GB1268054A - Improvements in or relating to a circuit arrangement for starting an electrical discharge lamp from an alternating current supply - Google Patents

Abstract

1,268,054. Supply circuits for discharge lamps. GENERAL ELECTRIC CO. Ltd. 23 Oct., 1969 [3 Oct., 1968], No. 47003/68. Heading H2H. A starting circuit for a discharge lamp comprises a capacitor which is charged through a diode from an A.C. source. A gate-controlled switching device is fired when the voltage across it reaches a given value, and the capacitor is thereby discharged into a pulse transformer, the resulting oscillations being superposed on the A.C. supply to start the lamp. In the circuit shown in Fig. 1, a capacitor C 1 is gradually charged through a diode D connected in antiparallel with a thyristor T. During non- conductive half-cycles, the voltage across the diode approaches the peak-to-peak supply voltage, and eventually a voltage sufficient to fire the thyristor appears at the centre tapping of a voltage divider R 1 , R 2 . When this happens, the capacitor C 1 discharges through a second capacitor C 2 and the primary L 1 of pulse transformer PT, several oscillations occurring before the thyristor is turned off by insufficient voltage at its gate. In addition to these oscillations, which are amplified by the transformer PT, higher frequency ringing oscillations occur due to the leakage inductance and self-capacitance of the transformer and leads. After the lamp 1 has started, the voltage across it -is insufficient to allow further operation of the thyristor. A power factor correction capacitor C p is connected across the supply, while a Zener diode may be connected to the gate of the thyristor, and a suppressor choke may be connected in series with the transformer PT. In modifications, Figs. 3 and 4 (not shown), the capacitor C 1 may be omitted and the second capacitor C 2 then stores the charge.