Multiple ballasts operable from a single DC bus

The present invention relates to the artificial illumination arts. It finds particular application in protecting lamp ballasts from open circuit load faults and will be described with particular reference thereto. It is to be appreciated, however, that the present invention is also applicable to protecting vital components of other circuits from voltage and current surges, and is not limited to the aforementioned application. A plurality of lighting ballasts () draw power from a single DC bus signal. A power factor correction circuit () rectifies and smoothes AC power to produce the DC bus signal. In order to prevent damage to the ballast () when a lamp () dies or is removed, the ballast () includes an AC switch that senses damaging conditions and responds by changing a resonant frequency of the ballast (). The AC switch operates in 2-3 second cycles. While it is operative, it shunts current away from inductors () of the ballast () causing a resonant frequency of the ballast () to change. At the end of the cycle, the switch turns off, but if a load fault is still present in the ballast () it activates again. Preferably, the AC switch has a response time of approximately 500 s.