GB2040485A

GB2040485A - Electronic Flash Unit

Info

Publication number: GB2040485A
Application number: GB8001275A
Authority: GB
Original assignee: Rollei Werke Franke und Heidecke GmbH and Co KG
Current assignee: Rollei Werke Franke und Heidecke GmbH and Co KG
Priority date: 1979-01-15
Filing date: 1980-01-15
Publication date: 1980-08-28
Also published as: DE2901399A1

Abstract

A flash tube 10 has a photoelectric time control 17, 18 and a supplementary time control 35 for switching off the tube after a predetermined time if the photoelectric control has not already done so. This is to ensure that the supply capacitor 13 does not become completely discharged. The recharging of the capacitor 13 which is only partially discharged, makes more efficient use of the current storage supply 11, reducing the need for frequent recharging of the current storage supply. Fig. 2 (not shown) shows an alternative form of supplementary control 35. <IMAGE>

Description

SPECIFICATION Electronic Flash Unit for Photographic Exposures The present invention concerns electronic flash units for photographic exposures in general and those flash units having a storage capacitor in particular.

As is generally known, the DC voltage transformer used for charging the storage capacitor of such a flash unit operates at different efficiencies depending on the load impedance.

This efficiency is much poorer at low load impedances than at high load impedances. In an electronic flash unit of the above-mentioned type the storage capacitor forms the load impedance for the DC voltage transformer. If the completely discharged storage capacitor is to be recharged by the DC voltage transformer, it signifies that the transformer initially operates in short circuit, until the load impedance of the storage capacitor begins to gradually rise and thus the load impedance for the DC voltage transformer also increases. The efficiency of the DC voltage transformer is therefore very poor during the initial charging cycle and improves gradually with the rising charge on the storage capacitor.

However, this variation in the degree of efficiency of the DC voltage transformer means that the charging of the storage capacitor to a constant energy level, for example 10 Ws, when said capacitor is completely empty or has only a slight charge, requires much more energy than in the case of a partially charged capacitor. Because the energy to charge the storage capacitor is taken from a current storage device contained in the flash unit (a battery or accumulator), this means that when the reservoir capacitor is constantly charged beginning with a very low residual charge voltage, the current storage device will be exhausted very rapidly. This may be acceptable in the case of flash units with rechargeable batteries, because it is necessary only to recharge the battery more often.With flash units operated exclusively with non-rechargeable batteries, the latter are used up at a very rapid rate and must be constantly replaced by new ones. The number of flashes obtained with such batteries is thus in most cases rather modest and often unsatisfactory for the photographer.

It is therefore the object of the invention to provide an electronic flash unitof the abovementioned type, wherein by virtue of the improved efficiency of the direct voltage transformer, the energy available in the power storage device, particularly in non-rechargeable batteries, is utilized with a significantly higher efficiency for the pure charging of the storage capacitor, so that an increased number of flashes may be obtained from the battery.

The above and other objects are achieved in accordance with the present invention wherein the electronic flash unit comprises: a flash tube, a storage capacitor connected in series with said flash tube; a means for charging the storage capacitor; a means for powering the charging means; and means for controllably discharging the capacitor through the flash tube to a preset partial discharge of the capacitor. In a preferred embodiment, a timing circuit started at the time of activation of the flash tube terminates conduction in the flash tube after a preset time interval which is shorter than the longest duration of the flash possible.

A more complete appreciation of the invention and the attendant advantages thereof will be more clearly understood by reference to the following drawings, wherein: Figure 1 is an electrical schematic of one embodiment of the electronic flash unit; Figure 2 is an electrical schematic diagram of a second embodiment of the electronic flash unit; Figure 3 is an electrical schematic of a variation of a portion of the diagram of Figure 2; and Figure 4 is an electrical schematic of a variation of a portion of the diagram of Figure 2.

In the case of the electronic flash unit according to the invention, at least during the execution of a series of flash exposures, the storage capacitor is not discharged completely, even when the photographer operates in the boundary range of the flash unit, i.e. at the greatest possible distance from the object of the exposure. The residual charge of the storage capacitor is chosen so that the DC voltage transformer will work with this impedance load of the storage capacitor at a reasonable efficiency.

