GB2110890A

GB2110890A - Frequency controlled excitation of a gas discharge lamp

Info

Publication number: GB2110890A
Application number: GB08234231A
Authority: GB
Inventors: Dr Rainer Koehne; Ralph Krauss
Original assignee: KRAUSS INNOVATRON; Innovatron Krauss and Co
Current assignee: KRAUSS INNOVATRON; Innovatron Krauss and Co
Priority date: 1981-12-07
Filing date: 1982-12-01
Publication date: 1983-06-22
Also published as: IT1153369B; FR2517922A1; GB2110890B; IT8224615A0; CH660826A5; DE3245219A1; ES8403003A1; ES517947A0; IT8224615A1; BE895278A; FR2517922B1

Abstract

An electric lighting system produces light by feeding an electric tension of high frequency generated in an oscillator (21) having a transformer (24) connected to electrodes of a gas discharge lamp (20). Thereby the gas in this lamp becomes ionized, emits light and exhibits plasma resonances. The frequency of the electric tension is kept on a resonance frequency of the ionized gas by using the operating gas discharge lamp and the secondary winding (27) of the transformer as a resonant tank circuit which determines the frequency of the oscillation. The oscillator operates at either a higher or lower frequency than the plasma resonance before the lamp starts. This higher or lower frequency is determined either by the saturation of the core (K) or by the inductance and stray capacitance of the primary winding (25) of the transformer. The transformer core has an air gap calculated so that the oscillator frequency may shift in the ratio of 1:4. The oscillator frequency is between 10 KHz and 1 MHz. <IMAGE>

Description

SPECIFICATION Frequency controlled excitation of a gas discharge lamp This invention relates generally to light emission of a gas discharge lamp excited by a high frequency bipolar electric tension applied to said lamp via a transformer, and more particularly concerns controlling the oscillation frequency of an oscillator generating this electric tension in such a way, that the whole system resonates on a plasma-frequency of the lamp, if the gas in this lamp is fully ionized.

The invention thus relates to a lighting device consisting of an oscillator, which is inductively coupled to the gas discharge lamp by means of a transformer, whereby the electrical conditions for resonance of the system are chosen in such a way, that a tank circuit formed by the fully ionized gas discharge lamp as condensor and an output winding of the transformer arranged in parallel determines the resonant feed back frequency of the oscillator by dominating over all other possible feed back frequencies of the oscillator circuit Gas discharge lamps are gas filled tubes having two electrodes, these two electrodes are identical and remain generally cold during operation of the lamp. The tubes are generally filled with some inert gas or mixture of gases, such as neon, argon or the like at low pressure, i.e. ten mbar.When sufficient voltage is applied across the two electrodes, the gas contained in the tube will become ionized and thus become a plasma. Fluorescent lamps having electrodes in form of electrically heated wires may be used as gas discharge tubes, too, thereby the wires serve as electrodes and remain cold during operation ofthetube. For initiating a glow discharge, thus starting the glow discharge lamp an electrical tension of 3000 V and more is required. If the lamp is operating and radiating, a lower tension is required, depending upon the type of gas and pressure emp loyed in the tube.

U.S. Pat. No. 3,525,900 by C. D. Skirvin et al. refers to frequency controlled enhancement of light emission and describes a total of four circuits for electrical lighting systems. According to this citation gas discharge lamps have a high efficacy if the electric tension applied to the electrodes contains a frequency of a resonance in the plasma. These resonances are attributed to the behaviour of the free and captive electron structure of the gas. The luminous output of the gas was found to be a function of the applied frequency and showed maxima at specific frequencies. For a neon lamp an example is given: At afrequency of 80 KHzthis lamp showed a pronounced minimum in current-consumption, thus showing a high resistance to the supply voltage.

Thereby the performance of the tube in terms of light output is at the same time increased.

