GB1575832A

GB1575832A - Operating circuit for a gaseous discharge lamp

Info

Publication number: GB1575832A
Application number: GB2315677A
Authority: GB
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1976-06-02
Filing date: 1977-06-01
Publication date: 1980-10-01
Also published as: DE2718151A1; NL7705830A; AU511603B2; MX147047A; AU2506977A; FR2354016B1; FR2354016A1

Description

(54) OPERATING CIRCUIT FOR A GASEOUS DISCHARGE LAMP (71) We, GENERAL ELECTRIC COMPANY, a corporation organized and existing under the laws of the State of New York, United States of America, of 1 River Road, Schenectady 12305, State of New York, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to operating circuits for gaseous discharge lamps, and more particularly concerns direct current operating circuits for sodium vapor discharge lamps.

It is a general object of the invention to provide an improved DC operating circuit for pulsed operation of loads in the form of gaseous discharge lamps, particularly of high pressure sodium vapor type, to thereby produce improved color properties of the lamp light output.

The present invention provides a load operating circuit for a gaseous discharge lamp, characterized by a DC power source, a commutating capacitor, a first inductor in series circuit with said commutating capacitor, said series circuit being connected across said DC power source and forming therewith a charging circuit for said capacitor, a second inductor connected across said capacitor and forming therewith a discharging circuit for discharging said capacitor, a unidirectional controlled power switch means in series with one of said inductors, said switch means being commutated by said capacitor and controlling power, when in the conducting state, to a gaseous discharge lamp load, said one inductor having a substantially smaller inductance than the other inductor, control means coupled to said controlled power switch means for repetitively operating the same at predetermined intervals, and means for connecting said gaseous discharge lamp load in series with either said charging or said discharging circuit.

In one embodiment, the controlled switch and the smaller inductor are in the charging circuit, whereas in another embodiment these components are in the discharging circuit.

In a typical embodiment of the invention, the lamp is of high pressure sodium vapor type, and the controlled switch is a silicon controlled rectifier.

The operating circuit of the invention may be used for applying DC pulses of predetermined duty cycle and repetition rate on the lamp for improving the color and other properties of the lamp. A method and apparatus for pulsed operation of high pressure sodium vapor lamps for improving the color rendition of such lamps are disclosed in our co-pending U.K. patent application No. 1220/77. (Serial No.

1575831).

As disclosed in this patent application, the high pressure sodium vapor lamp typically has an elongated arc tube containing a filling of xenon at a pressure of about 30 torr as a starting gas and a charge of 25 milligrams of amalgam of 25 weight percent sodium and 75 weight percent mercury.

The present invention seeks to provide an improved circuit for DC pulsed operation of such lamps in accordance with the method and principles disclosed in the aforesaid patent application. As there disclosed, pulses may be applied to the lamp having repetition rates above 500 to about 2,000 Hertz and duty cycles from 10% to 30 /". By such operation, the color temperature of the lamp is readily increased and substantial improvement in color rendition is achieved without significant loss in efficacy or reduction in lamp life.

In order that the invention may be clearly understood, preferred embodiments thereof will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram of a DC pulse operating circuit in accordance with an embodiment of the invention with the controlled switch and smaller inductor in the charging circuit; Figure 2 is a graphical representation of the voltage and current waveforms relating to the operation of the circuit shown in Figure 1; Figures 3 and 4 show modifications of the Figure 1 circuit wherein the lamp is located in different positions in the circuit; Figure 5 is a circuit diagram of a varied embodiment of the circuit of Figure 1;; Figure 6 is a circuit diagram of a DC pulse operating circuit in accordance with an embodiment of the invention with the controlled switch and smaller inductor in the discharging circuit; Figure 7 is a circuit diagram of a DC pulse operating circuit of the type shown in Figure 1 and including a starting aid circuit and a voltage protective circuit; Figure 8 is a circuit diagram of the starting aid circuit designated A in Figure 7; Figure 9 shows a number of voltage and current waveforms relating to the operation of the Figure 7 circuit; Figure 10 is a graph showing the relationship of lamp volts and watts characterizing the circuit of Figure 7; Figure 11 is a circuit diagram of the improvement shown in Figure 7 and applied to a pulse generating circuit of the type shown in Figure 6;; Figure 12 is a circuit diagram of the DC supply in conjunction with the pulse generating circuit of Figure l; Figure 13 is a circuit diagram of a DC pulse operating circuit of the type shown in Figure 6 and including another embodiment of a voltage protective circuit; Figure 14 is a graphical representation of the voltage and current waveforms relating to the operation of the circuit shown in Figure 13; Figure 15 is a graph showing the relationship of lamp volts and watts characterizing the circuit of Figure 13, and: Figure 16 is a circuit as in Figure 13 and applied to a pulse generating circuit of the type shown in Figure 1.