Even though the measures according to the invention reduce the conductance of the flash unit of the prior art (otherwise unchanged), this disadvantage is more than compensated for by the energy saved. Even if the flash unit had its previous conductance (by means of increasing the capacity of the storage capacitor), the savings in energy are still considerable and the number of flashes that may be obtained from a set of batteries significantly increased. The provision of a timing element and the corresponding time dependent disconnection of the flash tube following the onset of the flash discharge, create suitable conditions for the design of the simplest possible circitry for the flash unit of the invention.

Because the light measuring device in the camera or flash unit is required in any case, the flash unit according to the present invention comprises only a few additional electronic structural elements. The additional cost of the flash unit according to the present invention as.

compared with prior art units of a similar type, is minimal.

Referring now to the drawings wherein like reference characters designate like parts throughout the several views, Figure 1 depicts the electronic flash unit for photographic exposures having a flash tube 10, a power storage device in the form of a battery 11 and a DC voltage transformer 12, referred to hereinafter in the abbreviated form as the transformer 1 2. A storage capacitor 13 may be charged by means of the transformer 12 by the battery 11. The storage capacitor 13 is discharged by way of the flash tube 10, when ignition device 14 renders the tube conductive. The ignition device 14 is synchronized in the usual manner with the release of the camera, so that the time of the ignition of the flash tube 10 coincides with the time of the shutter release.

The flash unit is equipped with a control circuit 1 5 which automatically limits the duration of the flash, said control circuit comprising the major part of the electronic structural elements shown in Figures 1 and 2. The control circuit 1 5 includes a light measuring device 1 6 comprising a phototransistor 1 7 in series with an integrating condenser 1 8. This light measuring device 1 6 is connected with a source of direct voltage at the onset of the flash discharge, i.e. when the storage capacitor 13 begins to discharge by way of the flash tube 10. The source of direct voltage is condenser 19, which is charged by way of a resistance 20 and the anode-cathode path of a Zener diode 21, together with the storage capacitor 13.The Zener diode is connected in parallel to the light measuring device 1 6. During the duration of the flash, the condenser 1 9 is also discharged by way of the flash tube 10 and now in the reverse direction by way of the Zener dide 21 and the resistance 20. The other, essentially constant, DC voltage drop across Zener diode 21 serves as the feeder voltage for the light measuring device 1 6.

The control circuit 1 5 further comprises an extinguishing device 22 for interrupting the discharge of the flash upon receiving an external signal. The extinguishing device 22 comprises a series thyristor 23 arranged in the flash discharging circuit, i.e. in series with the flash tube 10 and the storage capacitor 1 3. The control electrode of the series thyristor 23 is connected with the ignition device 14 of the flash tube 10, so that the series thyristor 23 ignites and becomes conducting simultaneously with the flash tube 10. The series thyristor 23 is connected in parallel with a resistance 24. The extinguishing device 22 further comprises a quenching capacitor 25, which is connected, in series with an electronic switch 26, both in parallel with the anode-cathode path of the series thyristor 23.The connecting point of the quenching capacitor 25 and the electronic switch 26 is connected by way of a resistance 27 with the positive plate of the storage capacitor 13. The extinguishing device 22 upon receiving a signal at the ignition electrode of the electronic switch 26, interrupts the flash discharge. During this extinguishing process, the quenching capacitor 25 discharges in a known manner by way of the electronic switch 26 and the cathode-anode path of the thyristor 23, whereby the latter is blocked and the discharge of the storage capacitors 1 3 by way of the flash tube 10 is terminated.

To generate the signal arriving at the igniting electrode of the electronic switch 26, the control circuit 1 5 is provided with a signal generating device 28, which is connected with the light measuring device 1 6 and the extinguishing device 22. This signal generating device 28 comprises a thyristor 29, a condenser 30 and a pulse transformer 31. The primary coil of the pulse transformer 31 is connected in series with thyristor 29 and the condenser 30. The igniting electrode of the electronic switch 26 is connected with the secondary coil of the pulse transformer 31. The control electrode of the thyristor 29 is connected with the point of connection of the phototransistor 1 7 and the positive plate of the integrating condenser 1 8.The cathode of the thyristor 29 is connected with the variable voltage tap of a potentiometer 32, the potentiometer together with a resistance 33 forms a voltage divider, connected in parallel with the Zener diode 21. The condenser 30 of the signal generator device 28 is connected by way of a resistance 34 with the positive plate of the reservoir capacitor 13 and is charged through it. The cathode of the thyristor 29 is connected with the anode of the Zener diode 21 by means of a condenser 35a. The function of the condenser 35a is without importance in this context.

