ELECTRONIC PHOTOFLASH CONTROL CIRCUIT

Abstract: In order to provide a control circuit for a photoflash gun which is capable of fast recycling times, the circuit includes a flashtube (FT), a switching circuit (SW, L1) for initiating operation of the flashtube, a switch (CR1) in series with the flashtube and which is switched on at the same time as the flashtube in order to permit current flow through the flashtube, and an arrangement for resetting the switch (CR1) after a predetermined time to cut off current flow through the flashtube, the arrangement comprising an inductance (L2) and a capacitor (C2) coupled to the anode of the switch (CR1), so that when the flashtube is switched on, current discharges through the capacitor (C2) creating a back EMF in the inductance (L2). A voltage of negative polarity is thus developed across capacitor (C2) to switch off switch (CR1) after a predetermined time, thereby to switch off the flashtube. Further arrangements are disclosed wherein the switch off time is controlled by a light calculation circuit responsive to ambient light.

Description: ELECTRONIC PHOTOFLASH CONTROL CIRCUIT TECHNICAL FIELD The present invention relates to a control circuit for an electronic photoflash apparatus used in photographic applications. BACKGROUND ART Electronic photoflash guns have been extensively used for some years to provide extra illumination for photography in low ambient light conditions. A requirement of control circuits associated with photo¬ flash guns has been to time accurately the firing of the photoflash tube(s) and also to provide a definite cut-off or quench of the photoflash tube when sufficient light has been generated. This latter cut-off can be either fixed to provide one or more discrete levels of generated light (whereupon exposure settings within the camera may need to be varied to compensate) or may be automatically provided when a quantity of light sufficient for a predetermined exposure setting of the caaera has been generated by the tube. Prior control circuits for photoflash guns as will hereinafter be described operate on the principle of charging and discharging capacitors in order to switch thyristors in order to initially fire the flash gun and to cut off or quench the flash gun. However, operation O PΓ by charging and discharging capacitors can be slow, particularly where fast recycling times are required, for example in motor driven cameras where it may be necessary to take sequential exposures very rapidly 5 and a photoflash gun must be able to recycle in a very short time between exposures. SUMMARY OF INVENTION It is therefore an' object of the present 10 invention to provide a control circuit for a photoflash gun which is capable of fast recycling times. The present invention provides a control circuit for a photoflash gun, the circuit including a flashtube, a switching circuit for initiating operation of the l ζ flashtube, a switch means coupled with the flashtube which when operated permits current flow through the flashtube, and reset means for resetting the switch means after a predetermined time to cut off current flow through the flashtube, characterised in that the 20 reset means includes an inductance and a charge storage means coupled to the switch means such that when the switch means is operated current flow occurs through said charge storage means and said inductance to create a voltage across said charge storage means which 25 is of a polarity such as to reset the switch means. In accordance with the invention, the inductance and the charge storage means provide a resonant like cirucit which permits fast recycling time of operation. 30 BRIEF DESCRIPTION OF DRAWINGS Features and advantages of the present invention will become apparent from the following description of a prior arrangement and preferred embodiment of the invention, together with the accompanying drawings. 35 in which:- Figure 1 illustrates a basic block diagram of a previously proposed control circuit; / * & Figure 1A shows the electronic switch circuit of Figure 1 in greater detail; Figure IB shows the light-sensing calculation circuit of Figure 1 in greater detail; Figure 2 illustrates a block diagram of the preferred embodiment of the invention; Figures 2A and 2B show two forms of electronic switch circuit which can be used in the circuit of Figure 2; Figures 2C and 2D show two further developed forms of electronic switch circuit which can be used in the circuit of Figure 2; Figures 2E and 2F show two forms of light-sensing calculation circuit which can be used in the circuit of Figure 2; and Figure 2G shows a voltage controller circuit which can be used in the circuit of Figure 2F. BEST MODE FOR CARRYING OUT THE INVENTION Referring to Figure 1, a previously-proposed circuit includes a voltage source 10 arranged to charge an energy storage capacitor CM (and other capacitors elsewhere in the circuit) to