Referring now to the drawing, and particularly to Figure 1, there is shown a circuit diagram illustrating an embodiment of the DC pulsing circuit of the invention for operating a gaseous discharge lamp 1.

The lamp, which is typically a high pressure sodium vapor lamp such as described above, is connected at one side to the positive terminal of DC power source 2, which may have a voltage, for example, of about 180 volts. At its other side lamp 1, typically of 330 watts rating, is connected to series-connected inductor Ll, a thyristor such as a silicon controlled rectifier (SCR) 3 and capacitor 4 connected by conductor 12 to the negative terminal of DC power source 2 as shown. A second inductor L2 in series with diode 5 is connected by conductor 12 to the negative terminal of DC power source 2 as shown. The second inductor L2 in series with diode 5 is connected across capacitor 4. The operation of SCR 3 is controlled by an RC timing circuit contained within the dotted line designated B comprising, in the illustrated embodiment, capacitor 6 and resistors 7 and 8 connected across the SCR.

A voltage breakdown device 9, constituted by a diac in the circuit shown, is connected at one side to the junction of capacitor 6 and resistor 8 and at the other side to the control electrode (gate) 3a of SCR switch 3. Zener diode 10 is connected across capacitor 6 and resistor 8 of the timing circuit.

The inductance of inductor L2 is substantially higher than that of inductor Ll, and in a typical circuit for practicing the invention the L2 inductance would be about 10 times that of Ll. However, the ratio may be in the range of about 4:1 to about 50:1 or higher while still obtaining satisfactory results. In general, the L2 inductance should be sufficiently high to ensure proper charging of capacitor 4, while the upper limit of its value should be such as to provide for sufficient reversal of the capacitor charge to commutate the SCR switch.

It appears that the use of higher values of inductor L2 tends to reduce circuit losses.

Also, it has been found that with sufficiently high inductance of inductor L2, diode 5 may be omitted while still avoiding further reversals of the charge on capacitor 4 as explained below, it being understood that if diode 5 is dispensed with, the values of capacitor 4 and inductor L2 should be such that the pulse voltage available in the circuit is sufficient to re-ignite the lamp.

In a typical circuit, the following components would have the values indicated: Inductor LI 0.7 millihenries Inductor L2 7 millihenries Capacitor 4 3 microfarads Capacitor 6 .12 microfarad Resistor 7 41K ohms Resistor 8 7K ohms Zener diode 10 62 volts Diode 5 1K volts Diac 9 38 volts In the operation of the described circuit, when SCR switch 3 is triggered on by the RC timing circuit, DC current flows through lamp 1, inductor LI and SCR switch 3, thereby charging capacitor 4, which serves as an energy metering device in the circuit. The charge on capacitor 4 reaches a positive voltage substantially higher than the supply voltage, due to the voltage build up thereon as a result of the operation of the LC circuit comprising inductor Ll and capacitor 4.This causes the SCR cathode voltage to be more positive than its anode voltage, and when this voltage build up is complete and as th current attempts to reverse through the SCR, commutation and turn-off of the SCR occurs. In the absence of the shunt inductor L2, the charge would remain on capacitor 4, thereby preventing subsequent pulsing of lamp 1. In the circuit shown, capacitor 4 discharges and momentarily transfers its energy to inductor L2; subsequently this energy is returned to capacitor 4 but with the polarity of the voltage reversed, such that the upper electrode of capacitor 4 goes to a high negative potential. This negative potential is locked and stored on capacitor 4 by diode 5 and SCR 3. As a result, the voltage across SCR 3 assumes a positive voltage drop from anode to cathode higher than the supply voltage.Diode 5 is included in this LC circuit to inhibit oscillations. The next pulse is then provided by operation of the RC timing circuit, which is adjusted to trigger SCR 3 to produce pulses of the desired repetition rate for pulsing lamp 1 in the manner intended.

On subsequent cycles, the positive voltage drop across SCR 3 increases to even higher levels, until an equilibrium potential is reached as a function of the total resistive losses in the circuit. This equilibrium potential can assume values greater than twice the supply voltage. In an illustrative case, with a power supply voltage of about 180 volts the equilibrium voltage across SCR 3 typically reaches about 450 volts during operation. Such higher voltages, when imposed across lamp 1 during conduction of SCR 3, serve to ensure reionization and continued operation of the lamp, especially, when the pulse repetition rate is relatively low.

The operation of the RC timing circuit is such that capacitor 6 is charged at a rate determined by the combination of resistors 7, 8 and capacitor 6. When the potential on capacitor 6 reaches the breakdown voltage of diac 9, capacitor 6 discharges through the loop including SCR control electrode 3a and turns on SCR 3. While a diac is shown as the voltage breakdown device 9, other breakdown devices such as a silicon bilateral switch (SBS), a Shockley diode, a glow tube, or a series combination of certain of these devices, could be employed.

