US2938149A - Pulse circuit for arc lamp - Google Patents

Description

May 24, 1960 E. H. WILEY PULSE CIRCUIT FOR ARC LAMP Filed May 2, 1957 NAaMEr/z/m FORCE H Illllllllllllllll 5 9 g b F sill! 1 M W M w 7 a M M Mwfifi; kzumwsu 0 23 38w muazwm mwwb mi kw mfibwmwu w kfimwsw Q23 M United States Patent 2,9ss,149 PULSE CIRCUITFOR ARGL-AMP Emmett H. Wiley, Cleveland,,0hio, assignorto General Electric Company, .a corporation of New York Filed: May 2, 1957, Ser- No.'6'56,595

Claims. (Cl. 3151-443) This invention relates to ,an ,operating circuit for an electric dischargelamp providing a pulsed waveform to achieve a high instantaneous loading while maintaining the average energy input into ,the lamp within a rated level.

Although not limited thereto-in'its field of application, the invention is particularly useful with arclamps containing an inert rare gas of relativelyhigh atomic weight at a pressure not exceeding atmospheric. These lamps, frequently referred to as fiashtubes, require a high instantaneous loading in order to achieve a reasonably high luminous efficiency, that is in order to produce a reasonably highlight output in relation to the .energy input. When the instantaneous loading is high, the spectrum of the radiation produced contains, in addition tothe usual line spectrurnof the xenon gas obtained with low instantaneous loading, a continuum of radiation of substantial intensity whose over-all color is close to that of natural daylight. In flash or ,stroboscopic applications, the common method of 'energizetion of ,such lamps is'to charge a capacitor to a relatively 'high'voltage, usually several hundred volts; the capacitor is then discharged throughthe fiashtube conneeted'acrossit. The discharge through the flashtube is initiated by applying a'pulse to 'a triggering electrode which may take the 'form-of-a capacitiveelement disposed along the lamp, for instance a wire spiralled around the fiashtube. 5

Where continuous-pulsed operation of 'an arc lamp or fiashtube is desired, 'thecapacitor discharge method of operation has several limitations. In the'first-place it is basically ineflicient by virtueof the'losses in the system for charging the capacitor. In the second place, the residual ionization remaining in the lamp-limits the rate of flashing or the loading below the level at which continuous conduction through the lamp prevents -the.buildup of charge in the capacitor; otherwise the lamp conducts continuously at alow instantaneous loading, and therefore at low efiiciency. The use of a gating tube or thyratron in series with the flashtube willprevent premature firing or continuous conduction, but increases the complexity and cost of the circuit and reduces its overall efiiciency.

The object of the invention is :to provide a :new :and improved circuit combination iforoperatingan arclamp with high instantaneous loading whereby to achievehig'h luminous efiiciency without exceeding the rated average input which the lamp is capable ofstanding.

A more specific object .of the invention is to provide a circuit operable from the usual alternatingcurrentsupply to generate high current pulses for operating an arc lamp with high instantaneousloading and which will maintain a residual current through the lamp .to prevent its-deionization between pulses.

In-accordancewiththe-invention, an alternating current pulse generating circuit is provided using a saturating reactor in series with the lamp. The reactor has a highpermeability high-saturation flux density magnetic core material with a generally rectangularhysteresis loop char- 2,938,149 RatentedMey .24, 1.960

. 2 acteristic. ,The low. magnetizing current through the reac tor below saturation then provides .a residual current through the lamp to maintain ionization between high current pulses. .The high current pulses occur at the instants when the reactor corereaches saturation; by virtue of the maintenance of ionization through the'larnp, the current pulses need only be at the voltage necessary-to maintain conduction through the lamp and need not be at the high levels required were the lamp allowed to deionize between pulses. Thus the tendency of the lamp to remain ionized is used to advantage in the present circuit and ceases to be a limiting factor as will capacitor discharge circuits for repetitive flashing.

In a'preferred. embodiment, there isprovided .a'T-net- Work consisting of a current'limiting inductive reactance means such 'as an inductor, and 'a capacitorconnected .across'an alternating voltagesource'and a saturable reactor having a high-permeability high-saturation density core connected .in series with the arclamp across the capacitor. At saturation, the saturable reactor produces high-current pulses through thelamp resulting in high instantaneous loading. Below saturation, the magnetizing current through the saturable inductor provides a residual currentthrough the lamp maintaining it ionized and-thereby assuring conduction at the occurrence of the pulses without the need for excessively high voltages.

