I reverse engineered a couple more electronic ballasts I came across recently. Both are from those cheap Lights of America outdoor dusk till dawn wall lights. The first uses a 42W Fluorex compact fluorescent lamp which has an ordinary medium screw base but that contains only the resonant capacitor. The second uses a 50W high pressure sodium lamp. Both are nearly identical circuits, differing primarily in the DC rail voltage and the inductance of the resonant choke, both varieties appear to be wound on identical cores approximately 1" square. They are a series resonant half bridge design based around the UB2021P ballast IC. Both also use a rather unusual arrangement of two transistors to construct an SCR rather than using a real SCR. Must have been a few pennies cheaper I guess. Both ballasts had one or more shorted mylar capacitors, the HPS one I revived, the fluorescent one suffered a catastrophic failure that took out nearly every semiconductor on the board.