US3703662A

US3703662A - High speed quenching arrangement for a flash device

Info

Publication number: US3703662A
Application number: US882689A
Authority: US
Inventors: Rolf Dister Dennewitz; Lutz Breitkreuz
Original assignee: Loewe Opta GmbH
Current assignee: Loewe Opta GmbH
Priority date: 1968-12-11
Filing date: 1969-12-05
Publication date: 1972-11-21
Anticipated expiration: 1989-11-21
Also published as: DE1814856A1; GB1275658A; NL6918599A; FR2025884A1

Abstract

A pulsed flash unit is provided with a photoelectrically controlled, interruptible thyristor gate whose transconductive path is connected in shunt with the flash tube for instantaneously quenching the light output from the flash tube when a predetermined quantity of emitted light has been monitored and integrated by the unit. A threshold-regulated transistor coupled to the output of the light integrator supplies a twostate control voltage to the control electrode of the thyristor so that the latter selectively provides a short and an open circuit across the flash tube.

Description

United States Patent Dennewitz et al.

HIGH SPEED QUENCHING ARRANGEMENT FOR A FLASH DEVICE Inventors: Rolf Dieter Dennewitz; Lutz Brett- 7 kreuz, both of Berlin, Germany Assignee: Loewe Opta G.m.b.H., Berlin, Germany Filed: Dec. 5, 1969 App]. No.: 882,689

Foreign Application Priority Data Dec. 1l, 1968 Germany..., ..P 1 8 14 856.5

vs. C! ..315/159, 315/241 R, 315/241 P Int. Cl. ..H05b 37/02 Field of Search ..250/205; 307/252.51; 315/124, 315115145 159,178,182,183,192,228, 230, 232, 234, 241, 241 P, 322, 340

1451 Nov. 21, 1972 [5 6] References Cited 7 7 UNITED STATES PATENTS 3,033,988 5/1962 Edgerton ..250/205 3,509,422 4/1970 Kilgore ..315/232 X 3,519,879 7/1970 -o 'awa...; ..'.....315Il83 X 3,568,582 3/1971 Uchida ..315/241 X Primary Examiner-Roy Lake Assistant Examiner-Lawrence J. Dahl Attorney-Arthur O. Klein [5 7 ABSTRACT 3 Claims, 2 Drawing Figves l I l I I I 1 l l I 1 BACKGROUND THE'INVENTION In recent years various devices employing light integrating means-have been proposed for automatically adjusting the operating duration of a pulsed flash device by suitably quenching theflash aftera controllable quantity of light energy has been emitted therefrom. Examples of such devices are described, e.g., in U. S. Pat. No. 3,033,988'issued to H. E. Edgarton and in U. S.Pat. Nos. 3,350,603 and 3,350,604 issued to R. D. Erickson.

These devices generally employ aheavy-dutyquench tube of the gas discharge type which is connected in shunt withthe flash tube and triggered into conduction via an ignition transformer and associated components when the integrated light derived from theemission of the flash tube has reached a predetermined value. The quench tube when conductive, has an impedance which is far lower than that of the-flash tube toeffect the premature termination of the flash by causing the exciting energy for the tube e.g., from the flash capacitor) to bypass the flash tube and be dissipated in the shunting quench tube.

Such gas discharge quench tubes have several disadvantages. Firstly, they are susceptibel to premature ignition by stray electrstatic fields in the flash unit unless special space-consuming screening devices are provided. Also, the size and bulk of the quench tubeitself andof the associated ignition transformercircuitry is frequently too large to be conveniently accommodated in a ligthweight portable flash unit. Finally, suchheavyduty quenching devices and associated circuitry respond relatively slowly to a triggering control pulseat the output of the light integrating circuitry. The associated time delay between triggeringand the start of quench establishes a relatively large irreducible minimum duration of the flash pulse, which in turn fixes the minimum amount of light exposure to which an object may be subjected bythe flash. Since aportion of the light from the flash is employed to operate the light integrating circuitry, a corresponding irreducible minimum distance is required betweenthe flash device and the object to be illuminated, therebylimiting th flexibility of operation of the unit.

SUMMARY OF THE INVENTION The problems inherent in using such conventional quenching arrangements for a flash tube are alleviated by the arrangement of the instant invention. In one embodiment, a normally disabled, interruptible thyristor gate, which can be both opened and closed by pulses of respectively opposite states applied to a control electrode thereof, is connected in shunt with the flashtube. The control electrode of the thyristor is supplied with such two-state control pulses by means of a thresholdregulated transistor which is driven into conduction when the flash tube emits light energy.