Zener diode 10 connected to the junction of resistors 7 and 8 of the RC timing circuit stabilizes the frequency of the triggering operation by establishing a fixed clamping voltage toward which capacitor 6 is charged. Resistors 7 and 8 arranged as shown constitute a voltage divider, so that the use of a smaller Zener diode is made possible.

Figure 2 graphically shows the SCR voltage drops and current pulse waveforms achieved after equilibrium is reached in the operation of the described circuit. The initial positive SCR voltage drop shown (anode positive with respect to cathode) prevails before the SCR is gated on. When the SCR switch is turned on at point A, as determined by the RC timing circuit, the voltage across the switch immediately drops to zero, as indicated at point B. The voltage remains zero while the current flows through the SCR switch. During this period, as seen in the SCR current waveform, the current rises to a peak value and then drops to zero due to operation of the LC circuit.

The reversal of current is prevented by the SCR, and due to the large positive voltage build up on capacitor 4 as described previously, the SCR is reversed biased to achieve commutation. As the SCR ceases, to conduct, as indicated at point C, the voltage drop across the SCR is reversed, i.e., assumes a negative sense with its cathode voltage more positive than its anode voltage as indicated at point D.

Capacitor 4 then discharges through diode 5 into inductor L2, producing the SCR voltage waveform portion extending from D to E as the SCR cathode potential is pulled negative by the reversal of charge in capacitor 4 due to the operation of inductor L2. The positive voltage drop is held at level E by diode 5. The RC timing circuit starts the timing interval when the SCR voltage drop goes positive (point F) so that the interval from F to A is determined by the timing circuit.

It has been found that arranging lamp 1 in various places in the series circuit of Ll, SCR 3 and capacitor 4 will provide similar results, such as placing it between Ll and SCR 3, or between SCR 3 and the capacitor discharge loop, or in conductor 12. Lamp I may also be placed in the discharge loop in series with capacitor 4 across L2 as shown in Figure 3, or in series with inductor L2 across capacitor 4 as shown in Figure 4. Such modifications will produce varied but satisfactory results in accordance with the invention.

Inductor Ll may also be placed in various positions in the described series circuit while obtaining satisfactory results, and such modifications are intended to be included within the scope of the invention.

In the embodiment illustrated in Figure 5, lamp 1 is in series with inductor L2 and across storage capacitor 14, which is arranged in the discharge loop comprising capacitor 4, diode 5 and inductor L2 and forms another discharge loop with lamp 1 and current limiting inductor L3 in series therewith. In the operation of this circuit, the energy on capacitor 4, similar to the operation previously described, is transferred through inductor L2 to capacitor 14, resulting in a charge reversal on capacitor 4. When the voltage on capacitor 14 reaches the ionization potential of lamp 1, the energy stored on capacitor 4 and capacitor 14 flows to the lamp through inductor L3. Capacitor 14 and inductor L3 may be selected to provide a desired current waveform for pulsed operation of the lamp.

This arrangement provides for isolation of the lamp from the pulse generating circuit, allowing increased freedom in lamp current wave shaping.

In the circuits thus far described, the charging inductor Ll was in series with the SCR 3 and placed between the source 2 and the capacitor 4. However referring to Fig. 6, it is also possible to reverse the circuit and place the inductor L2 as the charging inductor between the source 2 and the capacitor 4 and use the inductor Ll in series with the SCR 3 as the discharging circuit.

The inductance of L2 is again substantially higher than that of inductor LI, as before.

The circuit of Figure 6 would operate in accordance with the curves heretofore described in Figure 2. In Figure 6, the components are identified in a manner as in Figure 1 with the addition of a filter capacitor 15 connected across DC power supply 2 to provide a filtered DC voltage supply for the pulse generating circuit.

Filter capacitor 15 is typically an electrolytic capacitor, which in comparison to other types of capacitors provides a large capacitance in a relatively small size. In contrast to the previous embodiments where such a filter capacitor may be subjected to high pulse currents generated by the firing of the SCR switch and leading to excessive heating of the electrolytic capacitor which may unduly shorten its operational life, filter capacitor 15 in the present circuit is isolated from the SCR pulsing circuit and thereby avoids the foregoing disadvantage.

Lamp 1 may be arranged in various places in the discharge circuit of Ll, SCR 3 and capacitor 4, or in series with inductor L2 and diode 5. Such modifications will produce varied but satisfactory results in accordance with the invention.

Inductor Ll may also be placed in various positions in the described discharge circuit while obtaining satisfactory results, and such modifications are intended to be included within the scope of the invention.

Referring now to figure 7 there is shown an improvement over the previous embodiment by including a circuit which serves to limit the peak voltage across the SCR 3 and thereby avoid undesirable firing of the SCR, due to excessive anode voltage.

