Electronic flash device for camera

The enclosed electronic flash device for a camera includes a booster circuit connectible to an electrical power source or battery, a high voltage rectifier diode for rectifying the output of the booster circuit, and a storage capacitor. A main power supply switch and an auxiliary power supply switch are connected in series between the negative poles of the power source and capacitor. The auxiliary power supply switch is arranged to close when the flash device is attached to the camera. A signal terminal is connected to the negative pole of the storage capacitor through other circuit elements and another signal terminal is connected to one contact of the auxiliary power supply switch, which is connected in turn to the negative pole of the storage capacitor. The auxiliary power supply switch is connected in series with the main power supply switch at a point farther from the negative terminal of the battery than the main power supply switch.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to improvements in electronic flash devices for 
cameras having an auxiliary power supply switch (or "auxiliary switch") 
arranged to close when the device is attached to the camera. 
2. Description of the Prior Art 
Electronic flash devices of the kind described generally have an electrical 
circuit as shown in FIG. 1. The voltage of a battery 1 is converted to an 
alternating current of increased voltage by a booster I. Booster I 
includes a battery capacitor 2, an oscillating transistor 3, a bias 
capacitor 4, an oscillating transformer 5 having a saturated iron core, 
and a resistor 6. The output of booster I passes through a high voltage 
rectifier diode 7 to a main or storage capacitor 8. Use of a manually 
operated main switch 9 is generally sufficient to initiate operation of 
the booster circuit I. However, if the power supply is controlled only by 
the main switch 9, a problem arises. When the electronic flash device is 
not used for a long time and is detached from the camera, while leaving 
the battery switch 9 ON, the booster circuit I continues to operate, 
causing wasteful consumption of the electrical energy of the battery. One 
proposed, and utilized, way of avoiding this has been to provide this kind 
of electronic flash device with an auxiliary battery switch 10 arranged to 
open or close when the device is detached from or in use with the camera. 
FIG. 2 illustrates the construction of such an auxiliary switch 10. An 
embossed portion is formed on the inner surface of a casing of the 
electronic flash device. A print substrate 102 fixed by the fastener 
screws 101 is mounted on the embossed portion. A switch contact in the 
form of a plunger rod 10s passes through a fitted hole provided through 
the wall of the print substrate 102 and is urged by a coil spring 103 to 
move away from a fixed contact 10a when the device is detached from the 
camera. The coil spring 103, plunger 10s and a copper foil pattern 10c of 
the print substrate 102 are always electrically connected with one 
another, constituting a movable contact of the switch 10. When the device 
is attached to the camera, end "a" of the plunger 10s is brought into 
electrical connection with a terminal on the camera housing (not shown). 
At the same time, the opposite end of plunger 10s contacts the resilient 
contact member 10a. The auxiliary switch 10 is thus turned on. When the 
main switch 9 is then turned on with movable contact 9s in contact with a 
contact 9a, the booster circuit I starts to oscillate. When the device is 
then detached from the camera, auxiliary switch 10 is turned off and 
operation of booster circuit I is automatically stopped, even when the 
main switch 9 is still in the ON state. 
Current induced in the secondary winding S of the oscillation transformer 5 
flows in a closed circuit from the high voltage rectifier diode 7 through 
the storage capacitor 8, closed main switch 9, closed auxiliary switch 10, 
battery 1, capacitor 2 connected in parallel to the battery 1 and the 
emitter-to-base path of oscillating transistor 3 to the opposite end of 
the secondary winding S. With the electronic device on the camera, this 
closed circuit changes to the open position when the main switch 9 changes 
from its ON to OFF state, or when chattering takes place at the time of 
contact between the members 9s and 9a of the main switch. 
