CA1052440A - Fluorescent lamp dimming circuit employing an improved auxiliary circuit - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An adjustable dimming circuit is provided to control light output levels of a fluorescent lamp. The dimming circuit comprises an intensity selector circuit controlling an auxiliary circuit, and a ballast circuit which is con-trolled by the auxiliary circuit. The intensity selector circuit comprises a control switch and a power switch. The control switch is gated on by the intensity selector circuit and controls the operation of the power switch by providing gate current for the power switch after the control switch comes on until zero crossover current in the control switch.
The ballast circuit is used to limit current to the fluor-escent lamp and to provide starting pulses for the lamp. The power switch in the auxiliary circuit controls the timing of the application of the starting pulses applied to the lamp, and also provides a current path during the operation of the lamp.

Description

_~ 58-BD-6208 This invention relates to an improved dimming circuit for fluorescent lamps, and more particularly relates to an improved auxiliary circuit for a dimming system which is useful at all light levels.
In general, dimming circuits for fluorescent lamps utilize an auxiliary circuit in the form of a power switch which is controlled by an intensity selector. The power switch, which is usually a thyristor, supplies current to the lamps at various illumination or dimming levels by controlling the interval of current conduction through the lamps in each half cycle of the power supply. The intensity selector which controls the thyristor is made variable to provide continuous adjustment of illumination.
One of the major problems in the prior art dimming system has been flicker. Flicker is an instability or variability of light level and is very annoying to the viewer.
Flicker may occur at the higher or at the lower light levels.
Flicker can occur at high intensity light levels if a lamp is turned on during one half cycle of conduction much earlier or later than the next half cycle of conduction.
Under such circumstances, the amount of light produced varies ~rom one half cycle to the next.
Flicker can occur at low intensity light levels -if a timing circuit is unregulated. Also, variation in a component, such as a unijunction transistor, or fluctuation of line voltage can cause flicker to occur at low intensity light levels.
These types of flicker are somewhat overcome in the prior art by the use of high trim and low trim potentio-meters in conjunction with a master potentiometer, all of which affect the frequency of a controlling oscillator. The -~

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trim potentiometers could be used to adjust out the flicker by firing the thyristor after zero lamp current. This method of initial adjustment of the trim potentiometers is often misunderstood, however.
Another problem with prior art dimming circuits has been that the thyristor or auxiliary circuit required the use of a high wattage current-holding resistor to maintain conduction at low dimming levels. This resistor usually was connected in series with the thyristor, and was placed in the ballast circuit where it dissipated as much as five watts.
This resistor thus caused a power loss and substantial heating in the ballast, both of which are undesirable. `~
In some prior art dimming systems which used a control switch to control the power switch, another high wattage resistor was used as a voltage-dropping resistor so that full line voltage was not across the control switch. This resistor also caused heating in the dimming system.
It is desirable, therefore, to provide a circuit capable of dimming fluorescent lamps which does not require trim potentiometers to overcome flicker. It is further desirable to eliminate the need for a high wattage current-holding resistor and the need for a high wattage voltage-dropping resistor.
Accordingly, it is a general object of this in-vention to provide an improved dimming circuit Eor fluores-cent lamps.
Another object of the invention is to provide an ;~
improved dimming circuit which eliminates flicker in fluores-cent lamps at all light levels.

