Fluorescent lamp dimming adaptor kit

A conventional non-dimming ballast for fluorescent lamps is modified to achieve a 100:1 dimming ratio by connecting a switching module to the ballast. The switching module switches current to and from the fluorescent lamps under control of a level control, resulting in the desired light output. The switching module may be connected either in series or in parallel with the lamps.

The present invention relates in general to a fluorescent lamp dimming 
system and more specifically to modifying a conventional non-dimming 
ballast to a configuration which allows dimming of the fluorescent lamps. 
BACKGROUND OF THE INVENTION 
Specially designed ballasts for dimming fluorescent lamps are presently 
available and well known in the art. Many ballasts achieve dimming ratios 
of better than 1000:1. However, these systems operate as phase control 
systems, meaning that a large spike of current must flow along the power 
wiring to reignite the lamp when the lamp has not conducted for a period 
greater than about 1 millisecond. The resulting current and voltage spikes 
appear in the power line and may radiate from the lamps and building 
wiring causing electromagnetic interference (EMI) which may affect the 
operation of sensitive electronic equipment. It is especially important to 
keep transients off of the building wiring because it behaves like a 
transmission antenna and thus extends the range over which the EMI 
radiates. 
Using specially designed dimming ballasts to replace one of the over 600 
million U.S. installed 40 watt fluorescent lamps supplied by a non-dimming 
ballast in order to adapt an installation to a dimming system can be very 
expensive. Furthermore, these special dimming ballasts typically power 
only one lamp while most existing fluorescent lighting installations 
contain four or more lamps. Thus, it is desirable to be able to adapt the 
conventional non-dimming ballasts to a dimming configuration. 
OBJECTS OF THE INVENTION 
It is a principle object of the present invention to provide a new and 
improved fluorescent lamp dimming adaptor for modifying conventional 
non-dimming ballasts to a dimming configuration. 
It is another object of the present invention to provide a new and improved 
fluorescent lamp dimming circuit which generates reduced electromagnetic 
interference. 
It is yet another object of the present invention to provide a new and 
improved fluorescent lamp dimming circuit which achieves dimming while 
generating negligible net DC during a full waveform and hence no flicker. 
It is a further object of the present invention to provide a fluorescent 
lamp dimming adaptor which achieves a dimming ratio on the order of 100:1 
and which is field retrofitable with many different types of conventional 
ballasts. 
SUMMARY OF THE INVENTION 
These and other objects of the present invention are achieved by an add-on 
fluorescent lamp dimming adaptor which may be connected to a conventional 
non-dimming ballast for achieving dimming in a fluorescent lighting 
system. The adaptor comprises a switching module coupled to the lamp 
terminals for switching current from the lamp, and a level control coupled 
to the switching module for varying the duty cycle of the current in the 
lamp according to a dimming control signal supplied to the level control. 
The switching module may be connected to the ballast in parallel with the 
lamp, thereby shunting current from the lamps when the switching module 
closes, or may be connected in series with the lamp, thereby switching 
current from the lamp when the switching module opens. 
According to the present invention, the level control may control the 
switching module by providing high frequency switching signals, the timing 
of these signals being referenced to the ac voltage supplied to the 
ballast. Alternatively, the level control may provide pulse width 
modulated (PWM) signals to the switching module from a PWM generator which 
is synchronized by a zero crossing detector to the ac voltage provided to 
the ballast. A sawtooth oscillator operating at a high frequency may also 
be used to provide PWM signals. 
The features and advantages of the present invention will become apparent 
from the following detailed description of the invention when read with 
the accompanying drawings in which applicable reference numerals have been 
carried forward.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIGS. 1 and 2, a dimmable fluorescent lighting system is 
seen to include an AC voltage source 20 (typically a 50 or 60 hertz power 
line) and a conventional rapid-start ballast 21 coupling the AC source to 
fluorescent lamps 27 and 28. By way of example, an 8G1022W ballast 
manufactured by the General Electric Co. is schematically illustrated. It 
comprises an autotransformer 22, a capacitor 25 and a start capacitor 26. 