a voltage VI. Also connected across the storage capacitor CM are a series combination of energising switch SW and resistor Rl , an a capacitor Cl with triggering coil LI also connected across switch SW. A secondary of the triggering coil LI triggers a flash tube FT which is also connected to electronic switch circuit 11 for controlling cut-off of the flash tube FT. The switch circuit 11 is responsive to a light-sensing calculation circuit 12 which is connected to a light sensor LS. The operation of the Figure 1 circuit is broadly as follows. When the switch SW is closed (this switch being generally provided in the camera in association with the shutter), the trigger coil LI generates a pulse signal by virtue of the previously-charged capacitor Cl discharging through the switch SW and coil LI, the pulse signal firing the flash tube FT. The light- sensing circuit 12 calculates when sufficient light has been emitted by the photoflash and provides a quenching signal at terminal C which turns off the switch circuit 11 and hence the flash tube FT. Referring to Figure 1A for a more detailed explanation of the operation of the switch circuit 0 11, initially capacitor C3 is charged by voltage source 10. When switch SW is closed and the flash tube, triggered, th ristor switch CRl is turned on by current flow through capacitors C3 > C4 and resistor R4 which in turn provides a current path for the flash tube FT which emits light. When sufficient light has been 15 emitted, the quenching signal at terminal C turns on the thyristor switch CR2. The effect of this is that the junction between capcitor C3 and resistor R.6, which had previously been held at some positive voltage by virtue of the state of charge of capacitor C3 > is clamped 20 to zero volts (via the switch CR2) and the other side of capacitor C3 is left at a negative potential. This turns off switch CRl and stops the flash tube FT emitting further light. Figure IB will next be referred to in explanation 25 of the generation of the quenching signal at terminal C. The circuit acts to integrate the light-responsive .signal produced by a photodiode PD acting as photosensor. Initially, capacitor C6 is charged but, during the duration of light emission by the flash tube FT, the 30. capacitor C6 is discharged via resistor R7, flash tube FT and switch CRl. When the photodiode PD has received the required quantity of light, this will have effectively ■ been integrated by the capacitor C8 to a sufficient l c-1 to switch on the thyristor- CRJ and generate tht- 35 quenching signal (via capacitor C7) on terminal C. CM? - ~ This then acts to turn on thyristor CR2 as previously discussed with reference to Figure 1A. The means by which the flash tube of the preferred embodiment of the present invention is turned off differs in principle from that previously described, and provides an accurate and reproducible method of switching. This method relies on inductive resonant charging of a capacitor within the switching timing circuit to produce an opposite polarity voltage used to turn off the thyristor, rather than the clamping of an already-charged capacitor as previously described.' The circuit of Figure 2 is similar to that of Figure 1 with the exception that there are two connections E,F between the electronic switch circuit 14 and the light-sensing circuit 15, and a further winding from the trigger coil LI to the switch circuit 14. Figures 2A and 2B show two broadly similar forms of switch circuit 14, but in this case there is no automatic light sensing by a circuit such as the calculator 15, and turn-off of the flash tube is achieved a predetermined time after turn-on, i.e. a set quantity of light will be emitted, and the camera will need to be adjusted in respect of exposure settings dependent on the distance of the subject from the camera, etc. Referring to Figures 2A and 2B (in association with Figure 2) capacitors CM,C1 and C2 are charged when power is applied to the circuit from voltage source 10. When the switch SW is closed, charge in the capacitor Cl is discharged via the triggering coil LI which provides a triggering pulse to the flash tube FT and also to the switch circuit at terminal C. The pulse at terminal C triggers thyristor CRl (via diode Dl and resistor R2) * and accordingly current flows through the flash tube FT which emits light by discharge of the main capacitor CM. In addition, charge from capacitor C2 flows through coil L2 and thyristor CRl and the back e.m.f. in coil L2 generates inductive resonant charging of capacitor C2, with the current phase reversed by l8θ°. In other words, as shown in Figures 2A and 2B, the capacitor C2 would initially have been charged positively and, upon discharge via coil L2, would then have become charged to a negative potential. This negative potential is applied to the anode of thyristor CRl which causes the thyristor to turn off. Therefore the time