In the four circuits for an electrical lighting system according to said citation the voltage of high frequency for driving a gas discharge tube is produced in a unit having two stages, i.e. an oscillator and a power stage. The oscillator is a so-called RCoscillator. Throughout the discription R is used for resistor, C is used for a capacitor and L is used for an inductance. The frequency of this RC-oscillator can be adjusted by means of a variable resistor. In the power stage a semiconductor switching element is used, especially a thyristor. This semiconductor switch is controlled by the oscillator and is connected on its output side with a primary winding of a transformer. This transformer has one further winding, a secondary winding, to which the gas discharge lamp is connected.A condensor is arranged in parallel to the primary winding, thus forming a resonant tank circuit with this primary winding. The frequency of this tank circuit is according to the principles of this citation initially adjusted (as for example by varying the capacitance of the condensor), so that the resonant frequency of this tank circuit is as near as possible to the resonant frequency of the type of gas tube constituting the load. The circuit according to this citation and for supplying a high frequency voltage to a gas discharge lamp does operate on a con stantfrequency. Now the resonant frequency of a gas discharge lamp may vary as a function of temperature etc. To compensate for such frequency drifts, the wave-shape of the supply voltage is modified by varying the angle of the decay following the maximal positive amplitude.Thus the shape of the timevoltage plot over each cycle is modified and the number and the factor of the harmonics, whereas the basic frequency remains constant.

Furtheron electrical lighting systems having an oscillator directly coupled with a gas discharge tube are known, see for example the printed publications of the German Patent applications 3 525 900 and 2442091. In both circuits the oscillator does not oscillate on a frequency of the plasma. The object of these known circuits is to convert a low-voltage DC-tension into a high voltage AC-tension. Thus the energy for operating a gas discharge lamp can be taken from a car-battery or the like. For operating a gas discharge lamp a mains-supply is thus not needed.

It is an object of the invention to provide an electrical lighting system in which the oscillator is locked to the frequency of a plasma resonance.

It is a further object of the invention to provide an electric lighting system, in which the frequency of the driving oscillator automatically adjusts to the type of gas used and to different tubes in a rather large frequency range, i.e. 20 to 80 KHz.

A further object of the invention is to provide an electrical lighting system, in which no initial adjustment of a resonant tank circuit is needed.

A still further object of the invention is to provide but one electrical unit capable of exciting various different gas discharge lamps, i.e. neon-lamps, argon-lamps, lamps having gas mixtures or the like.

A still further object of the invention is to provide a cheap and easy circuit for supplying a gas discharge lamp.

A further object of the invention is to provide an electric lighting system, in which the oscillator resonates on a first frequency, if the gas discharge tube is not yet started and automatically drifts to a resonance frequency of the ionized gas as soon as the gas discharge tube is started by means of the output voltage produced during oscillation of the oscillator on the first frequency.

According to the present invention the gas discharge tube is a frequency determining element for the oscillation frequency, as it forms a tank circuit with a secondary winding of the transformer of the oscillator. This secondary winding being inductively coupled with a feed back winding of the same transformer. The feed back winding is connected to the input of the amplification element of the oscillator, so that the amplification element, normally a transistor, is controlled by the resonant oscillations in the tank circuit formed by the gas discharge lamp and the secondary winding. This tank circuit is dominant only if the gas discharge lamp is operating, as the unstarted gas discharge lamp forms a capacitor of very low capacitance and thus a poor tank circuit with the secondary winding.

In accordance with the principles of the present invention the oscillator of the electrical lighting system oscillates on a first frequency determined by at least one of the windings connected to thetransistor, if the gas discharge lamp is not yet started. The oscillator operates on a second frequency determined by the operating gas discharge lamp and the secondary winding, which form a parallel resonant circuit, if the gas discharge lamp is started and radiating. As soon as a plasma is formed in the gas discharge lamp, the capacitance of the gas discharge lamp and between its two electrodes is increased and a tank circuit of high quality (high O) is formed.This tank circuit produces a resonance signal that is picked up by the feed back winding and is higher than signals from other resonance circuits picked up by the feed back winding, thus the signal from the tank circuit including the gas discharge lamp dominates and is the frequency of oscillation.

