A $5 variable voltage power supply

Inside each of these car phone chargers is a sophisticated DC/DC converter that can be easily modified to provide a variable output power supply.

This design uses a slightly modified car phone charger. The idea is based on a Circuit Notebook contribution from Timo Mahoney in the November 2003 issue of SILICON CHIP.

Most car phone chargers use a DC-DC switching power supply to reduce the voltage from the car’s 13.8V to what ever the phone requires. However, it’s easy to modify the PC board to give an adjustable voltage output.

Apart from the car phone charger (the electronics are normally built into the cigarette lighter plug), all you need is a 10kΩ potentiometer. Car phone chargers are available in secondhand shops, at the tip and in the junk rooms out the back of phone retailers, while a 10kΩ pot can be scrounged from some old gear or purchased from your local electronics parts retailer.