US2928990A

US2928990A - Starting circuit for electric discharge lamps

Info

Publication number: US2928990A
Application number: US629337A
Authority: US
Inventors: Gosta E V Siljeholm; Bjorkman Ake Kag
Original assignee: Lumalampan AB
Current assignee: Auralight AB
Priority date: 1955-12-21
Filing date: 1956-12-19
Publication date: 1960-03-15
Anticipated expiration: 1977-03-15
Also published as: CH349698A; DK95487C; FR1232913A; DE1080227B; AT198374B; GB848548A

Description

March 15, 1960 e. E. v. SiLJEHOLM ETAL 2,928,990

STARTING CIRCUIT FOR ELECTRIC DISCHARGE LAMPS File d Dec. 19, 1956 3 Sheets-Sheet 1 STARTING PERIOD sr-zc z 5 March 15, 1960 Filed Dec; 19, 1956 G. E. V. SILJEHOLM El" AL STARTING CIRCUIT FOR ELECTRIC DISCHARGE LAMPS 3 Sheets-Sheet 2 Fig.3

B &

G. E. v. SILJEHOLM EIAL 2,928,990

March 15, 1960 STARTING CIRCUIT FOR ELECTRIC DISCHARGE LAMPS 3 Sheets-Sheet 3 Filed Dec. 19, 1956 WU figb 255 1 STARTING PERIOD 5P SEC 1,0

33 34 ,32 H Tf STARTING CIRCUIT FOR ELECTRIC DISCHARGE LAMPS cam is. v. Siljeholm, Bromma, and like KAG. Bjiirkman, Alvsjo, Sweden, assignors to Lumalampan Aktiebolag, Stockholm, Sweden, a corporation of Sweden Application December 19, 1956, Serial No. 629,337 Claims priority, application Sweden December 21, 1955 v 6 Claims. (Cl. 315-07) This invention refers to a starting circuit for electric discharge lamps without using magnetic relays, thermal or glow switches.

In electric fluorescent lamps especially it is-desirable to provide for an instant start, like that provided in case of electric incandescent lamps. A known device for instant start of fluorescent lamps includes preheatable glow electrodes fed by an inductance-capacitance circuit at resonant frequency, the heating of the glow electrodes being efiected either by the resonance current passing through the glow electrodes connected in series or by an inductance coil, called oscillatory choke, provided with glow current coils for heating the glow electrodes. As a rule, up to now the oscillatory choke was made as a leakage transformer to prevent the glow current coils from interfering with the resonance voltage.

In circuits of the type above referred to, the change in the resonance position is small during the preheating period, and the voltage drop across the discharge lamp produced by resonance is practically constant and independent of the temperature, to which the .glow electrodes are heated. To ensure the starting of the discharge lamp, this resonance voltage must be chosen relatively high, e.g. 300 volts for a 40 watt fluorescent lamp, which, however, is so high that it can cause cold-start of the fluorescent lamp, i.e. start at a premature stage of the preheating period when the glow electrodes are insufficiently heated. Such cold-start involves a very great stress on the glow electrodes, so that the life of the fluorescent lamps are considerably shortened. If, on the other hand, the resonance voltage is chosen so low that no cold-start is to be fearede.g. 265 volts in this case the discharge lampwill not start under all conditions without failure. Due to these risks of cold-start and failure to start, respectively, the circuits mentioned above did not find any proper practical use.

This invention involves a radical departure from the principles described above, granting a sure start of, as well as a long life to the discharge lamp. The new starting circuit is intended for electric discharge lamps with glow electrodes that are preheatable, have resistances depending on the glow temperature, and are fed by glow current coils in a transformer section of a resonancetuned LC-circuit for one or seveal discharge tubes. Characteristics of the invention are that the glow current coils are coupled to the primary coil of the transformer, so that the change in the resistance of the preheatable glow electrodes can be utilized for transforming the resonance voltage applied to the discharge lamp or lamps, from a low value precluding cold-start during the preheating of the glow electrodes, to a value ensuring the start of the discharge lamp or lamps in the final preheating stage.

