| 摘要 |
PURPOSE:To prevent a discharging lamp from flickering by allowing an output voltage to drop abruptly to the zero level if the terminal voltage of a smoothing capacitor drops below a specific voltage when a power supply switch is turned off. CONSTITUTION:While the power supply switch 42 is on, a voltage stepped down by a transformer 43 is rectified and smoothed by an element 46 to obtain a waveform shown in a figure (a) between a plus and a minus terminal. Through the operation of a constant voltage diode 49, this waveform is made into a voltage A in a figure (b) by a transistor TR47 having an invariable constant base- emitter voltage. When the power supply switch 42 is turned off at time t1, the capacitor 45 is discharged and then the voltage between the plus and minus terminal drops gradually. When this voltage drops below the voltage C obtained by adding a voltage drop to the breakdown voltage of the diode 49 by a resistance 50, a TR51 is turned off through the diode 49 and no voltage is developed between output terminals 48 and 52, thus causing the output voltage to drop to the zero level. |
