K0900E70 Datasheet, PDF (189/224 Pages) Teccor Electronics – Thyristor Product Catalog

AN1010
10
Thyristors for Ignition of Fluorescent Lamps
Introduction
One of the many applications for Teccor thyristors is in fluores-
cent 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 fluores-
cent 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 igni-
tion eliminates any audible noise or flashing off and on which
most mechanical starters possess.
Standard Fluorescent Circuit
The standard starter assembly is a glow switch mechanism with
option small capacitor in parallel. (Figure AN1010.1)
Line
Input
Starter Assembly
Ballast
Lamp
Figure AN1010.1 Typical Standard Fluorescent Circuit
The glow switch is made in a small glass bulb containing neon or
argon gas. Inside the bulb is a U-shaped bimetallic strip and a
fixed post. When the line input current is applied, the voltage
between the bimetallic strip and the fixed post is high enough to
ionize and produce a glow similar to a standard neon lamp. The
heat from the ionization causes the bimetallic strip to move and
make contact to the fixed post. At this time the ionization ceases
and current can flow through and pre-heat the filaments of the
fluorescent lamp.
Since ionization (glowing) has ceased, the bimetallic strip begins
to cool down and in a few seconds opens to start ionization
(glowing) again. The instant the bimetallic ceases to make con-
tact (opens), an inductive kick from the ballast produces some
high voltage spikes 400 V to 600 V, which can ignite (strike) the
fluorescent lamp. If the lamp fails to ignite or start, the glow
switch mechanically repeats its igniting cycle over and over until
the lamp ignites, usually within a few seconds.
In this concept the ballast (inductor) is able to produce high volt-
age spikes using a mechanical switch opening and closing, which
is fairly slow.
Since thyristors (solid state switches) do not mechanically open
and close, the conventional fluorescent lighting circuit concept
must be changed in order to use thyristors. In order to ignite
(strike) a fluorescent lamp, a high voltage spike must be pro-
duced. The spike needs to be several hundred volts to quickly ini-
tiate ionization in the fluorescent lamp. A series ballast can only
produce high voltage if a mechanical switch is used in conjunc-
tion with it. Therefore, with a thyristor a standard series ballast
(inductor) is only useful as a current limiter.
Methods for Producing High Voltage
The circuits illustrated in Figure AN1010.2 through Figure
AN1010.5 show various methods for producing high voltage to
ignite fluorescent lamps using thyristors (solid state switches).
Note: Due to many considerations in designing a fluorescent fix-
ture, the illustrated circuits are not necessarily the optimum
design.
One 120 V ac circuit consists of triac and diac thyristors with a
capacitor to ignite the fluorescent lamp. (Figure AN1010.2)
This circuit allows the 5 µF ac capacitor to be charged and added
to the peak line voltage, developing close to 300 V peak or 600 V
peak to peak. This is accomplished by using a triac and diac
phase control network set to fire near the 90° point of the input
line. A capacitor-charging network is added to ensure that the
capacitor is charged immediately, letting tolerances of compo-
nents or temperature changes in the triac and diac circuit to be
less critical. By setting the triac and diac phase control to fire at
near the 90° point of the sinewave, maximum line voltages
appear across the lamp for ignition. As the triac turns on during
each half cycle, the filaments are pre-heated and in less than a
second the lamp is lit. Once the lamp is lit the voltage is clamped
to approximately 60 V peak across the 15 W to 20 W lamp, and
the triac and diac circuit no longer functions until the lamp is
required to be ignited again.
©2002 Teccor Electronics
Thyristor Product Catalog
AN1010 - 1
http://www.teccor.com
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