GB2052896A

GB2052896A - Lighting unit and circuit

Info

Publication number: GB2052896A
Application number: GB8018101A
Authority: GB
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1979-06-13
Filing date: 1980-06-03
Publication date: 1981-01-28
Also published as: BE883820A; AU5924980A; GB2052896B; FR2458972B1; NL8003456A; FR2458972A1; IT8022696A0; BR8003745A; HU182689B; MX148106A; ES492441A0; AU522807B2; KR830002716B1; DE3021209A1; ES8101832A1; KR830004385A; IT1131300B; JPS563992A; SE8004394L

Abstract

A lighting unit uses a discharge lamp (11) as a main source of light supplemented by a filamentary source (12), serving as a resistive ballast. The main lamp is a small, low wattage metal halide lamp of high efficiency. The unit includes a rectifier (15) for conversion of 60 hertz ac to dc, and a dc energized operating network, including a ferrite transformer (20) and a transistor switch (19) driven by an oscillator transistor (30) for starting the discharge lamp. The network has an output adapted to various lamp conditions, including provision of a power boost during glow to arc transition and means for sustaining the arc during transients. The filamentary source provides standby illumination which is substantially constant until proportionately reduced as the main lamp reaches full output. In response to main lamp current and voltage, dc energization is provided during warm-up and normal operation, and energization at above audible frequencies during starting. A dimmer switch (18) is provided. <IMAGE>

Description

SPECIFICATION Lighting unit BACKGROUND OF THE INVENTION Field of the Invention The present invention deals with a lighting unit designed for functional similarity to an incandescent light source, and more particularly with a lighting unit in which the principal source of light is an arc discharge lamp supplemented by a standby filamentary light source, and which includes a compact "high frequency" power supply unit for supplying the needed energization from a conventional 110 volt 60 hertz source.

Background of the Invention The present invention is an outgrowth of earlier efforts to produce an energy efficient and comparatively low cost replacement unit for the incandescent lamp. The incandescent lamp converts most of the electrical energy supplied into heat, a small percentage always less than 10% being converted into visible light. With the cost of energy rising, a need has arisen for a lighting unit whose conversion of electrical energy into light was substantially higher. Known lighting units such as fluorescent units have had double or triple the lighting efficiency of an incandescent light. A property of such devices, which has limited their more general application, has been the high initial cost of the ballast for powering such devices and their elongated configuration.Another possible alternative has been the high pressure discharge lamp having up to six times the efficiency of an incandescent lamp.

High pressure metal vapor lamps have been available in high power units requiring costly power supplies, restricting their use to street lighting and commercial as opposed to home lighting. Recently, smaller, low wattage, metal halide laps having efficiencies approaching those of the larger size have been invented.

Such lamps are a potential energy efficient replacement for the incandescent lamp provided that convenient low cost provisions can be made for standby illumination and for supplying the diverse electrical requirements for the two light sources.

The power supply of the present lighting unit represents an outgrowth of earlier high frequency power supplies in which a ferrite transformer, normally controlled for non-saturated operation and a transistor switch are the most significant elements. Such power supplies have been termed static inverters in deference to the fact that "dc" quantities are converted to ac through static or non-moving parts. Patents dealing with inverters of this class and ferrite transformers having the saturation avoidance feature include the U. S.

Patents 3,914,680, 4,002,390 and 4,004,251.

SUMMARY OF THE INVENTION The objects of the invention are achieved in a novel lighting unit utilizing an energy efficient metal vapor discharge lamp as the main source of light, supplemented by a standby filamentary light source, the filament thereof serving as a resistive ballast for the discharge lamp. The lighting unit also includes a dc power supply and an operating network for converting 1 20V 60 hertz energy into the forms needed for operating the main and standby lamp. The main lamp and standby filament are contained in a single glass enclosure and the dc supply and operating network are contained in a small case to which the glass enclosure is attached and which has an "Edison" base for inserting the lighting unit into a conventional lamp socket.In short, the novel'lighting unit is functionally similar to an incandescent lamp but produces light with a more efficient use of power.

The dc power supply of the lighting unit comprises a rectifier, typically a bridge, for converting ac to dc and a capacitor for reducing the ripple.

The operating network of the lighting unit comprises a resistance element exhibiting a substantial resistance increase with applied voltage (it is the filament of the standby lamp), a transformer, a transistor switch, and means responsive to the electrical state of the main lamp for maintaining the switch in one condition (off) during warm-up and normal operation of the main lamp, and in a second condition (intermittent operation), during other states of the main lamp.

The elements of the operating network are interconnected with the dc supply for the supply of dc current to the resistance element and the main lamp in series for energizing and ballasting the main lamp when the switch is off. When the switch is intermittently operated, current is coupled in pulsating form to the resistance element for the production of standby light, and in an alternating form to the input of the transformer for starting the main lamp.

Sensors of both voltage and current conditions in the operating network, reflective of the state of the main lamp, maintain the intermittent switching operation through preignition, ignition and the glow to arc transition of the main lamp. When thermionic emission (warm-up) occurs, intermittent switch operation terminates, and the dc current flow through the standby lamp and main lamp starts.

The operating network maintains a substantially constant standby illumination through the starting procedure into early warm-up of the main lamp and supplies the widely disparate requirement of the main lamp from preignition to final operation. The compactness of the unit arises through the use of above audible electrical frequencies, which permit efficient, and small ferrite transformers, and the compactness of the efficient solid state circuit now to be further described.

The elements of the operating network are inter-connected with the dc supply for the supply of dc current to the resistance element and the main lamp in series for energizing and ballasting the main lamp when the switch is off. When the switch is intermittently operated, current is coupled in pulsating form to the resistance element for the production of standby light, and in an alternating form to the input of the transformer for starting the main lamp.

Considering the circuit of the operating network more particularly, the resistance element and the switch are serially connected across the dc supply. A first capacitor, the main primary winding of the transformer and the switch are serially connected with the switch across the de supply. The first resistance element also shunts the serially connected first capacitor and main primary winding. Intermittent operation of the switch provides the pulsating current for standby illumination and the alternating current in the transformer input for the main lamp. The first capacitor prevents direct current flow in the transformer primary and reduces unnecessary dissipation.

The transformer has a second output winding connected at one end to the first winding, and connected at the other end through a second capacitor to the anode of the gas discharge lamp. A diode is poled to permit dc current flow from the dc supply through the filamentary resistance and into the main lamp when the switch is off. When the switch is intermittently operated, the output circuit rectifies the transformed alternating output coupled to the lamp.

The main lamp responsive means includes main lamp current sensing, voltage sensing and a trigger oscillator responding to a difference in sensed quantities for controlling the switch. The sensing elements are a lamp current sensing resistance connected between the lamp cathode and the reference (- ) terminal of the dc supply and a voltage divider connected between the winding interconnection and the dc reference terminal. The voltage appearing at this interconnection reflects the loading produced on the input circuit by the main lamp during intermittent operation of the switch and the lamp voltage when the switch is in the off state. The sensed voltages are then coupled respectively to the base and emitter electrodes of a junction transistor, which is connected in a relaxation oscillator configuration with a capacitor across the input junction.The period of the oscillator is a function of the sensed voltage difference which affects the charging rate of the capacitor. The relaxation oscillator generates a trigger pulse, peaked with the aid of transformer feedback, and turns on the solid state switch, also a junction transistor.

The transistor switch is turned on by a trigger pulse provided from the trigger oscillator and turns itself off after conducting for a fixed interval. Self turn-off is achieved by a pair of feedback windings coupled to the switching transistor which provide feedback which reverses from conduction aiding to conduction inhibiting when a predetermined flux level is achieved in the transformer core, the flux level being due to switch controlled current in a main transformer winding.

The trigger oscillator controls the transistor switch in response to main lamp conditions.

When the lighting unit is first energized, the second lamp current is zero, the sensed voltage maximum and the trigger oscillator is turned on, causing intermittent switch operation. Intermittent switch operation continues until the lamp current reaches the larger initial value, corresponding to earlier warm-up, and the voltage at the winding interconnection plummets to the low voltage corresponding to early warm-up.

The switch is sustained in an off position from the warm-up to final run conditions so long as the current in the lamp does not fall below an arbitrary value substantially less than normal operating current, and the voltage does not rise an arbitrary value above the normal voltage. Such a departure from normalcy may occur when the lamp respnds to a transient low voltage line condition, and the trigger is restarted to prevent lamp extinction.

In addition to the foregoing means for establishing when the operating network should be in the intermittent state and when it should be in the dc state, the operating network has means for distinguishing between the pre ignition period and the glow to arc transition of the main lamp for further adaptive response to the dissimilar needs of the standby filament and the arc discharge lamp. The response of the operating network involves a change n the switching rate from 50 KHz during preignition to 35 KHz during the glow to arc region. The higher rate establishes a higher average on time for the transistor switch, which sets the energization of the standby filament at a desired light output. During the glow to arc transition, the frequency is reduced to 35 KHz, which increases the off time of the switch during which power is transferred to the main lamp.The reduction in frequency substantially increases the available power to the main lamp with a minimum and short duration reduction in light output from the standby filament. This increased available power meets the needs of the metal vapor lamps.

The term "discharge lamp" or "arc discharge lamp" is used interchangeably to characterize a lamp in which a discharge occurs through an ionizable gas and/or a vaporizable metal, and/or a vaporizable metal salt. While certain features of the invention are addressed to the use of a metal halide lamp, the principles of the invention are applicable to a larger class of arc discharge lamps.