At the start of the flash, theoutput of the conductive transistor is such as tocause avoltage of a first stateto be applied to the controlelectrode of the thryristor to maintain the latter nonconductive and thus ineffective to affect the flash. When the control signal developed in the light-integrating portion of the flash unit has reached a value sufficient to overcome a threshold level at the input of the transistor, the latter is cut off to reverse the state of the voltage on the control electrode of the thyristor, thereby switching the latter into conduction, to short-circuit the flash tube and terminate the flash.

Such a flash tube quenching arrangement is much faster in operation, and is smaller and lighter, than the gas discharge quench tubes and associated ignition circuitry employed in the prior art.

Moreever, the instant arrangement is'not susceptible tovpremature triggering by stray electrostatic fields and thus does not require separate shielding.

BRIEF DESCRIPTION OF THE DRAWING DETAILED DESCRIPTION Referring now to the drawing, FIG. 1 depicts a pulse-.

excited flash unit 3 including a conventional flash tube 7 (illustratively a gas filled envelope). The tube 7 is provided with anode and cathode terminals 8 and 9,

respectively. A primary winding 12 (FIG. 2) of a transformer 14 is connected between the cathode 9 and a common terminal 10 of the flash unit. A flash capacitor 4 (FIG. 1) is connected between the terminals 8 and The capacitor 4 may be selectively charged in a known manner with a relatively high potential of the polarity shown from a conventional DC-DC converter 2, which is energized from a relatively low potential battery 1 through a switch 50.

The flash tube 7 is further-provided with a trigger electrode 6 which is coupled to a'secondary winding 51 of an ignition transformer 5. The electrode 6 may be excited via the transformer S, by applying a suitable shorting pulse through a cable 53 and a capacitor 54 to a primary winding 56 of the the transformer 5.

The voltage of the charged flash capacitor 4 is not normally sufficient by itself to cause emission of light energy from the tube 7. However, in a conventional manner, such tube will emit light upon the concurrent application of an igniting pulse to the trigger electrode 6 through the ignition transformer 5. The resulting flash of light energy produced in the tube 7 is emitted through the walls of the tube envelope to a suitable object (not shown). A portion of the light reflected from the object is detected by a photosensitive element 57 (FIG. 2), illustratively a suitable resistor, which conv trols a light integrating circuit 80 in the manner described below.

The stored voltage on the flash capacitor 4 (FIG. 1) is also coupled to the primary winding 56 of the ignition transformer 5 through (A) a resistor 61 of an adjustable voltage divider 61-62 coupled across the capacitor 4;(B) a second pair of

resistors

63 and 64; and (C) the capacitor 54. Thus, prior to' ignition of the tube 7 a high positive voltage is stored with the polarity indicated across the capacitor 54.

The voltage across the capacitor 4 also operates a conventional indicator claim 66 which indicates readiness for flash in a known manner. The indicator lamp 66 is coupled as shown between the upper terminal of the resistor 61 and a variable tap 67 of the resistor 62.

Ignition of the tube 7 is accomplished upon the application of a shorting pulse to the cable 53, which shortcircuits the primary winding 56 and the capacitor 54 to cause the capacitor 54 to discharge through the winding 56. The resultant transient through the primary winding 56 causes the application of an igniting pulse to the trigger electrode 6 via the secondary winding 51 of the transformer 5. The resulting combination of the igniting pulse voltage on the electrode 6 and the flash capacitor voltage appearing across the terminals 8 and 9 of the flash tube 7 drived the latter into its conductive state and causes it to emit light energy toward the object to be illuminated.

The sudden conduction of the tube 7 causes a pulse of current through the primary winding 12 (FIG. 2) of the transformer 14 in series therewith to generate a corresponding voltage pulse across a secondary winding 70 of the transformer 14. Such voltage pulse is coupled through a filter 71 to establish a voltage across a capacitor 72, such voltage being stabilized by a Zener diode 73. The stabilized capacitor voltage is applied through a resistor 74 to the collector-emitter path of a threshold-regulated NPN transistor 75, which conducts when a suitable positive potential is simultaneously applied to its base electrode.

A first voltage divider consisting of a pair of

biasing resistors

76 and 77 is connected across the capacitor 72. The resulting positive potential appearing at a tap of the divider i.e., a junction 78 of the resistors 76 and 77) is coupled to the base of the transistor 75 through a resistor 79, so that the transistor will normally conduct whenever a voltage appears on the capacitor 72. Thus, the collector of the transistor will normally be at essentially zero potential with respect to the common terminal 10.

