US3919594A

US3919594A - Computer flash apparatus with series-connected control switches

Info

Publication number: US3919594A
Application number: US480501A
Authority: US
Inventors: Gotthard Mahlich; Heinz Engelstatter
Original assignee: Braun GmbH
Current assignee: Braun GmbH
Priority date: 1974-06-18
Filing date: 1974-06-18
Publication date: 1975-11-11
Anticipated expiration: 1992-11-11

Abstract

A first and second thyristor each have an anode-cathode circuit connected in series with the flash tube. The required voltage rating corresponds to only 1/2 the voltage across the capacitor furnishing the energy for the flash. To assure ignition of both thyristors, the gate of each is connected through a resistor to a separate secondary winding of a transformer having a primary winding connected in the ignition circuit. To achieve rapid switching from the conductive to the non-conductive state, a resistor is connected in series with the cathode of each thyristor and the other terminal of each resistor is connected to the opposite terminal of the secondary winding connected to its gate. The resultant negative bias on the gate becomes effective at low thyristor currents to increase the switching speed to the non-conductive state.

Description

United States Patent 1191 I I Mahlich et al. 4

[4 Nov. 11, 1975 [5 COMPUTER FLASH APPARATUS WITH SERIES-CONNECTED CONTROL 1 SWITCHES [75] Inventors: Gotthard Mahlich, Kronberg; Heinz Engelstiitter, Bad Soden, both of Germany [73] Assignee: Braun Aktiengesellschaft, Kronberg, Taunus, Germany 22 Filed: June 18, 1974 [21 App]. No.: 480,501

52 us. or 315/241 P; 315/241 R 51 Int. (:1. HOSB 39/00 58 Field of Search 315/241 P [56] References Cited UNITED STATES PATENTS 3,809.95] 5/1974 Vital et a1 315/241 P X 3.809.954 5/1974 Engelstzitternn 315/241 P $814,985 6/1974 Pecher et a1. .1 315/241 P Primary E.raminerNathan Kaufman Attorney, Agent, or Firm-Michael S. Striker 1 5 7 1 ABSTRACT A first and second thyristor each have an anodecathode circuit connected in series with the flash tube. The required voltage rating corresponds to only /2 the voltage across the capacitor furnishing the energy for the flash. To assure ignition of both thyristors, the gate of each is connected through a resistor to a separate secondary winding of a transformer having a primary winding connected in the ignition circuit. To achieve rapid switching from the conductive to the nonconductive state, a resistor is connected in series with the cathode of each thyristor and the other terminal of each resistor is connected to the opposite terminal of the secondary winding connected to its gate. The resultant negative bias on the gate becomes effective at low thyristor currents to increase the switching speed to the non-conductive state.

4 Claims, 1 Drawing Figure US. Patent Nov. 11, 1975 COMPUTER FLASH APPARATUS WITH SERIES-CONNECTED CONTROL SWITCHES BACKGROUND OF THE INVENTION This invention relates to computer flash units. In particular it relates to such flash units when a flash tube is connected in series with electronic switches which interrupt the flash tube current. The so-formed series circuit is connected in parallel with a capacitor which furnishes the energy required for the flash.

Circuits of this type are known wherein the anodecathode circuit of a thyristor connected in series with a flash tube serves to interrupt the current through the flash tube thereby terminating the flash. Specifically, the thyristor is ignited, that is switched to the conducting state, substantially simultaneously with the flash tube by the application of an ignition signal generated in the ignition circuit to the gate of the thyristor. The thyristor is switched to the non-conducting state at the end of the exposure time by the discharge of a terminating capacitor which furnishes a current to the thyristor in opposition to the current furnished by the flash tube. This current is maintained for a sufficiently long time that all charge carriers in the cathode-anode circuit of the thyristor are eliminated. When, therefore, current flow through the thyristor ceases, it is again in the non-conductive state and the flash has been terminated.

Apparatus of this type has the disadvantage that, since the flash tube requires a high voltage for ignition, a thyristor connected in series therewith must have a correspondingly high voltage rating, that is must be able to withstand a breakdown voltage exceeding the voltage across the capacitor furnishing the energy for the flash. Further, in order that the capacitor which stores the energy for the terminating current be small, these thyristors are also required to have high reverse current ratings.

SUMMARY OF THE INVENTION It is an object of the present invention to furnish an automatic flash apparatus having a series switch arrangement for terminating the flash which does not have the above disadvantages, that is wherein the voltage rating of the switches (thyristors) can be decreased to approximately half the voltage of the energy storing capacitor and also having a lower power rating.

The present invention resides in a computer flash apparatus having energy storage means for storing the energy required for the flash. It comprises a series circuit including flash generating means responsive to an ignition signal for furnishing said flash and a first and second switch each having a main conducting circuit connected in series with said flash generating means and having, respectively, a first and second control elec* trode, each of said switches switching from a nonconductive to a conductive state in response to a control signal at said first and second control electrode respectively. It further comprises first and second connecting means for connecting said first and second control electrode, respectively, to said ignition circuit in such a manner that said ignition circuit furnishes said control signals to said control electrodes substantially simultaneously with said furnishing of said ignition signal to said flash generating means.

In a preferred embodiment of the present invention the first and second switch are each a thyristor.

In a further preferred embodiment of the present invention, the first and second connecting means are, respectively, a first and second. ignition resistor. Each of the ignition resistors serves to maintain the control voltage at the control electrode sufficiently long to assure ignition of the second thyristor to be switched when the switching of both does not take place exactly simultaneously. This decreases the time during which the second thyristor to be ignited has a high resistance, thereby decreasing the undesired power loss in said thyristor.

