Monday, May 16, 2011

Fixing USB car chargers from eBay

A few months ago, in an accessory buying frenzy for my then-new N8, I ordered a couple of USB car chargers from eBay. The first one did not specifically say it was for the N8, but it didn't say it was for Apple either, so I took a chance with it. Disappointingly, it did not work. I ordered another one, this time in a bundle with a wall adapter and 3 micro USB cables. There was a choice of colours, so I ordered a pink one because that goes well with Subies. This model was advertised to work with the N8, but of course it didn't work. Some time passed. Recently, I helped put together two Minty Boost USB chargers, which showed that for some devices, pins 2 and 3 need to be shorted together. Inspired, I disassembled the pink one and soldered the pins together. Sadly, the N8 still wouldn't charge from it. Luckily I spotted this in a forum:
I found a similar problem: USB cigarette ligher (an expensive one) charger, and the phone (Nokia C7) won't charge. The solution was somewhat surprising. Some background info: in the USB plugs, there are 4 wires: +5 V, Ground, +Data, -Data. Only the first two is required for charging, but you have to do something with the rest. The spec says, they should be connected together either directly or through a maximum of 200 ohm resistor. However, in my charger (optimised for iPhone perhaps?) the +Data was connected to the +5 V through a 24K resistor, and likewise the -Data to the Gnd through a 24K resistor. Removing them and connecting the + and - Data through a 100 ohm resistor, the charger works perfectly. Too bad though, that you need a soldering iron for this...
Aha! After removing two resistors tying pins 2 and 3 to the power rails (and of course soldering pins 2 and 3 together), the charger works. The voltage between pins 1 and 4 is 5.24 V, and between 1 and 2-3 it's now 2.15V (it used to be ~3V). Mr Pink's circuit board. Green: solder bridge. Red: removed resistors. The other USB charger was trickier to disassemble because it is encased in a stainless steel sleeve, but after some prying it came apart. Same problem, same resolution, same results. Top: Mr Pink's resistors. Bottom: Mr Steel's resistors. There's no doubt in my mind that Mr Pink's components were placed and soldered by hand :(


  1. Excellent post. While looking for a solution to a non charging Nokia C7 via a cheapo auto 12v to USB charger, I came across your blog. When I looked into my cheapo charger, I found the same 24K resistors. Why are they built this way if it keeps the unit from charging?