The transistor 75 may be selectively cut off by the action of the light integrating arrangement 80. The latter includes a second voltage divider which consists of the serial combination of a pair of adjustable resistors 81 and 82 and the photosensitive element 57; the second voltage divider is also connected across the capacitor 72. A tap of the divider i.e., a junction 15 between the resistors 81 and 82) is coupled to the junction 78 of the

resistors

76 and 77 through a capacitor 58.

The resistance of the photosensitive element 57 is substantially changed in response to reflected light from an object illuminated by the flash tube 7, thereby establishing a charging voltage between the junction 15 and the common terminal 10. The resulting charging of the capacitor 58 lowers the potential on the junction 78. When the charging of the capacitor 58 has persisted for a duration long enough to allow the voltage accumulated on the capacitor 58 to reach the threshold value oringinall established at the junction 58 by the voltage divider 76-77, the transistor 75 will be rapidly cut off to raise the potential at its collector electrode from an essentially zero value to the relatively high positive value appearing across the capacitor 72.

In accordance with the invention, a conventional interruptibled thyristor gate 83 has its transconductive path serially connected through a low impedance resistor 84 between the terminals 8 and 10 for shunting the flash tube 7. The thyristor 83 is thus energized by the flash capacitor 4 through the resistor 84. This thyristor may be of the type used in various power circuits in television practice.

In the particular arrangement shown in FIG. 2, the interruptible thyristor 83, which may be of the type 40379 supplied by The Radio Corporation of America will remain nonconductive during the time that the potential at its control electrode is zero or negative (a first state) and will be conductive when the polarity at its control electrode is positive (a second state).

The required two-state control voltage for the thyristor 83 is supplied in the manner described below by means of a lead 86 extending between its control electrode and the collector of the transistor 75.

The operation of the above-described arrangement is as follows: Prior to the start of light emission from the tube 7 (as when no excitation appears on its control electrode 6) no current flows in its anode cathode path 8-9 and the primary winding 12 of the transformer 14 will be quiescent. Thus, no voltage appears across the capacitor 72 and the collector-emitter path of the transistor 75 is deenergized, so that the tube does not conduct. The resultant zero potential on the collector of the transistor is coupled through the lead 86 to the control electrode of the thyristor 83. Such zero potential is ineffective to switch the thyristor into its conducting state and the thyristor will appear as an open circuit across the flash tube 7.

When an ignition pulse is applied to the ignition electrode 6 of the flash tube 7 to cause it to emit light, the resulting pulse of current through the primary winding 12 of the transformer 14 will result in a pulse of positive voltage across the capacitor 72 through the secondary winding and the filter circuit 71. The voltage across the capacitor 72 is coupled via the resistor 74 to the collector-emitter path of the transistor 75. Such voltage is also coupled via the resistor 76, the junction 78 and the resistor 79 to the base of the transistor to cause the transistor to conduct. Since the transistor exhibits a low impedance during conduction, the previously existing zero voltage on the collector will be maintained, The thyristor 83 accordingly remains non-conductive and initially continues to exhibit an open circuit across the now-conductive flash tube 7.

When the emission of light from the tube 7 has persisted for a duration long enough to allow the build-up of positive charge on the capacitor 58 to yield a voltageeequal to the threshold voltage on the junction 78, the transistor 75 will be rapidly cut off to establish a high positive voltage on its collector. Such voltage, coupled to the control electrode of the thyristor 83 through the lead 86, drives the thyristor 83 into conduction so that the latter presents a short circuit across the flash tube 7. Such short circuit effectively quenches the flash to stop the emission of light from the tube 7.

The resulting deexcitation of the primary winding 12 of the transformer 14 disable the collector-emitter path of the transistor 75 so that its collector potential again drops to substantially zero. As a result, the thyristor 83 too has a zero potential at its control electrode so that it again presents an open circuit across the now-disabled flash tube 7 since the potential between anode and energy emitted by the flash unit. The totallight emission from the tube 7 can therefore be regulated by ad- I justing thethreshold value on the junction 15, as by suitably choosing the values of the resistors 82 and 81. Once the appropriate adjustment is made, the duration of the flash interval of the tube 7 is automatically regulated to maintain the desired amount of light energy emission from the flash tube 7, dependent on the value of the light-sensitive resistor 57.

It will be observed that with the quenching arrangement of the invention, no large and bulky gas discharge device with itsaccompanying heavy-duty ignition transformer and electrostatic shielding is required to quench the flash. In addition, because of the inherently high switching speed of the solid state thyristor,- a flash unit equipped with the above-described arrangement may safely operate with object-to-flash-lamp distances so small as to normally causeexcessive exposure of the object to be illuminated when prior art gas discharg quench tubes are used.

