| 摘要 |
PURPOSE:To obtain a stable constant cold cathode tube brightness even through the input voltage is changed, by providing a pulse width modulating circuit to modulate the pulse width of the output of a switch element at a duty ratio depending on a DC voltage, and to maintain the mean voltage output to a self-exciting type resonant oscillation circuit constant through the switch element. CONSTITUTION:The pulse output E of the collector of a transistor 25 is divided by resisters 39 and 40, smoothed by a capacitor 38, and made into a positive input F to a comparator 37. This positive input F is made equal in the mean voltage to the negative input G to a capacitor 37, which is the voltage limited by a Zener diode 32 and divided by resisters 33 and 34. In such a way, the voltage VO made by averaging the pulse presented in the collector of the transistor 25 during a cold cathode tube 28 is lighted is dependent only on the resistance values of the resisters 33, 34, 39, and 40, and the property of the Zener diode 32, and receives no influence from input voltage. As a result, an average pulse voltage is applied to a self-exciting type resonant oscillation circuit 50 as the input. Consequently, the brightness of the cold cathode tube 28 can be obtained in a stable condition to the variation of the input voltage. |