The circuit design described heretofore, identical in both examples of embodiment, is shown. The operation of this circuit assembly will be briefly outlined, for the sake of completeness.

In an electronic flash unit ready for operation the storage capacitor 13, the quenching capacitor 25 and the two condensers 19 and 30 are charged. When the camera release is activated, the ignition device 14 emits a signal each to the flash tube 10 and the series thyristor 23. The flash tube 10 and the series thyristor 23 start conducting and the storage capacitor 13 is discharged by way of the flash tube 1 0 and the series thyristor 23. The flash tube 10 as is known begins to emit light.

Simultaneously, the condenser 1 9 discharges by way of the flash tube 10, the series thyristor 23, the Zener diode 21 and the resistance 20. At the Zener diode 21 a constant DC voltage drop thereacross is maintained, as noted earlier, and forms the power source for the light measuring device 1 6 and simultaneously provides a threshold value for the signal generator device 28.

The light striking the object of the exposure from the flash tube 10 is reflected and arrives at the phototransistor 1 7. In this manner, a current flowing through the phototransistor 1 7 is generated, which gradually charges the integrating condenser 1 8. When the voltage on the condenser 1 8 and thus the trigger of thyristor 29 exceeds the threshold value potential applied by way of the potentiometer 32 to the cathode of the thyristor 29, the latter ignites and the condenser 30 discharges abruptly through the primary coil of the pulse transformer 31. This induces a pulse in the secondary coil of the pulse transformer 31, forming the signal for the extinguishing device 22.

The extinguishing pulse ignites the electronic switch 26 and, as previously noted, the discharge of the storage capacitor 13 through the flash tube 10 and the series thyristor is interrupted. At the instant of the disconnection of the flash tube 10 the residual charge on the reservoir capacitor 13 may vary very greatly. If the distance between the phototransistor 1 7 and the object of the exposure is great, the storage capacitor 1 3 may be entirely or almost entirely discharged. With shorter distances, the luminous intensity of the light striking the phototransistor 1 7 is substantially higher. The integrating condenser 1 8 is charged at a higher rate and the threshold value attained at a much earlier point in time.The residual charge remaining on the storage capacitor 13 at the instant of the interruption of the flash would be substantially higher in these instances.

The present invention improves upon the electronic flash unit. According to the invention, circuit means 35 are provided which, independently of the light from the object measured by the light measuring device 16, generates a signal for the extinguishing device 22, whenever the charge of the storage capacitor 13 declines to a predetermined residual value as the result of the flash discharge. The circuit means 35 comprises in Figures 1 and 2 at least one timing element 36, having a time constant smaller than the longest possible flash period. This dimensioning of the timing element insures that the storage capacitor cannot be discharged completely.Even when the control circuit is not activated because of an excessive distance between the object of the exposure and the phototransistor 17, a signal will be generated after the predetermined constant period of time which begins at the onset of the flash emission, and ends with the termination of discharge of the storage capacitor.

In Figure 1, the circuit means 35 is connected with the signal generating device 28. The circuit means 35 comprises an electronic switch, here represented by a transistor 37. While the base of the transistor 37 is connected with the timing element 36, the principle conduction path of the transistor 37 is connected between the control electrode of the thyristor 29 and a voltage higher than the threshold voltage taken off the potentiometer 32. The timing element 36 consists of a series arrangement of a condenser 38 and a variable resistor 39. Herein, the condenser 38 is connected with the cathode of the Zener diode 21 and the resistance with the anode of the Zener diode 21. The base of the transistor 37, which in one embodiment is a PNP transistor is connected with the junction of the condenser 38 and the resistance 39.The emitter of the transistor 37 is located at the junction between the resistance 33 and the potentiometer 32 and the collector of the transistor 37 is connected with the control electrode of the thyristor 29.

