WO1997021327A1

WO1997021327A1 - Improved starting device for fluorescent lamps

Info

Publication number: WO1997021327A1
Application number: PCT/IL1996/000168
Authority: WO
Inventors: Shimon Vainer; Zvi Vainer (Gregory)
Original assignee: Shimon Vainer; Vainer Zvi Gregory
Priority date: 1995-12-04
Filing date: 1996-11-26
Publication date: 1997-06-12
Also published as: IL116250A0

Abstract

The invention relates to a device and method for starting a fluorescent lamp (1) connected to an AC voltage supply (22, 23). The device comprises a current limiter (9) connected to the AC voltage supply and serially to a first filament (2) of the lamp, a voltage rectifier (36) for rectifying the AC voltage signal and for outputting the rectified voltage signal to the second filament (3) of the fluorescent lamp, switching means (30, 18, 19) controlled by timing means (34, 25, 27) for periodically cutting of said rectified signal from the second filament, and timing means for controlling the activation of said switching means. The method comprises providing to the electrodes of the lamp pulses of high voltage followed by current injections at least one time during each half-cycle of the AC electric network. Preferably, the rate of the current injection increases as the starting period progresses and several high voltage pulses are provided to the lamp during each half-cycle, and the number of pulses during each half-cycle reduces as the starting period progresses.

Description

IMPROVED STARTING DEVICE FOR FLUORESCENT LAMPS

The invention relates to lighting means. More particularly, the invention relates to fluorescent lamps, such as neon lamps. Specifically, the invention relates to a method and device for a fast and smooth start of fluorescent lamps.

Background of the invention

Fluorescent lamps are widely used for all lighting purposes. They are characterised by the low power consumption, bright illumination, and low heat emission. However, fluorescent lamps are still suffering from some drawbacks that prevent their use from being more widespread. For example the production cost of a fluorescent lamp is higher than that of an incandescent lamp, as its structure is more complicated, and fluorescent lamps require special starting means, which further increases the cost.

The prior art starting means for fluorescent lamps (hereinafter also shortly referred to as "starters") suffer from several drawbacks such as: a. They are capable of illuminating the lamps after a relatively long"starting period", normally in the range of up to 2-3 seconds and typically about 2.5 seconds (this period being referred to hereinafter as the "starting period", is the period from the first application of voltage to the lamp, until a stable illumination is obtained); b. They provide a stable illumination after a starting period which is accompanied by several annoying flashes; c. The most common starters comprise of a glow switch element, the contacts of which tend to wear out fast, which frequently causes damage to the iamp, and shortens the life of the lamp tube.

A typical fluorescent lamp comprises of a tube tilled with argon or krypton gas, and mercury. The interior surface of the tube is coated with a fluorescent powder, such as phosphor, which converts the ultraviolet light produced by the discharge, i.e., due to ionisation process of the gas, into a visible light. The lamp further comprises at ieast one, and normally two electrodes serving as a cathode or an anode alternatively, as the lamp is normally operated by an alternating current (AC). Each one of said electrodes comprises a filament, for starting an ionisation process of the gas. The ionisation process is normally activated by providing heating current to one filament, while the other electrode T serves as an anode. Electrons are emitted from the hot filament, ionisation process starts, and colliding electrons excite mercury atoms. This process is associated with ultraviolet and visible radiation, which when absorbed by the phosphor, is converted into a visible light. The visible light exists as long as voltage is provided to the lamp. Typically, the connection to each filament is made via two pins, extending from the interior of the tube to its exterior. Regardless of their structure, fluorescent lamps need control means for activating the above mentioned ionisation process of the gas, and thereafter keeping a stabilised arc.

A commonly used starter (switching device) of the prior art type is shown in Fig ] . The fluorescent lamp 1 filled with gas 4, has four contact pins 5,6,7 and 8. The two filaments 2, and 3 are located between two adjacent pairs of said contact pins, i.e. between pins 5 and 6, and between pins 7 and 8. The starting device further comprises of a bi-metal component 1 1 , usually contained within a small plastic canister, and a current limiting inductor 9.