BRIEF DESCRIPTION OF THE DRAWINGS The novel and distinctive features of the invention are set forth in the claims appended to the present application. The invention itself, however, together with further objects and advantages thereof may best be understood by reference to the following description and accompanying drawings in which: Figure 1 is an illustration of a novel lighting unit suitable for connection to a standard lamp socket using an arc discharge lamp as the principal light source, a standby light source and a compact power supply unit; Figure 2 is an electrical circuit diagram of the lighting unit; Figure 3 is a table of the five states of the lighting unit in a normal lighting sequence listing the conditions of the arc discharge lamp and the standby filamentary lamp, and the corresponding energization requirements; and Figure 4 is an illustration of a ferrite transformer forming a portion of the power supply unit.

DESCRIPTION OF PREFERRED EMBODI MENT Referring now to Fig. 1, a novel lighting unit for operating from a conventional low frequency (50-60 Hz) alternating current power source is shown. The lighting unit comprises a lamp assembly which produces light, and a power supply unit which supplies electrical power to the lamp assembly, with certain elements of the lighting unit having dual light production and ballasting functions.

The lamp assembly includes a glass enclosure 9 which contains a high efficiency arc discharge lamp 11 and filamentary resistance elements 1 2 and 1 3. The resistance elements 1 2 and 1 3 are an electrical part of the power supply being used to ballast the arc discharge lamp, while the element 1 2 in particular is a functional part of the lamp assembly becoming a supplemental light source. The power supply unit includes a rigid case 10 attached to the glass enclosure 9 and a screw-in plug 14. The plug 14 provides both electrical connection and mechanical attachment of the lighting unit to a conventional ac lamp outlet.

The unit develops the required energization for the arc discharge lamp during starting and operating conditions, including immunity to certain line transients, and produces an even illumination during starting by use of the supplemental light source.

The lighting unit provides an efficient and readily controlled light source which is economical in design and suited for household lighting. Efficiency in operation arises from the use of an arc discharge lamp as the principal source of light. The light output in lumens per unit of electrical power of an arc discharge lamp is typically 4 to 6 times greater than that of an incandescent lamp.

When electrically inefficient resistive ballasting is employed, as in the present unit, the efficiency is still comparable to that of a home fluorescent unit. By the selection of a minimum number of low cost, mass produced parts, the initial cost of the unit is comparable to a conventional fluorescent unit. When compared to an incandescent lamp, the savings in power during the lifetime of the new lighting unit more than compensates for the higher initial cost.

The novel lighting unit, as seen in Fig. 1, has the dimensional convenience of an incandescent lamp. The power supply unit occupies the space between the screw-in base 1 4 and the lamp assembly. In an incandescent lamp, this space corresponding to the neck of the lamp is normally allocated to the filament supporting structure. The glass enclosure 9 of the lamp assembly is approximately cylindrical. The lighting unit has approximately the same height and maximum diameter as an incandescent lamp. Light projection of the unit is over a solid angle slightly less than that of an incandescent lamp being reduced from a full sphere by the angle subtended by the power supply unit.

The lighting unit may be switched on, restarted, or turned off with the same convenience as an incandescent lamp and is insensitive to certain line transients normally affecting arc discharge lamps. The delays in production of light normally attendant upon the starting of an arc discharge lamp have been made less objectionable by the use of the supplemental incandescent element 12, packaged within the enclosure 9. In all stages of lamp operation, the light produced by the lamp assembly will appear to originate from the same approximate location and will retain approximately the same intensity and color balance. This feature is of particular interest for the half minute periods that it may take for the arc discharge lamp to reach full brightness after a cold start or the longer periods required for a hot restart. Finally, the power supply contains a line transient protection circuit.Should the line voltage be temporarily depressed, as by the starting of an electric motor also connected to the line, the supply will deliver energy to the gas discharge lamp in a form sufficient to maintain ionization until the transient is over. If the transient is long enough to cause cooling below thermionic temperatures, then a hot restart may be required.

The disposition of the elements of the lamp assembly are best seen in Fig. 1. The arc discharge lamp 11, the 60 watt filamentary resistance 12, and the 40 watt filamentary resistance 13, are all installed inside the single large glass envelope 9. The elements 11 through 1 3 are supported on leads sealed into the base of the lamp assembly. The gas filling the envelope 9 is an inert gas suitable for a conventional incandescent lamp. The discharge lamp 11 is shown with the positive electrode or anode down (near to the base) and the negative electrode or cathode up (remote from the base). The two electrodes are in turn sealed into the ends of a small quartz vessel whose outer contour is cylindrical except for a small central region of larger cross section, of less than 1/2" in diameter.

The interior of the arc lamp, which is not specifically illustrated, contains a spherical or elliptical central chamber filled with an ionizable mixture: argon, an ionizable starting gas, mercury, which is vaporized when hot, and vaporizable metal salt such as sodium and scandium iodides. When operating, an arc is formed between the electrodes which creates illumination throughout the chamber. Small, low power lamps of the type just described are referred to as metal halide or metal vapor lamps.

Light production is shared between the discharge lamp 11 and the filamentary resistance 12, while the latter and the filamentary resistance 1 3 provide resistive ballasting for the arc discharge lamp. In normal "final run" operation, the filamentary resistance 1 2 (and 1 3 if dimmed) conduct the current flowing in the discharge lamp but primary light generation occurs in the discharge lamp. In starting or restarting and warm-up of the main discharge lamp, the filamentary resistance (12 primarily) produce supplemental illumination.

In dimmed operation, the current levels and therefore the brightness of the discharge lamp is reduced by the imposition of resistance 1 3 in the current path.

A further feature of the lighting unit is the protection against accidental ultraviolet emission. The discharge is normally productive of substantial quantities of ultraviolet illumination. Since the electrode temperatures in the discharge lamp must be quite high, the enclosure must be of quartz. Quartz permits higher temperature operation but also transmits ultraviolet. Ultraviolet emission is then prevented by the use of a glass enclosure which is absorptive of ultraviolet.In the event that the glass enclosure is fractured, the possibility for continued operation of the discharge lamp and continued radiation of ultraviolet is precluded by the serial connection of the arc discharge lamp with the resistive filaments 1 2 and 1 3. The filaments are operated at sufficiently high temperatures during lamp operation that any destruction of the protective atmosphere as by a fracture of the glass enclosure destroys the filaments, preventing further lamp operation. Thus, the user is protected from ultraviolet emission in the case of fracture of the glass shielding by a nearly instantaneous extinction of the main lamp.

The arc discharge lamp exhibits several distinct states in conventional use and each active state requires distinct energization.

From a practical viewpoint, the arc discharge lamp has three essentially active states denominated Phases I-Ill and in inactive state.

In Phase I, "ignition" occurs. The duration of ignition is normally no longer than a second or two and often much shorter. It is the time required for a suitably high voltage to cause "electrical breakdown" of the gas contained in the arc discharge lamp to initiate a falling maximum lamp voltage. This latter condition is also referred to as the establishment of a "glow discharge". For purposes of definition, ignition is to be distinguished from pre-ignition. Pre-ignition is an interval preceding ignition, whose duration is predictable for a given discharge lamp and power supply unit, and is the period during which ignition is unprobable, normally due to non-optimum physical conditions in the lamp. Pre-ignition will be discussed below.

The ignition period consists of a delay period constituting most of the ignition period, distinguishable in principle from the pre-ignition period, and the much shorter microsecond-millisecond duration rise time associated with the initial discharge. The ignition delay assumes that the lamp is at standard ambient conditions, and is a period having a statistical average value, which by design is not longer than a second or two. The ignition delay is partly attributable to the random, isolated, natural creation of ions which instantaneously reduce the potential of the discharge, and is partly attributable to the nature of the ignition voltage. Should the ignition voltage potentials be sustained, a lower ignition delay is predicted than for pulsed ignition, and a lower voltage may be used.When the ignition voltage is pulsed, a coincidence between the applied voltage and random spontaneous ionizations will define the ignition instant. The probable time delay for such a coincidence will increase as the duration of the ignition pulse shortens.

As indicated above, the ignition delay should be less than a second or two for practical certainty in starting. An increase in the ignition potentials, or an increase in the duration of the ignition pulse will shorten the ignition delay. In the event that minimum voltage and minimum duration ignition pulses are desired, irradiation of the arc discharge lamp from a second light source can produce a drop of several hundred volts in the required voltage and facilitate the substitution of ignition pulses of a microsecond duration for a more sustained dc potential.

The rise time of the discharge is the short terminal portion of ignition. The arc discharge lamp will break down at the 1000-2000 volt ignition voltage, causing a sudden drop in lamp voltage to typically 1 5 volts and then the lamp may re-fire a second time, generally at a lesser voltage as the ionization level of the contained gases increases and entrance is made to the "glow to arc transition". In Phase I, lamps of the design herein contemplated require 1000 to 2000 volts using pulses of microsecond duration for ignition.

The power required for the ignition period is small.

Phase Il-the glow to arc transition--ex- tends from one-tenth of a second to perhaps two seconds and is characterized by a more sustained ionization level and a lower maximum voltage. As Phase II begins, the discharge is typically unstable, swinging between a maximum and a minimum value, with the voltage of the discharge falling continuously toward a lower maximum with a recurring minimum near 1 5 volts. As the average level of gas conduction increases, the maximum lamp voltage falls, the consumed power increases, and the temperatures inside the lamp also increase. As the maximum arc voltage falls through valus near 200-400 volts, a more substantial energy (typically 2-4 watts) is required by a metal vapor lamp.

