GB1594429A

GB1594429A - Ballast system for fluorescent tube

Info

Publication number: GB1594429A
Application number: GB1863478A
Authority: GB
Original assignee: Honeywell Ltd Canada
Current assignee: Honeywell Ltd Canada
Priority date: 1978-05-10
Filing date: 1978-05-10
Publication date: 1981-07-30

Description

(54) BALLAST SYSTEM FOR FLUORESCENT TUBE (71) We, HONEYWELL LIMITED, a Canadian Company of 740 Ellesmere Road, Scarborough, Ontario, Canada, do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to a ballast system for fluorescent tubes.

Typical fluorescent tubes comprise a sealed cylinder of glass having a heating filament at either end and filled with a gas such as mercury vapor. The filaments are heated by a supplied voltage to cause thermionic emission so that an arc can be struck across the tube causing the gas to radiate. The radiation given off by gases such as mercury is short-wave ultraviolet radiation and thus produces little visible light. In-order to provide visible light, the inside of the tube is coated with a suitable phosphor which is activated by the ultraviolet radiation and emits visible light of a color that is characteristic of the particularly phosphor or mixture of phosphors employed to coat the tube.In order to sustain the arc across the tube, the filament voltage must be maintained to a predetermined level which poses a problem when the fluorescent tube is to be used in a light dimming arrangement.

As one answer to this problem, the prior art developed a three-wire system where the filament voltage was separated from the variable current used to dim the fluorescent tube. In such a system, a first wire was used to supply variable current to the fluorescent tube. A second wire was used to supply constant filament voltage to the filaments of the tube. The third wire was used as a return. From a cost of installation standpoint, a two-wire arrangement for dimming fluorescent tubes is preferable.

The prior art has developed two-wire fluorescent tube dimming arrangements but these provide only a limited dimming capability. The problem with the prior art two-wire dimming arrangements is that the filament voltage to the tube is varied as the dimming current supply to the fluorescent tube is varied. Thus, as the current supplied to the fluorescent tube is reduced to dim the tube, the filament voltage is also reduced resulting in stripping the emission coating from the cathode, and when the filament voltage has been reduced below the level to sustain the arc across the tube, the tube will extinguish.

According to the invention there is provided a ballast system for a fluorescent tube the system comprising first and second terminals for connection to a source of variable power; an inductive coil having one end connected to said first terminal and the other end for connection to one side of the fluorescent tube; means for connecting the other side of the fluorescent tube to said second terminal; and means for supplying a voltage to the filaments of the tube, said voltage supply means being connected to an intermediate tap on the inductive coil, the tap being positioned so that the voltage supply to the filaments is maintained substantially constant even when the source power is reduced for dimming the tube.

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 shows a three-wire prior art arrangement for dimming a fluorescent tube; Figure 2 shows a prior art two-wire arrangement for dimming a fluorescent tube; Figure 3 shows the voltage vs. the controlled current through the tube for each side of the choke coild of Figure 2 and for the tap on the choke coil of Figure 4; Figure 4 shows a two-wire ballast system according to the invention; Figure 5 shows a ballast system of the present invention for supplying two fluorcscent tubes; Figure 6 shows an alternative ballast system according to the present invention for supplying two tubes, and Figure 7 is a further variation of a ballast system according to the invention.

The fluorescent dimming arrangement 10 shown in Figure 1 is a three-wire system comprising wires 11, 12 and 13 connected to respective input terminals 14, 15 and 16.

First wire 11 connects terminnl 14 to choke coil 17 the other side of which is connected to fluorescent tube 18 by line 19. Line 20 connects the other side of tube 18 to terminal I6. Connected across terminals 15 and 16 is primary winding 21 of filament transformer 22. Terminal 14 supplies variable power or current to fluorescent tube 18 for dimming, and transformer 22 connected to terminals 15 and 16 supplies constant filament voltage to fluorescent tube 18.

Thus, secondaries 23 and 24 of transformer 22 are connected to respective filaments within fluorescent tube 18. The starting stripe 25 which is accomplished by the fixture is associated with fluorescent tube 18 and is connected to ground terminal 26 by line 27.

Since the filaments of the fluorescent tube must be maintained at their specified voltage to keep them at their temperature and to sustain an arc across the tube, the prior art recognized that a constant voltage source for the filaments of tube 18 was desirable to increase the range over which it could be dimmed as controlled by the variable power or current supplied by terminal 14. To this end, the prior art arrangement as shown in Figure 1 comprised a separate filament voltage source (connected to terminals 15 and 16), not associated with the variable supply terminal 14, for supplying constant voltage to the tubes' filaments. However, this system is a three-wire system and the prior art has also recognized that a two-wire system would reduce the cost and complexity of installation. Thus, the prior art developed the type of system shown in Figure 2.

In Figure 2, first wire 30 connects variable current or power source terminal 31 to choke coil 32 the other side of which is connected by line 33 to fluorescent tube 34.

