| 摘要 |
PROBLEM TO BE SOLVED: To reduce light or put out light of a discharge lamp in the last stage of its service life in lighting without using a complex protecting circuit, by controlling in starting the output frequency of a high-frequency generating means according to full lighting and light regulating lighting so as to satisfy a specified relation. SOLUTION: A high-frequency generating means 2 and a control means 4 control frequencies, so that when natural resonance frequency of a load circuit 3 is f0, working frequency f1 in full lighting and working frequency f2 in light regulating lighting satisfy the relations f0/5<=f1<=f0/3 and f0/3<=f2<=f0. The high-frequency generating means 2 has a DC power source 1 connected to the input end and has a pair of switching means 2a, 2b for alternately conducting switching operation and a drive circuit for controlling the switching means as the main component. The working frequency, set at about 50 kHz, for example if frequency f1 in full lighting, is audible frequency or higher, the working frequency f2 in light regulating lighting is set so as to satisfy the relation shown in the formula according to the frequency f1 in full lighting. |