From theoretical treatings of plasma-oscillations it is known, that the excitation voltage applied to the electrodes gives rise to a reciprocating displacement of the system ofthe electrons and the system (or "cloud") of the ions. From this a frequency v amounting to n n .02 V2 = m'-:epsO whereby n = concentration of the charge carriers, e = charge of an electron, m = mass of the charge cariers, and reps, = absolute dielectric constant, is calculated.

If a concentration n='l0"cm-3 is assumed and the frequency of ionic oscillations is calculated, the frequency v amounts to about 100 KHz. This value is quite close to the frequencies of operation of the device according to the present invention and measured experimentally. Thereby of course the oscillator operates on the second frequency, i.e. the gas discharge lamp is ionized. In conclusion it is assumed, that the resonances in the plasma and during the excitation may be attributed to the elastic ionic vibration in the plasma.

As in the known electric lighting device according to US. Pat. No. 3,525,900 the excitation frequency is adjusted once and thus unchanged during future operation, the excitation frequency will no longer be an optimum frequency if the concentration of car riers changes. The a.m. formula shows, that the frequency of plasma oscillations is dominated by the value of the concentration of the charge carriers.

This in turn depends from the pressure of the gas inside the tube. As the gas pressure cannot be maintained constant over a long period of time, the known device falls off in efficacy, as the plasma frequency shifts away from the constant frequency of oscillation.

In a preferred embodiment of the invention, the oscillator is built up as a Meissner-oscillator and oscillates, if the gas discharge lamp is not yet started or disconnected, on a higher (first) frequency than during the normal operation of the gas discharge lamp. The frequency of the oscillator shifts to lower frequency after starting the gas discharge lamp and becomes equal to the frequency of the tank circuit formed by the gas discharge lamp and the secondary winding. The frequency of the oscillator before the starting of the gas discharge lamp or if this lamp is disconnected is determined by the inductance of the primary winding and unavoidable stray capacitances and the capacitance of this winding, forming the tank circuit.As the Q of this tank circuit incorporating the primary winding is rather low, it only determines the resonance frequency of the oscillator, if the tank circuit incorporating the gas discharge lamp is not yet operating properly.

In another embodiment of the invention the oscillator oscillates on a lower frequency compared to the frequency of the oscillator, if the gas discharge lamp is excited. Thereby this lower, first frequency is determined by the saturation of the core of the transformer due to the current in the primary winding. If this current in the primary winding reaches a certain value producing a saturation flux in the core, the flux in the core cannot rise with further rising current in the primary winding. This gives rise to a signal in the feed back winding, so that oscillation is controlled by the saturation. This known effect is used in flux controlled converters.

According to a further principle of the invention the shape oftheexcitation voltage of the gas discharge tube is formed in such a way, that it containes short pulses of high voltage which unable the starting of the gas discharge lamp in every cycle. These high-voltage peaks are produced by a network containing a diode, a resistor and a condensor arranged in the output circuitry of the oscillator.

The drawing shows a schematic diagram of an electric lighting device including a power supply unit, an oscillator, a gas discharge tube and a protecting circuit delivering a switch-off signal.

In the drawing a gas discharge lamp 20 is con nected to an oscillator 21, delivering a voltage of some KVand of high frequency, i.e. 10 KHzto 1 MHzto this lamp 20. A DC-voltage is supplied to the oscil lator 21 from a power supply unit 22.

An npn-transistor Q1 is the amplification element in the oscillator 21. A base B of this transistor Q1 is on one hand connected via a feed backwinding 23 of the transformer 24 to a center tap A of a voltage divider for the base voltage formed by two resistors R1 and R2 and on the other hand to the cathode of a diode D1 who's anode is on zero potential. The DC-voltage at the center tap A is equal to or exceeds the base-emitter-saturation voltage of the transistor Q1.This DC-voltage determines the zero signal operating point and is chosen in such a way, that the amplification of the transistor Q1 is sufficient to pro duce oscillations of the oscillator 21, if a secondary winding 27 of the oscillator 21 is unloaded, i.e. not connected to a gas discharge lamp or connected to a gas discharge lamp 20 which is not yet started. On zero potential is an emitter E of the transistor 01.

