If you want to power or charge USB-based gadgets like MP3 player in the car or without a computer, you need a 5V power supply with an output to a standard USB socket. Simply using a widely available linear voltage regulator like 7805 is not a good idea, because by powering the charger from the car 12V battery, the linear regulator would dissipate (12V - 5V)· 0.5A = 3.5W of power. This is too much for it, even if it is mounted on a large heatsink.
This project is based on the article Build a charger for your iPOD or MP3 player published in December 2007 issue of Everyday Practical Electronics, pp. 52 - 56. It is basically a step-down voltage converter and this is much more efficient solution that the trivial one mentioned above. I repeated it without any changes in the schematics (just eliminated a power switch). However, I put it into a smaller enclosure and that's why needed to redesign the PCB. My PCB in the Eagle format can be downloaded here.
| Modified PCB | Layout | |
The enclosure I used is a 3"x2"x1" project box available at RadioSchack. Another slight difference with the original design is that I did not wind the coil, just used a ready 220μH power inductor 2116-H-RC manufactured by J.W. Miller/Bourns (Digi-Key part #M8813-ND).
The charger can be powered from a 9.5 - 15VDC supply, in particular the car lighter, and can deliver up to 650mA current at 5V. The output voltage drops on about 0.07V when a 500mA load is attached. In this case the working frequency of the voltage converter is about 70KHz. I tested it in this mode and noticed no hot parts. Just the switching diode gets a little warm. I would estimate it surface temperature as 35°C (100°F).
| Charger's enclosure | Assembled device | |
Last modified:Mon, Jan 23, 2023.