Schematic Reference

El-Cheapo Fluoro Ballast

This simple circuit can start small (15W or less) fluorescent tubes such as those used in PC board exposure and EPROM ultraviolet erasure boxes. As you can see, the tube’s filament heaters are not used. Instead, ignition is provided by a voltage tripler formed by diodes D1-D3 and the two 6.8nF 2kV capacitors. At switch on, C1 charges up via R1 until the gas in the tube breaks down (around 700V). C1 then discharges through the tube, lowering the resistance enough to sustain continual AC current flow. C1 then continues to act as the ballast, with R1 included to prevent the three diodes shunting the tube on positive mains half-cycles.