US3612947A

US3612947A - Electronic timing apparatus for controlling the duration of light emission of a flash unit

Info

Publication number: US3612947A
Application number: US874935A
Authority: US
Inventors: Rolf Dieter Dennewitz
Original assignee: Loewe Opta GmbH
Current assignee: Loewe Opta GmbH
Priority date: 1969-01-28
Filing date: 1969-11-07
Publication date: 1971-10-12
Anticipated expiration: 1988-10-12
Also published as: NL7000206A; DE1905223A1; FR2029532A1

Abstract

An improved arrangement for adjusting the operating duration of a flash tube includes an interruptable thyristor gate in series with the main electrodes of the flash tube for selectively coupling operating voltage from the flash capacitor to the tube. When an ignition pulse is applied to the trigger electrode of the flash tube, a selectable reference voltage derived from the ignition pulse is applied to a Schmitt trigger circuit to generate a voltage of a first polarity. This voltage opens the thyristor gate to energize the main electrodes of the flash tube, thereby causing the tube to emit a flash of light. A portion of the corresponding light reflected from an object illuminated by the flash tube is detected by a photosensitive element and integrated to form a control signal. The control signal is applied to the input of the Schmitt trigger for a duration sufficient to overcome the reference input voltage and thereby reverse the polarity of the output voltage. This latter voltage disables the thyristor gate and terminates the flash.

Description

United States Patent [72] Inventor Rolf Dieter Dennewitz 1,350,603 10/1967 Erickson 315/151 Berhn Germany Primary Examiner-Raymond F. Hossfeld [21] Appl' 874935 Att0rneyArthur O. Klein [22] Filed Nov. 7, 1969 [45] Patented Oct. 12, 1971 [73] Assignee Loewe Opta G.m.b.H.

Berlin, Germany [32] Prlomy 1969 ABSTRACT: An improved arrangement for adjusting the [33] Germany operating duration of a flash tube includes an interruptable {31] P19 05 223-4 thyristor gate in series with the main electrodes of the flash tube for selectively coupling operating voltage from the flash [54] ELECTRONIC TIMING APPARATUS FOR capacitor to the tubehwhen an ignition pulse is applied to the CONTROLLING THE DURATION OF LIGHT trigger electrode of t e tube, a selectable reference volt- EMISSION 0F AFLASH UNIT age derived from the ignition pulse lsfapphed to a Schmltt 7 Claims, 1 Drawing Fig trigger circuit to generate a voltage 0 a first polarity. This voltage opens the thyristor gate to energize the mam elec- U.S. trodes of the tube thereby causing the tube to emit 3 250/205, 31 flash of light. A portion of the corresponding light reflected [51] Int. Cl ..l-105b 41/32 f an Object illuminated by the fl h tube is detected by a [50] Field of Search 250/205; photosensitive element and integrated to f a m 315/151, 209 R, 209 240 signal. The control signal is applied to the input of the Schmitt 56 R f d trigger for a duration sufficient to overcome the reference 1 e erences I e input voltage and thereby reverse the polarity of the output UNITED STATES PATENTS voltage. This latter voltage disables the thyristor gate and ter- 3,033,988 5/1962 Edgerton 315/151 X minates the flash.

4 27 i 29 I0 l I or 2 9 x l WWI/l I I mum J l 1 5: 7 [Z 7 l l l l l fl'fiM/Il" [Vi/06M lvl'lll'l'l l5 ,2 1/

ELECTRONIC TIMING APPARATUS FOR CONTROLLING THE DURATION OF LIGHT EMISSION OF A FLASII UNIT BACKGROUND OF THE INVENTION ln recent years various devices employing light integrating means have been proposed for automatically adjusting the operating duration of a flash device to obtain a controllable quantity of light energy therefrom. Examples of such devices are described, e.g., in U. S. Pat. No. 3,033,988 issued to H. E. Edgerton; and in U. S. Pat. Nos. 3,350,603 and 3,350,604 issued to R. D. Erickson.

