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Small 12V-to-USB charger

Why

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.

Charger reverse engineering

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

Modifications

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

MicroUSB "tail"

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

TODO


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