Phase Ill begins with the establishment of the "arc" which occurs when a portion of the cathode has reached thermionic emission temperatures. At the marked transition from Phase II to Phase Ill, the voltage of the discharge loses its unstable quality and holds to an initial value of about 1 5 volts. In Phase III, a sustained low lamp impedance is exhibited, and a current limiting ballast is required to prevent excessive heating. At the beginning of Phase Ill, the lamp dissipation is set to be between 10 and 1 5 watts and significant light production starts.

The warm-up period, which is the initial portion of Phase Ill, normally lasts from 30-45 seconds. During the warm-up period, the lamp reaches full operating temperature and the contained gases reach their high, final operating pressures. The voltage across the lamp increases to a value of typically 87 volts with an accompanying reduction in lamp conductance. When the final run condition occurs, the lamp absorbs maximum power (typically 32 watts) and the maximum light output is produced.

The pre-ignition period is a variable period having a nominal minimum value of zero at standard ambient conditions and a maximum value between 45 seconds and 4 minutes if there has been a failure of the arc and a hot restart is required. If the lamp is de-energized in the course of normal operation, the lamp will be at an elevated temperature and at a high gas pressure for a short while. To restrike the arc when the lamp is hot, the potential required may be in excess of an order of magnitude more than for a cold start (e.g., 10-30 KV). The thermal time constants of the lamps are such that the time required for cooling from a hot operating condition to the point where a conventional (1-2 KV) voltage will restrike an arc may be from 45 seconds to 4 minutes.

Supplemental illumination is particularly important to the user during warm-up and the pre-ignition period for a hot restart. Assuming a normal cold state, pre-ignition and ignition last for a second or two and because the arc discharge lamp is producing neglible light, standby illumination is desirable. The glow to arc transition period approaches two seconds and supplemental illumination is desirable for the same reason. During warm-up, which lasts from 30-45 seconds, the light output of the discharge lamp increases from a very low to the normal value and supplemental illumination, preferably modulated in an inverse sense, is essential. In the final run condition, no supplemental illumination is needed.

Should a hot restart be required, the period required for restoration of an arc may take up to 4 minutes, and supplemental illumination is also essential. While standby illumination is particularly needed during warm-up and "hot restart", smoothness in illumination throughout the starting or restarting procedure is also desirable, and the standby illumination is adjusted for approximate constancy from preignition to early warm-up.

Suitable operating power for the arc discharge lamp and the standby light producing filament is provided by the power supply illustrated in Fig. 2. When the discharge lamp is in the final run condition, the power supply provides dc energy at approximately 145 volts with resistive ballasting. The ballasting reduces the voltages at the lamp to 87 volts, and sets the operating power at 32 watts.

During this period current is flowing through the flament 12 sufficient to hold it at an elevated temperature but insufficient to produce significant light in the filament.

In pre-ignition and ignition, the power supply produces a succession of high frequency unidirectional pulses with a high frequency alternating component. The unidirectional pulses, which initially occur at a 50 KHz repetition rate, provide substantially full power to the filamentary resistance. At the same time the alternating component, transformed and rectified, is applied to the arc discharge lamp for ignition. It is typically at 1 600 V peak to peak and at a low power level.

In the glow to arc transition, high frequency pulses for full power standby illumination continue, while the high frequency power available for the arc discharge lamp, as it crosses through a 200-400 volts maximum, is increased to 9 watts. This increase in available power is achieved by a lamp condition responsive reduction to 35 KHz in the pulse repetition rate. The increase in available power insures a reliable transition of the metal vapor lamp to warm-up operation.

When warm-up occurs, high frequency operation of the power supply unit ends and the power supply now produces a dc output, subject to 1 20 Hz ripple. This dc output is applied to both the arc discharge lamp and the filamentary resistance. The role of the filamentary resistance at the beginning of the warm-up is a joint one, producing a full light output while also ballasting the arc discharge lamp. Ballasting holds the initial dissipation in the arc discharge lamp to about 12 watts as the arc voltage falls to about 1 5 volts. As warm-up continues, the light produced by the filamentary resistance falls to a neglible value as the light produced by the arc lamp increases. The brightness of the filament during hot restart, ignition, glow to arc transition and initial warm-up are preferably set to approximately the same value.This has the subjective advantage of preventing sudden changes in brightness of the lighting unit during starting or restarting.

The lighting unit whose electrical circuit diagram is illustrated in Fig. 2, has as its principal components the arc discharge lamp 1 1, a dc power supply (14, 15, 16) for converting the 120 volt 69 Hz to dc, an operating network (17-36) for converting electrical energy supplied by the dc power supply into the forms required for operation of the lamp assembly and finally two filamentary resistances (12 and 1 3) which perform a ballasting function in the operating network, and one (12) of which enters into the production of standby light. The lighting unit has five active conditions characterized by the states of the discharge lamp, the standby light source, and the operating network. These states, which summarize the preceding discussion, are illustrated in Fig. 3.

The dc power supply circuit of the generating network is conventional. Energy is supplied from a 1 20 volt 60 hertz ac source via the plug 1 4 and two input connections to the ac input terminals of a full wave rectifier bridge 1 5. The positive output terminal of the bridge becomes the positive output terminal of the dc supply and the negative output terminal of the bridge becomes the common or reference output terminal of the dc supply.

The filter capacitor 1 6 is connected across the output terminals of the dc supply to reduce ac ripple. The output of the dc power supply during normal run operation of the arc discharge lamp 11 is 145 volts at about 1/3 amperes current, producing an output power of approximately 50 watts of which 32 watts is expended in the lamp. The power required of the dc power supply by the lighting unit during a hot restart is approximately 60 watts and the maximum required during warm-up of the arc discharge lamp is approximately 75 watts.

The operating network, which derives its power from the dc supply, and in turn supplies energy to the lamp assembly, comprises the elements 17-35 (optionally 1 2 and 13) connected together as follows: The filamentary resistance 1 2 and 13, diode 17, arc discharge lamp 11 and lamp current sensing resistance 33 are serially connected in the order recited between the positive terminal and the common terminal of the dc supply. A switch 18 shunts the filamentary resistance 1 3 producing dimming of the discharge lamp when open and undimmed operation when closed. The diode 17, which is poled for easy current flow from the dc source to the discharge lamp, has its anode coupled to one terminal of the resistance 1 3 and its cathode coupled to one terminal of the gas discharge lamp 11. The discharge lamp, which has a required polarization, has its anode coupled to the cathode of the diode 1 7 and its cathode coupled to one terminal of the current sensing resistance 33.

Continuing with a description of the operating network, a triggered monostable solid state switch is provided, constituted of a power transistor 19, a step-up transformer 20, and passive components 28, 29. The power transistor has base, emitter and collector electrodes. The step-up transformer 20 has a ferrite core for high frequency operation ( > 20 KHz), a main primary winding 21, a main second winding 22, a primary control winding 23 and a secondary control winding 24, all associated with the core. The control windings, as will be described, provide a transistor conduction control whose sense is responsive to the magnetic state of the ferrite core and produce monostable action, avoiding full core saturation.The main primary windings 21 has its undotted terminal coupled through the capacitor 25 to the positive source terminal and its dotted terminal connected to the interconnection terminal 26 between filamentary resistances 12 and 13. The main second winding of transformer 22 has its undotted terminal connected to the terminal 26, and its dotted terminal connected through the capacitor 27 to the anode of the discharge lamp 11. The emitter of the switching transistor 1 9 is coupled to the unmarked terminal of the primary control winding 23.

The marked terminal of the primary control winding 23 is connected to the cathode of the arc discharge lamp 11. The base of transistor 1 9 is coupled to the cathode of a clamping diode 28, whose anode is coupled through resistance 29 to the common dc terminal. The secondary control winding 24 has its unmarked terminal coupled to the base of transistor 1 9 and its marked terminal connected to the emitter. The base of transistor 1 9 is the point for application of a trigger pulse for initiating each conduction cycle.

The operating network is completed by the transistor 30 which, with its associated components, forms a triggering oscillator for recurrently turning on the solid state switching transistor 1 9. The trigger oscillator is turned on and off and also shifted in frequency in response to electrical conditions attributable to the electrical state of the arc discharge lamp.

The transistor 30 has its emitter coupled to the emitter of transistor 19, its base coupled through the capacitor 31 to the base of transistor 19, and its collector connected through the resistance 32 to the inter-connection terminal 26. A voltage sensing voltage divider is provided consisting of resistance 34 connected between the anode of diode 1 7 and the base of transistor 30 and resistance 35 connected between the base of transistor 30 and the common source terminal. During warm-up and final run operation, both dc states of the lighting unit, the diode 1 7 is forward biased, and the divider output voltage, at the base of transistor 30, is a direct measure of the lamp voltage.During the high frequency states of the lighting unit, the diode 1 7 is reversely biased when power is delivered to the lamp, so that the voltage on the voltage divider reflects the loading effect of the arc discharge lamp upon the transformer circuit and is an indirect measure of the lamp voltage. The connection of the emitter of transistor 30 to the non-referenced terminal of the resistor 33 in series with the gas discharge lamp 11, makes the trigger oscillator responsive to lamp current in the form of the voltage proportional to lamp current developed in resistance 33. The trigger oscillator is connected to respond in the manner noted above to the difference in sensed voltages. A position sensitive switch 36 is provided connected in shunt with the resistance 35 for preventing operation of the lamp unless it is in a vertical position.