The circuit comprises terminals 2 of a source of alternating current, and induction coil L4 connected at one side to one of the source terminals and at the other side to an input terminal of full wave bridge rectifier 16, which comprises diodes Dl, D2, D3 and D4 arranged in conventional manner as shown, the other input terminal of rectifier 16 being connected to the other source terminal 2. Filter capacitor 15 connected across the DC supply circuit provides a filtered DC voltage supply for the pulsing circuit described hereinafter and increases the average voltage supplied thereto.

Induction coil L4 serves to limit current to the lamp at the starting and warm-up stage.

The DC pulsing circuit for lamp 1 comprises inductor Ll which is connected between the lamp and the upper terminal of filter capacitor 15. Lamp 1 is connected at its other side to series-connected controlled thyristor switch 3 such as a silicon controlled rectifier (SCR), and capacitor 4 is connected by conductor 12 to the other terminal of filter capacitor 15.

Transformer 17 is provided in the pulsing circuit with its primary winding L2 in series with diode 5 across capacitor 4 and its secondary winding L3 in series with diode 18 across filter capacitor 15, the windings being arranged out of phase with one another as indicated in the drawing.

Preferably, the transformer is characterized by low leakage reactance, that is the windings should be tightly magnetically coupled.

The operation of SCR 3 is controlled by a timing and triggering circuit B as heretofore described, and a starting aid circuit A connected to inductor Ll and across lamp 1 serves to apply sufficiently high voltage pulses to lamp 1 for starting it. Starting circuit A, which is shown in detail in Figure 8, is of the type disclosed in U.S. patent to Nuckolls 3,917,976 issued November 4, 1975. As seen in Figure 8, this high voltage pulse generator circuit comprises capacitor 19 and resistor 20 connected in series across lamp 1 and a voltage sensitive symmetrical switch 21, such as a triac, connected between a tap on inductor L I and the junction of capacitor 19 and resistor 20.

Gate electrode 21a of the triac is connected to a voltage sensitive triggering device 22 such as the silicon bilateral switch (SBS) shown. The firing of triac 21 is controlled by an RC timing circuit comprising capacitor 23 and resistor 24 connected in series across the triac, with SBS 22 connected to the junction thereof. In the operation of this circuit, capacitor 19 is initially charged by DC current flowing from the DC supply through inductor Ll and the circuit including capacitor 19, resistor 20, the SCR control circuit B, diode 5, and inductor L2 back to the DC supply. Capacitor 23 is charged through inductor Ll and resistor 24 until the voltage across it reaches the breakdown level of SBS 22 at which time triac 21 is triggered on.When this occurs, capacitor 19 discharges through the tapped turns of inductor Ll at its output end, inducing a high voltage, e.g. 3,000 volts, in inductor LI acting as an autotransformer.

Pulses of this high voltage level are produced across lamp 1 by repeated charging and discharging of capacitors 19 and 23 in the described starting circuit until the lamp ignites. Upon starting of the lamp, the described high voltage ignition circuit ceases to operate as a result of the voltage clamping action of the ignited lamp load, and therefore the voltage build up across capacitor 23 does not reach the breakdown level of voltage sensitive switch 22.

Typical values for the circuit of Figure 8 are as follows: Capacitor 19 .1 microfarad Resistor 20 33K ohms Resistor 24 2.2 megohms Capacitor 23 .12 microfarad SBS 22 9 volts (GE2N4992) Triac 21 400 volts (RCA 40669) The operation of the circuit in Figure 7 is similar to that described for Figure 1. The provision of transformer 17 serves to limit the peak voltage across SCR 3 and thereby avoids undesirable firing of the SCR, especially if it is of low voltage capability, due to excessive anode voltage. Such inadvertent firing may cause degradation of the SCR, and results in an interaction with inductor L2 to cause intermittent 60--80 ampere peak currents to go through lamp 1, which may cause lamp degradation and prevent normal ignition of the lamp.

In the operation of the described circuit, and assuming the turns ratio of the transformer windings to be 1:1, when SCR 3 fires at time to (see Figure 9) the voltage across capacitor 4 rises to a peak value at time t,. At this point SCR 3 turns off, while current has begun to flow through inductor L2. At time t2, the voltage across inductor L2 (and across capacitor 4) reaches the magnitude of the power supply voltage Vp, but is of negative polarity. It will be noted that the voltage across the secondary winding L3 is identical to the voltage across primary winding L2 (insofar as the leakage reactance is negligible).When the voltage across winding L3 reaches the power supply voltage, i.e., the voltage across filter capacitor 15, diode 18 becomes forward biased and current begins to flow through L3 and diode 18 into the power supply constituted by capacitor 15, and at the same time current ceases to flow in primary winding L2. While there is a rapid change of currents in windings L2 and L3, the magnetic field in transformer 17 is continuous and not rapidly varying. The speed with which current in L2 ceases and current in L3 begins depends on the leakage reactancebetween the two windings. In this way, the negative voltage appearing on capacitor 4 at time t3 is limited in magnitude to the supply voltage V and thus the maximum voltage across SCR 3 is limited to twice the supply voltage.As a result, SCR's with a voltage rating of 600 volts can be used in place of 1000 volt SCR's, thereby effecting a substantial saving in the cost of this component. Similarly, a lower rating and less expensive diode D13 may be used.