These devices conventionally employ a controllable switch (generally a quench tube of the gas discharge type) which is connected in shunt with the flash tube and made conductive when the integrated light derived from the emission of the flash tube has reached a predetermined value. Such excitation of the quench tube whose impedance is then far lower than that of the flash tube, effects 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.

This arrangement has several disadvantages. Firstly, the dumping" of a portion of the exciting energy into the shunting quench tube represents a loss of efficiency of the apparatus since such shunted energy does not result in useful light output. Moreover, since the flash capacitor may be completely discharged through the flash tube and later, through the quenching tube, the flash capacitor must be completely recharged from a completely discharged or nearly completely discharged state to its maximum voltage between successive device and increasing its recycling time.

SUMMARY OF THE INVENTION The speed and efficiency of a flash-unit employing light-integrating, automatic flash duration adjustment facilities may be improved with the arrangement of the present invention. A normally disabled gate, which can be switched both on and off by pulses of respectively opposite polarity applied to a control electrode thereof, is interposed in the main energizing path of the flash tube for selectively coupling operating voltage (e.g., from the flash capacitor) to the tube when an ignition pulse is applied to its trigger electrode.

A photosensitive element associated with the flash tube output cooperates with a capacitor to integrate a portion of the resulting light energy from the flash tube and to generate a control signal that varies in proportion to the amount of light integrated. A Schmitt trigger or other suitable bipolar control device coupled to the photosensitive element is initially excited with a reference voltage derived from the ignition pulse for opening the gate in the energizing path of the flash tube to permit the flash capacitor to discharge through the tube, whereupon the latter emits a flash of light. When the control signal developed in the integrator in response to such light has reached a predetermined value sufficient to overcome the reference voltage at the input of the Schmitt trigger, the output polarity of the trigger reverses to reclose the gate in the flash tube energizing path, thereby terminating the flash.

Such a flash termination technique is effective to instantaneously stop the discharge of the flash capacitor so that the latter does not have to be completely recharged from zero after each flash termination; thus, the recycling time of the unit is improved. Moreover, since only the exact amount of flash capacitor energy needed to produce useful light output from the device is consumed, the arrangement is highly efficient.

In an illustrative embodiment, the gate in the flash unit energizing circuit is an interruptable thyristor whose transconductive path is interposed in series with the main electrodes of the flash tube. The control electrode for this type of thyristor is responsive to signals of opposite polarities for switching the transconductive path into or out of its conductive state, respectively.

BRIEF DESCRIPTION OF THE DRAWING The nature of the invention will appear more fully from the following detailed description taken in conjunction with the appended drawing, whose single figure shows an illustrative arrangement for automatically adjusting the operating duration of a flash device in accordance with the invention.

DETAILED DESCRIPTION Referring now to the drawing, there is illustrated a conventional flash tube 5 (illustratively a gas filled envelope) which is provided with a pair of

main excitation terminals

21 and 22. The main terminals are coupled through a normally disabled, controllable gate 6 and a resistor 3 to the terminals of a conventional flash capacitor 2. The flash capacitor may be selectively charged in a known manner with a high voltage, of the polarity shown, from a conventional DC power source I through a diode 23.

The tube 5 is further provided with a trigger electrode 13 which is coupled to a secondary winding 24 of an ignition transformer 10. As described hereafter an igniting pulse may be applied to the electrode 13 through the transformer 10 by closing a pair of contacts 8 associated with a primary winding 9 of the transformer 10.

The voltage of the charged flash capacitor 2, which is coupled to the tube 5 when the gate 6 is open, is not normally sufficient by itself to cause emission of light energy from the tube 5. However, in a conventional manner such tube will emit light upon the concurrent application of an igniting pulse to the trigger electrode 13 through the ignition transformer 10. The resulting flash of light energy produced in the tube 5 is emitted through the walls of the envelope toward a suitable object (not shown). A portion of such light is detected by photosensitive element 14 and integrated by a capacitor 15 in the manner described below.

As is well known the maximum time duration of the flash of a conventional 'flash tube is essentially determined by the discharge characteristic of the flash capacitor 2 through the flash tube. As will be described presently, the instant invention automatically adjusts the flash duration in a more rapid and efficient manner than heretofore in those instances where the object to be illuminated requires less than the maximum amount of light energy obtainable from the tube in a single flash.

