(57) Two (or more) gas discharge lamps (10, 110), each in series with a choke ballast
(18,118) across A.C. supply terminals (16a, 16b) are started by a single starter circuit
(70), the lamps being connected to the starter circuit through respective rectifying
diodes (150, 152). The starter circuit (70) operates with rectified half-cycles of
the A.C. supply, and resistors (154,156) balance the cathode heating currents between
the two lamps. Preferably the starter circuit is rendered ineffective in a failed-lamp
condition, and to this end (per DE-OS 28 16 415) it includes a thyristor (26) triggered
by a control circuit which includes a capacitor (32) effective to increase the instantaneous
applied voltage which is required totriggerthe thyristor with successive cycles of
the applied voltage after initial switch-on of the circuit. A Zener diode (30) is
in series with the capacitor (32).
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