Connected to the primary winding 25 of the transformer 24 is a collector C of the transistor 01. The other end of the primary winding 25 is connected to a positive pole 26 of the power supply unit 22. The collector C is further connected to a cathode of a second diode D2, the anode of which is on zero potential. Furtheron the collector C is connected to a third diode which is in forward bias and paralleled by a resistor of low resistance, i.e. 100 Ohm. This parallel arrangement of the third diode D3 and the resistor R3 is on the other hand connected via a condensor C4 to zero potential (common).

The secondary winding 27 of the transformer 24 is connected by means of wires Z to the gas discharge lamp 20. This secondary winding 27 has a center tap M, which is wired to the input of the protecting cir cuit28. If a voltage different from zero appears bet- ween this center tap M and zero potential, this voltage is rectified by means of a forth diode D4. The rectified voltage is limited by a Zenerdiode Z1 and smoothed by a capacitor C5. The DC-voltage obtained appears on contacts 29 and 30 and is fed into a switch-off circuit (not shown).

Instead of arranging the diode D4 as shown in the figure it is possible to branch this diode between the upper terminals of the Zenerdiode Z1 and the condensor C4.

The power supply unit 22 has a bridge rectifier 31.

This unit 22 has input terminals 32,33, to which an AC-voltage from the mains supply is connected.

These terminals 32,33 are bridged via a first condensor Cl, which shifts the phase. The upper terminal 32 is connected via a high frequency choke 34, which suppresses high frequency signals and a second condensor arranged in series with this choke 34 to the upper terminal for AC-voltage of the bridge rectifier 31. The second condensor C2 forms a capacitive resistance and limits the current. This condensor may be replaced by a current limiting choke. The second input terminal 33 is directly connected to the lowerAC-input of the bridge rectifier 31. One output terminal of the bridge rectifier is on zero potential, the positive pole 26 is connected via a smoothing condensor C3 to zero potential.

The ratio of the numbers of turns of the windings 23,25,27 of the transformer is as follows: The ratio of primary winding to feed back winding amounts to ten to one. The ratio ofthe primary winding 25 to the secondary winding 27 is one to thirty.

The transformer 24 is a transformer having a flux shorting bar, thus a core K which is surrounded by the winding 23,25, 27 is not completely closed like a ring, but is open in an air gap of approx. three mil limeter. Core K is made of ferrit.

Due to this air gap the transformer is adapted to the negative caracteristic of gas discharge lamps 20.

If the voltage across the secondary winding 27 drops, the current in this winding 27 is increased.

The air gap is further calculated in such a way, that the oscillator can shift its frequency in a ratio of one to four, i.e. between 10 KHz and 40 KHz, to give an example.

At an AC-voltage of 220 V, for which the ratios of the winding given above are calculated, a positive DC-voltage of about 250 V appears at the positive pole 26. The potential at the center tap A of the base divider network rises to a value, which allows a certain amplification, so that the oscillator can start to swing. Thus a collector current is drawn, which flows through the primary winding. This leads to a magnetic flux in the transformer 24. An induced voltage appears across the feed back winding 23 in such a way, that the transistor Q1 further opens and draws more current.

The following explanations are based on the assumption, that the gas discharge lamp is not yet started or disconnected. Then the oscillator 21 starts oscillating as a Meissner-oscillator according to a first embodyment of the invention. The frequency is determined by the inductance of the primary winding 25 and unavoidable stray capacitances and the capacitance of the winding 25. In a second embodymenu the rise of the collector current is limited by flux saturation of the core K and a correspondingly induced signal in the feed back winding 23.

