Abstract: A lamp circuit is provided having a constant current-type AC power source and a plurality of isolation transformers coupled in series with the AC power source. The secondary circuit of each isolation transformer is connected to an electric lamp. The output voltage of the constant current-type AC power source is detected to determine the times when the output voltage crosses the zero voltage level. A disconnected lamp in the circuit results in a distortion of the output voltage waveform due to the magnetic-saturation characteristics of the isolation transformer coupled to the disconnected lamp. The output voltage waveform is integrated over time between the time periods when the output voltage signal crosses the zero axis. The result of the integration is compared with a reference value. When the integrated value exceeds the reference value, an alarm signal is generated. The number of disconnected lamps may also be determined and displayed.

Abstract: In a device for detecting broken filaments of lamps which are connected with secondary windings of transformers respectively provided for the lamps, primary windings of the transformers being connected in series across an AC power source through a power control device, there are provided a detector for detecting voltage and current delivered from the power control device, and another detector for detecting a phase difference between the voltage and the current thus detected by the voltage and current detector.