摘要 |
PROBLEM TO BE SOLVED: To control a piezoelectric transformer fully stably and precisely by sweeping the frequency of the drive voltage of a cold-cathode tube discharge lamp from high to low with the piezoelectric transformer until the current of the cold-cathode tube discharge lamp becomes within a set range, and determining the drive frequency of the piezoelectric transformer. SOLUTION: A feedback resistor 109 is connected in series to a cold-cathode tube 108, the voltage corresponding to the current flowing in the cold-cathode tube 108 is detected by a current detecting circuit 110, and it is compared with a set current value I1 by a comparing circuit 111. When the tube current is not within the range of an allowable current value I2 centering on the set current value I1, a control circuit 112 controls a variable oscillating circuit 101 to change the frequency of the drive signal so that the tube current nears the set current value I1. The changed drive signal is waveform-shaped into a sinusoidal wave by a waveform-shaping circuit 102, then it is voltage/current- amplified by a drive circuit 103 and fed to a piezoelectric transformer 104. The output signal is fed to the cold-cathode tube 108 and the feedback resistor 109 and is again detected by the current detecting circuit 110. |