In the foregoing, the invention has been described in connection with one preferred arrangement thereof. Since many of thevariations and modifications will now become obvious to those skilled in the art, it is desired that the breadth of the claims not be limited to the specific disclosure therein contained.

What is claimed is:

l. A lightweight compact apparatus for rapidly terminating the output of a pulsed flash tube when a controllable quantity of light energy has been emitted therefrom, which comprises:

photosensitive means for generating a control signal proportional to the integrated light energy contained in the emission from the flash tube;

a normally disabled interruptable thyristor gate having a transconductive path whose resistance is low relative to the resistance of the flash tube and a control electrode responsive to signals of one state for keeping the gate closed and responsive to signals of the opposite state for opening the gate;

resistive means for connecting the transconductive path of the thyristor gate directly across the flash tube to establish a shunting path for the tube, the resistance of the resistive means being sufiiciently low so that the resistance of the shunting path is lower than that of the flash tube;

normally disabled, threshold-regulated gating means having an input coupled to the output of the generating means, the gating means being triggerable into the one state when enabled and triggerable into the opposite state when subsequently excited by an input signal of a pre-determined value;

means for enabling the generating means and the gating means at the start of the flash; and

means for directly coupling the output of the gating means to the control electrode of the thyristor gate.

2. Apparatus as defined in claim 1, in which the gating means comprises a transistor and in which the apparatus further comprises, in combination;

a transformer having a primary winding In series with the flash tube and a secondary winding connected across the collector-emitter path of the transistor; rectifying means including a first capacitor connected across the secondary winding; a first voltage divider connected across the first capacitor;

a second voltage divider including a photosensitive element connected across the first capacitor;

means for coupling a tap of the first voltage divider to the base of the transistor; and

a second capacitor connected between the tap of the first voltage divider and a tap of the second voltage divider.

3. In combination with an electronic flash apparatus including a pulsed flash tube, photosensitive means operative at the start of emission of a light flash from the tube for generating a control signal proportional to the integrated light energy contained in the emission from the tube, and gating means operatively coupled to the output of the generating means, which gating means exhibits an output of one state at the start of the flash and exhibits an output of the opposite state when the control signal has reached a value representative of a selected quantity of lightenergy emitted from the tube, an improved means for terminating the output of the tube when the selected quantity of light has been emitted therefrom, which comprises:

a normally disabled interruptable thyristor having a transconductive path whose resistance is low relative to the resistance of the tube and a control electrode responsive to the signals of the one state for keeping the thyristor closed and responsive to signals of the opposite state for opening the thyristor;

resistive means for connecting the transconductive path of the thyristor directly across the tube to establish a shunting path for the tube, the resistance of the resistive means beind sufficiently low so that the resistance of the shunting path is lower than that of the tube; and

meansfor directly coupling the output of the gating means to the control electrode of the thyristor.

Claims

1. A lightweight compact apparatus for rapidly terminating the output of a pulsed flash tube when a controllable quantity of light energy has been emitted therefrom, which comprises: photosensitive means for generating a control signal proportional to the integrated light energy contained in the emission from the flash tube; a normally disabled interruptable thyristor gate having a transconductive path whose resistance is low relative to the resistance of the flash tube and a control electrode responsive to signals of one state for keeping the gate closed and responsive to signals of the opposite state for opening the gate; resistive means for connecting the transconductive path of the thyristor gate directly across the flash tube to establish a shunting path for the tube, the resistance of the resistive means being sufficiently low so that the resistance of the shunting path is lower than that of the flash tube; normally disabled, threshold-regulated gating means having an input coupled to the output of the generating means, the gating means being triggerable into the one state when enabled and triggerable into the opposite state when subsequently excited by an input signal of a pre-determined value; means for enabling the generating means and the gating means at the start of the flash; and means for directly coupling the output of the gating means to the control electrode of the thyristor gate.

2. Apparatus as defined in claim 1, in which the gating means comprises a transistor and in which the apparatus further comprises, in combination: a transformer having a primary winding in series with the flash tube and a secondary winding connected across the collector-emitter path of the transistor; rectifying means including a first capacitor connected across the secondary winding; a first voltage divider connected across the first capacitor; a second voltage divider including a photosensitive element connected across the first capacitor; means for coupling a tap of the first voltage divider to the base of the transistor; and a second capacitor connected between the tap of the first voltage divider and a tap of the second voltage divider.