The filaments of lamps 27 and 28 are heated by power supplied by windings 
closely coupled to the primary of autotransformer 22. Lamp voltage is 
provided by the secondary of autotransformer 22. While a rapid-start 
ballast (characterized by two series lamps and cathode heating) is shown 
in the drawings, the present invention is applicable to other 
conventional, non-dimming ballasts (both with and without filament 
heating) as will become apparent herein. For example, FIG. 1A shows a 
portion of a fluorescent lighting system having a single lamp 27' and 
being connected to the switching module portion 30 of the adaptor of the 
present invention. 
The add-on fluorescent dimming adaptor of the present invention 
controllably varies the average lamp current to achieve brightness 
control. The adaptor is comprised of switching module 30 and a level 
control 31. Switching module 30 is a controllable switch for switching 
current from a lamp. It is possible to connect switching module 30 in 
series with the lamps, in which case lamp current is interrupted by 
opening switching module 30. With switching module 30 connected in 
parallel with the lamps, lamp current is diverted through the parallel 
path formed by closing switching module 30. Level control 31 causes 
switching module 30 to become conductive or non-conductive in a manner 
which dims the lamps. Specifically, a pulse of current from level control 
31 causes switching module 30 to switch current from the lamps for a 
portion of the half cycle of the source voltage. The out-time of the lamp 
current is short enough to avoid deionization of the lamps, but may have a 
variable duration or a variable number of repetitions per half cycle to 
provide variable lamp brightness. 
A first embodiment of the add-on fluorescent lamp dimming adaptor of the 
invention will now be described with reference to FIG. 1. In this 
embodiment, switching module 30 is connected in parallel with 
series-connected lamps 27 and 28 by electrically connecting the output of 
switching module 30 to lamp terminals 23 and 24 of ballast 21 which carry 
current to and from the lamps during an arc discharge. In this way, lamp 
current is controllably diverted from lamps 27 and 28 as switching module 
30 becomes conductive, while normal lamp current flows in lamps 27 and 28 
when switching module 30 is non-conductive. Level control 31, responsive 
to a dimming control signal D.sub.sig, is coupled to switching module 30 
for controlling the conduction state of switching module 30. Several 
schemes for providing appropriate switching signals from level control 31 
to switching module 30 will be discussed later. 
An embodiment of switching module 30 specially adapted to operate in 
parallel with fluorescent lamps 27 and 28 is shown in FIG. 3. A gate 
controlled semiconductor switch or insulated gate transistor (IGT) 34 
connected across the output terminals of diode bridge rectifier 33 
provides switching module 30 with a conducting state or a non-conducting 
state depending on the gate voltage of semiconductor switch 34. Switch 34 
is shown to comprise an IGT, although other devices may be used, e.g. 
power field effect transistors (power FETs) and gate turn-off thyristors 
(GTOs). The IGT is preferred since its somewhat slower switching time 
results in lower EMI and lower voltage spike generation. 
A gate circuit connected to semiconductor switch 34 comprises a resistor 
40, a blocking diode 41 and a zener diode 42 connected in series between 
the collector and emitter of semiconductor switch 34. A capactitor 43 is 
connected between the gate of semiconductor switch 34 and the junction of 
diode 41 and zener diode 42. A phototransistor 44 has its collector 
connected to the junction of diode 41 and zener diode 42 and its emitter 
connected to the emitter of semiconductor switch 34. The zener diode holds 
a substantially constant voltage level across phototransistor 44 when the 
phototransistor is nonconductive. The base of phototransistor 44 is 
photocoupled to the output of level control 31, shown in FIG. 1. R.sub.g 
represents the leakage resistance of semiconductor switch 34 and may also 
comprise an additional resistor, if desired. 