Since thyristors of high power ratings are extremely expensive, the savings created by the use of two thyristors of lower rating more then compensates for the additional circuit elements required for the present invention over those circuits utilizing only a single thyristor.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS The single FIGURE shows a circuit diagram of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of the present invention will now be described with reference to the drawing.

Shown in FIG. 1 is a storage means, namely a storage capacitor 1. Connected in parallel with storage capacitor 1 is the flash generating means, namely a flash tube 2 connected in series with a first and second switch namely a thyristor 3 and 4. Ignition circuit means, which include a switch 5 which, subject to external activation, causes a pulse to appear across a transformer 6 which is coupled to the flash tube 2 and furnishes an ignition pulse when switch 5 is closed. The ignition circuit further has a transformer 7 which has a first secondary winding 71 and a second secondary winding 72. The first terminal of secondary winding 71 is connected through a resistor 81 (one embodiment of a first ignition resistor) to the gate 31 of thyristor 3, while the first terminal of secondary winding 72 is connected through the second ignition resistor 82, to the gate 41, of thyristor 4. An exposure measuring circuit which, as is well known, contains a photosensitive element which is responsive to the light reflected from the object being photographed furnishes a terminating signal when the so-received light has reached a predetermined light quantity. The terminating signal is applied to an electronic switch 9. When switch 9 switches to a conducting state in response to this terminating signal, a previously charged terminating capacitor 10 is discharged. The polarity on capacitor 10 is such that the current through thyristors 3 and 4 resulting from the discharge of this capacitor is in a direction opposing the current from the flash tube 2. The capacitance of capacitor 10 is such that this current is maintained sufficiently long so that all charge carriers in the thyristors are eliminated. In other words, the current through thyristors 3 and 4 becomes zero and these thyristors are thus effectively in the blocked state until such time as the control voltage is again applied to the gates. When thyristors 3 and 4 are in the blocked state no current can flow through flash tube 2 and the flash is of course terminated. The speed of switching back to the nonconductive state can further be increased by the action of series resistors 110 and 111 each connected to the cathode of one of the thyristors in such a manner that the current flow therethrough creates a negative bias on the gate of the corresponding thyristor. Of course any increase in the switching speed results in lesser loading of the thyristors and is thus very desirable.

It is seen that the circuit of the present invention serves to terminate the flash reliably while requiring thyristors having decreased ratings from the thyristors normally employed thereby effecting the substantial decrease in the cost of the equipment.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge adapt it for various applications without omitting features that, from the standpoint of prior art fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.

1. In electronic flash apparatus having storage means for storing the energy required for said flash, said storage means having storage output tenninals furnishing a predetermined voltage when said storage means is charged, in combination, a series circuit connected across said output terminals, said series circuit comprising flash generating means for furnishing said flash in response to an ignition signal, and a first and second switch each having a main conducting circuit connected in series with said flash generating means and having, respectively, a first and second control electrode, each of said main conducting circuits switching from a non-conductive to a conductive state in response to a control signal at the respective control electrode; ignition circuit means coupled to said flash generating means for furnishing said ignition signal upon external activation; first and second connecting means for connecting said ignition circuit to said first and sec- 0nd control electrodes in such a manner that said control electrodes receive said control signals substantially simultaneously with the furnishing of said ignition signal to said flash generating means thereby switching said main conducting circuits to the conducting state;

and terminating circuit means connected to said main conducting circuits, for blocking the current flow therethrough thereby terminating said flash at the end of the exposure time.

2. Apparatus as set forth in claim 1, wherein said storage means comprise a capacitor; wherein said series circuit is connected in parallel with said capacitor; and wherein said first and second switch are, respectively, a first and second thyristor, each of said thyristors having a gate constituting said control electrode and an anode-cathode circuit constituting said main conducting circuit.

3. Apparatus as set forth in claim 2, wherein said ignition circuit means comprise a transformer having a first and second secondary winding; and wherein said first connecting means comprise a first ignition resistor connected between said first secondary winding and said gate of said thyristor and said second connecting means comprise a second ignition resistor connected between said second secondary winding and said gate of said second thyristor.

4. Apparatus as set forth in claim 3, further comprising a first and second series resistor connected in said series circuit, each having a first terminal connected to the cathode of a corresponding one of said thyristors, and a second terminal; wherein said first secondary winding is connected between said first ignition resistor and said second terminal of said first series resistor and said second secondary winding is connected between said second ignition resistor and said second terminal of said second series resistor; and wherein said terminating means comprise a normally charged capacitor, means for initiating the discharge of said capacitor at the end of said exposure time, and means for connecting said capacitor to said switches in such a manner that discharge of said capacitor blocks said current flow through said main conductucting circuit thereby switching said switches to the non-conducting state and terminating said flash.

Claims

1. In electronic flash apparatus having storage means for storing the energy required for said flash, said storage means having storage output terminals furnishing a predetermined voltage when said storage means is charged, in combination, a series circuit connected across said output terminals, said series circuit comprising flash generating means for furnishing said flash in response to an ignition signal, and a first and second switch each having a main conducting circuit connected in series with said flash generating means and having, respectively, a first and second control electrode, each of said main conducting circuits switching from a non-conductive to a conductive state in response to a control signal at the respective control electrode; ignition circuit means coupled to said flash generating means for furnishing said ignition signal upon external activation; first and second connecting means for connecting said ignition circuit to said first and second control electrodes in such a manner that said control electrodes receive said control signals substantially simultaneously with the furnishing of said ignition signal to said flash generating means thereby switching said main conducting circuits to the conducting state; and terminating circuit means connected to said main conducting circuits, for blocking the current flow therethrough thereby terminating said flash at the end of the exposure time.