Often it is necessary to charge various instruments - camera, phone, etc., or power a USB-powered device, in a workshop where there's a plenty of 12V outlets and no USB.
Originally, a stepdown converter with a LM2586 was planned to be made. However, it was decided to adapt an inexpensive USB car lighter charger instead.
A cheap off-the-shelf charger was obtained. It features a car lighter plug interface and a single USB connector on its face, together with a blue LED indicator.
The charger's housing was carefully cracked open; it was possible to pry it apart with a screwdriver.
Inside there is a small circuitboard with SMD circuitry with three discrete parts - a pair of aluminium
electrolytic capacitors and a coil. The electronics is based on the cheap and good MC34063
step-up/step-down converter chip and the build quality is fairly good, though the coil seems to be
a little small.
The downside is using of crappy lead-less solder (as the RoHS label warned) that oxidizes under the soldering iron and becomes lumpy. A drop of real lead-containing solder replacing the joints rectifies this and makes them nice and shiny and reliable and easier to visually spot bad wetting.
The circuit was reverse-engineered and found to be decent.
 Original adapter |  Original adapter |  Inside |  Inside |
 Inside |
The contact springs for the lighter were removed. A 5.5/2.1mm barrel jack female was soldered to the board with its positive end and its negative end was connected via a short jumper wire.
The single USB plug was removed and replaced with a double one. For durability the plug was hot-glued to the board and also soldered by two pieces of wire to the board from the sides. A hole was drilled in the board where the negative-terminal spring of the car lighter connector was, and wires between the copper-side of the board and the USB terminals were pulled through it.
 Power connector replacement |  Power connector replacement |  USB connector replacement |  USB connector replacement |
 USB connector replacement |  USB connector attachment |  USB connector attachment |  USB connector attachment |
The halves of the plastic housing were drilled on the end to fit the barrel connector in. For tighter fit there was a heat-shrink tube applied around the connector's body. The face and top of the housing were cut to accommodate the larger-size double-USB connector.
 Modified housing |  Modified housing |  Modified housing |  Modified housing |
A transparent large-diameter heat-shrink tube was applied over the finished assembly to hold the housing together.
 Final assembly |  Final assembly |  Final assembly |  Final assembly |
As many things (cellphone, ebook reader, next to everything these days) uses the microUSB connector for powering/recharging, after a few cycles of searching-finding-using-misplacing the charger cable it was decided to attach a short pigtail with a microUSB cable directly to the unit.
A connector was taken from the stock of parts, the D+ and D- lines were connected together, and pulled up from 5 volts via a 470 Ω SMD 1206 resistor (a 0804 size would be better, but was not available at the moment); this pull-up idea was borrowed from the original charger's schematics. A drop of hot-melt glue was used to protect the resistor, then a few layers of heat-shrink tubes were placed over the connector to act as its body and strain relief.
 MicroUSB connector with 470Ω D+/D- pullup |  MicroUSB connector, housed and attached |  FinalModification |