The embodiment of switching module 30 shown in FIG. 3 is designed such that 
IGT 34 is nonconductive in the long term absence of high frequency 
signals, in optical form, from level control 31 of FIGS. 1 and 2. Thus, a 
failure of level control 31 does not result in a permanent short-circuit 
across lamps 27 and 28. When relatively high frequency switching signals, 
in optical form, from level control 31 are present, IGT 34 is operated on 
an AC basis since capacitor 43 has a value higher than the effective gate 
capacitance. When a switching signal, in optical form, is provided by 
level control 31 in order to dim lamps 27 and 28 shown in FIG. 1, the 
junction between diode 41 and zener diode 42 is grounded by 
phototransistor 44, which turns on. The gate of IGT 34 is pulled down and 
IGT 34 does not conduct. When phototransistor 44 turns off, charge is 
supplied to the gate of IGT 34 by capacitor 43 turning IGT 34 on. If the 
signal from level control 31 remains high or low (due to some malfunction) 
or if phototransistor 44 shorts out between its collector and emitter, 
then any charge of the gate IGT 34 will eventually leak off through 
R.sub.g. This drops the IGT gate-to-emitter voltage to zero, turning off 
IGT 34 and causing lamps 27 and 28 to operate at full brightness. 
As shown by FIG. 2, switching module 30' may alternatively be connected in 
series with lamps 27 and 28. Where the fluorescent lighting system being 
modified includes filament heating, a transformer 29 is used so that full 
filament power is continuously supplied regardless of the conduction state 
of switching module 30'. Although transformer 29 is shown connected 
between lamps 27 and 28, it will be understood that transformer 29 may be 
connected to any filament. 
In the series-connected embodiment of the fluorescent dimming adaptor shown 
in FIG. 2, switching module 30' bridges transformer 29 so that lamp 
current may be controllably interrupted without affecting filament power. 
For a fluorescent lighting system having lamps without filaments, 
transformer 29 is not needed and switching module 30' is connected in 
series with the lamps by insertion into the ballast circuit in a 
convenient location. The input to switching module 30' is coupled to level 
control 31 and is controlled according to the switching schemes which are 
discussed below. 
FIG. 4 shows switching module 30' having a gate circuit which specially 
adapts switching module 30' shown in FIG. 2, to series operation, wherein 
diode 41 and capacitor 43 have been replaced by direct connections, zener 
diode 42 has been removed from the circuit, and resistor 45 has been 
added. When there is no optical signal from level control 31, lamp current 
will turn on IGT 34 because resistors 40 and 45 form a voltage divider 
providing sufficient gate voltage to turn on IGT 34. When an optical 
signal is received from level control 31, phototransistor 44 will ground 
the gate of IGT 34 to the emitter of IGT 34. This turns off IGT 34 and 
lamp current as well. 
FIGS. 5-7 show level control circuitry for effecting several different 
switching schemes for switching module 30 of FIGS. 1-4, any embodiment of 
which may be used in either the series or the parallel connection of 
switching module 30 or 30' in the circuits of FIG. 1 or 2, respectively. 
Level control 31 must switch fast enough so that lamps 27 and 28 do not 
have time to de-ionize between conduction periods, thus avoiding 
re-ignition problems, e.g. high voltage spikes. Furthermore, by switching 
at a frequency substantially higher than the AC line frequency, one may 
avoid the resonant frequency of the ballast. A high switching frequency is 
also desirable because the conventional rapid-start ballast acts like a 
low-pass filter, preventing EMI-producing voltage transients from reaching 
the AC line. Transients are further controlled by the grounding of the 
metal fixture of the lighting installation. Preferably, level control 31 
causes switching module 30 to switch at a rate of 300 to 3000 cycles per 
second and higher. In addition, level control 31 should be synchronized to 
the AC line so that there is negligible net DC in lamps 27 and 28 during 
each full waveform of the AC line voltage to avoid the appearance of 
flicker resulting from different light production on one half cycle of 
line voltage then on the next half cycle. 
