GB2083300A

GB2083300A - Electronic flash equipment

Info

Publication number: GB2083300A
Application number: GB8021165A
Authority: GB
Original assignee: JESCAL SCIENT Ltd
Current assignee: JESCAL SCIENT Ltd
Priority date: 1979-06-28
Filing date: 1980-06-27
Publication date: 1982-03-17
Also published as: GB2083300B

Abstract

The unit comprises a charging control circuit which is a solid state switching circuit effective to delay the recharging of main flash tube capacitor or capacitors, after discharge through the flash tube A, to allow the flash tube to quench completely. When capacitors B discharge through the tube A, capacitor C discharges rendering transistor G conductive and transistor F nonconductive so that triac D cannot conduct in the negative phase of the AC cycle. The similar components C', G' and F' prevent triac D conducting in the positive phase of the cycle thus the main storage capacitors B are not recharged until the charge on C, C' is dissipated. The same solid state switching circuit can be used to stabilise the voltage on the flash tube capacitors B at any of a number of desired charge levels. The flash tube A can be mounted with a tungsten-halogen lamp 33 in a single housing. <IMAGE>

Description

SPECIFICATION Electronic flash equipment DESCRIPTION This invention relates to electronic flash equipment, particularly to such equipment for stu dio use in which a rapid rate of firing of the electronic flash tube may be required so that rapid recharging is called for. In photographic studios it may be desirable to use motordriven camera shutters which can operate at up to 5 exposures per second. It is an object of the invention to provide electronic flash equipment which recharges reliably even within such a short period, which provides a good level of illumination to the subject for a given power consumption, and which is reliable and inexpensive in construction.

The invention provides electronic flash equipment in which at least one flash tube capacitor is arranged to discharge through a flash tube to provide the flash, wherein a solid state switching circuit is provided to delay the recharging of the flash tube capacitor or capacitors, after dicharge through the flash tube, to allow the flash tube to quench completely.

It is perhaps surprising that a reliably more rapid rate of recharging is attained by delay ing the commencement of the charging for a predetermined period. However the use of a solid state switching circuit enables the delay to be relatively short, yet sufficient to allow the flash tube to quench completely. Thus there is no after-burn of the flash tube after discharge even though the recharging current may be only a few milliseconds.

The invention also provides such equipment in which the same solid state switching circuit also acts as a voltage control circuit for limit ing the maximum charge held by the flash tube capacitor or capacitors. Preferably two such capacitors are provided, connected in a voltage doubling circuit and positioned in se ries across the flash tube terminals. The use of relatively low cost solid state switching devies in the switching circuit enables a separate swiching circuit to be provided for each of the two capacitors. The charging of the two flash tube capacitors is thus stablized separately and independently, so that neither is allowed to charge to a voltage higher than the other (which would otherwise happen in practice).

The solid state switching circuit to be incorporated into the equipment according to the invention includes an auxiliary transistor which is effective to hold OFF a switching transistor when it is desired to discontinue the charging of the associated flash tube capacitor. Preferably the same pair of auxiliary transistor and switching transistor is used both for the charge delay during quenching and for the voltage stabilization of the flash tube capacitor. To provide the quenching delay, the base of the auxiliary transistor is preferably biased by a charge decaying from a control capacitor which has held a charge while the flash tube capacitor has been fully charged.

To provide the voltage stabilization, the base of the auxiliary transistor is preferably biased by a potential applied through a zener diode from a potential dividing circuit across the flash tube capacitor. By appropriate selection of the potential dividing circuit it is possible to stabilise the voltage of the flash tube capacitors at any desired level.

Advantageously the switching transistor of each of two solid state swiching circuits (one for each flash tube capacitor) is used to control the gate voltage of a triac in the common line of the voltage doubling circuit. Thus one switching circuit controls the triac in the positive phase of the AC power supply, and the other switching circuit controls the triac in the negative phase.

Instead of a triac, the same function can if desired be achieved by using a pair of opposed thyristors or silicon-controlled rectifiers.