To obtain proper operation of the described circuit, the inductance of transformer primary winding L2 must be substantially higher than the inductance of inductor Ll, and preferably the inductance ratio of L2 to Ll should be at least about 10:1. The maximum inductance of winding L2 should be such, for a particular pulse frequency, that the voltage induced in secondary winding L3 to cause conduction therein as intended.

A further benefit afforded by the described circuit is that it provides for control of the lamp watts-lamp volts relationship, whereby a flatter curve for this relationship may be obtained, as depicted in the graph of Figure 10. In the graph, Curve A in interrupted lines shows the variation in lamp watts with lamp volts in a circuit without the feedback arrangement of the present embodiment, while Curve B shows the watts-volts relationship characterizing the circuit of the present embodiment. In the circuit of Curve A, the increase in lamp volts, which typically occurs over the operating life of lamps such as here involved, is accompanied by a substantial increase in lamp watts, which tends to shorten lamp life due to excessive heating.

In contrast, it is evident that Curve B is substantially flatter than Curve A and that accordingly the lamp watts remains relatively constant with increase in lamp volts, resulting in longer life of the lamp and more nearly uniform illumination during its operating life.

The relatively steep rise at the initial portion of Curve B results from the return of energy to the power supply as described above, and typically the lamp wattage stabilizes at the top of this portion of the curve in about one-half minute after lamp ignition.

To obtain the benefits of this improvement, the turns ratio of secondary winding L3 to primary winding L2 should be at least 1:1, and may as high as 5:1 or higher.

As will be understood, the higher the turns ratio, the higher the voltage developed across secondary winding L3 and the sooner the voltage of the latter reaches the power supply voltage and begins to conduct current to the power supply, thereby clamping the L2 voltage at some voltage lower than the power supply voltage depending on the turns ratio. Thus, the higher the turns ratio, the flatter will be the watts-volts curve.

In a typical circuit in accordance with this improvement wherein the circuit has a pulse repetition rate of 1200 Hertz and a duty cycle of 20%, the tightly coupled secondary and primary windings of transformer 17 have a turns ratio of 1:1, and the inductance of the windings will be in the range of 7-25 millihenries.

Figure 11 shows another embodiment of this improvement wherein the pulsing circuit used has an arrangement similar to that of Figure 6 wherein the SCR 3 and Ll are on the other side of the capacitor 4. In Figure 11 the DC supply circuit to which filter capacitor 15 is connected has been omitted for the sake of brevity, transformer 17' comprising primary winding L2' and secondary winding L3' is arranged with primary winding L2' connected in series with diode 5 across capacitor 15 and with secondary winding L3' connected in series with diode 18 also across capacitor 15, as shown. Inductor LI, lamp 1 and controlled thyristor switch 3 are connected in series across capacitor 15. Starting aid circuit A and trigger circuit B are connected to the circuit in the manner and for the purposes described above in connection with the Figure 6 embodiment.The Figure 11 circuit operates in a manner substantially as described in connection with the Figure 7 circuit, being characterized by the various waveforms shown in Figure 9 and producing the benefits of the previously described circuit.

Although various types of DC supplies can be utilized a particular useful DC supply having a low ripple factor for use with the pulsing circuits as described, has been found especially effective. This circuit provides for gradual increase in power applied to the discharge lamp during the starting interval and thereby avoids instability lamp operation during that stage.

Referring now to Figure 12 there is shown a circuit diagram of this embodiment comprising terminals 2 of a source of alternating current, and induction coil L4 connected at one side to one of the source terminals and at the other side to an input terminal of full wave bridge rectifier 16, which comprises diodes Dl, D2, D3 und D4 arranged in conventional manner as shown, the other input terminal of bridge rectifier 16 being connected to the other source terminal 2. Auxiliary induction coil L5 is inductively coupled to main induction coil L4 such as by arrangement of the two coils on a common magnetic core on opposite sides of a magnetic shunt. Such an arrangement of inductively coupled coils is shown, for example, in our U.S. patent to Willis 3,873,910, issued March 25, 1975.

Auxiliary induction coil L5 is connected at opposite sides respectively to the input terminals of another full wave bridge rectifier 25 constituted by diodes D5 and D6 co-acting with diodes D2 and D4 to provide full wave rectification of the current from auxiliary coil L5. Capacitor 26 connected between auxiliary coil L5 and the input terminal of bridge rectifier 25 is selected such that in conjunction with the leakage reactance existing between induction coils L4 and L5, it serves to provide the necessary phase shift and power factor. If induction coil L5 and capacitor 26 are selected so that the portion of the magnetic core associated with coil L5 is saturated, a higher degree of lamp wattage regulation is achieved for a wide range of input voltage.

