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Ignition of Fluorescent Lamps  

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One of the many applications for Thyristors is in fluorescent lighting. Standard conventional and circular fluorescent lamps with filaments can be ignited easily and much more quickly by using Thyristors instead of the mechanical starter switch, and solid state Thyristors are more reliable. Thyristors produce a pure solid state igniting circuit with no mechanical parts in the fluorescent lamp fixture. Also, because the lamp ignites much faster, the life of the fluorescent lamp can be increased since the filaments are activated for less time during the ignition. The Thyristor ignition eliminates any audible noise or flashing off and on which most mechanical starters possess.

Figure illustrates a circuit using a SIDAC (a simpler Thyristor) phase control network to ignite a 120 V ac capacitor is charged and added to the peak line voltage, developing greater than 200 V peak or 400 V peak to peak. Since the SIDAC is a voltage breakover (VBO) activated device with no gate, a charging network is essential in this circuit to charge the capacitor above the peak of the line in order to break over (turn on) the SIDAC with a VBO of 220 V to 250 V. As the SIDAC turns on each half cycle, the filaments are pre-heated and in less than 1.5 seconds the lamp is lit. Once the lamp is lit, the voltage across it clamps to approximately 60 V peak (for a 15 W to 20 W lamp), and the SIDAC ceases to function until the lamp is required to be ignited again.

The circuits illustrated in Figure use 15 W to 20 W lamps. The same basic circuits can be applied to higher wattage lamps. However, with higher wattage lamps the voltage developed to fire (light) the lamp will need to be somewhat higher. For instance, a 40 W lamp is critical on line input voltage to ignite, and after it is lit the voltage across the lamp will clamp to approximately 130 V peak. For a given type of lamp, the current must be limited to constant current regardless of the wattage of the lamp.

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    Thyristors for Ignition of Fluorescent Lamps


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