When the voltage is applied, a glow discharge is created along the bi-metal contacts of the glow starter. The contacts warm up and close, closing the starting circuit, and allowing a current to flow from terminal 10, through the current limiting inductor 9, via the first cathode tube filament 2, through the closed bi-metal component 1 1 and the second filament, and back to terminal 14. Within a second or two, the cathode filaments are warm enough to emit electrons, and a glow is seen in the two ends of the tube. At this stage, the starter-switch bi-metal contacts 12 and 13 open (because of the glow discharge, which previously caused the to heat and close, ceases when they touch, and they cool and open) and this stops the filaments preheating current flow, causing a collapse of the magnetic energy, stored in the core of the inductor (choke) ballast 9, and producing a high voltage pulse (600V- 1000V) across the fluorescent tube, sufficient to strike the arc, and set up the electric discharge through tube 4. The process is repeated several times, normally 3 to 10 times. Once the tube arc has been struck, the current through the tube gradually builds up, causing an increase of the current through the tube. Λs this happens, the voltage across inductor 9 also increases, and the voltage over the tube falls. The inductor is designed so, that when the tube and the inductor current rise to a value determined by the inductor design, the circuit stabilises, and a stable illumination is provided.

Several other means have been developed in an attempt to overcome the above drawbacks, however, with no satisfactory results. For example, it has been suggested to provide to the fluorescent lamp a high frequency voltage, in the range of 30-60 kilo-hertz. High-frequency based starters involve performing 50hz-to-DC-to-high-frequency conversions are usually expensive and occupy a relatively large space. Therefore, these means are now being used mostly by low power, DC operated fluorescent emergency lamps, where no 50hz-to-DC is required. Other starting means either do not sufficiently shorten the starting period, or do not eliminate flashes during the starting period. Further more, some of the prior art devices have problems in starting tubes of different structures, or tubes which are filled with different compositions of gases.

Other starters that have been suggested, were found to be expensive, cumbersome, or not effective. For example, some starting devices normally provide a reliable start, but they do not shorten the starting period, or do not eliminate the annoying flashes. Some other means include a transformer instead of the bi-metal element, but these means are heavy, large and expensive, and still provide unsatisfactory results. Other starting devices are also known, in which the duration of the start depends on the voltage level across the lamp, which in turn depends on the ionisation level in the lamp. Those devices do not prevent repeated starts as well.

It is an object of the present invention to provide a method and device which significantly shorten the starting period of fluoiescent lamps

It is another object of the invention substantially to eliminate flashes during the starting period

It is a further object ot the invention to increase the duration of the life time of fluorescent lamps

It is still another ob|ect of the invention to piovide low-cost, reliable and simple starting means, occupying a relatively small space

It is still a further object of the invention to provide a device which is compatible in size and shape with existing starters, and which is capable of replacing prior art devices without the need for special adaptations

It is still a further object ot the invention to provide a starting device which is capable of starting a fluorescent lamp having any conventional size, shape or gas composition

It is yet a further object of the invention to provide a starting device which makes it possible to use a simpler structure of fluorescent lamps More particularly, to provide a device which can eliminate the need for filaments in the lamps

Other purposes, objects and advantages of the invention will become apparent as the description proceeds

Brief Description of the Drawings

Fig 1 illustrates a starting device according to the prior art,

Fig 2 shows a block diagiam form of a device according to one embodiment of the invention,

Fig 3 shows the device of Tig 2 in greater detail,

Fig 4 illustrates one possible structure of the switching element, in which two transistors are connected in series and act as a switch,

Fig 5 (a)-(d) illustrate signals over different points of the device, according to the embodiment of the invention shown in Fig 4 (a) Shows the AC voltage of the electπcal power, (b) shows the signal over points a and b during the starting period, (c) shows the signal over the two electrodes of the lamp during the starting period and (d) shows the signal over the two electrodes of the lamp during normal illumination;

Fig. 6 shows another possible structure of the switching element, according to another preferred embodiment of the invention, in which a pulse generator is used; and

Fig. 7 (a)-(d) show signals over different points of the device of Fig. 6 . (a) Shows the AC voltage of the electrical main power, (b) shows the signal over points a and b during the starting period, (c) shows the signal over the two electrodes of the lamp during the starting period, and (d) shows the signal over the two electrodes of the lamp during normal illumination;

Detailed Description of Preferred Embodiments

The method according to the invention is characterised in that the lamp is heated by forced pulses of current, while high voltage is provided to the electrodes of the lamp between those current pulses. The method therefore enables reduction of the starting period to a minimum, in the range of 0.1 of a second, with no repeated starts, or annoying flashes. Further, two devices based on said method are disclosed. These starting devices are simple, reliable, light weighted, cheap, comprise only electronic components, and are capable to light fluorescent lamps having any conventional structure, size, or gas composure.