Connected across the thus described DC supply circuit to the common output terminals of bridge rectifiers 16 and 25 is a lamp pulsing circuit including the gaseous discharge lamp, particularly of high pressure sodium vapor type, as described above.

By virtue of the described DC supply circuit, the direct current supplied to the lamp by main induction coil L4 via bridge rectifier 16 is substantially out of phase with the direct current supplied to the lamp by auxiliary coil L5 and capacitor 26 via bridge rectifier 25. As a result, the average current through the lamp and the voltage across the lamp is substantially increased over the average magnitude of current and voltage which would be applied in the absence of auxiliary coil L5 and its associated rectifier circuit, and therefore the tendency of the lamp to drop out because of de-ionization at current zero is largely prevented, and at the same time a sufficiently high re-ignition voltage is thereby provided to maintain operation of the lamp.In the operation of the circuit, main induction coil L4 also serves as a current limiting reactance to limit current flowing through the lamp after it starts and thereby provides a ballasting function.

A DC supply circuit of the above described type is disclosed in U.S. patent specification No. 4,045,708.

In the embodiment of the present invention illustrated in Figure 12, filter capacitor 15 connected across the DC supply circuit provides a filtered DC voltage supply for the pulse generating circuit and increases the average voltage supplied thereto. The type of pulse generating circuit employed in the present invention for pulsed operation of the lamp is shown in Figure 1, however any of the other pulse generating circuits as shown in Figure 3-5 or the reverse type of circuit shown in Figure 6, or variations thereof could also be used. The pulse generating circuit shown in Figure 12 includes the RC circuit B as was heretofore described and shown in Figure 1, as well as the starting aid circuit A shown in Figure 8.

It has been found that high intensity gaseous discharge lamps employed in pulsing circuits of the described type are subject to the disadvantage of unstable operation during the starting interval under conditions in which the lamp reaches its operating wattage too rapidly, i.e., without an adequate warm-up period being provided to enable a gradual increase of power to be supplied to the lamp during the starting interval. It has further been found, that the combination of the above-described DC supply circuit with a pulsing circuit of the type shown and described above will provide a relatively slow warm-up period, such that when the lamp reaches its steady state operating wattage, its operation is relatively stable and there is little or no risk of lamp drop-out due to excessive power being applied to the lamp at its start.

In the typical circuit shown in Figure 12, the following components would have the values indicated: Inductor L4 390 turns Inductor L5 468 turns Capacitor 26 7.5 microfarads Capacitor 15 120 microfarads Inductor LI 0.7 millihenries Inductor L2 7 millihenries Capacitor 4 3 microfarads Diode 5 1K volts SCR 3 600 volts 25 amps.

In a further embodiment to be hereinafter described, there is also provided a unidirectional circuit means connected across the series connected inductor and SCR for limiting the voltage across the SCR.

Referring now to Figure 13 there is shown a circuit utilizing the DC supply circuit heretofore described in connection with Figure 12, and the pulse generating circuit shown in Figure 6, wherein the SCR switch 3 and the smaller inductor Ll are in the discharge circuit. The operation of the circuit would be as heretofore explained with regard to Figure 2.

In accordance with the present embodiment, a feedback branch comprising series-connected diode 27 and inductor L6 is connected across the described discharging circuit comprising serially connected inductor Ll, lamp 1 and SCR 3.

The provision of this feedback branch serves to limit the peak voltage across SCR 3 and thereby avoids undesirable firing of the SCR and affords desirable control of the lamp watts-lamp volts relationship as explained more fully below.

The provision of the above-described feedback branch in accordance with this embodiment serves to limit the peak voltage, across SCR 3 during the lamp starting interval when the lamp voltage is low, and thereby avoids undesirable firing of the SCR during this period due to excessive anode voltage, especially if it is of low voltage capability.

Such inadvertent firing not only may cause degradation of the SCR but also may result in higher peak currents through lamp 1, causing higher lamp wattage and consequent shorter lamp life. The feedback branch functions to transfer energy from the cathode side of SCR 3 to the power source side of the discharge circuit combination of inductor Ll and SCR 3. As a result, the voltage across the SCR prior to switching on of the latter is limited to an acceptable maximum level.

Figure 14 graphically shows the SCR voltage and current pulse waveforms achieved after equilibrium is reached in the operation of the described circuit. The initial positive SCR voltage drop shown (anode positive with respect to cathode) prevails before the SCR is gated on. When the SCR switch is turned on at point A, as determined by the RC timing circuit, the voltage across the switch immediately drops to zero, as indicated at point B. The voltage remains zero while the current flows through the SCR switch. During this period, as seen in the SCR current waveform, the current rises to a peak value and then drops to zero due to operation of the LC circuit comprising inductor L2 and capacitor 4.