During the time that the gate 6 remains closed, the stored voltage on the flash capacitor 2 is coupled to the primary winding 9 of the ignition transformer through (I) a voltage divider consisting of a pair of

resistors

27 and 28, the latter being adjustable; (2) a pair of

additional resistors

29 and 31; and (3) a first capacitor 7. As a result, prior to ignition of the tube 5 the high positive voltage across the flash capacitor 2 is also stored with the polarity indicated across the capacitor 7. The flash capacitor voltage also operates a conventional in dicator lamp 4, which lights up indicating readiness for flash in a known manner. The indicator lamp 4 is coupled as shown between the upper terminal of the resistor 27 and a wiper 32 of the adjustable resistor 28.

The gate 6 may be a conventional interruptable" thyristor of the type used in various power circuits in television practice. Such an interruptable" thyristor differs from ordinary thyristors of the thyratron type, in which a pulse applied to a control electrode thereof, after initiating conduction in the transconductive path of the device, is thereafter ineffective to return the device to its nonconductive state. in an "interruptable thyristor, by contrast, the transconductive path, once rendered conductive by the application of a pulse of the required polarity to its control electrode, may subsequently be cut off (or interrupted") by the application, to the same control electrode, of a pulse of the opposite polarity. In the particular arrangement shown in the drawing, the interruptable" thyristor gate 6 will close to render the tube 5 inoperative during the time that the control electrode (designated 33) of the thyristor is negative, and will open to couple the energizing voltage from the flash capacitor 2 to the tube 5 to condition the latter for operation when the control electrode 33 is positive.

The control electrode 33 of the thyristor gate 6 is coupled to the output of a bipolar gating device 16 (illustratively a Schmitt trigger circuit) whose output polarity is normally negative to maintain the gate closed. The input of the Schmitt trigger 16 is arranged to be excited at different times by the ignition transformer 10 and the output of the photosensitive element 14. In particular, an adjustable tap point 34 on the primary winding 9 of the ignition transformer 10 is coupled through a diode 11 and a resistor 12 to the input of the Schmitt trigger l6 and to the junction of the photosensitive element 14 and the integrating capacitor 15. Electrical excitation for the integrating path including the photosensitive element l4 and the capacitor 15 is taken across the resistor 3, which generates an operating voltage only when the flash tube 5 is in its conductive state.

In operation, the trigger contacts 8 are initially open and the input of the Schmitt trigger circuit 16 is quiescent. The resulting steady negative output of the Schmitt trigger l6 maintains a steady negative potential on the control gate 33 of the thyristor gate 6. The gate 6 is thus closed and prevents operating voltage from the flash capacitor 2 from exciting the

main electrodes

21 and 22 of the flash tube 5.

When the trigger contacts 8 are momentarily closed, the primary winding 9 of the transformer 10 is short-circuited through the storage capacitor 7 which has been initially charged from the DC power source 1, like the flash capacitor 2. The capacitor 7 thereupon discharges through the winding 9. The resulting negative voltage at the tap point 34 of the winding 9 is coupled through the diode 11 and the resistor 12 to negatively charge the capacitor and to excite the input of the Schmitt trigger 16, thereby reversing its output polarity. The resultant positive impulse applied to the control electrode 33 of the gate 6 opens the latter and couples operating voltage from the flash capacitor 2 to the

main electrodes

21 and 22 of the flash tube 5. The gate 6 will temporarily remain in its conductive state because of the negative voltage stored on the capacitor 15. Such stored voltage, which has a magnitude determined by the setting of the tap point 34, will be hereafter referred to as the reference voltage.

As indicated before, the presence of the operating voltage across the tube 5 is not sufficient in and of itself to cause it to emit light energy. However, the capacitor discharge caused by closure of the contacts 8 also causes the application of an ignition pulse to the trigger electrode 13 of the flash tube 5 via the ignition transformer 10, so that the combination of this ignition pulse and the now-present operating voltage across the

main electrodes

21 and 22 drives the tube 5 into its conductive state and causes it to emit light energy. The conduction of the tube 5 also causes current flow through the resistor 3, which generates excitation voltage for the integrating circuit consisting of the photosensitive element 14 and the capacitor 15.

