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.
Thyristors for Ignition of Fluorescent Lamps
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