The reversal of current is prevented by the SCR, and due to the large negative voltage (with respect to ground) on capacitor 4 as described previously, the SCR is reverse biased to achieve commutation.

As the SCR ceases to conduct, as indicated at point C, the voltage drop across the SCR is reversed, i.e. assumes a negative polarity with its cathode voltage more positive than its anode voltage, as indicated at point D. Capacitor 4 then charges through inductor L2 and diode 5, as well as through feedback branch L6diode 27. The RC timing circuit starts the timing interval when the SCR voltage goes positive (point F) so that the interval from F to A is determined by the timing circuit.

The described dual charging operation produces the portion of the SCR voltage waveform from D to E as the SCR anode potential is made positive by the resulting reversal of charge on capacitor 4. At point E, capacitor 4 continues to be charged only through inductor L2 and diode 5, producing a more gradual charging rate of the waveform portion from E to A, and at point A the SCR anode potential is at its maximum.

Figure 15 graphically illustrates the additional benefit afforded by the described circuit in providing for control of the lamp watts-lamp volts relationship, whereby a flatter curve for this relationship may be obtained. In the graph, Curve A in interrupted lines shows the variation in lamp watts with lamp volts in a circuit without the feedback arrangement of the present embodiment as described above, while Curve B shows the watts-volts relationship characterizing the circuit of the present embodiment. In the circuit of Curve A, the increase in lamp volts, which typically occurs over the operating life of lamps such as here involved, is accompanied by a substantial increase in lamp watts, which tends to shorten lamp life due to excessive heating.In contrast, it is evident that Curve B is substantially flatter than Curve A and that accordingly the lamp watts remains relatively constant with increase in lamp volts, resulting in longer life of the lamp and more nearly uniform illumination during its operating life.

Figure 16 shows a different embodiment of feature wherein the charging circuit connected to filter capacitor 15 and DC power source 2, (shown in simplified form) is of the type shown in Figure 1 and comprises the series combination of SCR 3, smaller inductor LI and lamp I, whereas the discharging circuit connected across metering capacitor 4 comprises serially connected diode 5 and large inductor L2. In this embodiment, as in the Figure 13 embodiment, the feedback branch comprising inductor L6 and diode 27 is connected across the series combination of inductor Ll, lamp 1 and SCR 3 to produce similar results.However, in this case, in reference to the description of the waveforms in Figure 14 at point D capacitor 4 discharges through diode 5 and inductor L2, as well as discharging through the feedback branch L6-diode 27. This dual discharging operation produces the portion of the SCR voltage waveform from D to E, and at point E, capacitor 4 continues to discharge only through inductor diode 5 and inductor L2, producing the waveform portion E to A. This embodiment also provides the desirable lamp watts-lamp volts relationship shown in Figure 15.

In a typical circuit such as shown in Figure 14, the values previously given for the other embodiments are the same with the additional components having the values as follows: Inductor L5 .3 millihenries Diode 14 > 1Kvolts A snubber circuit of conventional RC type (not shown), if found necessary or desirable, may be connected across diode 5, diode 27 or SCR 3 to reduce voltage spikes across those components.

Lamp 1 may be arranged in various places in either the charging or discharging circuit of either of the Figure 13 and Figure 16 embodiments. Such modifications will produce varied but satisfactory results in accordance with the invention.

While an SCR is disclosed throughout this description as the unidirectional controlled switch in the described circuit, it will be understood that other equivalent switch devices may alternatively be employed in accordance with the invention. For example, a triac or a transistor switch may be employed in combination with a diode to provide unidirectional operation, and as used herein the expression "unidirectional controlled switch means" is intended to include all such equivalent switch devices or arrangements.

The power supply may be any suitable source of DC voltage, such as a battery or a rectified AC source different from that described above. Preferably, the DC supply is at least about 150 volts in order to achieve the desired improvement in color properties of lamp 1 (assuming the lamp to be of 250--300 watt variety).

While a diode has been disclosed in series with the larger inductor L2 this diode may be dispensed with if inductor L2 has sufficiently high inductance.

WHAT WE CLAIM IS: 1. A load operating circuit for a gaseous discharge lamp, characterized by a DC power source, a commutating capacitor, a first inductor in series circuit with said commutating capacitor, said series circuit being connected across said DC power source and forming therewith a charging circuit for said capacitor, a second inductor connected across said capacitor and forming therewith a discharging circuit for discharging said capacitor, a unidirectional controlled power switch means in series with one of said inductors, said switch means being commutated by said capacitor and controlling power, when in the conducting state, to a gaseous discharge lamp load, said one inductor having a substantially smaller inductance than the other inductor, control means coupled to said controlled power switch means for repetitively operating the same at predetermined intervals, and means for connecting said gaseous discharge lamp load in series with either said charging or said discharging circuit.