The operating network, as earlier indicated, supplies the complex energy requirements of the lighting unit. The operating network acts in response to the sensed state of the main arc discharge lamp and assumes the states summarized in the chart of Fig. 3. The chart is not complete in that it does not specifically include dimmed operation of the lamp, nor "transient catch" operation. The final run state of the operating network will be taken up initially.

In the final run state of the lighting unit, the operating network provides direct current energization of the arc discharge lamp with resistive ballasting; permits dimming of the arc discharge lamp by user choice; and monitors the current and voltage of the arc discharge lamp to sense signs of incipient failure of the arc. The dc supply voltage has 15%-20% of 120 Hz ripple. During final run operation (with the lamp undimmed), the voltage of the arc is 87 voltage with 32 watts being dissipated in the arc lamp and 1 8 watts being dissipated primarily in the filamentary resistance 1 2. The light output is 2200 lumens, approximately that produced by a 1 50 watt three way incandescent lamp.In the undimmed mode, the current for the arc discharge lamp supplied by the 1 45 volt dc source (14, 15, 16), flows through a serial path including the filamentary resistance 12, the closed dimming switch 18, the diode 17, the lamp 11 itself and current sensing resistance 33.

The operating point of the lamp is established in the final run state at an approximate current of one-third ampere, a voltage of about 87 volts, and a power of 32 watts as mentioned above. These settings are determined primarily by the ballasting resistance 12, the properties of the arc discharge lamp and the output voltage of the dc supply. The diode 1 7 and small (2) resistance 33, which are the other serially connected elements in the circuit, have a negligible effect upon the current and may dissipate a fraction of a watt.

The lamp exhibits a long term negative resistance of less than 20 ohms in the final run state, and will tend to reach too high a dissipation if the current is not properly limited. Excessive lamp dissipation is prevented by selecting a ballasting resistance having a sufficiently large positive resistance. The filamentary ballast resistance 1 2 has a value of 10 ohms when cold and approximately 200 ohms when at the temperatures produced by normal lamp operating currents. The 200 ohm ballasting resistance establishes an operating point which is stable, preventing excessive lamp dissipation over normal ranges of supply voltage.

Line voltage variation, which affects the dc supply voltage, presents the primary danger of accidental dropout of the arc lamp. Excessive line voltage, if of a long term nature, may cause excessive heating, which is normally not serious, but reduced line voltage, particularly short term, may extinguish the arc. Under operating condition, the dc supply provides a dc output voltage with a 15% to 20% ripple. This produces a roughly 50% ripple in the lamp current and an opposite 8% to 10% variation in ac lamp voltage. Under these conditions, the power dissipation may fluctuate on an instantaneous basis, but if the average is appropriate, the instantaneous variation is of little consequence.Should the ac line voltage drop 20% and the ac lamp voltage climb 10%, an instant may occur at which the lamp arc may require more voltage than is available from the supply, and the lamp will go out. When the ac waveform of the line voltage contains a 1 20 cycle ac ripple, the supply voltage minima are sharp dips and the maxima are smooth. The corresponding curve representing the voltage demanded by the lamp contains sharp upward peaks with gradual slopes before and after the peaks. The peaks are delayed with respect to the minima of the supply voltage for a small fraction of the 1 20 Hz cycle. The curves of the dipping ac supply voltage and the rising ac lamp requirement may cross through a common value and yet not coincide at the same instant in time, a condition which would force lamp extinction.Should the line voltage fall a further 20% for a few cycles due to a transient load condition on the line, then the curves may be expected to intersect even though minima and maxima are mutually displaced. Should an intersection occur, the arc will be momentarily extinguished, and if not restruck before de-ionization takes place, the discharge lamp will go out. This possibility is reduced by the transient catch circuit, which re-ignites the lamp before de-ionization has occurred. The transient catch circuit, which will be taken up subsequently, must be set to respond to both a dip in lamp current, typically to 50 milliamperes and an elevation in lamp voltage of 10 volts, both conditions being present when the lamp is near extinction.

The sensitivity of the circuit to drop out may be reduced by increasing the size of the filter capacitor 1 6. The present capacitor size (50fd) is dictated in part by econonic considerations and in part by the need for a small total size. In the event that these considerations are unimportant, some increase in capacity may be desirable. An increase of more than a factor of 10 is normally not indicated in view of the stresses that are placed on the input side of the circuit.

The ballasting resistance (12) is held at a sufficiently high temperature to produce a glow, which in the event of perforation of the envelope produces destruction of the filament and a discontinuance of the current to the arc discharge lamp, thus protecting the user from UV radiation as earlier noted. In the dimmed mode, the switch 1 8 is open and the filamentary resistance 1 3 becomes a part of the principal current path. In the dimmed mode, the current setting of the gas discharge lamp is reduced for one-third to one-quarter ampere by the effect of the additional series resistance and the light output falls by a factor of approximately two.In the dimmed mode, the current levels are sufficiently high to cause destruction of one of the filamentary resistances in the event of damage to the glass envelope and energization of the arc lamp is also stopped.

Warm-up conditions of the arc discharge lamp mark a distinct break with the preceding Phase II, but a gradual transition into final run conditions. In Phase II, (i.e., the glow to arc transition), ionization has been established but the average current, dissipation and light output of the lamp are low and the discharge unstable. With the beginning of warm-up, the discharge stabilizes, bring about an increase in average current, dissipation and light output. The transition to the final run condition is smooth with the voltage gradually increasing from about 1 5 volts to the 87 volt final voltage, the power dissipation in the lamp gradually increasing from 1 2 to 32 watts and the light output, initially low, gradually increasing to its final value.Within the lamp during warm-up, the electrodes, the envelope and the contained gas reach the final operating temperatures and the gas pressure increases to the final value. Warm-up typically lasts from 30 to 45 seconds.

In the warm-up mode, the operating network produces a dc output subject to substantial (120 Hz) ripple, at an initially high, but moderating power. The dc electrical output during warm-up represents a sharp change from the high frequency output produced in the glow to arc transition, but the change from warm-up to the final run output, both dc outputs, is gradual. The electrical circuitry applicable to warm-up is the same as that discussed in connection with final run, with the changes in electrical energization and standby illumination being a smooth response of the operating network to electrical changes in the lamp.

During warm-up, the operating network goes through gradual electrical change in response to current in the main lamp. Excessive dissipation in the main lamp is prevented and the standby illumination is transitioned for a maximum to a minium lumens output as the main lamp illumination increases from a low to its final high value. At the onset of warmup, the gas discharge has stabilized at a low voltage and the lamp current tends to increase. The serially connected filament 1 2 is already in the low conductance state having been energized previously during the glow to arc transition. The large serially connected filamentary resistance prevents the initial arc current from exceeding a pre-determined value (6/l0ths of an ampere) and the initial main lamp dissipation from exceeding 1 2 watts.At the same time, the dissipation in the filamentary resistance is initially approximately 63 watts and a maximum power of 75 watts is required from the dc power supply. The initial conditions produce approximately 800 lumens of light in the standby filament, and as will be shown, continue the level of standby illumination at approximately the same level as during Phases I and II. As warm-up continues, the voltage in the main lamp climbs, the current falls, and the dissipation climbs. In the filamentary resistance, the initial dissipation of 63 watts falls with the fall of current in the main lamp and gradually reduces to 1 8 watts. The initial standby light output of 800 lumens gradually diminishes to the negligible output produced at the 1 8 watt dissipation level in final run.

The filamentary resistance 1 2 thus establishes the maximum dissipation during warmup and helps to produce the desired inverse modulation in standby illumination. It also sets the desired final run dissipation of the main arc discharge lamp. A filamentary resistance whose value is approximately 200 ohms when hot (10 ohms cold) provides the foregoing essential settings.

In pre-ignition, ignition and glow to arc transition, the transformer 20, the transistor switch 1 9 and the trigger oscillator (30, etc.) of the operating network assume an active role generating a high frequency output. This is in contrast to the passive role played during warm-up and final run when the energization produced is primarily dc. The sharp change in electrical output occurring between the glow to arc transition and warm-up is in response to conditions in the main lamp. The changes in electrical output of the operating network between pre-ignition and ignition and between ignition and the glow to arc transition are gradual, and are also in response to conditions in the main lamp.

In pre-ignition, ignition and the glow to arc transition, the operating network produces short duration, high voltage pulses from ignition of the arc discharge lamp, the voltage falling to a lower value in response to lamp loading in the glow to arc transition. During pre-ignition, the unidirectional high voltage pulses have substantial ringing, and they occur at a rate of 50 KHz. In the glow to arc transition the ringing is reduced and the frequency shifts to 35 KHz. The downward shift in frequency produces a shorter transistor conduction duty cycle, which increases the energy supplied to the lamp in the glow to arc transition. The operating network also supplies enough current to the filamentary resistance 1 2 to produce approximately 800 lumens of standby illumination through preignition, ignition and glow to arc transition.

The filamentary energization is primarily a series of unidirectional pulses at the 50-35 KHz rate.