Another object of the invention is to provide such a dimming circuit which eliminates the need for a high wat-tage current~holding resistor for maintaining conduction of the power switch in the auxiliary circuit.
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-` 58-BD-6208 Another object is to eliminate the need for a high wattage voltage-dropping resistor for operating a control switch.
Still another object of the invention is to provide a dimming circuit which eliminates the need for trim potentio-meters in the intensity selector circuit.
In accordance with one form of this invention there is provided a gaseous discharge lamp dimming circuit includ-ing a bilateral current conducting control switch having a control electrode, and a step-down transformer connected between the control switch and a source of A.C.voltage. A
gating signal is provided to turn on the control switch and the control switch will conduct until its current goes to zero crossover. A bilateral current conducting power switch having its own control electrode is further provided. The power switch control electrode is connected to the control switch and is continually gated by the control switch even when the current in the power switch falls below the holding current level.
Because the power switch is continually gated to its "on" condition, even if the lamp current goes through zero, the need for a high trim potentiometer in series with the master potentiometer in the intensity selector circuit and the need for a high wattage holding resistor for the power switch are eliminated. Furthermore, the need for a high wattage voltage-dropping resistor, which connected the control switch to the A.C. source, is eliminated by the inclusion of the step-down transformer.
The subject matter which is regarded as the invention is set forth in the appended claims. The invention itself, however, together with further objects and advantages thereof, can be better understood by referring to the ~ollowing _ 3 -~... . . . :
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description taken in conjunction with the accompanying drawings in which:
FIG. 1 is a schematic circuit diagram of one embodiment of the dimming circuit of the invention, including an intensity selector circuit, an auxiliary circuit, and a ballast circuit.
; FIG. 2 is a diagram of the waveforms of a prior art dimming system.
For an explanation of high intensity flicker in a prior art dimming system, reference is made to FIG.2. In FIG.2 are shown waveforms of input voltage, lamp current and timing pulses for a prior art dimming system operating near full intensity. A timing pulse gates the thyristor on ;
at point "a" in the lamp current half cycle, and the thyristor ;` latches on until the lamp current goes to zero. It is desir- `
; able that a timing pulse gate on the thyristor as early in the negative half cycle as occurred in the positive half cycle, i.e. at point "a", in order to operate at nearly full in-tensity. However, because the timing circuit operates from - 20 leading input voltage, the timing pulses do not occur at identical times in the half cycles. FIG.2 shows a timing pulse occurring at point "c", which was meant to occur after the beginning of the negative half cycle. This pulse does not turn on the thyristor in the negative half cycle, and as soon as the current waveform reaches zero, the thyristor turns off. The thyristor does not come on again until another timing pulse starts it at point "d". The lamp thus conducts for a longer time in the positive half cycle and a shorter time in the negative half cycle, such unbalanced operation producing flicker. This type of flicker does not usually occur at low intensity levels, because the timing pulses are i not applied near zero current crossover.

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Referring now to FIG. 1, the dimming circuit 10 of the invention comprises an intensity selector circuit 11, an auxiliary circuit 12 and a ballast circuit 13. The intensity selector circuit controls the auxiliary circuit. The auxiliary circuit, to which this invention is primarily directed, controls the ballast. The ballast controls the starting of a gaseous discharge lamp which, in this embodiment, is a fluorescent lamp and it also controls the current through the lamp.
The intensity selector circuit 11 utilizes a timing circuit to initiate lamp dimming. This timing circuit includes a programmable unijunction transistor or PUT Ql having anode, cathode, and gate electrodes. A capacitor Cl is connected to the anode of PUT Ql and provides anode-to-cathode current to PUT Ql A charging path for capacitor Cl is provided by a series circuit comprising resistors Rl and R~ and diodes D
and D2. A transformer Tl is connected to diodes Dl and D2 and to an A.C. power supply to provide power for the intensity selector circuit. To provide voltage regulation for the in-tensity selector circuit/ a Zener diode D3 is connected across a series circuit of R2 and PUT Ql Resistors R4~ R5 and Rll form a voltage divider. The gate of the PUT is connected to the junction of the divider between resistor R4 and parallel resistors R5 and Rll. The parallel circuit includes variable resistor Rll which is known as the master potentiometer, since it controls the frequency of the timing circuit. The frequency of the timing circuit ultimately controls the dimming level of lamp L. To this end an SCR Q2' which has its gate electrode con-nected to the cathode of PUT Ql' forms the output of the inten-sity selector circuit and is connected to the auxiliary circuit 12.
The illustrative intensity selector circuit 11, as described above, is somewhat simplified for the sake of ease of understanding. A more detailed description of various embodi-ments of an intensity selector circuit useful in the present dirnming circuit is provided in U.S.Patent No.3,767,940 - dated : . : . . . .
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October 23, 1973 - Herzog, et al and assigned to the General Electric Company, assignee of the present invention.
The auxiliary circuit 12 is connected to the out-put of the intensity selector circuit. The auxiliary circuit includes a series circuit consisting of resistor R6, capacitor C2, and primary winding 16 of transformer T3. Resistor R6 ;~
and winding 16 provide a charge path for capacitor C2 from Zener diode D4. Primary winding 16 also provides a discharge path for capacitor C2 when SCR Q2 is turned on. The secondary winding 15 of transformer T3 is connected to bilateral current conducting control switch 14. Resistor Rg is connected across a secondary winding 15 to eliminate false gating o~ the con-trol switch. The control switch comprises a diode bridge D5 and an SCR Q3, which is connected across the output of the diode bridge. This switch arrangement provides full wave alternating current through itself.
Power for the control switch is obtained from step down transformer T2 having a primary winding 17 and a secondàry winding 18. In some prior art circuits a high wattage volt-age dropping resistor was used to supply power to the controlswitch at a voltage which was near or below the rating of the control switch. By providing a step down transformer, this resistor is no longer necessary.
The control switch 14 acts as a latch. That is, once it is gated on by the voltage induced by secondary 15, it ;
stays on until its zero current crossover. The output of -the control switch is connected to the gate electrode of power triac Q4~ Capacitor C4 and resistor R7 are connected across power triac Q4 in order to overcome dv/dt which may cause false triggering of the power triac. The power triac serves to switch current through the ballast circuit 13 and the fluorescent lamp L. Since the control switch has its output connected to the gate of power triac Q4 and serves as a latch, .. .

