US Patent 7902764 Control device for discharge lamp

A driving method for a discharge lamp having two cathodes includes providing a supply input voltage for providing an alternating voltage at the terminals of the cathodes, monitoring a condition of each of the cathodes and measuring a first direct voltage signal of the waveform of the voltage of the lamp that develops when the lamp approaches an ageing condition, deactivating the alternating voltage when a variation of the first direct voltage signal occurs, and supplying a second direct voltage signal proportional to the supply input voltage for deactivating the alternating voltage when a variation of the first direct voltage signal occurs in relation to the second direct voltage signal.