The operating network produces the high frequency electrical energization described above as a result of high frequency switching of the monostable transistor switch. Intermittent switching of the transistor switch produces an alternating component in the main primary winding 21 of the step up transformer 20, a stepped up alternating component in the transformer output and a pulsating current in the filamentary resistance 1 2 which is primarily unidirectional.

Alternating current flow in the main primary winding takes place in the following manner.

Assuming that the transistor 1 9 has been turned on by a suitable trigger signal coupled to its input junction, a displacement current path is completed between the positive and common terminals of the dc supply. That path comprises in order the capacitor 25, the main primary winding 21, the NPN switching transistor 1 9 (collector and emitter electrodes respectively), the primary feedback windings 23 and the current sensing resistance 33. The switching transistor presents a low impedance when conducting, and the capacitor 25, the primary feedback winding 23 and the resistance 33 are also low impedances.As the current in the circuit increases, the primary feedback winding 23, which is inductively coupled to the secondary feedback winding 24, produces regenerative feedback in the input circuit of the transistor and turns it on more strongly. Accordingly, when the transistor conducts, the current rapidly builds up in the transformer primary winding, limited primarily by the primary inductance. The current build up continues, however, until a prescribed flux level is reached in the core of the power transformer. At that point, by a mechanism which will subsequently be explained, feedback is inverted to become degenerative, turning off the transistor 1 9 before full core saturation is reached.The discontinuance of conduction through transistor 1 9 opens the prior path for current flow through the primary winding and allows a portion of the energy stored in the circuit to dissipate in the form of a reverse current through the filamentary resistance 1 2. Thus, the current flow, which was initially out of the dotted terminal of the primary winding when transistor 1 9 was conducting, reverses and the current now flows into the dotted terminal.

The transformed version of the high frequency alternating voltage appearing across the transformer primary winding during preignition, ignition, and the glow to arc transition appears at the terminal of the winding 22, remote from winding 21. The output is coupled from winding 22 by means of the capacitor 27 to the anode of the discharge lamp 11. The output takes the form of unidirectional pulses by virtue of the presence of the diode 1 7 whose anode is coupled through filamentary resistance 1 3 (or the closed switch 18) to the undotted terminal of the secondary winding and whose cathode is coupled to the anode of the arc discharge lamp.The diode 1 7 is poled to permit application of a steppedup secondary voltage to the arc discharge lamp developed during the reverse current flow in the transformer primary circuit and to suppress application of the secondary voltage developed during forward current flow when the switching transistor is conducting. With the indicated parameters, and assuming substantial ringing, the available pre-ignition potential is the 1 600 volts peak to peak referred to earlier. Pre-ignition is nominally zero when the lamp is cold and from 45 seconds to 4 minutes when the lamp is hot.

The transformer 20 is essentially an autotransformer although in certain respects it may be regarded as a conventional transformer with separate primary and secondary windings. The windings 21 and 22 are serially connected, and wound in the same sense and the input is applied across the primary winding 21. When transistor 19 is conductive, the common terminal (point 26) between the primary and second windings is at reference potential and the voltage developed in the second winding reflects the primary to second turns ratio 500/140, with the diode 17 providing a short circuit and precluding the application of an output voltage to the main lamp.When the transistor 1 9 is nonconductive, stored energy developed across the winding 21 and referenced through capacitor 25 to the B + terminal of the power supply is released, and the device appears as an autotransformer with the transformer ratio being 640/140. Thus, during the critical period when the transformer is delivering energy to the arc discharge lamp, the transformer is in an auto-transformer configuration.

The current for standby illumination during pre-ignition, ignition, and the glow to arc transition is also produced by high frequency switching of the transistor switch. At the instant that the transistor switch becomes conductive, a direct current path is completed between the positive and common terminals of the de supply. The dc path includes the standby light producing filamentary resistance 12, the transistor 1 9 (collector and emitter electrodes, respectively), the primary feed back winding 23 and the current sensing resistance 33. The transistor 1 9 presents a low impedance, when conducting, and the primary feedback winding 23 and the resistance 33 are also low impedances. At the start of preignition, the resistance of the filamentary resistance may also be low, and a large initial current ensues.Self-heating is rapid, and the resistance quickly reaches a relatively stable, larger value near 200 ohms, which persists throughout the balance of the starting proce- dure. The heat dissipation in the filamentary resistance during pre-ignition is set primarily by its own relatively large resistance, the duty cycle of the transistor switch and the dc voltage available from the dc power supply, and may be increased by adjustment of these parameters.

In addition to the intermittent current supplied to the filamentary resistance in the dc path just described, the return portion of the alternating current flowing in the primary winding 21 of the transformer also flows through the filamentary resistance as discussed earlier. During pre-ignition, with the secondary winding of the transformer 20 being substantially open-circuited, the heating effect of the reverse current in the primary circuit is negligible. During the glow to arc transition, when the lamp draws the more substantial energy, the alternating current adds significantly to the total dissipation in the filament, in which pulsating dc energization is reduced. The glow to arc transition is of a sufficiently short duration to neglect this momentary variation in filamentary dissipation, and the standby light output appears to transition smoothly into the dc warm-up period.

The operating network is responsive to the electrical state of the arc discharge lamp to produce the outputs previously characterized during pre-ignition, ignition and the GAT period. The means by which this responsiveness is accomplished includes the triggering oscillator (transistor 30, etc.), lamp current sensing resistor 33 and the voltage sensing resistors 34, 35.

The trigger oscillator causes active operation of the transistor switch 1 9 during preignition, ignition and the GAT period and controls the transistor duty cycle to supply additional energy to the arc discharge lamp during the GAT period. Since the transistor switch is monostable, each trigger pulse supplied from the trigger oscillator initiates a conduction sequence.

The trigger oscillator is normally activated at the time the operating network is first energized, and remains energized through the preignition, ignition and glow to arc transition.

During pre-ignition, there is no lamp current, while during ignition and the glow to arc transition, the lamp current increases to one fifth of an ampere peak in short pulses. The voltage developed in the transformer primary winding at point 26 is high ( > 300V) during pre-ignition, falls appreciably under the loading affect of the lamp during ignition, and the glow to arc transition, and consists of a series of pulses initially with substantial ringing.

The foregoing current and voltage conditions reflecting the lamp condition during preignition, ignition and glow to arc transition are sensed in the operating network and combined differentially at the input junction of the oscillator transistor, and used to activate the trigger oscillator. Any lamp current flowing in the lamp current sensing resistance, to which the emitter electrode of the junction transistor 30 is coupled via the low impedance feedback winding 23, produces a voltage in a sense tending to back-bias the input junction. (The lamp current is zero at the start and remains small during these lamp conditions.) The voltage at the interconnection point 26 is applied across the voltage divider 34, 35, the output tap of which is coupled to the base electrode of the transistor 30.The voltage appearing at the interconnection point 26 is positive and a fraction (1/1 81th) of that voltage is applied to the base electrode. Here the voltage is in a sense tending to forward bias the input junction. During pre-ignition, the voltage at 26 is a maximum and sufficient, assuming time has been allowed for the capacitor 31 to charge up, to forward bias the transistor 30 and initiate oscillation.

The trigger oscillator operates as a relaxation oscillator with the capacitor 31 recurrently charged through the passive elements of the operating network and re-currently discharged by the transistors 19, 30. The charging period of capacitor 31 is determined primarily by the value of capacitor 31, the value of resistor 35 and, as will be shown, the differential voltage applied to charge the capacitor 31.The capacitor 31 has one terminal coupled to the base of the transistor 30, the output tap on the voltage divider 34, 35, and the other terminal coupled to the base of the switching transistor 1 8. The other capacitor terminal is led to ground through one path involving the back-biased resistor 38 serially connected with resistance 39, and a second path including the serially connected, low resistance, feedback windings 24, 23 to the unreferenced terminal of the lamp current sensing resistance 35. The discharge of the capacitor 31 starts when transistor 30 begins to conduct, and is completed after the transistor switch 1 9 is turned on by transistor 30.

With both transistors conducting, both terminals of capacitor 31 are coupled through a conducting junction to a common point, discharging the capacitor 31, and removing the forward bias on transistor 19, turning it off.

As will be seen below, the turn off action of the transformer 20 leaves a residual inverse voltage on the capacitor at the end of switch conduction.

As an examination of the circuit will show, when sufficiently high potentials are present at the interconnection point 26 and assuming a low lamp current, the oscillator will start to conduct when the capacitor 31 reaches the value required to forward bias the input junction of the transistor 30 (+ 0.6 volts) as indicated above. The voltage on the capacitor is determined by the difference between the voltage at the voltage divider output and the voltage due to lamp current in resistor 33.

The charging resistance for capacitor 31, regarding the voltage divider and current sensing resistor 33 as serially connected generators, is primarily the value of resistor 35 since resistor 34 is electrically in parallel and much larger than resistor 35. Resistor 33 is negligible because it is in series and much smaller than resistor 35. Thus, the time constant of the relaxation oscillator is determined primarily by the capacitor 31 and resistor 35. Regarding the voltage divider as one generator, its charging voltage is derived from the interconnection point 26. The voltage divider represents a high impedance load for the primary circuit and thus reproduces as charging current the voltage waveform at 26.In short, the equivalent charging circuit viewed from the divider side may be represented by an attenuator of 181 to 1 from the tap point 26 and by a source resistance of 1800 ohms. From the capacitor terminal coupled to current sensing resistance 33, the charging network is represented by a voltage source (a source of low internal impedance) equal to the cathode current times the two ohm resistance of resistor 33 and, as stated above, the charging resistance is essentially 1000 ohms.