2. A circuit as claimed in Claim 1, characterized in that the ratio of inductance of said other inductor to said one inductor is at least 2:1.

3. A circuit as claimed in Claim 2, characterized in that said ratio is at least 10:1.

4. A circuit as claimed in Claim 1, and further characterized by a first diode in series with said other inductor.

5. A circuit as claimed in Claim 4, characterized in that said unidirectional power switch means comprises a second diode and an induction coil in series therewith.

6. A circuit as claimed in any one of the preceding claims, further comprising a gaseous discharge lamp connected to said connecting means.

7. A circuit as claimed in Claim 6, characterized in that said gaseous discharge lamp is a high pressure sodium vapor lamp.

8. A circuit as claimed in Claim 6, characterized in that said gaseous discharge lamp contains mixed metal vapors.

9. A circuit as claimed in any one of the preceding claims, characterized in that said controlled power switch means comprises a silicon controlled rectifier.

10. A circuit as claimed in Claim 1, characterized in that said connecting means is in series with said capacitor across said other inductor.

11. A circuit as claimed in Claim 1, characterized in that said connecting means is in series with said other inductor across said capacitor.

12. A circuit as claimed in Claim 1, characterized in that said connecting means is arranged between said one inductor and said unidirectional controlled power switch means.

13. A circuit as claimed in any one of the preceding claims, characterized in that said control means comprises an RC timing circuit.

14. A circuit as claimed in Claim 13, further characterized by a Zener diode connected across said RC timing circuit for stabilizing the frequency of operation of said timing circuit.

15. A circuit as claimed in Claim 1, further characterized by a storage capacitor connected in said discharge circuit across said first mentioned capacitor, said connection means being connected across said storage capacitor and forming a second discharge circuit therewith.

16. A circuit as claimed in Claim 15, and further characterized by current limiting impedance means arranged to be connected in said second discharge circuit in series with said gaseous discharge lamp load.

17. A load operating circuit as claimed in claim 1, further characterized by a capacitor connected across said power source, a transformer having a primary winding and a secondary winding, first unidirectional conducting means connected in series with said primary winding across said power source, and second unidirectional conducting means connected in series with said secondary winding across said power source.

18. A load operating circuit as claimed in Claim 17 and characterized in that the primary winding serves as one of said first or second inductors.

19. A circuit as claimed in Claim 17, characterized in that said primary winding and said secondary winding are arranged so as to be out of phase with one another.

20. A circuit as claimed in Claim 17, characterized in that said series-connected second unidirectional conducting means and said secondary winding are arranged to conduct current to said power source when the voltage on said secondary winding during operation of said transformer exceeds the voltage of said power source.

21. A circuit as claimed in Claim I, characterized in that said DC power source

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. source of DC voltage, such as a battery or a rectified AC source different from that described above. Preferably, the DC supply is at least about 150 volts in order to achieve the desired improvement in color properties of lamp 1 (assuming the lamp to be of 250--300 watt variety). While a diode has been disclosed in series with the larger inductor L2 this diode may be dispensed with if inductor L2 has sufficiently high inductance. WHAT WE CLAIM IS:

1. A load operating circuit for a gaseous discharge lamp, characterized by a DC power source, a commutating capacitor, a first inductor in series circuit with said commutating capacitor, said series circuit being connected across said DC power source and forming therewith a charging circuit for said capacitor, a second inductor connected across said capacitor and forming therewith a discharging circuit for discharging said capacitor, a unidirectional controlled power switch means in series with one of said inductors, said switch means being commutated by said capacitor and controlling power, when in the conducting state, to a gaseous discharge lamp load, said one inductor having a substantially smaller inductance than the other inductor, control means coupled to said controlled power switch means for repetitively operating the same at predetermined intervals, and means for connecting said gaseous discharge lamp load in series with either said charging or said discharging circuit.

21. A circuit as claimed in Claim I, characterized in that said DC power source

comprises input terminals for connection to an AC current source, a first induction coil connected to said input terminals, an auxiliary induction coil inductively coupled to said first induction coil, first rectifier means connected to the output of said first induction coil, and second rectifier means connected to the output of said auxiliary induction coil.

22. A circuit as claimed in Claim 21, characterized in that a filter capacitor is connected across said DC power source.

23. A circuit as claimed in Claim 1, and further characterized by high voltage lamp starting means including a portion of said one inductor for providing high voltage starting pulses on the gaseous discharge lamp.

24. A circuit as claimed in Claim 23, characterized in that said high voltage lamp starting means comprises a charging capacitor and a resistor connected in series across said connecting means, and voltage sensitive switch means having a predetermined breakdown voltage connected across said charging capacitor and said portion of said one inductor and forming a discharge loop therewith for generating high frequency starting pulses.

25. A load operating circuit for a gaseous discharge lamp, substantially as hereinbefore described with reference to and as shown in the accompanying drawings.