Thursday, September 20, 2012

Using mini iPhone chargers with Android devices

It's possible you have find these little USB chargers in eBay and similar places. They are very cheap, you can get one even under one euro (shipping included). But, Can they be used with your Android device?

After playing with more than twelve chargers I can say the answer is yes, but some care must be taken before using them.

The charger

The charger is a very common one. It is based on the MC34063 switching regulator and it's schematics is almost a clone of the schematics present in the datasheet. The charger is labeled 5V, 1000mA (more on this later) and it is supposed to be adapted to the iPhone charging interface: D+ and D- USB pins are shorted and a voltage is applied to those pins using a voltage resistor divider:

Most Android devices detect a charger looking for direct electrical connection between D+ and D- pins (DC resistance 200 ohms maximum). If they found it, the device draws the maximum current allowed and battery charging is quite fast. If not, they assume they are connected to a computer USB port and charging current is usually quite low, taking an eternity to charge the battery or even no charging at all.

Looking at the diagram it should be clear what modification is needed to adapt the charger to Android devices: Those two resistors must be removed. They are placed very near to the USB socket pins (yellow circles):

In some chargers these resistor are already omitted, like the one at the photo. If not, simply remove them. Be careful to not remove the other 4.7k and 1.5K resistors: They are the ones that sets the output voltage.

Output current

Although the charger is stated 1 amp output current the truth is it will never be able to deliver such high current. Looking at the datasheet and the charger you can verify there is no external switching transistor, and Rsc is a 0.22 ohms resistor. What does this means?

In few words, the maximum output current is around 500-600 mA. During heavy load work, both IC and coil get very hot, and when they are hot, output current is further reduced. I tested some chargers with a Galaxy Mini, who draws 500mA maximum while charging and worked perfectly. Other phones or devices who need more than 500 mA can have problems or exhibit a longer charging time using these chargers.

Common problems

Looking in detail at the above image you will see soldering is made with quite poor tin. Many chargers are defective just by poor soldering. Sometimes there are short circuits because of excess of tin solder. My advice is to remake all soldering before apply voltage to the charger.

Other common problem is the negative connection to the car's 12V socket. The connection is made by one of the plates touching the USB socket shield (red circle). This contact will loosen up with time, producing heat and / or charger malfunction, or even not working at all because the lack of electrical contact.

One solution is to bend the plate at the USB side to force more pressure over the USB socket shield. Soldering would be a much better solution but the plates can't be soldered with tin, even after scratching them with sandpaper.

The last of the problems I found is inside the green circle. When the negative plates loosen up, they can make contact with the central U shaped plate (+12V) making a wonderful short circuit. As you can see, there is no fuse so it will probably fuse your car's socket fuse. A solution is to bend that plate end in a way never make contact with the central plate or even better, insert between them a small insulator. A piece of insulating tape or even a cardboard will make the job.


Nice charger for the money, but never forget the maximum output current is around 500 mA and take into account you will need to disassemble it to check for bad soldering before use it.

Never use one of these chargers without testing it first, or you can kill your device.

Check for bad soldering, and make sure the negative plates make electrical contact only where they are supposed, and only there. Output voltage must be between 5.0 and 5.3 - 5.5 volts maximum.

And remember: You can't ask more for one euro.

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