A first embodiment of level control 31 comprises a zero crossing detector 
50 shown in FIG. 5A driving a PWM generator 70 shown in FIG. 5B. Level 
control 31 generates pulse width modulated signals for controlling 
switching module 30 or 30' to switch current from lamps 27 and 28. FIGS. 
8A and 8B illustrate the output signals of level control 31, shown in 
FIGS. 5A and 5B, for a lesser and a greater amount of dimming, 
respectively. In both instances, a PWM output signal 142 in the form of 
optical pulses is shown to be synchronized with AC voltage 141 from AC 
source 20. 
Zero crossing detector 50 is coupled to AC source 20 by a voltage sensing 
transformer 51 with a center-tapped secondary. The center-tap is connected 
to ground. Each end of the transformer secondary is connected to a 
separate input, respectively, of a separate comparator circuit, 
respectively, for detecting when each rectified half-wave of the source 
voltage exceeds one diode drop (across diodes 58a and 58b, respectively). 
A comparator 52 has its non-inverting input coupled to one end 51a of the 
secondary of transformer 51 through a series-connected diode 54a and 
resistor 55a and coupled to the center-tap of the secondary through a 
resistor 56a. The inverting input of comparator 52 is coupled to a source 
of constant DC voltage +V.sub.DC through a resistor 57a and coupled to 
the center-tap of the secondary through a diode 58a. DC voltage +V.sub.DC 
may be supplied in any convenient way, e.g. the regulated output of a 
rectifier connected to ac source 20. A pullup resistor 59a is coupled 
between the output of comparator 52 and DC voltage +V.sub.DC. The output 
of comparator 52 is also coupled to the base of a transistor 60 through a 
series-connected diode 62 and resistor 64. The noninverting and inverting 
inputs of a comparator 53 are coupled to the other end 51b of the 
secondary of transformer 51 and to DC voltage +V.sub.DC, respectively, by 
circuitry identical to that described for comparator 52. The output of 
comparator 53 is connected to pullup resistor 59b and is coupled to the 
base of transistor 60 through a series-connected diode 63 and resistor 64. 
Thus, diodes 62 and 63 constitute input circuitry, and resistor 64 the 
output circuitry, of an OR gate. The collector of transistor 60 is coupled 
to +V.sub.DC through a resistor 61 and is coupled to PWM generator 70 
through an output 65. The emitter of transistor 60 is connected to ground 
and to the transformer center-tap. 
When an AC voltage is supplied to voltage sensing transformer 51, 
comparators 52 and 53 will each sense voltages at their noninverting 
inputs greater than the voltage drops across diodes 58a and 58b, 
respectively, during each half cycle of opposite polarity, respectively. A 
high output from either comparator 52 or 53 turns on transistor 60, 
shorting output 65 to ground. When there is no signal from either 
comparator 52 or 53, i.e. in the vicinity of the zero crossings of AC 
source 20, output 65 goes high. The resulting pulses on output 65 are 
provided to PWM generator 70. 
PWM generator 70 may comprise a commercially available integrated circuit. 
A pulse width modulation control circuit TL494C manufactured by Texas 
Instruments Incorporated is shown in FIG. 5B as pulse width modulation 
control circuit chip 69. All the circuit elements external to TL494C chip 
69 and some of the internal circuitry of chip 69 are shown in FIG. 5B. Pin 
12 of chip 69 is provided with a positive DC voltage +V.sub.cc. Pin 7 is 
grounded. 
The pulse repetition rate of PWM generator 70 is determined by the values 
of a capacitor 71 and a resistor 72 connected to the oscillator between 
pins 5 and 6 of chip 69. As previously discussed, these values should be 
selected to provide from 300 to 3000 pulses per second and higher from the 
oscillator. The oscillator in chip 69 is synchronized to AC source 20 by 
stopping the oscillator and restarting it with each pulse received from 
output 65 of zero crossing detector 50. This is done by shunting capacitor 
71 with a series-connected diode 74 and FET 73. The gate of FET 73 is 
coupled to output 65 of the zero crossing detector shown in FIG. 5A. By 
synchronizing PWM generator 70 to AC source 20 in this way, only 
negligible net DC may be generated across lamps 27 and 28, shown in FIGS. 