It is well known that a reversal of polarity at the output of the Schmitt trigger 16 may be effected upon the reversal of polarity at its input. Such input polarity reversal, in the illustrated configuration, may in turn occur when a positive voltage equal to the negative reference voltage on the capacitor 15 is applied thereto through the photosensitive element 14. Such positive voltage is generated during the interval that the flash tube 5 emits light, since the resistance of the photosensitive element 14 is lowered sufficiently in a well-known manner by the influence of the incoming light reflected from the object to be photographed to charge the capacitor 15 in a positive sense from the voltage source represented by the potential across the resistor 3. When such flash duration has persisted for a duration long enough to allow the buildup of positive charge on the capacitor 15 to yield a voltage equal to the stored reference value, the polarity at the output of the Schmitt trigger 16 will again be reversed to apply a negative disabling voltage to the control electrode 33 of the gate 6. The resulting closure of the gate 6 stops the discharge of the flash capacitor 2 through the tube 5, so that the latter ceases to emit light energy. The resulting cutoff of the tube 5 disables the energizing voltage generated across the resistor 3 for exciting photosensitive element 14. Since the positive voltage built up on the capacitor 15 during the light flash is indicative of the 5 quantity of light integrated via the photosensitive element 14,

and since such voltage is also equal to the reference voltage determined by the adjustment of the tap point 34 on the primary transformer 9, it is evident that the output quantity of light emitted from the tube 5 before the flash is terminated can be adjusted to a desired value by appropriately setting the tap point 34 on the ignition transformer 10. Once this adjustment is made the duration of the flash interval of the tube 5 is automatically regulated to maintain the corresponding output quantity of light energy.

It will be noted that unlike prior art light-integrating flash duration controls, the instant arrangement employs only so much of the energy stored on the flash capacitor 2 that is necessary to produce useful emitted light from the tube 5. In most instances, this will represent an incomplete discharge of the capacitor 2. Thus time necessary to restore full voltage on the capacitor 2 from the power supply 1 in the interval between successive flashes can be relatively short compared to the time necessary to fully recharge the capacitor 2 from a completely discharged state which is manifested in prior art arrangements by the use of quench tubes in parallel with the flash tube. Hence rapid and efiicient operation of the flash unit is assured with the instant arrangement.

In the foregoing, the invention has been described in connection with a preferred arrangement thereof. However, since many other variations and modifications will now become obvious to those skilled in the art, it is accordingly desired that the breadth of the claims not be limited to the specific disclosure herein contained.

lclaim:

1. In combination with a flash device having a transconductive path and a trigger electrode and operable to emit a flash of light upon the application thereto of operating voltage through the transconductive path and the simultaneous application .of an igniting pulse to the trigger electrode, an apparatus for automatically adjusting the operating duration of the flash device to obtain a controllable quantity of light ener gy therefrom, which comprises:

normally disabled switching means interposed in the transconductive path for coupling the flash device to a source of operating voltage; v

detecting means for integrating a portion of the light energy contained in the emission from the flash device and for generating control signal that varies in proportion to the amount of light integrated;

means for selectively applying an igniting pulse to the trigger electrode; and

means coupled to the detecting means and to the applying means for enabling the switching means when an igniting pulse is applied to the trigger electrode and for subsequently disabling the switching means when the control signal has reached a predetermined value.

2. In an electronic timing apparatus wherein the duration of 'light energy emitted from a flash tube having a trigger electrode and a source of operating voltage is regulated to produce a predetermined quantity of emitted light energy:

I detecting means for integrating a portion of the emitted light to produce a control signal that varies in proportion to the amount of light integrated;

a controllable gate serially connected with the main electrodes of the flash tube and the source of operating voltage and operative when open to apply operating voltage to the main electrodes, the gate having a control electrode responsive to signals of one polarity for opening the gate and responsive to signals of the opposite polarity for closing the gate;

means for applying an igniting pulse to the trigger electrode of the flash tube;

bipolar control means coupled to the applying means and to the detecting means for initially generating an output signal of the one polarity when an igniting pulse is applied to the trigger electrode and for subsequently terminating the last-mentioned signal and generating an output signal of the opposite polarity when the control signal has reached a predetermined value; and

means for coupling the output signal of the bipolar control means to the control electrode of the gate.

