab doesn't recognize the charging device as "native charger".


What is "native" charger?

The native charger for smartphone or tablet often have a special voltage signature on USB data pins to let the device recognize the charger and figure out the maximum charging current it can consume from the power source. The intent is twofold. First, it is stopping the device from consuming too much current from the charger. Second, it is preventing the charging from unrecognized power sources. The "unrecognized" is the keyword here, as profiting from selling additional device accessories (chargers) is definitely a business strategy.


Samsung devices

I have figured the simple solution that will make the Samsung Galaxy pad device charge from USB port, see the schematic below. The R1/R2 voltage divider is providing the 1.2V voltage on the D+ and D- pins of the USB connector to be recognized as Samsung native charger. That's it. Make sure you are using the resistors with 1% tolerance. The only issue is that completely discharged Galaxy tab battery might draw even more then 500 mA from the USB port, when the USB 2.0 specification limiting the maximum current load to 500 mA. Just be aware.

If you would like to make your own, you can download the Eagle 6.x project files.


The latest Samsung Galaxy S4 and Galaxy Tab charger ETA-U90JWS is using slightly divider values with R1=300K and R2 =100K and the same ratio (0.25).


Motorola Droid charger

Motorola phones require a windows driver to be installed on your PC to let them charge from USB port. Even with the driver installed the charging is taking much longer then using dedicated Motorola wall charger as it limiting the charging current to 100 mA. Taking apart the Motorola charger reveals the following: the D+ and D- pins are shorted. It looks like it is related to USB Power Specification, when dedicated USB charging port can be recognized when the D+ and D- pins are interconnected. Charge the Motorola phone from USB port is simple, see schematic below. You can you the PCB above, just do not install the resistors.


Apple iPhone and iPad devices

Apple is using multiple voltage configurations to charge the devices. The regular iPhone charger applies 2.0V and 2.7V to D+ and D- data lines. However, the charging current in this mode could be up to 1A, way more then USB port can provide. To make things worse, the iPad charging profile with 2.7V and 2.0V applied to D+ and D- allows devices charging at 2.1A.The workaround is just to use the 500 mA charging profile applying 2.0V to both D+ and D-, see below.