The other side of tube 34 is connected by line 35 and by second wire 37 to second supply terminal 36. Connected between first wire 30 and second wire 37 is primary winding 38 of filament supply transformer 39. Transformer 39 has secondary winding 40 connected to one filament of tube 34 and second secondary winding 41 connected to the other filament of tube 34. The starting stripe 42 associated with tube 34 is connected by line 43 to ground terminal 44.

This arrangement, although it reduces the cost and complexity of installation of a dimming system for-controlling the intensity of a fluorescent tube, does not provide a system for supplying constant filament voltage to tube 34 and, therefore, the operating range for the dimming of tube 34 is vcry limited.

As can be seen from Figure 3, as the current through the choke coil 32 is varied from 0% to 100% by variable supply terminal 31, the voltage at point B increases whereas the voltage at point E on the opposite side of the choke coil decreases due to the negative residence behaviour of the tube. Thus, in the arrangement. of Figure 2, as the dimming current supplied to tube 34 decreases, the voltage supplied to the filaments -of tube 34 also decreases.

In order to avoid the problems associated with the circuit of Figure 2 but yet retain the advantages of a two-wire system, the present ballast arrangement connects the primary winding of the filament supply transformer to an intermediate tap on the choke coil selected to give relatively constant voltage: In Figure 3, the voltage VT represents the voltage at the tap of the choke coil which is selected to be connected to the primary of the filament supply transformer and is a constant or substantially constant voltage as the dimming current is varied.

In Figure 4, choke coil 50 of ballast 51 has a tap T selected to produce this substantially constant voltage. First wire 53 connects choke coil 50 to variable current or power supply terminal 52. Primary winding 54 is connected between intermediate tap T on choke 50 and second wire 56. Filament supply transformer 57 has secondary coil 58 connected across filament 59 of fluorescent tube 60. Filament 59 is also connected by line 61 to the other side of choke coil 50. A second secondary winding is comprised of tap 62 of primary winding 54 which tap is connected to one side of.filament 63 of fluorescent tube 60. The other side of filament 63 is connected by line 64 and second wire 56 to terminal 55. Provision is made for starting stripe 65 connected by line 66 to ground terminal 67.

Since the voltage at tap T on choke coil 50 is substantially constant as the current therethrough varies, the filament transformer 57 will supply substantially constant voltage to filaments 59 and 63 of fluorescent tube 60.

In this manner, the operating range of the fluorescent tube is greatly extended without decreasing its life since the filament voltage and emission temperature will be maintained through the whole dimming range.

In order to dim fluorescent tube 6(1. a dimming circuit 70 is provided connected between a source of alternating current and terminals 52 and 55. Dimming circuit 70 is comprised of a solid-state semiconductor switch or triac 71 having one side connected to one side of the alternating current source ansd the other side to terminal 52. Connected across triac 71 is a series combination of variable resistance 72 and capacitor 73. A diac 74 is connected from the junction of variable resistance 72 and capacitor 73 to the gate terminal of triac 71. Resistor 75 is connected from the junction of triac 71 and terminal 52 to the junction of the other side of the alternating current source and terminal 55. The dimming control circuit 70 is a phase control circuit which controls the amount of current supplied to terminal 52 by the setting of variable resistance 72.

Figure 5 shows a variation of the circuit of Figure 4 for supplying two fluorescent tubes 80 and 81. First wire 82 connects a variable power or current source terminal 83 to choke coil 84 of ballast 85. The other of the choke coil is connected to fluorescent tube 80. Primary winding 86 of filament supply transformer 87 is connected from intermediate tap T on choke coil 84 to second supply terminal 88 by second wire 89. Filament supply transformer 87 has first secondary winding 90 connected to a first filament of fluorescent tube 80, a second secondary winding 91, comprised of a tap on primary winding 86, connected to a first filament of fluorescent tube 81 and a third secondary 92 connected to the second filament of both fluorescent tubes 80 and 81.The starting stripe 93 associated with fluorescent tube 80 and starting stripe 94 associated with fluorescent tube 81 are connected to ground terminal 95 by line 96. Since tap T is selected as the substantially constant voltage point on choke coil 84, secondary windings 90, 91 and 92 will supply substantially constant filament voltage to fluorescent tubes 80 and 81.

The system in Figure 6 is substantially similar to the circuit of Figure 5 and, therefore, like elements have been assigned like reference numerals. The primary difference between these two circuits is that instead of secondary winding 91 of Figure 5 being comprised of a tap on primary winding 86, a separate secondary winding 91' associated with transformer 87 is provided for the first filament of fluorescent tube 81.

Also, a small, start-aid capacitor 100 is connected from the top of secondary winding 90 to the top of secondary winding 92.

The system of Figure 7 is designed to operate from low voltage (e.g. 200-277 volts) two-wire supply mains 110 and 111.

The step-up auto transformer 112 provides high voltage (e.g. 347 volts) thereacross for supplying filament heating voltage to secondaries 113, 114 and 115 for tubes 116 and 117.