In both embodyments, especially for an oscillator which operates with saturation of the flux in the core K, the frequency of the oscillator 21 before starting the gas discharge lamp 20 may be lower or higher than after starting the lamp 20. According to a principle ofthe present invention it is important, that the losses in the feed back system of the oscillator 21 are high, higher than for the feed back system determining the frequency of normal operation. As in the case of a Meissner-oscillator shown in the figure the resonant tank circuit for this condition of oscillation is formed by the inductance of the primary winding 25 and stray and winding capacitances the tank circuit so formed has a low Q and thus is a poor resonant circuit Its behaviour is further deteriorated by the second diode D2 and the network of the third diode D3, the third resistor R4 and the condensor C4.

Two conditions have to be met during the initial phase of oscillation of the oscillator 21: At first a voltage of sufficient amplitude should appear across the secondary winding 27, to allow a start of the gas discharge lamp 20 in cold condition, so that a plasma is formed in the lamp 20.

At second this initial phase of oscillation should be on a frequency for which the feed back system of the oscillator 21 shows a poor performance, so that the oscillator 21 shifts to the resonant frequency of the tank circuit incorporating the lamp 20 as soon as resonances in this tank circuit formed by the gas discharge lamp 20 and the secondary winding 27 start. Once the gas discharge lamp 20 is started and shows a plasma, this plasma start to oscillate.

The phenomena occuring then in the tank circuit formed by the secondary winding 27 and the lamp 30 are important for the inventive lighting system: Under the action of the voltage induced in the secondary winding 27 by the oscillator 21, the positive and negative clouds of change are displaced towards one another, i.e. the center of all negative charges (electrons) is spaced apart from the center of all positive (ions) charges. This displacement occurs along the direction ofthe electric field applied, i.e. along a line that links the two electrodes.

If for example the voltage induced in the secondary winding 27 is directed in such a way, that the positive pole of the voltage appears at the upper electrode (in the figure), the center ofthe negative charges lies closer to this positive electrode than the center ofthe positive charges. If the collector current diminishes, the voltage induced in the secondary winding 27 drops, too. Thus the two clouds of charge carriers of the plasma are no longer forced apart, but fly towards each other. Thereby they reach a neutral point, where they don't stop, but continue separating in the opposite direction according to the elasticity of the system. The opposite displacement of the centers of the total charges gives rise to a countervoltage across the secondary winding 27 thus the upper electrode is on negative potential compared to the lower electrode now.This counter-voltage gives rise to a current in the secondary winding 27, which induces a counter-voltage in the feed back winding 23. Thereby the potential of the base B drops and the transistor Ol becomes nonconductive. The transistor Q1 is blocked until the clouds of the two charge carriers reverse their direction of movement.

The counter-current in the secondary winding 27 gives rise to a counter-voltage in the primary winding 25, too. The diode D2 impedes the potential at the collector C to reach negative values.

If the directions of movement and oscillation of the charge carriers have reversed and the cloud of negative charge carriers flies again towards the upper electrode (in the figure), the counter-voltage induced in the feed back winding 23 disappears and the transistor Q1 becomes conductive again. This gives rise to a current in the primary winding 25, which in turn induces a voltage in the secondary winding 27. This induced voltage enhances the separation of the clouds of charge carriers in the direction described, i.e. the negative cloud flies towards the upper electrode. In this phase energy is fed into the resonant tank circuit formed by the secondary winding 27 and the gas discharge lamp 20.

The frequency of the oscillator 21 shifts after a short period of initial oscillations on a first frequency to a resonantfrequency of the plasma, as soon as the plasma begins to form in the gas discharge lamp.

Claims

1. An electrical lighting system, having a gas discharge lamp and an oscillator, said oscillator a. producing an electric tension of high frequency to start and to keep radiating the gas discharge lamp, b. having output terminals connected to the gas dis charge lamp, c. oscillating at a frequency determined by a reson ant tank circuit, and d. having an amplification element and a trans former, said transformer having- a primary winding connected to the output side of the amplification element, - a secondary winding having two terminals, said terminals being connected to the gas discharge lamp, and - a feed back winding connected to the input side of the amplification element, said tank circuit determining the oscillation fre quency of the oscillator being formed by the started and radiating gas discharge lamp and the secondary winding.