The invention also provides a novel and more effective arrangement for the flash tube and a studio modelloing lamp in such equipment. The flash tube is preferably a linear flash tube in a specular trough reflector, with a tungsten-filament modelling lamp, perpendicular to the flash tube, inset into the trough reflector and provided with its own trough reflector in close proximity to the lamp surface. The use of the linear flash tube is preferred as it makes the most efficient use of the power that is available. In addition, the use of a tungsten-halogen lamp inset into the main reflector is useful to provide a continuous source of light so that the user may assess the lighting conditions. As this modelling lamp cannot be ideally positioned in exactly the same plane as the flash tube, it is preferably small and compact.The use of a tungsten-filament lamp achieves this object, and because the modelling lamp is fitted with its own reflector, this enshrouding reflector also serves to maintain the required operating temperature of the lamp envelope. This ensures that the halogen cycle is maintained, and avoids premature deterioration of the tungsten-halogen lamp when used at reduced voltages.

It is a further preferred feature of equipment according to this invention that there should be an audible alarm which sounds when the flash tube capacitor is charging but is extinguished when the charging is terminated. This is of particular advantage because the extinguishing of the alarm is achieved when the desired predetermined charge has been stabilised on the flash tube capacitors, as opposed to indicating simply when sufficient charge has been attained to fire the tube. Thus the audible alarm will sound for longer when the potential divider circuit has been set to charge the flash tube capacitors to full charge, providing full power for the flash discharge. The audible alarm may sound continuously or intermittently during the charging period, so that it is easy for the photographer to know with certainty when charging is complete.The use of such an alarm is particularly useful when more than one electronic flash unit is employed in a studio. It then becomes necessary for the photographer to listen only for the last tone to be silenced, to be assured that all of the flash units are fully charged and ready for use.

Drawings Figure 1 is a circuit diagram showing the charging and charging control circuits of flash equipment according to this invention; Figure 2 is a front view equipment according to this invention; and Figure 3 is a vertical section through the equipment of Fig. 2.

In the circuit diagram of Fig. 1, the principal components are indicated by reference letters A to J, whereas the parts and components of the associated circuitry have been given numerical references Fig. 1 is a circuit diagram of a charge source circuit for a flash tube A. In use a charge, for example 650 volts, is applied across the tube A from main flash tube capacitors B and B', and when it is necessary to fire the tube an ionizing field is applied to the tube itself to initiate the flash.

Voltage stablization of the charge held by the capacitors B and B' before the flash tube discharge, and the provision of a delay before recharging is commenced, to allow the tube to quench, is provided by the circuit of Fig. 1.

This circuit comprises a mains voltage AC power supply to lines 1 and 2,2' of a basic doubler circuit comprising the main flash tube capacitors B and B' each in series in one half of the circuit with a resistor 3 or 3' and a diode 4 or 4'. The diodes 4 and 4' are opposed so as to give the known voltage doubling effect between the lines 2 and 2'.

Considering first the bottom half of the circuit as illustrated in Fig. 1; between the lines 1 and 2 there are connected, in series, a zener diode 5, a resistor 6 and a diode 7. The zener value of the zener diode 5 is 1 2 volts, so that the junction of the zener diode 5 and the resistor 6 is at 1 2 volts and feeds a supply line 8 supplying low voltage power to two transistors as described below. A reservoir capacitor 9 between the lines 1 and 8 acts to maintain a generally consistent 1 2 volts in line 8 throughout the whole cycle of the AC supply.

The top half of the circuit has analogous components that have been given the same references except that each reference number is given a prime (').

In the line 1 is a triac D the gate 10 of which is connected to the emitter of a PNP transistor F and to the emitter of an NPN transistor F', the collectors of these transistors being fed by lines 8 and 8' respectively. A leakage resistor 11 is placed across the triac D to supply the initial current to operate the transistor power supply.

The base of the transistor F is fed by the line 1 (on the power supply side of the triac D) via a high impedance resistor E, and the base of the transistor F' is similarly fed via a resistor E'. In consequence in the negative phase of the AC power supply cycle the emitter of the PNP transistor F applies a negative bias to the gate 10 of the triac D and in the positive phase of the AC power supply cycle the emitter of the NPN transistor 11' applies a positive bias to the gate 10 of the triac D. Because a triac is rendered conductive by a gate bias of either polarity (with sufficient current) the triac D is initially conductive in both phases of the cycle, enabling the main flash tube capacitors B and B' to become charged.