For further objects and advantages and for a detailed description of a preferred embodiment of the invention and its mode of operation, attention is -now directedto the following description and accompanying drawing. The features of the invention believed to be novel will be moreparticularly pointed out inithe appended claims.

In the drawing:

Fig. '1 illustrates diagrammatically an elongated arc lamp-orfiashtube with-an operating circuit itherefor embodying the invention.

Fig. 2 illustrates the-rectangular'hysteresis loop characteristic of the core material ofthc saturable reactor used in the circuit of Fig. 1. s

Figs. 3a to d show cur-rentan'd voltagewaveforms in the circuit of Fig. 1.

Fig.4 is aschematic-diagramof a-modified circuit embodying the invention.

Fig. 5 shows schematically a -further variant of the invention.

Referring to the drawing and more particularly to Fig. 1, the circuit illustrated is designed-for the operation of an 'arc lamp or'flashtube F consisting of an elongated tubular quartz envelope 1 provided With'activated thermionic self-heating electrodes 2 at opposite ends. Each electrode may consist of atungsten wire helix '3 wound about the inwardly projecting end of the inlead and a thorium sliver disposed alongside. The inleads include foil portions 4 of molybdenum pinch-sealed through the end of the tube. The tube is filledwith 'an inert gas such as Xenon at a pressure below atmospheric pressure. As an example, are lamp F may consist of'a quartz tube 11" in over-all length, in diameter, and with a 'xenon filling at a pressure of'40mm. of mercury.

In the illustrated embodiment, the discharge current regulating circuit consists of reactive elements forming a T-network connected across input terminals T T adapted to be energized from a 240 volt, 60 cycle supply. Closing switch S connects current limiting reactor L and capacitor C in series across the voltage supply. L is a generally linear or non-saturating reactor and is shown as a winding 6 about the central leg of an E lamination 7 provided with an air gap at 8 next the end lamination 9 to prevent saturation of the core. A saturable reactor L =is connected across C in :series with the arc lamp or flashtube F.

sa a g eaetorlla comprisesa 'l'ieweund frequency high-voltage source.

about a toroidal core 12 consisting of grain-oriented silicon steel or nickel-iron strip continuously wound in the usual fashion to preserve the grain orientation parallel to the direction of magnetization. The core material is of a high-permeability, high-saturation flux density type having a rectangular hysteresis loop characteristic. Such a characteristic is illustrated in Fig. 2 showing the flux density B increasing very rapidly with the magnetizing force H up to the saturation levels S beyond which there is substantially no further increase in flux density. Below saturation the incremental permeability is given by the slope of dotted line 13, whereas beyond saturation the incremental permeability is substantially that in air and equal to l. permeabilities below and above saturation may be in the range of 10,000 to 1. Grain oriented silicon steel may be used for core 12 of L or nickel iron having a yet smaller and more rectangular hysteresis loop may be used if a narrower pulse and smaller circuit losses are desired. Nickel iron core material of this type is sold under the various trade names Deltamax, Orthonol, and Permeron.

In the operation of the circuit, when switch S is closed, capacitor C charges through linear reactor L until its voltage is sufficient to saturate saturable reactor L The charging voltage waveform is shown by curve 15 of Fig. 3a; the charging occurs in a generally positive direction from time t to t and in a generally negative direction from t to 1 although there may be some reversal during the early part of each half-cycle as illustrated. At the voltage level corresponding to t;, saturable reactor L saturates. At this instant, its incremental permeability may drop by a factor of 10,000and its inductive reactance falls through the same ratio. Accordingly, capacitor C discharges rapidly through the saturable reactor and lamp F, providing the current pulse shown in Fig. 3b by curve 16 at time t A similar pulse of opposite polarity occurs a half-cycle later at time t On a 60 cycle supply, the time intervals t to and t to t are M second, that is 8333 microseconds.

The ratioof incremental If the voltage-time integral of the charge across capaciv tor C reaches the saturation level of saturable reactor L early in the half-cycle of the supply frequency, the capacitor may recharge to the saturation level in time to generate one or more additional pulses during the half cycle, as shown by dotted line pulses 16', 16" trailing main pulses 16. The additional pulses are of smaller amplitude than the main pulses and do not produce as high instantaneous loading in the lamp so that light is produced at a lower efliciency.