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there is continual gating current for triac Q4 while control switch 14 conducts. Even if the auxiliary 12 is pulsed by the timing circuit before zero lamp current, triac Q4 will stay on after zero lamp current because of this continual `~
gating. Furthermore r this continual gating holds triac Q4 on even at low lamp current levels. This obviates the need for a high wattage current holding resistor for the power triac during low int~nsity levels and furthermore, obviates the need for a high trim potentiometer formerly required in the intensity circuit during operation at high intensity levels.
The ballast circuit includes primary coils 19 and 20 of transformer T4; coils 19 and 20 receive power from a 277 volt supply. When the ballast is operating at 120 volts, the coil 19 alone is used as a primary. Capacitor C5 and secondary coil 22 form a peaking circuit which provides a voltage spike to turn on lamp L. Resistor R8 is connected to secondary coil 22 and provides a charge path for capacitor C5.
The peaking circuit is further connected to power triac Q4 in the auxiliary circuit. Coil 22 and power triac Q4 provide a discharge path for capacitor C5 when power triac Q4 comes on. --Secondary coils 24 and 25 are magnetically coupled to theprimary coils 19, 20 and provide heating for the cathodes of the fluorescent lamp. Capacitor C6 is connected across primary - ~
19, 20 to provide power factor correction. Resistor R8, which -is a low wattage resistor dissipating one-half watt maximum, is connected in series with power triac Q4 to provide stabil-ization during the discharge time of capacitor C5. In the prior art a high wattage current holding resistor was used in place of resistor R8. This caused a power loss of up to five watts and heating in the ballast. By continually gating power triac Q4, a high wattage resistor is no longer needed.
A more detailed description of the ballast circuit ''~ '' ' ` '; ' ' ' ' ,' , . :
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is provided in my Canadian Application Serial No.203,937 filed July 3, 1974 and assigned to the ~eneral Eleetric Company.
The overall operation of the dimming eireuit is as follows: Variable resistor Rll, known as the "master potentiometer", is adjusted to provide the light level or dimming level desired by controlling the gate voltage of PUT
Ql Capaeitor Cl, having eharged through resistors Rl and R2, is discharged through PUT Ql when the PUT anode voltage exceeds its gate voltage. The discharge of capaeitor Cl turns on SCR Q2 Capacitor C2 is charged through diode D4 in series with resistor R6 and primary winding 16, and is discharged through primary winding 16 when SCR Q2 comes on. This causes ~-a pulse to ~e supplied to secondary winding 15, ~rning on SCR
Q3, which forms a part of control switch 14. A full wave current path to the control switch 14 is established through resistor R12, secondary winding 18 and ~he gate~anode path of power triac Q4. When SCR Q3 turns on, control switch 14 latches on until the next"eurrent zero" through the eontrol switeh occurs. Control switeh 14 gates power triac Q4 to its "on" condition. The gating on of the triac is independent of the lamp current, which lags the supply voltage. Capacitor C5,having been eharged through resistor R8 and secondary eoil 22, is discharged through seeondary coil 22 and power triae Q4 when power triac Q4 comes on. This discharge of capaeitor C5 through secondary coil 22 provides a peaking voltage to turn on the lamp L by inducing a voltage spi~e in secondary coils 21 and 23. A current path is pro~ided from one side L2 f the A.C. source, through primary coil 20, secondary eoil 23, lamp L,secondary coil 21 and power triac Q4 to the other side ~ -Ll of the A.C. souree.

