| 摘要 |
PURPOSE:To supply sufficient pre-heat current in filament electrodes by operat ing a time constant circuit utilizing the voltage appearing across a pre-heat electrodes so as to control a three-terminal switching element. CONSTITUTION:When an AC tower supply 1 is turned on, a transistor 8 is driven into conduction state by supplying current in the base of the transistor 8 through a resistor 10. Consequently, a pre-heat current flows in a closed circuit comprising a ballast 2, a pre-heat electrode 4, the transistor 8, a diode 7, and a pre-heat electrode 5. In this case, since there exists a voltage drop across the pre-heat electrode 4 due to the pre-heat current, the voltage charges a condenser 13 in a time constant circuit through a resistor 11. The pre-heat state continues until the voltage across the condenser 13 exceeds the voltage across a Zener diode 14, but when exceeding the voltage, since a current flows between the gate and the cathode of a thyristor 12 through a resistor 15, the thyristor 12 turns on, and the transistor 8 turns off, and then the current flowing through the ballast is interrupted so that a lamp 3 turns into the light-on state. |