A highly advantageous feature of the present pulsed operating circuit combined with a long gap arc lamp or flashtube as illustrated in Fig. 1 is the use of the residual magnetizing current through the saturable inductor to maintain a minimum ionization within the lamp during the intervals between high current pulses. In long gap arc lamps at relatively low pressures, the discharge may be described as wall stabilized and there occurs a rapid recombination of ions and electrons at the walls. Such lamps have an initial striking voltage much higher than the normal operating voltage after the discharge medium has become ionized, for instance a striking voltage ten times greater than the operating voltage. For this reason, whereas the circuit of Fig. 1 may be designed to provide current pulses at a high enough voltage to initially strike the lamp,'it would be uneconomical to do so since the circuit losses would then be excessive during normal operation. Moreover, the circuit elements would have to be designed for these high voltages with resultant higher cost. To start the lamp with the present circuit,

'it' may be initially ionized by bringing close to the envelope of the lamp an electrode connected to a high- Alternatively, a voltage pulse may be coupled into saturable inductor L by means of an auxiliary winding thereon into which a current pulse is supplied, as described and claimed in copending application Serial No. 656,679, filed of even date herewith, of Harry E. Schultz, entitled Starting Circuit for Are Lamp, and assigned to the same assignee as the present invention.

After the lamp has been started, there is a tendency for it to become extinguished by reason of the rapid de-ionization which occurs between pulses. This is particularly pronounced with long gap lamps of small discharge cross section. In conventional repetitive flash circuits, electrical breakdown or ionization of the gas is necessary for each pulse or flash and is achieved .by means of external triggering. Such arrangements are usually complex and subject to some instability of operation causing a time jitter of the pulses, that is, a random change in the time intervals between consecutive pulses. This produces a low frequency flicker component which is objectionable.

In accordance with the invention, the ionization of the lamp is preserved by providing sutficient average loading to maintain the electrodes thermionically emitting during the intervals between pulses coupled with a residual curquired to build up the flux therein below saturation, and

it is of low magnitude under one ampere, for instance 1 ampere as against a peak current of amperes during pulses. This residual current is seen in Fig. 3:: wherein the ordinate or current scale has been expanded and is indicated at 16a between the pulses occurring at times 2 t and The lamp, of course, has some voltage on it at all times as shown in Fig. 3d by curve 17, except at the instants of crossover of the zero voltage level at each half cycle. It will be noted that the ratio of the voltage peaks to the average voltage across the lamp is nowhere near as great as the ratio of peak current to residual current. The reason for this is that a minimum voltage is required across the lamp to maintain the residual current therethrough no matter how low the value of this current,

the lamp of course being a non-linear circuit element.

The successful operation of the circuit and lamp combination depends on maintaining a high enough electrode temperature in the lamp and sufiicient ionization of the gas filling therein between pulses to permit the low value of magnetizing current drawn by the saturable reactor during the intervals between pulses. This low value of current in turn is what maintains the degree of ionization required in the lamp and assures that the lamp will conduct the current pulses without requiring excessively high voltages to achieve reignition at each pulse. Thus the features and requirements of lamp and circuit are unusually complementary. Their combination in accordfor the various circuit elements and the values selected for a unit actually constructed and found to operate under test as previously described are as follows.

The size of capacitor C in combination with the sup- 'ply voltage determines the loading or watt input into the lamp. 'The voltage applied to terminals T T must, of course, be sufiicient to cause the discharge through the lamp to take place after it has been initially ionized. Regulating inductance L must allow the recharging of capacitor C .at'sucha rate that the voltagetime integral of the charge thereacross causes the flux in saturable reactor L to build up to the saturation level at each half-cycle. For operation on a 240 volt, 60 cycle supply, in the unit ,actually constructed C is 50 microfarads, and linear inductor L has a value of 58 millihenrys. Thesaturable inductor L consists of 160 turns on a toroidal core of grain-oriented silicon steel 1%" x 1%" in cross section, 3" inner diameter and 6 /2" outer diameter. The unit operatesthe lamp with 120 pulses per second of 125 amperespeak amplitude with a halfpeak duration of approximately 270 microseconds. The

energy input intothe lamp is 890 watts and the measured light output is 25,726 lumens, indicating an efiiciency of 28.8 lumens perwatt as regardsthe lamp and neglecting circuitlosses.

