Retro 12V fluorescent light driver with pre-heat

An unusual bit of circuitry in an interesting fluorescent lamp driver. To aid starting and give longer tube life, the fluorescent tube's heaters are energised before the tube is struck. That raises the electrodes up to a thermally emissive state reducing the voltage drop across them and reducing blackening and sputtering.

The circuitry with two zeners and a large capacitor on the 555's control pin is unusual. I can only guess it might be for frequency stability with fluctuating voltage.

While it would have been best to swap the faulty transistor with an identical one, I drew a blank on that component from my usual suppliers. I looked for alternatives but the need for a low saturation voltage, high frequency operation and similar gain pointed to various devices, but with no guarantee of compatibility. Just for fun I put in an STP36NF06L MOSFET and it works without getting too hot.

Thanks to Adam for sending this in.



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