Once the transistor 30 conducts, current flows in the primary feedback winding 23 and the strongly regenerative feedback action involving secondary feedback winding 24 and capacitor 31 produces a short duration trigger pulse for turning on transistor switch 1 9.

The initial starting conditions and charging interval for each oscillation of the relaxation oscillator are established by the operating network. The capacitor 31 is fully discharged when both transistors 1 9 and 30 become conductive. The capacitor 31 assumes a reverse charge as a result of the feedback reversal in windings 23 and 24 attributable to maximum conduction by transistor switch 1 9.

When conduction ends, a conduction inhibiting voltage of approximately 4 or 5 volts is produced on capacitor 31. The inverse voltage is limited by the serially connected diode 28 and resistor 29, and represents the starting point for each charging interval of the relaxation oscillator. While the transistor switch 1 9 is conducting, the virtual generators embodied by the voltage divider 34, 35 and the lamp sensing resistor 33 of the relaxation oscillator are inactive, precluding recharging of the capacitor 31, and precluding the starting of the next oscillation cycle.

Assuming that the lamp current has begun to flow and the voltage across the lamp has begun to increase, the differential voltage used to charge capacitor 31 falls on the average; increasing the period required to turn on the transistor 1 9 and initiate the next trigger pulse. As will be described in more detail below, this provides more time for the energy stored in the input circuit of the operating network to be released to the lamp. The waveshapes confirm that during the glow to arc transition, the cathode current decays before the succeeding trigger pulse occurs, indicating that the stored energy has been expended in the gas discharge lamp. Earlier in the starting cycle, the lamp cathode current may be truncated by the next conduction interval, and less stored energy is delivered to the lamp.The circuit has been designed so that the non-conduction period is maximum when the lamp voltage is in the glow region (approximately 200-400 volts), to maximize the output power at about 9 watts for metal vapor lamps.

The charging time constant is about 5 microseconds and provides some smoothing within each pulse, reducing the noise sensitivity, but negligible pulse to pulse averaging.

The principal function of the capacitor 31 is to serve as the integrating capacitor in the RC network used to time the off interval of the power transistor. During pre-ignition, ignition and the glow to arc transition region, high frequency operation continues, with the trigger oscillator recurrently turning on the transistor switch 1 9 while the transistor switch turns itself off through feedback reversal in the transformer 20. The trigger oscillator transistor 30 is turned off shortly after conduction by transistor switch 1 9 removes the conduction favoring charge on capacitor 31.

Transistor 30 remains quiescent through the balance of switch conduction. Turn on of the transistor switch is achieved through the coupling of the base electrode of transistor 30 through the capacitor 31 to the base of transistor 1 9, the interconnection of the emitters of transistors 1 9 and 30 together, and the shared connection of the transistors 1 9 and 30 to the transformer feedback windings 23 and 24. When transistor 30 becomes forward biased, and starts to conduct, collector current is developed in the primary feedback winding 23. This produces the regeneration needed to create a trigger pulse on the order of 1 /10th ampere and having a sub-microsecond duration at the secondary winding 24.The trigger current 23 flowing in the secondary winding 24 turns on the main switching transistor 19, initiating monostable switching action. Transistor 19 completes its conduction cycle, which is set by transformer design to be shorter than the interval between trigger pulses, and turns off in response to the reversal in feedback provided by the feedback windings 23, 24.

High frequency operation of the switch continues so long as the trigger oscillator generates trigger pulses.

Once the arc discharge lamp has reached therm ionic operation corresponding to warmup, the high frequency output produced by transistor switching is designed to stop and the dc state commences. The trigger oscillator 31, which triggers the monostable transistor switch 1 9 into active operation, remains reversely biased due to a new set of current and voltage conditions in the operating network and becomes inactive. The rectified high frequency voltage at point 26, previously applied across the voltage divider 34, 35 is replaced by a sustained dc voltage with some ripple, representing the lamp voltage. The dc voltage continues in a sense favoring conduction, but is lower by 1 or 2 orders of magnitude.The diode 17, now forward biased, connects the voltage divider across the lamp, and the voltage divider now senses 1/181to of the new lamp voltage, initially 1 5 volts. Simultaneously, a maximum initial lamp current of 6/10'his of an ampere occurs in resistor 33, developing a conduction inhibiting voltage of approximately 1.2 volts. The differential voltage produces a reverse bias on the input junction of the trigger oscillator, inactivating it, and with it, the transistor switch 1 9.

As warm-up continues into final run condition, the lamp voltage rises and the lamp current falls. The lamp condition sensors are set to keep the trigger oscillator inactive through warm-up and final run. In final run, the lamp reaches a current of 0.3 ampere and a voltage of 87 volts. Should the lamp voltage rise 10 volts above the normal value (e.g., 97 volts), and the current fall to 0.050 ampere, then the trigger oscillator will be reactivated as a safeguard against transient dropout.

The operating network is designed to meet the ine:reased power demand during the glow to arc transition without developing undue dissipation during a prolonged pre-ignition period. This accommodation of the operating network to the requirement of the lamp for additional energy during the glow to arc transition is achieved in part by the voltage and current responsive adjustment of the frequency of the trigger oscillator which affects the duty cycle of the switch and in part by optimizing the transformer design. The change in duty cycle also reduces the filamentary energization on the pulsating dc component as noted earlier.

During the pre-ignition and ignition, the operating frequency (pulse repetition rate) of the trigger oscillator is approximately 50 KHz and it falls to 35 KHz during the glow to arc transition in response to the differential voltage reflecting the voltage sensed by the volt age divider 34, 35 and the current sensed in resistor 33. During pre-ignition, the primary voltage is > 300 V peak to peak while the gas discharge lamps draws no current. In the glow to arc transition, the maximum primary voltage falls to a value of about 1 50 volts peak to peak under the loading effect of the discharge lamp. In this state, metal vapor lamps draw significant current (0.2 ampere peak current pulsed), and require the four watts of additional power referred to earlier.The voltage reduction on the voltage divider and the increase in sensed lamp current signals the need for more power.

The trigger oscillator responds to the reduction in the voltage across the voltage divider 34, 35 and the increase in lamp current during the glow to arc transition of the arc lamp by a decrease in the pulse repetition rate. The reduction in pulse repetition rate is attributable to the fact that the trigger oscillator is a relaxation oscillator. A reduction in voltage across the voltage divider 34, 35 reduces the voltage on the capacitor terminal associated with the base electrode and an increase in lamp current increases the voltage on the capacitor terminal associated with the emitter electrode.The differential voltage, which is the means for charging the capacitor 31 through the resistor 35, is thus reduced, and with it the rate at which the capacitor 31 charges to the potential required to forward bias the transistor 30, connected in shunt with capacitor 31, to start the next pulse. The decrease in frequency is a continuous variable, whose sensitivity may be adjusted to occur earlier or later in the glow to arc transition. The reduction to 35 KHz in the indicated circuit makes 9 watts of average power available for the main lamp when the main lamp is at approximately 250 volts with current peaks of about 0.2 amps.

The 50 to 35 KHz reduction in frequency of the trigger oscillator is accompaned by a decrease in the transistor conduction duty cycle, which, as will be seen, facilitates the described power increase to the arc lamp. The interval between the trigger pulses is greater than the conduction interval of the switching transistor 19, the latter being determined by the magnetics. Thus, when the interval between trigger pulses is shortened, the off time of the transistor is shortened, and a higher duty cycle results. At 50 KHz, transistor 1 9 conducts for an approximately 60% duty cycle, while at 35 KHz it conducts for an approximately 35 % duty cycle.

The reduction in duty cycle of the transistor switch caused by a reduction in the pulse repetition rate provides some of the desired increase in power supplied to the discharge lamp. Assuming that a given amount of energy has been stored in the reactive elements during conduction, and that it is delivered to the main lamp at a given rate during nonconduction, an upward increase in off time may permit more of the stored energy to be delivered. This is in fact the case and an upward adjustment of the switch off time from 11 to 1 7 microseconds facilitates the supply of 50% additional watts energy to the arc discharge lamp during the glow to arc transition.

While energy storage in the operating network may be affected by the operating frequency, the primary parameters which effect an increase in energy delivered to the discharge lamp during the GAT period are the decreasing pulse repetition rate (i.e., increasing pulse repetition period) and the falling arc voltage. Increasing the pulse repetition period, which contains a relatively fixed transistor conduction interval, increases the switch off time, during which stored energy may be delivered to the arc discharge lamp. An examination of the current waveforms of the arc discharge lamp indicates that during a shorter off time, the current supplied to the lamp is still flowing when the switch becomes conductive. Substantial current will flow to the lamp for up to 1 5 microseconds using the indicated circuit parameters.The slow decay in the lamp current waveform is controllable by the values of the capacitance 25, the inductance 21 and the filamentary reistance 12 in the transformer primary circuit, (e.g., the smaller the filamentary resistance, the longer the time required for the stored energy to dissipate in the primary circuit, and the more protracted the flow of current to the gas discharge lamp.) To achieve the desired increase in energization, the time constant of the discharge (6-7 microseconds) must be on the same order as the off times (7-15 microseconds) of the switching transistor.