' The lamp and circuit combination whichhas been described is particularlyuset'ulas a high-intensity elongated source of radiation with a broad continuum in the visible spectrum. One application of particular promise for such a lamp is as a radiation source for the reaction of heatsensitive copying paper. In this application, the slender tubular arc lamp F may be disposed along one focus of an elong ated reflector of elliptical cross section, the material to'be copied overlaid by the heat-sensitive paper being drawn slowly bythe otherfocus. The use of the present lamp and circuit combination results in much improved color sensitivity of the heat-sensitive copying paper. The copying paper responds much better to differences in color of the printed page than is the case when elongated tubular incandescent lamps are used, as has been the practice up to the present. It will be apprecuated that in responding to diflerences in color, the heat-sensitive paper responds by gradations of white to black and does not actually reproduce the color of the original printed page being copied.

Fig. 4 illustrates schematically a modified circuit embodying the invention for operation on the ordinary 120 volt, 60 cycle A.C. supply generally available. reference numerals denote corresponding parts with reference to the circuit of Fig. 1. For ease of illustration all the circuit elements have been symbolically illustrated. The circuit is modified by the provision of an auto-transformer 21 comprising a primary winding 22 connected in series with line switch S across terminals T T for energization from a 120 volt, 60 cycle A.C. supply. A high-reactance secondary winding 23 loosely coupled to the primary is connected in series therewith across capacitor C The primary winding forms the input circuit of the transformer whereas the primary and secondary windings in series form the output circuit. The output circuit provides the necessary voltage for operating the lamp, and at the same time the leakage reactance of secondary winding 23 serves the function of regulating reactor L previously described with reference to Fig. 1.

Fig. 5 illustrates a further variant of the invention for increasing the steepness and reducing the time duration of the current pulse. It may be used for increasing the instantaneous loading of the arc lamp or flashtube in order to achieve a higher luminous efiiciency, particularly where Xenon at a very low pressure is used for the lamp filling. Instead of connecting the lamp F immediately across the charging capacitor C and saturable reactor L another peaking stage is inserted comprising capacitor C and saturable reactor L Capacitor C is connected where formerly the lamp was connected (by reference to the circuit of Fig. 1) and saturable reactor L is connected in series with lamp F across capacitor C The circuit elements are proportioned so that when reactor L saturates and causes capacitor C to discharge into capacitor C saturable reactor L thereupon saturates even more quickly and in turn discharges capacitor C through the lamp. The transition from non-saturation to saturation in reactor L occurs more quickly Like" than it does in reactor L so'thata current pulse through the lamp offshorter t meshares anusor greater instantaneous amplitude i s obtained. Obviously the number of saturating stages may be increased to the extent desired to achieve the necessary steepness and intensity of current pulse. In such -a' combinatiom 'the inductors and capacitors intervening between the'input'terminals and the final peaking stage comprising the last capacitor across which the'last, saturableinductor-is connected in series with the lampptogether perform the function of the current limiting inductor, thatis L in Fig. 1.

' While the -invention has'been described in detail by reference to specific constructions embodying its principles, same are intended as illustrative examples and not in order to limit'the invention thereto. The appended claims are intended to cover such modifications as will occur to those skilled in the art and coming within the true spirit and scope oft he invention.

What I' c'laim as "new and desire to secure byLetters Patent of the'United Statesis:

1 The combination of an arc lampcomprising an envelope containing an inert gas at a pressure below atmospheric and having a pair of thermionic electrodes sealed into opposite ends, with an operating circuit comprising a current limitinginductive 'reactance means and a capacitor connected in series-across an alternating voltage supply of relatively low frequency and generally sinu soidal waveform, and a saturable inductor-having a highpermeabilitycore with a substantially rectangular hysteresis loop characteristic connected in series with said lamp across said capacitorfsaid current limiting inductive reactance means and capacitor'being proportioned with respect to'-the"voltage"supply such that the voltagetime integral of the charge into the capacitor causesthe saturable inductor to reach saturation at every half-cycle of the supply whereby to energize the lamp with high current pulses and with a residual current through the lamp to maintain ionization therein between pulses.