3. Apparatus as defined in claim 2 in; which the bipolar control means is a Schmitt trigger circuit.

4. Apparatus as defined in claim 2, in which the controllable gate is an interruptable thyristor having a transconductive path serially connected with the main electrodes of the flash tube, the transconductive path being rendered conductive upon the application of a voltage of the one polarity to its control electrode and rendered nonconductive upon the application of a voltage of the opposite polarity to its control electrode.

5. Apparatus as defined in claim 2, in which the detecting means comprises, in combination, a photosensitive element in operative association with the light emitted from the flash tube;

a first capacitor; means for coupling the photosensitive element to the first capacitor and to the input of the bipolar means; and means for electrically exciting the photosensitive element when the gate is opened.

6. Apparatus as defined in claim 5, in which the 'applying means comprises, in combination, an ignition transformer having a primary winding coupled across the gate and the flash tube through a second capacitor;

a secondary winding coupled to the trigger electrode of the flash tube; and

means for momentarily short circuiting the primary winding and the second capacitor.

7. Apparatus as defined in claim 6 in which the primary winding has an adjustable tap, and in which the apparatus further comprises means for coupling the adjustable tap to the input of the bipolar control means and to the first capacitor for establishing a reference voltage at the input of the bipolar control means.

Claims

1. In combination with a flash device having a transconductive path and a trigger electrode and operable to emit a flash of light upon the application thereto of operating voltage through the transconductive path and the simultaneous application of an igniting pulse to the trigger electrode, an apparatus for automatically adjusting the operating duration of the flash device to obtain a controllable quantity of light energy therefrom, which comprises: normally disabled switching means interposed in the transconductive path for coupling the flash device to a source of operating voltage; detecting means for integrating a portion of the light energy contained in the emission from the flash device and for generating control signal that varies in proportion to the amount of light integrated; means for selectively applying an igniting pulse to the trigger electrode; and means coupled to the detecting means and to the applying means for enabling the switching means when an igniting pulse is applied to the trigger electrode and for subsequently disabling the switching means when the control signal has reached a predetermined value.

2. In an electronic timing apparatus wherein the duration of light energy emitted from a flash tube having a trigger electrode and a source of operating voltage is regulated to produce a predetermined quantity of emitted light energy: detecting means for integrating a portion of the emitted light to produce a control signal that varies in proportion to the amount of light integrated; a controllable gate serially connected with the main electrodes of the flash tube and the source of operating voltage and operative when open to apply operating voltage to the main electrodes, the gate having a control electrode responsive to signals of one polarity for opening the gate and responsive to signals of the opposite polarity for closing the gate; means for applying an igniting pulse to the trigger electrode of the flash tube; bipolar control means coupled to the applying means and to the detecting means for initially generating an output signal of the one polarity when an igniting pulse is applied to the trigger electrode and for subsequently terminating the last-mentioned signal and generating an output signal of the opposite polarity when the control signal has reached a predetermined value; and means for coupling the output signal of the bipolar control means to the control electrode of the gate.

5. Apparatus as defined in claim 2, in which the detecting means comprises, in combination, a photosensitive element in operative association with the light emitted from the flash tube; a first capacitor; means for coupling the photosensitive element to the first capacitor and to the input of the bipolar means; and means for electrically exciting the photosensitive element when the gate is opened.

6. Apparatus as defined in claim 5, in which the applying means comprises, in combination, an ignition transformer having a primary winding coupled across the gate and the flash tube through a second capacitor; a secondary winding coupled to the trigger electrode of the flash tube; and means for momentarily short circuiting the primary winding and the second capacitor.