GB1397265A - Ballast unit for gas discharge lamps - Google Patents

Abstract

1397265 Supply circuits for discharge lamps E VOGELI 24 May 1972 [24 May 1971 16 May 1972] 24324/72 Heading H2H A ballast circuit for a discharge lamp 6 having pre-heatable electrodes 5, 7 comprises a semi-conductor switch 31 connectible in parallel with the lamp electrodes and a control circuit which triggers the switch during successive or alternate half cycles of an A.C. supply to enable heating of the lamp electrodes, the conduction period of the switch being gradually reduced during successive or alternate half cycles until the lamp fires. When the system is initially switched on, a high charging current for a capacitor 32 flows through a resistor 42, the voltage across which breaks down a diac 43 to trigger the triac 31 early in the half cycle of the supply voltage. The voltage induced in a choke 3a, 3b is insufficient to start the lamp but heating current flows through the electrodes 5, 7 via the triac. A rectifier circuit 34-37 prevents the capacitor discharging so that the capacitor is charged to a higher voltage, during successive half cycles thereby gradually increasing the time taken for the voltage across the resistor 42 to reach the breakdown voltage of the diac. The firing of the triac is therefore retarded in successive half cycles until the voltage induced in the choke is sufficient to start the lamp. Once the lamp has ignited the reduction in the voltage across the electrodes ensures that the voltage across the resistor 42 is insufficient to trigger the diac. The effects of higher frqeuency harmonics of the mains supply are eliminated by an RC circuit 38-41, the capacitors of which have low reactances at these frequencies. Modifications are described in which the triac is replaced by a thyristor and a circuit in which the electrodes of series connected lamps are supplied via saturable chokes is also described.