Patent ID: 6140779
Filing Date: 2000-10-31
Classification: H05B,Y10S

Abstract:
A lighting system for a discharge lamp having a pair of electrodes, comprising:(a) an inverter circuit having a pair of outputs to be connected respectively to a pair of electrodes of a discharge lamp for providing a variable frequency output voltage;(b) a resonant circuit connected to the inverter circuit and including a capacitor with which the lamp is to be connected in parallel, in order to cause an inversely frequency dependent voltage to be applied between the lamp electrodes according to a predefined resonance characteristic, the resonant circuit having a resonance frequency (f.sub.0) which is less than a discharge start frequency (f.sub.4) at which the lamp is to start glowing;(c) preheat timer means for providing a preheat signal indicative of a preheat period (T.sub.2 +T.sub.3) during which the lamp is to be preheated,(d) light up timer means for providing a lightup signal indicative of a lightup period (T.sub.4) during which the lamp is to be lit up;(e) preheat control means connected between the preheat timer means and the inverter circuit and responsive to the preheat signal for changing, during the preheat period, the frequency of the output voltage of the inverter circuit from a first frequency (f.sub.2) to a second frequency (f.sub.3) which is less than the first frequency, the first and the second frequencies being both higher than the discharge start frequency of the lamp and holding the lamp unlit; and(f) lightup control means connected between the lightup timer means and the inverter circuit and responsive to the lightup signal for changing, during the lightup period following the preheat period, the frequency of the output voltage of the inverter circuit from the second frequency to a third frequency (f.sub.5) which is less than the discharge start frequency (f.sub.4) of the lamp, the mean rate of frequency change during the lightup period (T.sub.4) being higher than the mean rate of frequency change during the preheat period (T.sub.2 +T.sub.3), in order that the lamp may start glowing by the time the output voltage of the inverter circuit reaches the third frequency;(g) whereby the voltage impressed across the lamp is lower at the beginning than at the end of the preheat period.