Fig. 2 shows in a block diagram form a basic structure of a device for providing a fast and smooth start of a fluorescent lamp according to one embodiment of the invention. The device basically comprises a current limiter 9 which is essentially the same as the one which exists in any conventional device, a rectifier such as a diode bridge 36, a charging unit 25, a switch controller 27, a switch 30 controlled by said switch controller 27, and an asymmetric current supplier 18 (the function and operation of the "asymmetric current supplier" will become apparent as the description proceeds).

Other elements appearing in the figure are essentially supporting means, which perform minor tasks or provide several conditions which are needed for the proper operation of the device and are well known to those skilled in the art, and therefore are not described herein, for the sake of brevity. According to one embodiment of the invention, durmg each half-cycle of the AC supply, i e 0 01 second in a 50hz network, the device provides the electrodes of the lamp one period of high voltage suppl}, followed by a peπod of current injection (the term cui rent injection" when used herein refers to a short peπod duπng which a high amount of current is applied, i e a pulse of current) The lengths of said two periods dynamically vary during the starting period, as will be further elaborated later

According to a preferred embodiment of the invention, the frequency of change between said two periods of current and high-voltage is higher, and therefore the two periods are shorter In other words there are several high voltage periods and current injection periods in each half-cycle of the AC supply during the starting period

Referring to the block diagram of Fig 2 , the closure of the main switch 20, connects the device to the main AC power supply, and the device is immediately activated The mam AC voltage passes through the current limiter inductor coil 9, and rectified by the full wave rectifier 36 Switch 30, is controlled by the switch controller 27 and by the asymmetric current supplier 18, both supply current towards the control point 40 and produce some varied potential at this point, which is generally high at the beginning of the starting period, and is gradually reduced The rectified signal from the diode bridge passes through resistor 1 also adds potential to point 40 and causes switch 30 to close when the potential is above a threshold point, and latei close to the end of the half-period, to open again Duπng the closing state of the switch, current is provided to the lamp, and when the switch opens a short and high voltage pulse is provided to electtodes of the lamp, for the same reasons as heretofore discussed for the pπor art element Therefore, alternating periods of current injections, followed by high-voltage periods are applied to the lamp electrodes The current pulses heat the filaments of the lamp, and cause emission of electrons The high-voltage supplies between any two current injections first cause an arc, and later an ignition of the gas The division between the two period types, namely, the current injection and the high voltage periods, continuously varies during the starting period Generally, in the beginning more time is given to the high voltage supply and less to the current injection Later, a shorter period is given to the voltage, and more to the current At the end of the process when a stable arc along the lamp is obtained the switch controller reduces the potential of point 40 to a low level, below the switch threshold level, and therefore no switch closure is possible any more Then, the electrodes just get a rectified AC voltage supply, enough to retain the lamp s light An important featuie of the device is the operation of the asymmetric current supplier This unit supplies current toward point 40 everv second half-period Therefore, every second half- period the closure of the switch occurs slightly earlier The asymmetry between any two sequential half-periods constantly causes some DC component of current to flow through the filaments duπng the starting period, in order to provide mote heat, which is obviously a necessary condition for ignition In the above mentioned device, element 30 in conjunction with elements 18 and 19 produce together switching means for heating the lamp using forced current pulses Element 30 is also a switching device by it's self, and is used to provide high voltage pulses in between the current pulses Element 18 being used as shown in Tig 2 , acts as a mean for temporary reducing the inductive resistance of current limiting inductor while a lamp is being heated Elements 34, 25 and 27 form together a switching timer In one of the embodiments of the starting device, element 18 can be replaced by additional switching device (not shown on this figure), controlled by switching timer This switching device is connected in parallel to one of the diodes of rectified bridge