The circuit means 35 in Figure 1 operates as follows: When the electronic flash unit is ready for operation the condenser 38 of the timing element 36 is discharged. Whenever the flash tube 10 is emitting light, an essentially constant DC voltage exists at the cathode-anode path. With the condenser 38 in the nearly discharged state, a relatively high potential is generated at the resistance 39 of the timing element 36, said potential being substantially higher than the potential on the emitter of the transistor 37. The condenser 38 is now being charged with the time constant determined by the capacity of the condenser 38 and the resistance value of the resistance 39. The voltage drop across the resistance 39 is gradually reduced.At the point in time when the potential at the base of the transistor 37 is smaller than the emitter potential of the transistor 37, the transistor 37 becomes conducting and the control electrode of the thyristor 29 will be connected with a potential which in every case is higher than the potential on the cathode of the thyristor 29. The thyristor 29 is ignited, and a signal is generated in the pulse transformer 31 such that the electronic switch 26 ignites, the extinguishing device 22 is activated and the discharge of the storage capacitor 13 is terminated in the above-noted manner. The time constant of the timing element 36 is chosen so that following the interruption of the flash discharge a sufficiently high residual charge remains on the reservoir capacitor.When the transformer 12 is then activated to recharge the storage capacitor 13, due to the partially charged state of the capacitor 1 3, it will operate with a sufficiently high load impedance ensuring a reasonable efficiency during the operation of the transformer 12. The energy contained in the battery 11 is utilized more to charge the storage capacitorl 3, than would be the case if the transformer 12 has to initially charge a completely discharged storage capacitor 13 and operated with the very poor efficiency associated with such a condition.

The variable resistance 39 of the timing element 36 may be used to adjust the conductance of the flash unit. As mentioned hereinabove, the conductance of the flash unit is reduced by the insertion of the circuit means 35, if the rest of the circuit layout of the electronic flash unit remains unaltered. Depending on whether the circuit means 35 interrupts the discharge of the storage capacitor 13 at an early point in time or not, conductance will become smaller or greater. It is thus logical that the conductance of the flash unit may be set exactly by means of the corresponding adjustment of the resistance 39.

In the embodiment in Figure 2, the circuit means 35 is connected with the light measuring device 1 6. Here again the timing element 36 of the circuit means consists of a series circuit of a resistance 40 and a condenser 41. The timing element 36 is connected in parallel to the light measuring device 16, but in a manner such that the resistance 40 is connected with the higher potential and the condenser 41 with the lower potential of the DC voltage drop on the Zener diode 21. The resistance 40 is thus connected with the cathode and the condenser 41 with the anode of the Zener diode 21.

The circuit means 35 comprises a semiconductor diode 42 with a predefined breakthrough voltage, inserted between the timing element 36 and the light measuring device 16, more accurately between the junction of the condenser 41 and the resistance 40 on the one hand, and between the phototransistor 17 and the integrating condenser 18. In one preferred embodiment the semiconductor diode 42 with the defined breakthrough voltage may be a Diac 43.

The circuit means 35 in Figure 2 operates in the following manner.

Prior to the initiation of an electronic flash the condenser 41 of the timing element 36 is completely discharged. At the beginning of the flash discharge, an essentially constant DC voltage drop occurs across the Zener diode 21.

The diode charges the condenser 41 by way of the resistance 40 at a rate controlled by the predetermined time constant. As soon as the voltage at the condenser 41 exceeds the breakthrough voltage of the Diac 43, the Diac 43 becomes conducting and current flows from the cathode of the Zener diode 21 by way of the resistance 40 and the conducting Diac 43 to the integrating condenser 1 8. Independently of the current generated in the phototransistor 1 7 by the incident light of the object, the integrating condenser 18 is charged very rapidly. In an exceedingly short time the potential at the integrating condenser 18 reaches a value which is higher than the threshold value taken off the potentiometer 32 by an amount at least equal tothe ignition voltage of the thyristor 29.The thyristor 29 ignites, the signal generator device 28 is activated and produces a pulse which is passed to the ignition electrode of the electronic switch 26 of the extinguishing device 22. The discharge of the storage capacitor 13 is terminated in the manner already described hereinabove.

In place of the Diac 43 of Figure 2, the semiconductor diode 42 with a defined breakthrough voltage may be a four layer diode 44 as shown in Figure 3. The above-described mode of operation of the circuit layout is not altered by inserting between the terminals A and B in place of the Diac 43, the four layer diode 44 shown in Figure 3.

