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Cheap Switch-Mode DC-DC ConverterThis circuit is based on mobile phone chargers. These chargers are based on the Motorola MC34063 switchmode IC. By changing the values of the feedback resistors (R1 & R2), the output voltage can be varied over a wide range. Just modify R1 and R2 according to the formula: Vout = 1.25 (1+R2/R1). The values shown give an output of 3V.
Circuit diagram:Author: Timo Mahoney - Copyright: Silicon Chip Electronics Magazine