Running a rare 15 W metal halide lamp

In 2011 Osram released the smallest commercial ceramic metal halide lamp on the market, the HCI Powerball 15W, which was also distributed in North America by OSI-Sylvania under the Metalarc Powerball brand. The intention was to provide an energy-efficient and long-life alternative to 50 W tungsten halogen lamps in the retail sector, but LEDs were beginning to make inroads into that market already. Because of the latter's success, Osram/OSI abandoned its 15 W metal halide system the same year of its introduction, thus making it an exceedingly rare lamp. Since its electronic ballast is practically impossible to find nowadays, the problem of properly operating the lamp is a rather difficult one.

I solved this challenge by modifying a Philips PrimaVision ballast intended for 35W CDM lamps, which actually runs at 39 W. Changing the system's current-sensing resistor from 1.5 to 4.4 Ohm lowers the regulated power to 15-16 W only. The picture above shows the end result with a Sylvania MC15TF running optimally. I came to that solution when I sought to modify this type of electronic ballast for running a 35 W SDW white sodium lamp properly, and those normally work at 33 W, so I had to find a way to lower the ballast output by 6 W, which is achieved by increasing that resistance value to 1.8 Ohm. I ended up testing various resistances, which led me to such low power level compatible with the rare 15 W MH lamp shown here (still amazed at the circuit's flexibility!). Additionally, varying that resistor value effectively enables dimming from an electronic ballast originally designed for a fixed power output, which is very convenient since to the best of my knowledge dimmable 35 W HID ballasts do not exist (and I needed one :). Although I had the MC15TF shown here for a couple years, I never ran it for fear of damaging it. So it was quite a treat seeing such unique and rare discharge lamp finally come to life.