during which the flash tube FT is emitting light is defined by 10 the component values in the circuit, particularly the time constant of the LC circuit including capacitor C2 and coil L2. Figures 2C and 2D show two further switch circuits which operate in a somewhat similar manner to those of 15 Figures 2A and 2B but include automatic flash quenching by the light sensing circuit 15- Turn-on of thyristor CRl and consequent light emission fro flash tube FT occurs exactly as previously described; however, when capacitor C2 has become charged to a negative potential 20 by inductive resonant charging, it cannot apply that negative potential to the anode of thyristor CRl because of the blocking action of diode D2. On the other hand, when a quenching signal is provided on terminal F (from the light sensing circuit 15) , the thyristor CR2 is 25 turned on allowing the reverse-phase current (at negative potential) to be applied to the anode of thyristor CRl, turning it off and hence stopping illumination of the flash tube FT. It will be seen that in all of the circuits of Figures 2A -2D, the negative potential which _ Q turns off thyristor CRl is caused by the inductive resonant effect which reverse charges the capacitor C2. This is in distinction to the previously-proposed circuits (e.g. as shown in Figure lA) where a negative potential is obtained by clamping or pulling down the 35 potential of one terminal of a previously-charged capacitor so as to leave it with an effectively opposite charge on the other terminal . Figures 2E and 2F show two forms of light sensing circuit ( 15 in Figure 2) which utilise bridge arrange- 5 ments rather than the integrating circuit of the previously-proposed device. The illustrated circuits .derive power from the charge across capacitor C2 (in Figures 2C and 2D) fed via terminal E to voltage control¬ ler VC providing two potentials El and E2. A capacitor 10 C3 connected across the light sensor LS (phototransistor " PT in Figure 2E and photodiode PD in Figure 2F) is charged by the potential El. The potential E2 is supplied to an amplifier circuit which comprises a suitable amplifying element CR3, such as a transistor, thyristor or uni- 15 junction transistor, and is compared to the potential El. When the circuit is in balance, no signal is provided on terminal F. When the appropriate light sensor receives a variation in light, the capacitor C3 discharges current 20 in accordance with that variation. The circuit goes out of balance, triggering the amplifier circuit and generating a quench signal at terminal F (which acts as previously described to stop illumination of the flash tube FT). 25 In the circuits of Figures 2E and 2F, the potentials _,, El and E2 are supplied from that on capacitor C2 (via terminal E) and are hence subject to the same phase reversal of l8θ°. Figure 2G shows one form of voltage controller VC 30 usable in the circuit of Figure 2F. A siailar controller could be used in the circuit of Figure 2E but with the polarity-sensitive components (e.g. diodes) reversed. Referring to Figure 2G, the two potentials El and E2 are derived from two series-connected zener diodes D4, D5 35 fed via resistors RIO, Rll and a blocking diode D3 from O f-I - δ - the potential on capacitor C2 (Figure 2D) via the terminal E. In operation, when the thyristor CRl turns on, inductive resonant charging caused by the back e.m.f. in coil L2 (as previously described) occurs in capacitor C2, with the current phase reversed by 180°. As a result of this, the voltage at terminal E goes initially negative, then rises along a charging curve via zero to a positive potential. At the negative impulse, a current path exists through voltage controller VC via zener diodes D5, D4 resistor RIO and diode D3- Thus the potential El is generated across zener diode D4 which charges capacitor C3, the potential remaining stored across capacitor C3 even when the negative impulse has ceased. Once the voltage at terminal E goes positive, a current path exists via resistor Rll and zener diode * D5, and the potential E2 is provided across zener diode D5. Referring back to Figure 2F, when the light sensit- ivity calculation circuit is in balance, i.e. when the potentials El and E2 are equal, there is no potential across resisotr R8. When the light sensing element PD receives a variation in light causing a change in its internal resistance or causing it to generate a current (as in the case of a solar cell), the capacitor C3 is discharged either by the change in resistance or by the generated current, in accordance with the variation in intensity of the light, -thereby lowering the potential El. Potential E2 is thus higher than potential El and -the circuit go&s out of balance. When the difference in potentials between El and E2 reaches a predetermined value causing resistor R8 to have a potential thereacross, thyristor CR3 is triggered and a quench signal is generated at terminal F acting to turn on thyristor CR2 as previous di=.cu*~.ed . O PI The advantages of the above-described arrangements are that adjustable and extremely fast recycling times are provided for firing and cut-off of the photoflash tube; thus such arrangements are very useful in association with motor driven cameras where it is otherwise possible to take sequential exposures very rapidly and a photoflash gun must be able to recycle in a very short time, between exposures. With the previously-described circuits, the recycling times can be made sufficiently short to provide extra illumination for movie cameras which may require the photoflash tube to be fired more than twenty times per second. OMPI