Fig 3 shows a more detailed description of the device according to the invention Numeral 30, which is symbolically marked as a switch, refers to an element function of which may be carried out by a variety of known solid state switching components, such as by a thyπstor, a transistor, etc Of course, element 30 may comprise several components Fig 4 shows an example of element 30 in which two transistors in series produce a suitable switch, having two contacts a and b, and a control input c Fig 6 shows another possible structuie for element 30, in which the switch is replaced by a pulse generator The followmg description relates to two embodiments of the invention, wherein in the first one element 30 contains the structure of Fig 4 , and in the second one, it contains the structure of Fig 6

According to one embodiment of the invention, when element 30 has the structure of Fig 4 , the closure of the main switch 20 initiates the follow mg process a The current limiter 21 starts to gradually allow an increased current supply to the filament 22, and through the device (or more particularly, the tectifier 36), to the second filament 18 The current limiter is so designed as to gradually increase the current up to a value suitable to maintain a stable illumination at the end of the starting period, as long as switch 20 is ON b AC voltage is provided to a full wave rectifier 36 Immediately after the application of the voltage supply to the rectifier 36, conditions exist to enable a closure of switching element 30 shortly after the beginning ofthe half-cycle, bv the provision of the signal coming to point 40 through resistor 19 These conditions exist as transistor 27 is essentially in a cut-off state at this stage It should be noted that a signal through resistor 19 to the switch control point 40 (point c of ele ent 30) causes the switch to close any time it passes a specific threshold level. Said switch opens again at the end of the half-cycle when the voltage decreases below said threshold level. Simultaneously, capacitor 25 starts to accumulate charge through resistor 24, and as the voltage over the capacitor rises, more current passes through the base of transistor 27, and the potential at the switch control point 40 decreases. As the potential decreases, closure of switching element 30 occurs after a longer time from beginning of each half-cycle of the AC voltage.

A current injection is provided to the filaments when switching element 30 is closed, and this, as described, occurs in the earlier part of each half-cycle. Also, the reopening of the switch causes a high voltage pulse between the two electrodes of the lamp. Therefore, for a time span of one second there are 100 periods of current injections to the lamp, and 100 periods of high voltage. However, normally there is no necessity for 1 second, because the illumination process is much faster in the range of 0.1 -0.2 of a second, i.e. about 5-10 cycles.

Every second half-cycle, some additional current passes through diode 15 and resistor 16 which form the "asymmetric current supplier". This current causes a larger potential in the switch control point 40 every second half-cycle, and therefore every second half-cycle there is a slightly earlier closure of the switching element 30. The asymmetry between the half-cycles causes to an average potential over the lamp electrodes which is not equal to zero volts, which assures at least some direct current through the filament. The total current supply to the device, which gradually increases during the starting period, causes an increasing DC voltage over inductor 21 , so, at the end of the starting period when the arc over the lamp stabilises, about 50% of the voltage falls over the inductor, and 50% over the lamp due to the current flow through the lamp. Later, when the potential over capacitor 25 accumulates above the threshold base-emitter voltage of transistor 27, transistor 27 reaches a state of saturation, and therefore there are no conditions in point 40 for allowing the closure of switch 34, and current can only pass through the lamp, and the starter is essentially disconnected. Fig. 5 is a timing diagram describing signals over different components of the starter according to this embodiment of the invention. Fig. 5 (a) shows the main AC supply, Fig. 5 (b) shows the voltage over points a and b during the starting period, Fig. 5 (c) shows the voltage over the lamp electrodes during the starting period, and Fig. 5 (d) shows the voltage over the lamp electrodes after the starting period and during a stable illumination. In the diagrams C indicates periods of current injections, and V indicates periods of high voltage pulses. It should be noted that experiments carried out according to the invention showed that illumination, depending on the type and structure of the lamp, can be achieved after about 0.2 of a second. The device as illustrated is cheap, and when incorporated in a standard size canister, can easily replace any standard starting device o the prior art, as it can be connected to any lamp by the same two existing connections. Further, it should be noted that the current limiting inductor (choke) according to any embodiment of the invention is the same standard unit which is currently used for igniting fluorescent lamps, and this inductor normally is not mounted within the canister of the device.