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`.b ' f In prior dimmi~g circuits it was necess~ry to use a high trim potentiometer in s~ries with the ma~ter potent-iometer in the intensi~y selector circuit in order to pr~vent flicker at the high intensity output lev~ls. Flicker occurred becau~e the power triac was turned on before zero lamp curre~t.
A high trim potentiometer was used to change the timing and thus ~ire ths triac after zero lamp current. By using applicant's method of continuously gating the triacr the necessity of the high trim potentiometer at high intensity 13 levels i~ obviated. Another result of continuously gating the power triac is to overcome the ne~d o~ the high wattage -current holding re~istor in series with the triac. The hi~h wattage, voltage dropping resistor for the control switch has also been el~minated by the step down tran~former. ' :~
The circuit a~ set forth in FIG. 1 has been built ~ -and operated with components having the ~ollowing values:
Resistor R1 - 1 K

R3 ~ 1 K
R4 ~ 15 K ~ .
~5 - 10 K
R6 -100 K .
R7 -100 ohms ~8 -220 K -:
Rg - 1 K
Rlo _ 2 ~eg.
Rll - 10 K pot R12 -100 ohms R13 -6B0 ohms R14 4.7 K
Diode Dl - 400 V, 1 A
D2 ~ 400 V, 1 A
D3 _ 10 V, Zener, 100 ,~
D4 _ 400 V, 1 A : -D5 - Full Wave Br~dge with 4 Diodes-400 V DC~ 1 A DC, 280 V RMS
PUT Ql - 2~6027 SGR Q2 2~4184 Q3 - C106Ba - G.E. ~o.

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TRIAC Q4 _ 500 V, 15 amp 85&
Transformer T
primary winding - 1690 turns, .0063 :-in. dia. secondary winding-2 windings (420 turns, .0063 in. dia.) Transfor~er T2 primary winding 17 - 1690 turns, .0063 in dia.
s~condary winding 18 - 132 turns, .010 in. dia.
Transformer ~3 primary winding 15 ~ 1000 turns, .0045 in. dia. :~
secondary winding 16 - 1000 turns ~0045 in. dia, Transformer T4 primary winding 19 - 1048 turns, .0071 in. dia.
primary winding 20 - 1241 turns, .010 in~ dia. ` :
~econdary winding 21 - 598 turns, .0119 in. diaO :
secondary winding 22 - 358 turns -`
.0071 in. dia. ::
secondary winding 23 ~ 373 turns, .0119 in. dia. ~ .
heating wi~ding 24 - 42 turn~, .010 in. dia.
heating winding 25 42 turns, .010 in. dia.
Capacitor C1 _ .047 uf C2 - .01 u~
C3 _ 70~7 U~
C4 _ .05 u :~
Cs _ .01 u~
C6 - 1.9 u~ , C7 _ .01 u~
C8 - .15 uf ~rom the foregoing description o~ the 0~b~iment of thQ invention, it will be apparent that many modifications may be made therein. It will be understood that this embod-iment of the invention is intended as an exemplification of the invention only, and that the invention is not limited thereto. Por example, i~ th~ auxiliary circuit shown, control switch 14 compri~es an SCR connectad across a full wave diode bridge. Other full wave latching devices may be employed as -10~ , "

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a control switch, such as back-to-back SCR'~ or a triac.
Also in the auxiliary circuit, a triac i3 disclosed as the power switch. Other power switches, such as a bilateral tran~istor, may also be e~ployed. It is to be understood, therefore, that it is intended in the appended claims to cover all such modifications which fall within the true spirit and scope of the invention.

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Claims (6)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows

1. A gaseous discharge lamp dimming circuit for connection to a source of AC voltage, comprising:
a bilateral current-conductive control switch having a control electrode;
means for connecting said control switch to said source of AC voltage;
timing means for applying a signal to said control electrode of said control switch to render said control switch conductive, said control switch being adapted to continue conducting current thereafter until the control switch current goes to zero crossover;
a bilateral current-conductive power switch having a control electrode;
said control switch being connected to said control electrode of said power switch for continuously gating on said power switch during conduction of said control switch to cause said power switch to remain on even if current through said power switch is below a holding current level; and a ballast circuit connected to said power switch for limiting lamp current.

2. The dimming circuit as set forth in claim 1, wherein said power switch comprises a thyristor.

3. The dimming circuit as set forth in claim 1, wherein said power switch comprises a triac.

4. The dimming circuit as set forth in claim 1, wherein said control switch comprises a full-wave rectifier bridge and a thyristor connected across said bridge.

5. The dimming circuit as set forth in claim 1, further comprising an isolation transformer for coupling said timing means to said control electrode of said control switch.

6. The dimming circuit as set forth in claim 1, wherein said means for connecting said control switch to said source of AC voltage comprises a step-down transformer.