A second factor which increases the power applied to the arc discharge lamp during the GAT period, has to do with a proper power match between the transformer and the arc tube. As the arc tube voltage falls, a better power match is realized when the arc voltage approaches a value approximately half the open circuit voltage of the transformer. If the arc tube voltage is appreciably below or appreciably above this value, less power will be delivered. The shape of the power curve is approximately quadratic in this region and the indicated turns ratio (140-640) produces an optimum power transfer at approximately 250 volts.

A third factor in achieving the ability to achieve both a large open circuit output voltage during pre-ignition and a high power transfer to the lamp during the glow to arc transition, and also influenced by the frequency shift, is the transformer leakage inductance. This is designed to be large by the selection of a large gap in the center leg of the magnetic structure, and the separation of the primary and secondary windings in sepa rate pies with the separation near the air gap.

The leakage inductance permits an increased resonant rise in the open circuit output voltage during pre-ignition. During the glow to arc transition, the effect of the leakage inductance, electrically in series with the lamp load, would be to reduce power in the lamp. However, by reducing the frequency of the waveform, the series reactance of the leakage inductance is reduced and more power is available for the main lamp.

The arc lamp voltage is continuously adjusting to a lower value throughout the GAT period, so it is conventional to apply a power specification to a single voltage point. Assuming conventional loads, and a conservative specification of that power point, the requisite power should be available throughout the glow to arc transition. The present supply has an approximately 9 watt capability at the 250 volt rating point, while the normal lamp requirement is 4 watts for a metal vapor lamp.

The foregoing discussion of the power "boost" to the arc discharge lamp during the glow to arc transition by reducing the repetition rate of the trigger oscillator and reducing the percentage of conduction time of the transistor switch, is not complete without a consideration of the effect of this change upon the energization provided to the standby filament. In overly simple terms, power is supplied to the dc filament when the transistor switch is on and none is supplied when the transistor switch is off. Similarly, power is supplied to the gas discharge lamp when the transistor switch is off and none is supplied when the transistor switch is on. Increasing the percentage of on time tends ta increase the supply of power to the filament, and increasing the off time tends to increase the supply of power to the arc discharge lamp.

The foregoing simplification provides the practical basis for changing the optimization of the power supply to the dissimilar needs of the standby lamp and the discharge lamp at different points in the starting procedure.

More particularly, the percentage of on time may be set to meet the requirements for a desired level of standby illumination during pre-ignition and reset to meet the increased power demands of the main lamp during the low voltage region (e.g. 250 volts) in the glow to arc transition. The optimization generally permits minimization of the size of the magnetics and other components to meet predetermined output criteria.

In practical terms, when the transistor switch is operated at the higher frequency (50 KHz), the percentage of on time of the switch is larger (although the "on" time of each conduction interval is substantially constant and set by the magnetics, allowing more energy to be provided to the standby filament from the dc source through the switch. Operating at a lower frequency (35 KHz) increases the percentage of off time of the switch and allows more energy to be provided to the arc discharge lamp since the energy is discharged at a finite rate. This energy increase meets the needs of the discharge lamp during the glow to arc region.During pre-ignition and ignition, little energy is needed by the main lamp, and the higher repetition rate is unobjectionable for the main lamp since it does not significantly compromise the ignition and pre-ignition process. The foregoing setting is therefore optimized at 50 KHz in accord with the need for sufficient filamentary energization (56 watts) for the desired level of standby illumination. During the glow to arc transition, when the lower repetition rate is employed, the percentage of on time of the switch is reduced, bringing about a reduction in the pulsating dc current fed from the dc source through the switch. However, circulating current in the primary circuit increases and generally offsets much of the loss in pulsating de current.The glow to arc transition period is short (less than 2 seconds) and any change in standby illumination is unimportant when compared to the major advantage of providing more glow to arc transition energy to the main lamp. The 50 KHz to 35 KHz frequency shift produces an approximately 50% increase in available glow to arc energy and results in a less than 5% reduction in standby illumination.

For the foregoing reasons, the duty cycle is readjusted from one value in which filamentary energization is optimized (56 watts) at a desired 800 lumens of standby illumination during pre-ignition and ignition to another value in which the power to the arc, lame is optimized in the glow to arc transition to provide a power boost to insure that the lamp transitions to warm-up smoothly and with a high degree of certainty.

The present invention has been shown in an embodiment wherein resistive ballasting is provided by a filamentary resistance which also provides standby illumination. One may also employ the operating network herein described in a configuration wherein the ballasting resistances do not also supply standby illumination. In such a case, reliability and convenience in starting still dictates that the resistive element exhibit a substantial resistance increase with applied voltage such as is exhibited by conventional incandescent fila ments. Phrased another way, it is particularly desirable that the resistance element exhibit a strong positive coefficient of resistance with current. A resistance having this property has several important advantages in a circuit of the type herein described. During the final run state of the discharge lamp, a positive-tesis- tance coefficient tends to stabilize the operat ing point of the arc, discharge lamp against variation in the line voltage or load. During cold starting, the resistance used for ballasting at a much higher value (""200 ohms) may have a low value, e.g., 10 ohms, for a substantial portion of the ignition process.

This low value provides more current and more power during the glow to arc transition in particular and shortens the starting procedure. Since most starts are cold starts, the effect of shortening the starting procedure is to increase the lifetime of the electrodes in respect to the expected lifetime from a more delayed starting procedure using a ballasting resistance having a larger fixed value. A third advantage of using a positive coefficient resistance arises in respect of the dimming resistance (13). During either a cold start or a hot restart, approximately the same power is available for the glow to arc transition in the dimming mode as in the bright mode.The power available for the glow to arc transition is not reduced in the dimming mode because the dimming resistance (filamentary resistance 13) is cold during ignition and pre-ignition and remains cold and thus at a lower resistance through the glow to arc transition. The resistance remains low enough to prevent a substantial reduction in the power available for the lamp during the glow to arc transition.

The dimming resistance is in the secondary circuit of the power transformer and experiences no significant current flow until the glow to arc transition produces the first significant lamp current. The glow to arc transition is sufficiently short, taking into account the current levels, to prevent significant heating of the filamentary resistance and it remains at a low value until warm-up occurs.

The power transformer 20 which is employed in the operating network is the invention of W. Peil and R J. McFadyen and the subject of the copending application Serial No. 969,381 referred to earlier. The core structure and the windings (21, 22, 23, 24) of the power transformer are illustrated in Fig.

4. More particularly, the core comprises two E cores arranged in an "8" core configuration with an air gap in the common leg. An aperture 41 is provided in the upper E core, as shown in Fig. 4, at the base of the common leg. The aperture is bounded by three contiguous regions, each defining a flux path and coilectively forming a small virtual toroid. The first region provides a path between the common leg and the upper left portion of the upper E core; the second region provides a path between the common leg and the upper right portion of the upper E core; and the third region provides a path between the upper left and upper right portions of the upper E core.

The primary and secondary power windings are wound on a bobbin 42 around the common branch of the core in three pies, one being assigned to the primary winding and two to the secondary winding, the primary winding occupying the upper portion of the bobbin in proximity to the aperture 41. The division separating primary and secondary windings occurs near the air gap to increase the leakage between primary and secondary windings. The division of the secondary winding into two pies minimizes the distributed capacity and reduces the voltage stresses in the secondary winding.

The feedback windings 23 and 24, which are wound through the aperture 41 and about the upper portion of the upper E core, provide a mechanism for monostable transistor operation. As explained in the earlier cited application, if the transformer 20 is connected to the transistor 1 9 in the manner generally illustrated in Fig. 2, with the secondary feedback winding coupled to the input junction and the primary feedback winding and the primary power winding connected to carry collector current, and further assuming a trigger pulse to initiate transistor conduction, then conduction will occur for a short period and terminate. The result is the production of an approximately rectangular output waveform with high overall efficiency.

The feedback windings 23, 24 provide monostable operation by supplying feedback to the transistor responsive to the magnetic state of the core which reverses in sense when a predetermined toroidal region becomes saturated from the build-up of collector current.

Initial regenerative feedback is provided between primary feedback winding 24 and secondary feedback winding 23, close coupling being provided by the virtual magnetic toroid.

When one region of the magnetic toroid saturates, direct coupling between the primary feedback winding and the secondary feedback winding is substantially reduced and feedback in another sense is applied to the secondary feedback winding 24 as a joint result of the sudden increase in reluctance coupling the two windings; the constant voltage effect of the transistor input junction acting with the inductance of the secondary winding to constrain the rate of change in flux in the outer region; and the stored charge at the junction which supports a reversal in current flow and whose removal completes the transistor turn off.The arrangement produces an automatic turn off of the transistor prior to the core achieving full saturation, resulting in increased transistor switching efficiency and more reliable operation of the transistor by avoiding the stresses which would automatically occur should the core be driven to saturation. The design also permits substantial reductions in the amount of ferrite material required for a predetermined power level.

In a practical embodiment of the present application, the winding counts are as illustrated in Fig. 2 and the core has the external dimensions of .760" x 0.64" x 0.1875", the aperture having a diameter of 0.040", spaced 0.0738" from the outer surface. The air gap of the center leg is 0.030". The outer leg is 0.9375" wide and the center leg is of double width. The E cores are made of the Stackpole 24B ferrite material, catalog 57.04340, the upper E core being modified by the introduction of the aperture and the shortening of the center leg. The bobbin 42 is designed to fit within the openings of the figure "8" core, with the windings themselves having a rounded rectangular outline generally approximately 3/8" X 3/8", the pie being 3/32" thick and separated by 0.030" spacers.