1 and 2, during any full cycle of AC source 20. 
The duty cycle of PWM generator 70 is determined by an error amplifier 83 
in chip 69. A reference voltage V.sub.ref is supplied by pin 14 to a 
dimming control 80. Potentiometer 81 and resistor 82 connected to the 
potentiometer tap supply a dimming control signal D.sub.sig to pin 1 of 
chip 69, connected to the non-inverting input of error amplifier 83. A 
second voltage, less than V.sub.ref, is provided to pin 2, connected to 
the inverting input of the error amplifier, through a resistance 75 from a 
voltage divider comprised of potentiometer 81 and a resistor 76. A 
feedback resistor 77 is connected between pin 2 and pin 3 for providing a 
constant gain for error amplifier 83. The duty cycle of PWM generator 70 
is thus controlled by varying the output of potentiometer 81. 
The PWM output of chip 69 internally operates an output transistor 84 with 
its collector connected to pin 11 and its emitter connected to pin 10. A 
resistor 78 and a light-emitting diode (LED) 79 are connected in series 
between +V.sub.cc and pin 11. Pin 10 is grounded. Thus, the PWM output 
modulates LED 79 which is optocoupled to switching module 30. 
A second embodiment of level control 31 produces pulses 142 of fixed width 
and frequency, as shown in FIGS. 9A and 9B, but for a controllably 
variable time (or chopping interval) centered in each half wave 141 of th 
AC power source. The pulses of FIG. 9B provide more dimming than the 
pulses of FIG. 9A since they are generated during longer intervals and 
thus result in a lower duty cycle of the current in the lamp. 
A circuit for level control 31 is shown in FIG. 6. Level control 31 is 
supplied from AC source 20 through an isolation transformer 90. Full-wave 
rectified voltage is provided at the output of diode bridge rectifier 100. 
A DC bus 101 is coupled to rectifier 100 through resistor 102 and diode 
103 in series. The DC bus voltage is smoothed by a filter capacitor 104. A 
parallel RC transient suppression circuit 105 limits voltage spikes 
appearing in level control 31. 
A dimming control 95 receives a DC reference voltage through a resistor 
106. Series-connected resistor 107 and zener diode 108 are connected 
across, and thereby tend to regulate voltage on, dimming control 95, thus 
providing dimming control 95 with partial compensation for line 
fluctuations. Dimming control 95 comprises a resistor 97 and a 
potentiometer 96 connected in series and energized from resistor 106. The 
tap of potentiometer 96 provides a variable DC output signal D.sub.sig. 
Potentiometer 96 may be replaced by a fixed voltage divider and a switch 
providing discrete levels of dimming. Small decoupling capacitors 98 and 
99 increase noise immunity. 
The tap of potentiometer 96 is coupled to the inverting input of a 
comparator 91. Full-wave rectified signals from a voltage divider 
comprised of series-connected resistors 110a and 110b connected across 
rectifier 100 are provided to the non-inverting input of comparator 91 
through resistor 111. A diode 109a provides a discharge path for noise 
filter 109b. Square wave signals are provided at the output of comparator 
91 which are high when the full-wave rectified voltage produced by 
rectifier 100 and sensed through resistors 100 and 111 is greater than 
D.sub.sig. Thus, the width of the high portions of square waves provided 
by comparator 91 may be varied under control of dimming control 95. 