The maximum voltage to which each capacitor B or B' can be charged is attained at an inverse exponential rate, so that interruption of the charging process at a selected instant enables a predetermined charge to be held in one or both capacitors. Thereafter only an intermittent and momentary continuation of the charging process is needed to compensate for leakage and maintain that predetermined charge. The circuit of Fig. 1 enables the voltage of the main flash tube capacitors B,B' to be selected and stabilized at three predetermined levels. Moreover each capacitor B,B' is controlled independently of the other, with a resulting increase in reliability of the system.

A potential divider circuit between lines 1 and 2 across the terminals of the (lower) capacitor B comprises three resistors 12, 1 3 and 14 in series, and a fourth resistor 1 5 swiched by a 3-position switch 1 6 selectively to the junction of 1 2 and 1 3 (as shown), to the junction of 1 3 and 14 or to the other terminal of resistor 14. A preset tapped resistor 1 7 can for the present be regarded as part of the resistor 1 5. In use a fixed proportion of the voltage across the capacitor B will exist between line 1 and the switch 16, the size of that proportion depending on the position of the switch and the resistance values of the resistors in the potential dividing circuit. A zener diode H with a zener voltage of, for example, 70 volts is connected between the switch 16 and the base of a PNP transistor G'. The values of the resistors in the potential dividing circuit are such that when this zener voltage is rached the capacitor B has its selected desired charge potential. When the zener diode H is conductive an appropriate bias is applied to the base of the transistor G', rendering that transistor conductive. Because the emitter of the transistor G' is connected directly to the base of the transistor F'. and because G' is a PNP transistor whereas F' is an NPN transistor, this effectively pins the base voltage of the transistor F' and renders it non-conductive.No bias is therefore applied to the gate of the triac D during the negativegoing half-cycles of the power supply, and the charging of the capacitor B is terminated immediately the zener voltage of H is reached.

A similar potential dividing circuit formed of resistors 12', 13', 14', and 15' and a 3position switch 16', a similar zener diode H' and a similar arrangement between an NPN transistor G and the PNP transistor F ensures a parallel but independent control and stabilization of the charge potential of the capacitor B'. The switches 1 6 and 16' are ganged so that they are moved in unison to select a condition of "full" charge, 4 charge or 2 charge on the capacitors B and B', to give full power, half power or quarter power firing of the tube A.Clearly the time taken for the capacitors B and B' to reach half charge is substantially less than that taken to attain "full" charge, so that this condition is selected for rapid firing of the flash tube A, for example when using a motor-driven camera taking up to 5 exposures per second.

A principal feature of the circuit of Fig. 1 is its ability to ensure that there is a delay, after firing the flash tube A, before the recharging cycle is commenced. The additional components which achieve this are a control capacitor C and resistor J in the lower half of the circuit as illustrated, and a corresponding control capacitor C' and resistor R' in the upper half. The control capacitor C and resistor R are connected in series between the 3-position swich 1 6 and the base of the transistor G, so that immediately before the flash tube A is fired a charge of about 70 volts (determined by the zener diode H') is held across the control capacitor C.

On collapse of the voltage across B and B', caused by firing the tube A, the 70 volt charge on the conrol capacitor C exhausts itself through the resistor J, producing sufficient energy to bias the base of transistor G for a finite time after the firing of the tube A. This maintains the transistor G conductive and does not allow the base potential of the transistor F to rise until the charge on C is sufficiently dissipated. Until then the transistor F is turned OFF and the triac D is not permitted to conduct in the negative phase of the AC cycle. Similarly the capacitor C' and the resistor R' prevent the triac D from conducting in the positive phase of the AC cycle for a finite period after firing of the tube A. In this way a delay is built into the circuit, preventing the commencement of the recharging cycle until the tube has completely quenched.This delay need be only of the order of milliseconds, but can be varied at will by suitable choice of C and J (and C' and J').

The preset tapped resistor 1 7 in the bottom half of the circuit, referred to earlier, controls an NPN transistor 20 for sounding an audible warning provided by a piezoelectric crystal 21. When the capacitor B is charging, the voltage on the 1 2 volt line 8 through a capacitor 22 biases the transistor 20 to conduction. When the capacitor B is fully charged the negative potential at the pick-off of the tapped resistor 1 7 is sufficient to turn the transistor 20 OFF, so that the audible warning stops. The warning therefore indicates that the charging cycle is in progress.