In order to attain this result there must be a tight coupling between the glow current coils and the primary coil of the transformer. This makes it possible to design the circuit so that the voltage across the discharge lamp has a low value, and that, by heating the glow electrodes, this voltage rises to a permissibly higher value, ensuring a reliable start under all external conditions. This result is attained by utilizing the change in, resistance of the glow electrodes caused by starting from an initial cold condition up to a final hot condition. In this process an approximately tenfold increase of resistance may take place with glow electrodes of the common type. By virtue of the coupling of the glow current coils and the oscillatory choke this change in resistance" affects the electric current in the oscillatory choke so that its impedance rises from a low value for cold electrodes to a high value for hot electrodes This hypothesisled to the invention. Later experience may possibly lead to a modified theory of the basic phenomenomthe steerage of the starting voltage by means of the glow electrodes. -i The invention is'clarified by the following description of a few examples shown in the attached diagrams. Fig. 1 shows a wiring diagram for operation of a discharge lamp according to the invention. Fig. 2 shows a sche matic diagram for certain data of this wiring diagram compared with data for the old wiring diagram described above. Fig. 3 shows a wiring diagram for operation of two discharge lamps according to the invention. Fig. 4 shows a schematic diagram for data of the wiring diagram given by Fig. 3. Fig. 5 shows one more wiring diagram for operation of two discharge lamps according to the invention.

In the wiring diagram according to Fig. 1 an A.C.-fed discharge lamp 1, a capacitor 2, and impedances 3 and 4 are matched to each other and to the frequency of the main voltage in such a way that resonance voltage is produced across the lamp and lights it. The choke 3 is in series with the lamp, while a line containing an oscillatory choke 4 and capacitor 2, shunts it. The choke 3, the capacitor 2 and the oscillatory choke 4 constitute a con-1 trol circuit for the discharge lamp 1. The oscillatory choke 4 is constructed as a transformer with two glow current coils, 5 and 6, for feeding the preheatable glow electrodes, 7 and 8, of the lamp 1.

By coupling the primary and the secondary coils of the transformer 4 as tight as possible and by inserting a capacitor of the proper size in series with the primary of the transformer, it is possible to choose a starting voltage suitable for the discharge lamp independent of the value of the main voltage, and to phase-compensate the circuit so that the currents through the discharge lamp and through thechoke are equal. I

The wiring diagram of Fig. 1 illustrates a starting circuit for the operation of a 40 watt fluorescent lamp. 1 .On the basisof his experience an expert can draw the necessary conclusions from this example for applying the invention to other cases.

When a voltage of 220 volts is applied to the connection terminals, a current will pass through the choke 3, the capacitor 2, and the

transformer windings

4, 5 and 6 with a magnitude determined by the geometric sum of the resistances of the respective components. 'If the choke is standard for a 40 watt lamp, the condenser and the transformer are designed for resonance, so that at most 200 ma. may pass through the circuit and so that the voltage drop across both of them'becoines 260 volts, i.e. the voltage drop across the tube will be 260 volts at the switching moment. The glow current C ils of the transformer are designed so that at that moment a glow voltage of 10 volts is set up across every glow electrode. In a case chosen as an example the transformer coils consist of a primary coil of 2000 turns of 0.22 mm. wire and two turn secondary coils of 0.43 mm. wire wound on a 20 watt mandrel. Whenthe lamp lights, a lamp current passes through the choke increas" ing its inductive reactance considerably, shifting consider;

ably the resonance position of the circuit and reducing the current through the primary coil of the transformer to about 30 ma. This current, which is capacitive, sets up a constant glow current of about 3 volts in the respective glow electrodes and has a phase-compensating effect upon the choke, so that the network current is maintained at the same value as that of the lamp current, i.e. at about 0.42 A. The condenser of 1.0-1.2 ltf. capacity has in the beginning a voltage of 330 volts and of approximately 80 volts in operation. The voltage across the lamp, when the cold resistance of the glow electrodes is 2 times 2.5 ohms, is 260 volts, as indicated above. While the electrodes are glowing they have a total resistance of 2 times 25 ohms, and the voltage across the lamp is approximately 300 volts.