The foregoing operating unit supplies the requisite electrical energy to the standby filamentary resistance and the arc discharge lamp for starting and normal operation, including "arc" maintenance during short reductions in the line voltage. The final output for operation of the main lamp is dc with a tolerable degree of 120 hertz ripple. The ripple is not necessary, but its removal beyond a certain point costly. The energization of the standby filamentary resistance during the warm-up period is of the same nature--de with 120 hertz ripple. During the earlier starting period, the standby energization is primarily unidirectional pulses at above audible frequencies arising from conduction in the transistor switch, and secondarily circulating or "alternating" currents in the primary circuit of the transformer, which primary circuit includes the standby filament.The term "pulsating" has been employed in a sense to include both elements of standby energization. The arc discharge lamp derives its power from these "alternating" voltages which are applied to the transformer primary for transformation. The transformed quantities are rectified and delivered to the discharge lamp primarily in the form of unidirectional pulses.

While a single circuit embodiment of the operating network has been shown, other variations lying within the scope of the invention exist. In the event that some increase in core size is unobjectionable, the filamentary resistance, the primary winding, the second winding, and the main lamp may be serially connected in the order recited between the B + and the reference terminals, respectively, with the transistor switch being connected between the winding interconnection and the reference terminal. An additional diode to complete the ac input circuit is connected between the B + terminal and the winding interconnection. The ballast and dimming resistance may be the first and second elements in the series path or the dimming resistance may be separated and introduced after the second winding.

In the event that a dc current through the transformer is undesirable, several circuit variations are also possible. In one circuit variation, the filamentary resistance, a pair of diodes, a dimming resistance and the lamp are serially connected in the order recited between the B + and reference terminals, respectively, and the transistor switch is coupled between the terminal of the first filamentary resistance remote from the B + terminal and ground. The transformer has one end terminal connected to the reference terminal, the other end terminal capacitively coupled to the point between the diodes, and the transformer tap is capacitively coupled to the switch and the first filamentary resistance interconnection.

A further variation in which direct current flow in the transformer is avoided, is one in which the ballasting filamentary resistance, the dimming filamentary resistance, the arc lamp and a pair of diodes are serially connected in the order recited between the B + terminal and the reference terminal. The transistor switch is connected between the interconnection between filamentary resistances and the reference terminal. The transformer has the end terminal of the primary winding connected to the B + terminal, the end terminal of the second winding capacitively coupled between the diodes, and the interconnection between windings capacitively coupled to the ungrounded electrode of the transistor switch.

A further variation having substantial similarity to the preferred embodiment is one in which the ballasting filamentary resistance, the dimming filamentary resistance, a diode pair and the arc lamp are serially connected in the order recited between the B + terminal and the reference terminal. The transistor switch is connected between the interconnection between filamentary resistances and the reference terminal. The transformer has one end terminal connected to the B + terminal, the other end terminal capacitively connected between the diodes and the winding interconnection point capacitively connected to the interconnection between filamentary resistances.

Claims

1. A lighting unit comprising: A. a dc power supply having two output terminals, one a reference terminal, B. a main discharge lamp requiring energization dependent on its electrical state, and C. an operating network comprising: (1) a first resistance element exhibiting a substantial resistance increase with applied voltage, (2) alternating electrical energy transforming means whose output is coupled to said main lamp, (3) a solid state switch, (4) interconnecting means for coupling current from said dc supply (a) in a dc form to said resistance element and said lamp in series for energizing and ballasting said main lamp, when said switch is in a stable first condition, and (b) in a pulsating form in said resistance element and in an alternating form in the input of said transforming means for starting said main lamp, when said switch is in an intermittently operated, second condition, and (5) means responsive to the electrical state of said main lamp for maintaining said switch in said first condition during warm-up and normal operation of said main lamp, and in said second condition during other states of said lamp.

2. A lighting unit as set forth in Claim 1 wherein said first resistance element is a filament reaching incandescent to supply standby light to supplement that from said main lamp.

3. A lighting unit as set forth in Claim 2 wherein A. said dc power supply comprsies rectification means and a filter capacitor for energization from a conventional ac main, B. a case is provided attached to said enclosure, said case containing said dc power supply and said operating network, less said filamentary first resistance element, and including electrical terminal means for supportive attachment of the lighting unit to an electrical outlet.

4. A lighting unit as set forth in Claim 2 wherein said intermittent, second switch condition occurs during pre-ignition, ingnition and glow to arc transistion, and said first switch condition is nonconduction, said first condition occuring during warm-up and normal operation of said main lamp.

5. A lighting unit as set forth in Claim 4 wherein the light output of said first resistance element, when said switch is intermittently operated, is substantially equal to that produced by said first resistance element during early warm-up of said main lamp.

6. A lighting unit as set forth in Claim 4 wherein said electrical energy transforming means is a transformer.

7. A lighting unit as set forth in Claim 6 wherein A. said first resistance element and said switch are serially connected across said dc supply, and B. said transformer has a main primary winding serially connected with said switch across said dc supply and said first resistance element is connected in shunt with said main primary winding, said intermittent operation of said switch prividing said pulsating current in said first resistance element, and said alternating current in said transformer input.

8. A lighting unit as set forth in Claim 7 wherein said transformer comprises: A. a second main winding, said output being derived from one terminal thereof and the other terminal thereof being connected to the terminal of said main primary winding remote from the connection to said first capacitor, and wherein said operating network comprises:: B. a second capacitor connected in the series path between said second winding and said main lamp for ac coupling and to prevent dc current flow in said second winding, and C. a diode connected in the series path between said first resistance element and said main lamp poled to permit said dc current when said switch is in said first condition, and in shunt with the series connected main second winding and second capacitor for rectifying the transformed alternating output coupled to said main lamp, when said switch in in said second, intermittent condition and poled to permit said alternating output instants in said second switch condition when it is non-conductive.

9. A lighting unit as set forth in Claim 1 wherein said main lamp responsive means comprises: A. a lamp current sensing resistance serially connected between said main lamp and said reference terminal in which a voltage proportional to lamp current is sensed, B. a voltage divider connected between the main winding interconnection and said reference terminal for sensing voltage which during said non-conductive instants in said second switch is indicative of lamp loading and which during said first switch condition is indicative of lamp voltage, and C. means connected between said current sensing resistance and the output of said voltage divider for response to the difference between said first and second sensed voltages.

1 0. A lighting unit as set forth in Claim 1 3 wherein A. said switch comprises a first transistor, and B. said lamp state responsive means is responsive to current and voltage conditions in said operating network dependent on the state of said main lamp, said means comprising a second transistor connected in an oscillatory configuration for coupling conduction inducing trigger pulses to said first transistor.

11. A lighting unit as set forth in Claim 7 wherein said main lamp responsive means initiates and sustains said second, intermittent switch condition when A. the current in said main lamp is sufficiently small, corresponding to pre-ignition, ignition or glow to arc conditions in said main lamp, and. B. the voltage between the main winding interconnection and said reference terminal is sufficiently high, during said nonconductive instants, said voltage reflecting the loading on said operating circuit by said main lamp during said three last recited main lamp conditions.

12. A lighting unit as set forth in Claim 11 wherein said main lamp responsive means terminates said second switch condition, initiating and sustaining said first switch condition when A. the current in said main lamp is suffici ently high, corresponding to warm-up and final run conditions of the main lamp, and B. the voltage between the main winding interconnection and said dc reference terminal is sufficiently low, corresponding to said two last recited main lamp conditions.

1 3. A lighting unit as set forth in Claim 1 2 wherein said main lamp responsive means initiates said second switch condition, when said switch is in said first condition, when A. the current in said main lamp falls below an arbitrary value, substantially less than said normal operating current, and B. the voltage between said main winding interconnection and said reference terminal rises an arbitrary value above said normal voltage, corresponding to a lamp condition likely to result in its extinction.

14. A lighting unit as set forth in Claim 8 wherein said main lamp is a metal vapor discharge lamp, and said operating network delivers pulsating dc energy to said first resistance element and stores electrical energy during conductive instants of said switch in said second intermittent condition, the energy storing elements including said transformer and said first capacitor, the transformer thereof transferring power to said main lamp during the non-conductive instants in said second switch condition, said power transfer capability being greater during said glow to arc transition than during pre-ignition or ignition.

1 5. A lighting unit as set forth in Claim 14 wherein A. the frequency of intermittent operation of said switch, which has one value during preignition and ignition, is reduced to a lower value during the glow to arc transition, and B. said transformer is designed with substantial leakage for resonant enhancement of the peak voltage when lightly loaded to facilitate lamp ignition, said lower frequency during the glow to arc transition minimizing the reduction in power transfer by reducing the reactance thereof.

1 6. A lighting unit as set forth in Claim 14 wherein said main lamp responsive means comprises A. means for distinguishing between preignition and the glow to arc transition, and B. means responsive to said distinguishing means for establishing a larger percentage of switch conduction time during pre-ignition to establish a desired level of standby illumination, and a reduced percentage of switch conduction time during the glow to arc transition to increase the power available for establishing the arc in said main lamp.

1 7. A lighting unit according to Claim 1 and substantially as herein described with reference to the accompanying drawings.