According to another preferred embodiment of the invention switching element 30 can be replaced by a miniature pulse generator. Experiments made by the applicants showed that the lamp ignition can be further accelerated, and the device can better start lamps of all structures and types when a pulse generator replacing the structure of Fig. 4 . The pulse generator as shown in Fig. 6 initiates pulses of at least several times the amplitude of the electric network. The pulse generator is designed to provide several pulses during every half-cycle. The frequency of the pulses is determined and controlled by the voltage level at point c. As in the previously discussed embodiment of the invention, this voltage level is determined by the operation of the switch controller 39, and by the asymmetric current supplier comprises of numerals 15 and 16. The device as shown therefore provides high voltage pulses, which occur several times during each half-cycle. Injection of the current is provided to the lamp during the periods between any two pulses (periods between pulses herein are electrically equivalent to the periods during which the switch is closed in the previous embodiment). Diode 15 and resistor 16 cause more pulses to be produced during the first half-cycle than in the following half-cycle, and therefore the asymmetric condition is kept for the same reasons as previously discussed. Fig. 7 is a timing diagram describing signals over different components of the starter according to this preferred embodiment of the invention. Fig. 7 (a) shows the main AC supply, Fig. 7 (b) shows the voltage over points a and b during the starting period, Fig. 7 (c) shows the voltage over the lamp electrodes during the starting period, and Fig. 7 (d) shows the voltage over the lamp electrodes after the starting period and during a stable illumination. In the diagrams C indicates periods of current injections, and V indicates periods of high voltage pulses. It should be noted that the diagrams of Figs. 5 and 7 are just principle diagrams, and the signal may vary according to the adaptation of the device. Furthermore, it has been found by the applicants that when a pulse generator is used, the device can light fluorescent lamps with no filaments at all. The condition of heat, which is needed during the starting period is provided by the spark. This fact can significantly simplify the structure of fluorescent lamps, reduce their production cost, improve their reliability, and extend their life duration.

While preferred embodiments of the invention have been described by way of illustration, it will be understood that the invention may be carried out with many variations, modifications and adaptations, without departing from its spirit or exceeding the scope of claims.

Claims

1 . A device for starting a fluorescent lamp connected to an AC voltage supply, comprising:

■ A current limiter connected to the AC voltage supply and serially to a first filament of the lamp;

■ A voltage rectifier for rectifying the AC voltage signal, and for outputting the rectified voltage signal to the second filament of the fluorescent lamp;

■ Switching means controlled by timing means for periodically cutting off said rectified signal from the second filament; and

■ Timing means for controlling the activation of said switching means.

2. A device according to claim 1 wherein the switching means is a pulse generator.

3. A device according to claim 2 wherein the pulse generator is a transformer based pulse generator, for providing a pulse with amplitude higher than the amplitude of the AC network.

4. A device according to claims 1.2 or 3 wherein the timing means controls the switching means.

5. A device according to claims 2 and 3 wherein current is provided to the lamp in the periods between pulses.

6. A device according to claim 1 wherein the switching means comprises of at least one transistor acting as a controlled switch.

7. A device according to claim 1 wherein the switching means comprises a thyristor.

8. A device according to claims 1 to 7 wherein the timing means assigns the closure timing of the switch.

9. A device according to claim 1 wherein the current limiter is of inducting type.

10. A device according to claim 1 wherein the voltage rectifier provides a full wave rectification.

1 1. A device according to claim 1 wherein the voltage rectifier is a diode bridge.

12. A device according to any of claims 1 to 1 1 wherein a current supplier supplies an extra current during every second half of the AC cycle, for allowing a continuos non-zero current through the filaments of the lamp during the whole starting period.

13. A method for starting a fluorescent lamp comprising providing to the electrodes of the lamp pulses of high voltage followed by current injections at least one time during each half-cycle of the AC electric network.

14. A method according to claim 13 wherein the rate of the current injection increases as the starting period progresses.

1 5. A method according to claim 13 wherein several high voltage pulses are provided during each half- cycle of the electric network, and wherein the number of pulses during each half-cycle reduces as the starting period progresses.

16. A starting device for fluorescent lamps according to claim 1 essentially as described and illustrated.

17. In combination, a system comprising of:

(a.) A device for starting a fluorescent lamp connected to an AC voltage supply, comprising:

■ Timing means for controlling the activation of said switching means; and

(b.) A fluorescent lamp which is filamentless or which comprises only one filament.