Between the terminals A and B in Figure 2, the series circuit of a Zener diode 45 and a rectifier diode 46 may also be inserted in the manner shown in Figure 4. The Zener diode 45 forms the threshold value, after which the integrating condenser 1 8 is being charged by way of the resistance 40. The rectifier diode 46 prevents the flow of current from the integrating condenser 1 8 to the condenser 41 of the timing elements 36.

The mode of operation of the circuit layout is not altered by the insertion between the terminals A and B, in place of the Diac 43, the series circuit comprising the Zener diode 45 and the rectifier diode 46, as shown in Figure 4.

Although the invention has been described relative to a specific embodiment thereof, it is not so limited and many modifications and variations thereof will be readily apparent to those skilled in the art in light of the above teachings. It is, therefore, to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

Claims

1. An electronic flash unit comprising: a flash tube; a storage capacitor connected in series with said flash tube; means for charging said storage capacitor; means for powering said charging means; and means for controllably discharging said capacitor through said flash tube to a preset partial discharge.

2. An electronic flash unit according to claim 1, wherein said means for controllably discharging comprises: means, responsive to the charge on said capacitor, for providing a signal; and means, responsive to said providing means, for terminating conduction in said flash tube.

3. An electronic flash unit according to claim 2, wherein said providing means further includes means for measuring light and for producing a signal indicative of said light.

4. An electronic flash unit according to claim 3, wherein said providing means further includes means, responsive to said flash tube, for providing a timing signal having a time constant less than the longest possible flash duration.

5. An electronic flash unit according to claim 4, wherein said timing signal is connected to said terminating means and causes termination of conductance in said flash tube after a predetermined time period.

6. An electronic flash unit according to claim 5, wherein said providing means further includes means, responsive to at least one of said timing signal means and said measuring means, for generating a signal provided to said termination means.

7. An electronic flash unit according to claim 6, wherein said generating means comprises: a thyristor having a control electrode, said control electrode connected to both said timing signal means and said measuring means; a charged capacitor; and a pulse transformer, the primary of said pulse transformer is connected in series with said thyristor and said charged capacitor such that a signal applied to the control electrode causes said charged capacitor to be discharged through said pulse transformer and the secondary of said pulse transformer is connected to said termination means.

8. An electronic flash unit according to claim 7, wherein said timing signal means comprises: a timing element; and means, responsive to said timing element, for connecting a signal to said control electrode.

9. An electronic flash unit according to claim 8, wherein said connnecting means is a transistor, the base of which is connected to said timing element.

10. An electronic flash unit according to claim 9, wherein said providing means includes means for supplying an essentially constant DC voltage to said light measuring means, and said timing element comprises a condenser and a resistor forming a series circuit, said series circuit connected in parallel with said light measuring means.

11. An electronic flash unit according to claim 10, wherein said condenser and said resistance form a junction, said junction is connected with said base of said transistor, said providing means further including a voltage divider, said emitter and collector of said transistor connected between said voltage divider and said control electrode.

12. An electronic flash unit according to claim 10 or 11, wherein said resistance is a variable resistor.

1 3. An electronic flash unit according to claim 6, wherein said generating means comprises: a thyristor having a control electrode connected to said measuring means; a charged capacitor; a pulse transformer, the primary of which is connected in series with said charged capacitor and said thyristor such that a signal applied to the control electrode of said thyristor will discharge said charged capacitor through said pulse transformer, and the secondary of which is connected to said termination means; and a semiconductor diode with a predetermined breakthrough voltage, said diode connected between said timing signal means and said control electrode.

14. An electronic flash unit according to claim 13, said timing signal means comprises: a timing element; and means for supplying an essentially constant DC voltage to said light measuring means, said timing element comprising a condenser and a resistance forming a series circuit, said supplying means comprising a Zener diode, said series circuit connected in parallel with said Zener diode and said light measuring means, and said light measuring means comprises a phototransistor and an integrating condenser connected in series, the junction between said phototransistor and said integrating condenser being connected to said control electrode of said thyristor.

1 5. An electronic flash unit according to claim 1 3 or 14, wherein said semiconductor diode is a Diac.

1 6. An electronic flash unit according to claim 13 or 14, wherein said semiconductor diode is a four layer diode.

1 7. An electronic flash unit according to claim 13 or 14, wherein said semiconductor diode is a Zener diode in series with a rectifying diode.