'Qrily y m king the tran f rm r with a tigh c up ing it ifs possible to utilize the change in resistance of the glow electrodes y ting t em f om ro m o slo temperature. This change may be considerable, eg. 2.5:25 ohms. Due to this change in electrode resistance an increase in load is created in the secondary circuit of the transformer, which during the critical period between the switching on of the circuit and the lighting of the lamp with its full intensity reacts on the primary of the transformer.

tion, as shown in Fig. 2, to obtain by changes of impedance of the transformer a voltage drop across the fluorescent lamp of only approximately 260 volts in the moment of connection, which successively increases to approximately 300 volts with the temperature of the glow electrodes rising from room temperature to approximately 800 C. In Fig. 2 the time for lighting in seconds (r is set off as abscissa, the cathode temperature in C., and the voltage across the lamp in volts (curves 9 and 10) as ordinates. In practice, by this invention a valuable technical effect is obtained, implying not only that cold-start is precluded, but also that the high impedance of the transformer precludes every influence of the condenser on the form of the curve of the lamp current. By measuring the form factor of the described scheme it was found to be approximately the same as that of an ordinary choke connection, e.g. 1.46.

According to the diagram in Fig. 2 initiation of the discharge occurs at point 11, corresponding to a glow electrode temperature of 700 C. or higher. The voltage across the lamp that a leakage transformer would supply is constant during the whole preheating interval, as indicated by the dash-dot line 12. Premature initiation of the discharge occurs at point 13 on the temperature curve, corresponding to 500 C., which is too low; but still more unfavourable cold-starts can occur due to the fact that such a high voltage has to be applied to the lamp.

The invention is, as indicated above, not limited to starting circuits for operation of single lamps but can also be used to operate several discharge lamps. 3 shows an example of such use for initiation and operation of two

lamps

14 and 15 by a LLC-circuit. In this case the oscillatory impedance 16 has three glow current coils 17, 18 and 19, all coupled with theprimary coil 20 in such a way that the change in its self-inductance occurs according to desire, at the start causing the voltage across the discharge lamps to rise from a low value that does not cause a cold-start, to a high value that releases the ignition without fail. The specific characteristics .of the invention show up advantageously, particularly in such a circuit with several glow current coils. It ,is important to bring about the change .of impedance desired in the primary coil 20, so that the voltage change mentioned'takes place across the discharge lamps. Herein itisnecessary to consider'carefull-y all dimensions and 186019 it-that thec'ouplings between the different coils are adjusted inrsuch away that noundesirable change inthe;sclf-inductance.of.thelprimarycoilmayiariset By suitable choice of the sizes of the components contained, it is possible according to the inven- 4 can easily arise, if the change of the inductance is so great that the resonance position of the circuit is displaced too much. In Fig. 4 curve 21 shows such an undesirable case for voltage change across one, respectively several lamps, whereas curve 22 shows the result aimed at with a correct design.

Operating two 40 watt fluorescent lamps in parallel according to the wiring diagram shown in Fig. 3, the components had on one occasion the following data: the L-choke 2'3 consisted of 1700 turns of 0.43 mm. wire, the air gap of the mandrel being 2.0 mm. The LC-choke 24 had 1320 turns of 0.43 mm. wire and an air gap of 1 mm, the C-condenser 25 connected to it in series having a capacity of 3.7 f. The condenser 26 of the shunting line had a capacity of 2.0 f, and the primary coil of the oscillatory impedance 16 con sisted of 1850 turns of 0.25 mm. wire. Each secondary with; a d 1.9 te t ng one le t-wiles? the C and of he blame. respecti l s n s sd 9? 1 -9 tu n o 0-3 mm. wire, while the secondary coil 17, feeding the two remaining electrodes in parallel connection, consisted of lIOturns of 0.40 mm. wire. The condenser 27 between the L and the C-lamp had a capacity of 0.5 f.

