| 摘要 |
PROBLEM TO BE SOLVED: To eliminate detection of wasteful lamp current due to performing feed back control for the full range of light adjustment signals, widen a dynamic range, and enhance detection precision of lamp current. SOLUTION: A current detection circuit 5 detects current which flows an electric discharge lamp 4 and outputs it as a lamp current signal A to a feed back control circuit 7. The feed back control circuit 7 takes the difference between the lamp current signal A and a light adjustment signal B and amplified it and outputs an amplified signal D to an adder 12. The adder 12 adds the amplified signal D and the light adjustment signal B, constitutes a control signal E, and feed backs it to a high-frequency power supply 2, however, if a comparator circuit 15 detects that the light adjustment degree against the full light of the light adjustment signal B is 40 to 60% or more, it is disconnected by means of a switch 13 provided between an error amplifier circuit 9 and the adder 12 to stop feed back control, the range in which feed back control is performed for the full light adjustment range of the light adjustment signal B is limited, the detection range of the lamp current signal is narrowed, a dynamic range is widened relatively, and the detection precision of the lamp current signal is enhanced. |
