| 摘要 |
PURPOSE: To provide a ballast circuit for a gas discharge lamp having a pair of resistive heating type cathodes. CONSTITUTION: This ballast circuit has converters S1 , S2 for supplying a bi- directional current, according to a direct current bus voltage, to a resonance load circuit including a gas discharge lamp 415, a resonance capacitor CR, and a resonance inductor LR, and has a circuit for keeping a lamp voltage below a fixed level so as to prevent the lamp from being lighted during a cathode preheating period. This circuit has circuits 450, 455 for holding a first cathode 415B of the lamp at a nearly constant voltage and a circuit for clamping a second cathode 415A of the lamp below a fixed level. This clamp circuit is connected to the second cathode of the lamp and has an impedance element 480 having a positive temperature coefficient(PTC) which is connected in series to a bus conductor via a clamp diode D1 polarized in the positive direction and connected in series to an earth via a clamp diode D2 polarized in the negative direction. |