The wiring diagram'in Fig. 5 refers to connection in series. Using it to operate two 20 watt fluorescent lamps on 220 volts, the following data may serve as an example: the choke 28 had 1550 turns of 0.40 mm. wire and l rnrn air gap. The condenser 29 in the line shunting the two lamps had a capacity of l.8/;rf., and the primary coil 31 of the oscillatory impedance in series with it had l 0 turns of 0.25 mm. wire on a 20 watt mandrel. Each of the two

coils

32 and 33 out of the three

secondary coils

32, 33, and 34 of the transformer, feeds one electrode of the respective lamp, and the third coil 34 the two remaining electrodes in parallel connection. The coil 32 as well as 33 had turns of 0.35 mm. wire and coil 34 turns of 0.40 mm. wire. If, according to the idea of the invention, sufficient control of the voltage across the discharge lamp is .obtained merely through one or a few glo'w electrodes by way of the respective glow current coils, it is permissible to have the remaining glow current coils with a looser coupling or even to have a leakage transformer. Thus, for example in the circuit with two discharge lamps in series as described above with reference to Fig. 5, the

coils

32 and 33 of the transformer 30 can be so tightly coupled that a voltage across the fluorescent lamp which changes according to curve 22 in Fig. 4 is obtained. The ,coil 34 may then, if so desired, be coupled very loosely to the primary coil 30. The ignition course desired according to the invention is nevertheless obtained. In such an arrangement of multilamp connections one glow electrode in each discharge lamp should suitably contribute to the steerage of the starting voltage from a low to a high value in the way intended.

What we claim is:

1; A starting circuit for electric discharge lamps comprising a source of alternating current, a lamp control circuit connected across said source, said lamp control circuit including a choke coil, a capacitor and a primary coil, an electric discharge lamp supplied by said source and having a coupling connection with said primary coil, said electric discharge lamp having preheatable electrodes the resistances of which are positively variable according to temperature, said couplingconnection including a pair of glow coils, each of said glow coils being connected across one of said 'preheatable electrodes and at least one of said glow coils being tightly coupled to said primary coil whereby any change of resistance in the preheatable electrode which is connected across said closely coupled glow coil is transferred as change of impedance'to said primary coil in the lamp control circuit, said'lamp control circuit being-normally capacitive and adapted tobe resonant whereby any increase in the impedance pf said primary coilwill-cause the current therein to increase, the resulting increase in the voltage drop across said primary coil being applied to the 'j dis'- charge lamp by said coupling connection as said electrodes are heated.

2. A starting circuit for electric discharge lampsQ-as claimed in claim 1, wherein there is provided a second electric discharge lamp having a pair of preheatable electrodes the resistances of which are positively variable according to temperature, said second electric discharge lamp being disposed between the first mentioned lamp and the other side of said alternating current source, one of the electrodes in said second electric discharge lamp being series connected with one of the electrodes in said first mentioned electric discharge lamp. J

3. A starting circuit for electric discharge lamps as claimed in claim 2 wherein there is provided a third glow coil, said third glow coil being arranged in tight coupling with said primary coil, one of the electrodes in said second electric discharge lamp being connected across one of the glow coils for the first mentioned lamp and the other electrode in said second discharge lamp being connected across the third glow coil. 2

4. A starting circuit for electric discharge lamps as claimed in claim 1 wherein there is provided a second electric discharge lamp having a pair of preheatable electrodes the reistances of which are positively variable according to temperature, a second choke coil connected to one side of said alternating current source, a second capacitor and one of the electrodes in said second choke discharge lamp being'series connected to said second choke coil, a third glow coil coupled to said primary coil, said one electrode in the second electric discharge lamp being also connected across said third glow coil, and the other electrode .in said second electric discharge lamp being connected across one of the glow coils for the first mentioned lamp and to the other side of said alternating current source.

5. A starting circuit as claimed in claim 4 wherein all of said glow coils are arranged in tight coupling with said primary coil.

6. A starting circuit as claimed in claim 5 wherein a third capacitor is connected between a pair of electrodes each of which are therefor disposed in one of said electric discharge lamps.

References Cited in the file of this patent UNITED STATES PATENTS