12 Volt Fluorescent Lamp circuits

  
Here is the schematic diagram for a 20 Watt driver. I developed this circuit in 1985, and used it to build a lamp that found much use both as camping light and as emergency light during the then-frequent power outages. The two transistors work as a push-pull oscillator. To avoid crossover conduction, during operation the bases are self-biased to a slightly negative voltage by the 1R5 resistor. The 220uF capacitor gives strong positive bias during power-up, in order to get the oscillator started. 2k2 resistor's only purpose is discharging the capacitor when the circuit is switched off, so it will restart the next time too!
12 Volt Fluorescent Lamp circuits - schematic

12 Volt Fluorescent Lamp circuits - img1

12 Volt Fluorescent Lamp circuits - img2

I used a ferrite core salvaged from an old 12" black and white TV flyback transformer. I wound the primary and feedback windings on one leg of the core, and the windings going to the tube on the opposite leg. This arrangement is crucial for the circuit to work! The only ballasting action in this circuit is the imperfect magnetic coupling between the primary and secondary coils! You may be wondering about the "ionization antenna" in the schematic. This is simply a piece of wire that runs along the entire length of the tube. You can stick it there with tape, or you can string it from socket to socket, like I did. Striving for the highest possible efficiency, I came up with the design shown in this schematic diagram. A CMOS hex inverter is used as an oscillator and driver for a MOSFET, which drives a high voltage autotransformer that feeds the fluorescent tube via a ballast capacitor. The circuit is very simple and small, uses just 8 components, yet it is extremely efficient (I could not measure the loss, it seems to be below 3%), and it drives the tube with true AC, despite the simplicity! The tube filaments are not explicitly heated. On powerup, the gas ionizes from the high voltage alone, and the small discharge that starts has enough anodic heating effect to warm up the filaments. Soon the discharge increases, generates a runaway effect, and less than a second after applying power the tube is fully on, working at the...



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