WHAT WE CLAIM IS: 1. A ballast system for a fluorescent tube, the system comprising first and second terminals for connection - to a source of variable power; an inductive coil having one end connected to said first terminal and the other end for connection to one side of the fluorescent tube; means for connecting the other side of the fluorescent tube to said second terminal; and means for supplying a voltage to the filaments of the tube, said voltage supply means being connected to an intermediate tap on the inductive coil, the tap being positioned so that the voltage supply to the filaments is maintained substantially constant even when the source power is reduced for dimming the tube.

2. The ballast system of Claim 1, wherein said constant filament voltage supply comprises a transformer having a primary winding, said primary winding having a first end connected to said tap on said coil and a second end connected to said second terminal, said transformer having first and second secondary windings for supplying filament voltage to said fluorescent tube.

3. The ballast system of Claim 2, wherein said first secondary winding comprises a separate winding magnetically coupled to said primary winding and connected in use to a first filament of said fluorescent tube said second secondary winding comprising a tap on said primary winding, and a second filament of said fluorescent tube connected in use across said tap on said primary winding and said second terminal.

4. The ballast system of Claim 2 or 3, wherein said transformer comprises a third secondary winding for use in a two tube system, said first secondary winding connected in use to a first filament of one fluorescent tube, said third primary winding connected in use to a first filament of a second fluorescent tube and said second secondary winding connected in use to second filaments of both fluorescent tubes.

5. The ballast system of claim 4 wherein said first secondary winding comprises a separate winding magnetically coupled to said transformer, said second winding comprises a separate winding magnetically coupled to said transformer and said third secondary winding comprises a tap on said primary winding.

6. The ballast system of claim 4 wherein said first secondary winding comprises a separate winding magnetically coupled to said primary winding, said second winding comprising a separate winding magnetically coupled to said primary winding and said third secondary winding comprising a separate winding magnetically coupled to said primary winding.

7. The ballst system of claim 2 or any

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. between a source of alternating current and terminals 52 and 55. Dimming circuit 70 is comprised of a solid-state semiconductor switch or triac 71 having one side connected to one side of the alternating current source ansd the other side to terminal 52. Connected across triac 71 is a series combination of variable resistance 72 and capacitor 73. A diac 74 is connected from the junction of variable resistance 72 and capacitor 73 to the gate terminal of triac 71. Resistor 75 is connected from the junction of triac 71 and terminal 52 to the junction of the other side of the alternating current source and terminal 55. The dimming control circuit 70 is a phase control circuit which controls the amount of current supplied to terminal 52 by the setting of variable resistance 72. Figure 5 shows a variation of the circuit of Figure 4 for supplying two fluorescent tubes 80 and 81. First wire 82 connects a variable power or current source terminal 83 to choke coil 84 of ballast 85. The other of the choke coil is connected to fluorescent tube 80. Primary winding 86 of filament supply transformer 87 is connected from intermediate tap T on choke coil 84 to second supply terminal 88 by second wire 89. Filament supply transformer 87 has first secondary winding 90 connected to a first filament of fluorescent tube 80, a second secondary winding 91, comprised of a tap on primary winding 86, connected to a first filament of fluorescent tube 81 and a third secondary 92 connected to the second filament of both fluorescent tubes 80 and 81.The starting stripe 93 associated with fluorescent tube 80 and starting stripe 94 associated with fluorescent tube 81 are connected to ground terminal 95 by line 96. Since tap T is selected as the substantially constant voltage point on choke coil 84, secondary windings 90, 91 and 92 will supply substantially constant filament voltage to fluorescent tubes 80 and 81. The system in Figure 6 is substantially similar to the circuit of Figure 5 and, therefore, like elements have been assigned like reference numerals. The primary difference between these two circuits is that instead of secondary winding 91 of Figure 5 being comprised of a tap on primary winding 86, a separate secondary winding 91' associated with transformer 87 is provided for the first filament of fluorescent tube 81. Also, a small, start-aid capacitor 100 is connected from the top of secondary winding 90 to the top of secondary winding 92. The system of Figure 7 is designed to operate from low voltage (e.g. 200-277 volts) two-wire supply mains 110 and 111. The step-up auto transformer 112 provides high voltage (e.g. 347 volts) thereacross for supplying filament heating voltage to secondaries 113, 114 and 115 for tubes 116 and 117. WHAT WE CLAIM IS:

1. A ballast system for a fluorescent tube, the system comprising first and second terminals for connection - to a source of variable power; an inductive coil having one end connected to said first terminal and the other end for connection to one side of the fluorescent tube; means for connecting the other side of the fluorescent tube to said second terminal; and means for supplying a voltage to the filaments of the tube, said voltage supply means being connected to an intermediate tap on the inductive coil, the tap being positioned so that the voltage supply to the filaments is maintained substantially constant even when the source power is reduced for dimming the tube.

7. The ballst system of claim 2 or any

one of claims 3 to 6 as appendant to claim 2 wherein said transformer is a step-up auto transformer.

8. A ballast system for a fluorescent tube, substantially as herein described with reference to any one of Figures 4 to 7 of the accompanying drawings.