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Thyristor starter for fluorescent tubes

The Type Y1112 fluoractorTM form Taxes instruments a specially designed thyristor for starting fluorescent tubes. Its main advantages over a standard thyristor are its much greater hold current. its ability to cope with rapidly rising anode-cathode voltages (high du/dt value), and its overvoltage protection. These properties make it very suitable to replace a traditional electro-mechanical starter for reliable flicker-free starting and a long life.

A fluorescent tube does not switch on as easily as an incandescent lamp because it can only start at a voltage much higher than the mains, after which it will remain lit at the mains voltage. The level of both the starting voltage and the working voltage depends on the temperature of the tube.

Normally, the high starting voltage is obtained by interrupting the current through a choke. This is usually done by the starter, which also ensures that a fairly large current flows through the filaments of the tube. This heats the ends of the tube, which makes starting easier.

These tasks of the starter are taken over by the fluoractorTM, Th2. When the mains is switched on, Th9 will start via RI and R2. A fairly large current will then flow through the filaments of the tube so that this will be preheated.

Because of the potential drop across diodes D7 and D8. capacitor C1 will be charged via resistors R4 and R5. When, after a few milliseconds, C1 is charged to a certain value (during which time the current through Th2 is appreciably larger than the hold current), thyristor Th1 is switched on, whereupon the gate current of Th2 is interrupted. The sudden discontinuation of this fairly large current causes the choke to produce a back-e.m.f. that is high enough to start the fluorescent tube.
*This special thyristor remains on. however, until the current through it has dropped to the hold value of about 200 mA.

Thyristor starter for fluorescent tubes Schematic diagram

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