Current flowing through the closed circuit is divided into two parts, one 
of which serves as a charging current for the storage capacitor 8. Another 
part flows through the emitter-base path of oscillating transistor 3, 
becoming a positive feedback current to booster circuit I. Because the 
opening of this closed circuit results in an increased impedance between 
the lines A and B, a surge voltage is produced for a very short time 
across lines A and B. The higher the impedance of the circuit beginning 
with the base of oscillating transistor 3 with resistor 6 and terminating 
at the main switch contact 9c, inevitable for actuating booster circuit I, 
the more prominent the surge voltage becomes. Such surge voltage does not 
create any problem provided that the synchronizing switch for controlling 
the timing of firing of the electronic flash device is in the form of a 
mechanical switch 11, as shown in FIG. 1. This prevails in the prior art. 
For cameras employing a CMOS type integrated circuit of low withstand 
voltage from which an actuating signal for firing the device is obtained, 
however, the production of a surge voltage across lines A and B becomes a 
serious problem, because the surge voltage is applied backwards through 
the interconnection terminal leading from line B to the CMOS type 
integrated circuit and from there to line A. It is therefore unavoidable 
that the integrated circuit and associated parts therewith are broken as 
the withstand voltage is exceeded. 
To prevent such withstand voltage damage, a diode of high withstand voltage 
may be connected in such a direction to not apply the surge voltage 
directly to the integrated circuit. With the reduced driving voltage for 
the integrated circuit, however, the output signal of the diode is lowered 
by the voltage loss (about 0.6 volts) in the forward direction to a value 
depending on the ambient temperature and an alternative problem of 
securing the accuracy and reliability of control results. The use of such 
a diode cannot be said to be an advantageous solution. 
SUMMARY OF THE INVENTION 
A first object of the present invention is to eliminate the above-described 
problem and to provide an electronic flash device for a camera capable of 
preventing the withstand voltage damage to the circuitry of the camera 
which would otherwise result from the application of the surge voltage 
backward to the camera when the main power supply switch is turned off 
while the device is in use on the camera. 
To achieve this, according to the present invention, the auxiliary switch 
connected in series to the main switch is positioned farther from the 
negative terminal of the battery than the main switch, and one of the 
contacts of the auxiliary switch which is connected to a signal line, is 
connected to the negative pole of the storage capacitor so that a circuit 
connecting the signal line to another signal line is not cut off when 
either of the main or auxiliary switches is turned off. 
A second object of the invention is to provide or electronic flash device 
of the construction described above having an arrangement for preventing 
an indicator lamp from being lit to give a spurious signal representing 
that a full charging of the storage capacitor has occurred which would 
otherwise result when the storage capacitor is fully charged, the main 
switch is turned off and the device is shortly thereafter detached from 
the camera. 
To achieve this object, according to the present invention, in the 
electronic flash device of the construction described above, a one-way 
semiconductor element is connected in parallel to the aforesaid auxiliary 
switch. The one-way semiconductor element is oriented so that its forward 
direction coincides with the direction in which the full charging 
indicator lamp is short-circuited. Accordingly, even when the auxiliary 
switch is in the OFF state, the indicator lamp is maintained 
short-circuited by the one-way semiconductor element. 
Other objects of the invention will become apparent from the following 
description of embodiment thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention will next be described in connection with an 
illustrated embodiment thereof. 
FIG. 3 illustrates an embodiment of the present invention where the same 
reference characters have been employed to denote similar parts to those 
shown in FIG. 1. In particular, a feature of the embodiment of FIG. 3 is 
that to achieve stabilization of the charging characteristics of the DC-DC 
converter comprising the booster circuit I and high voltage rectifier 
diode 7, a semiconductor element such as a Zener diode 17 is connected to 
a circuit for compensating the initial actuation of the booster circuit I. 
The use of the Zener diode 17, however, tends to intensify the surge 
voltage between a line A and another line B. FIG. 4 illustrates variation 
with time of the surge voltage produced in a case where, instead of the 
Zener diode 17, a diode is used. FIG. 5 illustrates another case where the 
Zener diode 17 of FIG. 3 has a Zener voltage of 6 volts. FIG. 6 
illustrates still another case where the Zener diode 17 of FIG. 3 is 
short-circuited. As illustrated in FIGS. 4 to 6, so long as either the 
main battery switch 9 or the auxiliary switch 10 is provided in between 
the lines A and B, though the peak value varies with the impedance of the 
compensating circuit for the initial stage of operation of the booster 
circuit I, it is impossible to reduce it to zero because of the basic 
characteristics of the DC-DC converter. 
