| 摘要 |
A plurality of lighting ballasts (12, 14, 16) draw power from a single DC bus signal. A power factor correction circuit (10) rectifies and smoothes AC power to produce the DC bus signal. In order to prevent damage to the ballast (12) when a lamp (18) dies or is removed, the ballast (12) includes an AC switch that senses damaging conditions and responds by changing a resonant frequency of the ballast (12). The AC switch operates in 2-3 second cycles. While it is operative, it shunts current away from inductors (38, 40) of the ballast (12) causing a resonant frequency of the ballast (12) to change. At the end of the cycle, the switch turns off, but if a load fault is still present in the ballast (12) it activates again. Preferably, the AC switch has a response time of approximately 500 mus.
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