(57) Starter circuits are disclosed for a fluorescent discharge tube (10) of the type
having a pair of heater cathodes between which the discharge is struck and which are
connectable to the mains supply (V.) through a ballast inductor (L). The starter circuit
(20) includes a thyristor (SCR) that conducts current through the ballast inductor
and heaters on half-cycles on one polarity. A gate control circuit (30) includes a
capacitor (C1)that receives a controlled (by C2, Z1) bias voltage increment in each such half-cycle to eventually terminate triggering
of the thyristor (SCR). Preferably an automatic switching arrangement (C3, Z2) changes the value of capacitance to give a lesser rate of increment upon a certain
bias level being reached to provide a two phase start operation that eventually times
out. Alternatively, the maximum bias may be clamped and a positive temperature coefficient
resistor (PTC) carrying the heater current terminates the second phase upon heating
to a predetermined resistance. The starter circuit (20) also includes a non-linear
dielectric element (NLDE) that cooperates with the inductor to generate striking voltage
for the tube and that is connected in an ignition circuit including a capacitor (Cs) connected to the thyristor (SCR) to charge and maintain the NLDE quiescent during
normal operation but to be discharged by the thyristor (SCR) during starting to allow
the NLDE to achieve saturation and coact with the ballast inductor (L).
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