Figs. 2 and 3 illustrate a complete flash unit according to the invention. The unit comprises a rectangular housing 30 in which is recessed a specular trough reflector 31. The use of a specular trough reflector with a linear flash tube A is known to make the most efficient use of the power that is available.

The reflector can be made with an abrasionresistant highly polished specular surface that does not need special protection so that the unit can be open at the front at 32, rather than protected with a glass cover. This is advantageous in that it greatly assists heat dissipation in use.

A studio modelling light 33 is provided at the centre of the front opening of the housing 30, inset into the reflector 31. This is a tungsten-halogen lamp, and has its own vertically arranged trough reflector 34 in close proximity to the bulb surface. This has two advantages. First, the tungsten-halogen lamp 33 is small and compact and, even when enshrouded by its own reflector 34, it does not obscure a substantial proportion of the direct light from the flash-tube A. Secondly, the enshrouding reflector 34 serves to maintain the operating temperature of the envelope of the tungsten-halogen lamp 33 at the intended working temperature, so that the halogen cycle is maintained and premature deterioration of the lamp is avoided.

A further advantage of the flash unit of Figs. 2 and 3 is that the pins of the tungstenhalogen lamp 33 are constrained always to be below the lamp in the permitted working range. When the unit is horizontal as shown the lamp 33 is vertical, and a mounting cradle 35 allows pivoting only within the range 80 upwards to 70 downwards.

At the rear of the housing 30 are various controls, such as on/off switches and a knob controlling the 3-position switch 16, 16' of Fig. 1. These are however conventional and will not be described further.

Claims

1. Electronic flash equipment in which at least one flash tube capacitor is arranged to discharge through a flash tube to provide the flash, wherein a solid state switching circuit is provided to delay the recharging of the flash tube capacitor or capacitors, after discharge through the flash tube, to allow the flash tube to quench completely.

2. Equipment according to claim 1, wherein there are flash tube capacitors connected in a voltage doubling circuit and positioned in series across the flash tube terminals.

3. Equipment according to claim 2, wherein the solid state switching circuit comprises, for each flash tube capacitor, a control capacitor which holds a charge when the other flash tube capacitor is charged but has a charge decay route, on discharge of the flash tube capacitors, past an auxiliary transistor which is effective to prevent the recharging of the flash tube capacitor while the control capacitor is discharging.

4. Equipment according to claim 3, wherein the auxiliary transistor is effective to hold OFF a swiching transistor while the control capacitor is discharging to prevent recharging of the flash tube capacitor.

5. Equipment according to claim 4, wherein the swiching transistor is in a control circuit for controlling a gate voltage at a solid state switching device in the voltage doubling circuit, through which charge has to flow to charge the flash tube capacitor.

6. Equipment according to claim 5, wherein the sold state switching device is a triac.

7. Equipment according to claim 6, wherein a single triac is used for controlling the charge flowing to the two flash tube capacitors in the different phases of the AC supply.

8. Equipment according to any of claims 3 to 7, wherein the solid state switching circuit also comprises, for each flash tube capacitor, a voltage control circuit for limiting the maximum held by the flash tube capacitor.

9. Equipment according to claim 8, wherein the voltage control circuit comprises a potential divider circuit across the flash tube capacitor, and a zener diode chosen to reach its zener voltage when the desired maximum charge is attained by the flash tube capacitor, and thereafter to bias the base of the associated auxiliary transistor for as long as that charge is carried by the flash tube capacitor, to prevent further charging of the flash tube capacitor.

10. Equipment according to claim, 9 wherein the potential divider circuit can be switched to select different preselected voltages, across the flash tube capacitors, which are effective to apply the zener voltage to the zener diode.

11. Equipment according to any preceding claim, having an audible alarm which sounds when the flash tube capacitor is charging but is extinguished when the charging is terminated.

1 2. Equipment according to claim 1, conrolled by a charging control circuit substantially as illustrated in Fig. 1.

1 3. Equipment according to any preceding claim wherein the flash tube is a linear tube in a specular trough reflector.

14. Equipment according to claim 13, wherein a tunsten-filament modelling lamp, perpendicular to the flash tube, is inset into the trough reflector and is provided with its own trough reflector in close proximity to the lamp surface.

15. Equipment according to claim 14, substantially as described herein with reference to the drawings.