Claims: CLAIMS : 1. A control circuit for a photoflash gun, the circuit including a flashtube (FT), a switching circuit (SW,LI) for initiating operation of the flashtube, a switch means (CRl) coupled with the flashtube which when operated permits current flow through the flashtube, and reset means (C2) for resetting the switch means after a predetermined time to cut off current flow through " the flashtube, characterised in that the reset means includes an inductance (L2) and a charge storage means (C2) coupled to the switch means such that when the switch means is operated, current flow occurs through said charge storage means and said inductance to create a voltage across said charge storage means which is of a polarity such as to reset the switch means. 2. A control circuit as claimed in claim 1, characterised in that "the switch means (CRl) comprises a thyristor with the con-trol electrode connected -to said switch circuit and with said storage means (C2) connected to the anode of said thyristor. 3. A control circuit as claimed in claim 1, characterised in that a rectifier means (D2) is provided to permit current flow through said inductance and charge storage means until said charge storage means develops a voltage of said polarity whereupon said current flow ceases, and a switch (CR2) is provided which is operative after a predetermined time to permit the voltage across the charge storage means to be operative to reset the switch means (CRl ) . 4. A control circuit according to claim 3 > characterised in that said diode is connected in series with said inductance and said switch is connected in parallel with said inductance and diode 5. A control circuit as claimed in claim 3 characterised in that a control electrode of said switch is connected to a light calculation device (PT,C3,CR3) for operation thereof. 6. A control circuit as claimed in claim 5 , characterised in that said light calculation device comprises a light sensor (PT) coupled to a second charge storage means (C3), means for comparing the voltage developed across said second charge storage means with a further voltage (C3,R6-R8,CR3) , and for providing a signal to operate said switch when the further voltage and the voltage across said second charge storage means have a predetermined relationship to one another. 7. A control circuit as claimed in claim 6, characterised in that said comparing means comprises a bridge circuit with said second charge storage means and said light sensor forming a first arm of said bridge, and said further voltage is developed across a second arm of said bridge in which a further switch (CR3) is disposed, said further switch being controlled by the voltage which is developed across a third arm of said bridge (R8). 8. A control circuit according to claim 7 characterised in that third and fourth arms of the bridge comprise respective impedances (R7, R8) , said first arm is connected between said second arm and said third arm, said second arm is connected between said first arm and said fourth arm, and the connection point between said third and fourth arms is coupled to the control electrode of said further switch. 9- A control circuit according to claim ' 6 characterised by means (D3,D4,D5) coupled to said first mentioned charge storage means for providing a voltage to said second charge storage means when the voltage across said first mentioned charge storage means is of a first polarity, and for providing a voltage to said comparing means when the voltage across said first mentioned charge storage means is of a second polarity. 10. A control circuit according to claim 9 characterised in that said voltage providing means is connected across said first mentioned charge storage means and includes a diode (D3) and a first zener diode (D4) connected between said second charge storage means and a nodal point, and a second zener diode (D5) coupled between said voltage comparison means and said nodal point and pδled ' in the opposite sense to said first zener diode. OMPI

Inventors: LAM SHEIR CHUN, LAM, SHEIR, CHUN
Applicants: LAM SHEIR CHUN, LAM, SHEIR, CHUN
Application year: 1984
Main classification: H05B41
IPC codes: G03B15, H05B41