2. The electrical lighting system set forth in claim 1, wherein the amplification element is a transistor.

3. The electrical lighting system set forth in claim 1, wherein the oscillating frequency of the oscillator has a first frequency value in a first operation condition of the oscillator, in which the gas discharge lamp is not yet started and a second frequency determined by the tank circuit formed by the operating gas discharge lamp and the secondary winding, said second frequency being different from the first frequency and being the frequency of the oscillator if the gas discharge lamp is started and radiating.

4. The electrical lighting system set forth in claim 3, wherein the oscillation frequency of the oscillator has a higher value if the oscillator operates in the first condition than if the oscillator operates in the second condition.

5. The electrical lighting system setforth in claim 3, wherein the resonant frequency of the tank circuit being formed by the primary winding and unavoidable capacitances as stray capacitances and the capacitance of the said primary winding is higher in frequency than the resonant frequency of the reson anttankcircuit being formed by the operating gas discharge lamp and the secondary winding arranged in parallel.

6. The electrical lighting system set forth in claim 1, wherein the ratio of the numbers of turns of the transformer windings and for a DC-supply voltage for the oscillator of 250 V amounts two.

a. ratio of the turns of the primary winding to the turns of the feed back winding being between seven to one and thirteen to one, the ratio ten to one is preferred, and b. ratio of the turns of the primary winding to turns of the secondary winding being between one to fifteen and one to fifty, ratios between one to twenty and one to forty being preferred.

7. The electrical lighting system set forth in claim 1, wherein the transformer has a core being surrounded by the three windings of the transformer, said core forming a nearly closed loop, the loop being open but in an air gap.

8. The electrical lighting systemsetforth in claim 2, wherein the transistor is an npn-transistor having a base, an emitter and a collector, - the base being on one hand connected via the feed back winding to a first and a second resistor arranged in parallel and forming a voltage divider and on the other hand to a first diode having a cathode connected to said base and an anode on zero potential, - the emitter being on zero potential, and - the collector being connected via the primary winding to a positive pole of a DC-voltage supply unit.

9. The electrical lighting system set forth in claim 7, wherein the collector of the transistor is furtheron connected to a cathode of a second diode, said second diode having an anode on zero potential and to an anode of a third diode and a third resistor, said third diode and said third resistor being arranged in parallel and being connected with their other connection point via a capacitor to zero potential.

10. The electrical lighting system set forth in claim 2, wherein the transistor is connected at its base via the feed back winding to a voltage divider being formed by a first resistor and an IR-light emitting diode operated in forward bias.

11. The electrical lighting system setforth in claim 2, wherein the DC-potential of the base being determined by a voltage divider equals the baseemitter-saturation voltage of the said transistor.

12. An electrical lighting system having a gas discharge lamp and an oscillator, said oscillator a. producing an electric tension of high frequency for starting and keeping in operation the gas discharge lamp, b. having two output terminals connected to the gas discharge lamp, and c. incorporating a transistor having a base, an emitter and a collector and a transformer, said transformer having - a primary winding connected to the collector of said transistor, - secondary winding having said two outputter- minals, and - a feed back winding connected to the base of said transistor, said oscillator oscillating on a first frequency determined by at least one of the windings connected to the transistor, ifthe gas discharge lamp is not started, and operating on a second frequency determined by the operating gas discharge lamp and the secondary winding arranged in parallel, if the gas discharge lamp is started and radiating.

13. The electrical lighting system set forth in claim 12, wherein the elements determining the first frequency and the second frequency show electrical tosses, the losses of the elements determining the first frequency ofthe oscillator being higher than the electrical losses of the tank circuit being formed by the gas discharge lamp and the secondary winding and forming the elements determining the second frequency.

14. The electrical lighting system set forth in claim 12, wherein the electric tension produced by the oscillator contains needle like pulses of very short duration and enabling the starting of the gas discharge lamp.

15. The electrical lighting system set forth in claim 12, wherein the resonant frequency of the tank circuit formed by the secondary winding and the operating gas discharge lamp is between ten KHz andone MHz.