2. The combination of an arc lamp comprising an envelope containing an inert gas at a pressure below atmospheric and having a pair of self-heating thermionic electrodes sealed into opposite ends, with an operating circuit comprising a current limiting inductor and a capacitor connected in series across an alternating voltage supply of relatively low frequency and generally sinusoidal waveform, and a saturable inductor having a highpermeability core with a substantially rectangular hysteresis loop characteristic connected in series with said lamp across said capacitor, said current limiting inductor and capacitor being proportioned with respect to the voltage supply such that the voltage-time integral of the charge into the capacitor causes the saturable inductor to reach saturation at every half-cycle of the supply where by to energize the lamp with high current pulses achieving high instantaneous loading of the lamp, and said saturable inductor being proportioned to pass a residual magnetizing current not over 1% of the peak amplitude of the current pulses and serving to maintain ionization Within the lamp between pulses.

3. The combination of an arc lamp comprising an elongated envelope containing an inert gas at a low pressure and having a pair of self-heating thermionic electrodes sealed into opposite ends, with an operating circuit comprising a current limiting inductor and a capacitor connected in series across an alternating voltage supply of relatively low frequency and generally sinusoidal waveform, and a saturable inductor having a high-permeability core with a substantially rectangular hysteresis loop characteristic connected in series with said lamp across said capacitor, said current limiting inductor and capacitor being proportioned with respect to the voltage supply such that the voltage-time integral of the charge into the capacitor causes the saturable inductor to reach saturation at every half-cycle of the supply whereby to energize the lamp with high current pulses, and said satu '7 table inductor having a ratio of non-saturated to saturated impedance providing a residual current through the lamp not exceeding one percent of the peak pulse current therethrough whereby to maintain ionization within the lamp between pulses with resultant greater stability in the time intervals between pulse occurrences.

4. The combination of an arc lamp comprising a slender elongated envelope containing xenon at a pressure below atmospheric and having a pair of self-heating thermionic electrodes sealed into opposite ends, with an operating circuit comprising a current limiting inductor and a capacitor connected in series across an alternating vltage supply of relatively low frequency and generally sinusoidal waveform, and a saturable inductor having a a high-permeability core with a substantially rectangular hysteresis loop characteristic connected in series with said lamp across said capacitor, said current limiting inductor and capacitor being proportioned with respect to the voltage supply such that the voltagcvtime integral of the charge into the capacitor causes the saturable inductor to reach saturation at every half-cycle of the supply whereby to energize the lamp with high current pulses providing an instantaneous loading much greater than the average loading, said saturable inductor having a ratio of non-saturated to saturated impedance providing a residual current through the lamp not exceeding one percent of the peak pulse current therethrough, and the average loading of said lamp being proportioned to maintain the electrodes at a temperature of thermionic emission between pulses and, combined with the residual current therethrough, serving to maintain ionization therein whereby to allow conduction with relatively low voltage pulses and with high stability in the time intervals between pulse occurrences.

. 5. The combination of an arc lamp comprising an elongated envelope containing an inert gas at a low pressure and having a pair of, self-heating thermionic electrodes sealed into opposite. ends, with an operating circuit comprising a transformer having an input circuit connected to an alternating voltage supply of relatively low frequency and generallyfsinusoidal waveform, and an output circuit including a high reactance secondary winding, a capacitortconnected across said output circuit, and a saturable inductor having a high-permeability core with a substantially rectangular hysteresis loop char acteristic connected in series with said lamp across said capacitor, said output circuit and said capacitor being proportioned with respect to the voltage supply such that the voltage-time integral of the charge into the capacitor causes the saturable inductor to reach saturation at every half-cycle of the supply whereby to energize the lamp with high current pulses, and said saturable inductor having a ratio of non-saturated to saturated impedance providing a residual current through the lamp not exceeding one percent of the peak pulse current there through whereby to maintain ionization within the lamp between pulses with resultant greater stability in the time intervals between pulse occurrences.

References Cited in the file of this patent UNITED STATES PATENTS 2,482,894 Bird Sept. 27, 1949 2,487,092 Bird Nov. 8, 1949 2,509,188 Feinberg May 23, 1950 2,585,963 Ranney Feb. 19, 1952

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