The output signal of comparator 91 is supplied to the inverting input of an 
operational amplifier 92. Operational amplifier 92 is connected, in 
conventional fashion, to act as an oscillator when the output of 
comparator 91 is high. Thus, a timing capacitor 112, resistors 113 and 114 
and a diode 115 control the switching frequency of operational amplifier 
92, which is selected to produce pulses at a frequency of 300 to 3000 
pulses per second or higher. Thus, high frequency pulses are provided at 
the output of operational amplifier 92 during periods that D.sub.sig is 
lower than the full-wave rectified voltage provided to the non-inverting 
input of comparator 91, i.e. symmetrically around the center of each 
half-wave. If D.sub.sig is greater than this full-wave rectified voltage 
then no high frequency pulses are provided by operational amplifier 92. 
The non-inverting input of an operational amplifier 93 is coupled to the 
output of operational amplifier 92 through diode 117 and to bus 101 
through a resistor 121. The inverting input of operational amplifier 93 is 
connected to a DC voltage equal to the DC voltage on bus 101 reduced by a 
voltage divider comprising resistors 118 and 119. When the output of 
operational amplifier 92 goes low, diode 117 is forward biased and 
provides a low input voltage to the noninverting input of operational 
amplifier 93. A high output from operational amplifier 92 causes a high 
input voltage to operational amplifier 93. Thus, operational amplifier 93 
isolates operational amplifier 92 and provides increased current, high 
frequency pulses during the center portion of each half-wave of voltage 
provided by rectifier 100 (but only when the voltage from rectifier 100 is 
greater than D.sub.sig) and provides a low output signal otherwise. 
An LED 120 is connected between the output of operational amplifier 93 and 
ground. High frequency switching signals are provided to switching module 
30 (FIGS. 3 and 4) through the optocoupling of LED 120 and phototransistor 
44. 
A further embodiment of level control 31 will now be described with 
reference to FIG. 7. FIG. 7 shows a simplified method for providing PWM 
signals which, however, are not synchronized to AC source 20. A diode 
rectifier 134 is coupled to AC source 20 through an isolation transformer 
135. Rectifier 134 supplies a sawtooth oscillator 130 which operates at a 
frequency greater than about 2000 hertz. The output of sawtooth oscillator 
130 is supplied to the non-inverting input of a comparator 131. A 
potentiometer 132 is connected between a constant DC voltage +V and 
ground. The output of potentiometer 132 provides D.sub.sig to the 
inverting input of comparator 131. The output of comparator 131 provides 
PWM signals to an LED 133 which are optocoupled to switching module 30 of 
FIGS. 3 and 4. It is possible to operate the circuit shown in FIG. 7 
without synchronization with AC source 20 because of the high frequency of 
sawtooth oscillator 130 which avoids the generation of any significant net 
DC voltage across the lamps over any full cycle of AC source 20. 
With any of the above described embodiments of level control 31 it is 
possible to control the dimming of more than just the lamps connected to 
one ballast. A plurality of switching modules can be connected to a like 
plurality of conventional ballasts and can be optocoupled to a single 
level control by providing a plurality of LEDs (either in series or in 
parallel) in the level control. 
From the foregoing, it is apparent that the invention achieves the objects 
of a wide range of dimming and low electromagnetic interference generation 
in a dimming adaptor for modifying conventional non-dimming ballasts. The 
inductance of the ballast prevents large current surges from being 
transferred to the AC line. Further, in the rapid-start ballast, the 
starting capacitor provides over-voltage control. In the parallel 
arrangement of the switching module, voltage transients are limited to 
about 200 volts because the lamps will undergo an arc discharge and 
ignite. Thus, the low EMI generated by the dimming adaptor of the present 
invention will allow lamp dimming without affecting the operation of 
nearby electronic equipment. 
While preferred embodiments of the present invention have been shown and 
described herein, it will be obvious that such embodiments are provided by 
way of example only. Numerous variations, changes, departures, 
substitutions and partial and full equivalents will now occur to those 
skilled in the art without departing from the invention herein. 
Accordingly, it is intended that the invention be limited by the spirit 
and scope of the appended claims.