To prevent such surge voltage from being transferred to the interior of the 
camera, in the embodiment of FIG. 3, the auxiliary switch 10 is connected 
in series to the main switch 9 but positioned farther from the negative 
terminal of the battery 1 than the main switch 9. Further, the one of the 
contacts of the auxiliary switch which is connected through a pole 10s to 
an earth or signal terminal "a", namely the contact 10c, is connected with 
the line B. 
Another feature is that the main switch 9 includes an additional fixed 
contact 9b arranged so that when the switch 9 is turned off, that is, when 
the pole 9s is moved away from the first fixed contact 9a, the pole 9s 
touches the second fixed contact 9b, and is connected to the anode of a 
neon lamp 14. Auxiliary switch 10 is provided with a diode 16. The anode 
and cathode of diode 16 are connected to the contacts 10a and 10c of the 
auxiliary switch 10 respectively. Thus, the forward direction of 
connection of the diode 16 coincides with the direction in which the neon 
lamp 14 is short-circuited. 
In operation, when the electronic flash device is slipped into the 
accessory shoe of a camera, the circuit earth terminal, "a", charging 
completion signal terminal, "b", and flash lamp triggering signal 
terminal, "c", on the casing of the electronic flash device, are connected 
with respective signal terminals, a', b' and c' on the housing of the 
camera. At the same time, the plunger or pole 10s of the auxiliary switch 
10 moves to contact fixed contact 10a, thereby turning the auxiliary 
switch on. When main switch 9 is then turned on by manually bringing the 
pole 9s into contact with the first fixed contact 9a, current from the 
positive terminal of battery 1 flows through the feedback winding F of 
oscillation transformer 5, Zener diode 17 for matching the oscillating 
characteristics, a parallel circuit of resistor 6 and capacitor 18, 
auxiliary switch 10 and main switch 9 to the negative terminal of battery 
1, while a base current is allowed to flow to the oscillating transistor 
3. Therefore, booster circuit I starts to operate, and oscillating 
transistor 3 is in the ON state. Primary winding P of the oscillation 
transformer 5 is thus supplied with electrical power for a period during 
which the oscillation transformer is capable of reaching magnetic 
saturation, while an alternating voltage of magnitude the ratio of turns 
times as high is induced in the secondary winding S of oscillation 
transformer 5. Booster circuit I continues performing such oscillating 
operation. This alternating current is converter to a direct current by 
the high voltage rectifier diode 7, which is applied to charge storage 
capacitor 8. 
Voltage stored on the capacitor 8 increases with time. When it reaches a 
value necessary to maintain the amount of flash light emitted from flash 
discharge tube 13 at a proper level, the neon lamp 14, which acts as the 
pilot lamp for indicating when the storage capacitor 8 is fully charged, 
is discharged through a resistor 19, and lit. At the same time, a 
potential appears across resistor 15 and a base current flows to 
transistor 20, thereby rendering the collector-emitter path of transistor 
20 conductive. A light-emitting diode 12 positioned in the field of view 
of a finder (not shown) is then energized with current flowing from 
battery 21 through resistor 22. As it gives off light, the photographer 
looking through the finder is informed of the fact that the flash device 
is ready to fire. 
Meanwhile, a trigger capacitor 23 is charged in proportion to the voltage 
on the storage capacitor 8 through resistors 19 and 24 and the primary 
winding of a trigger transformer 25. When the neon lamp 14 is lit, the 
voltage stored on capacitor 23 is kept at a level for sustaining the 
discharge of the neon lamp 14. 
When a shutter release is then actuated, a correct exposure value is 
computed from the automatically metered object distance and the preset 
value of guide number to adjust the period of actuation of a shutter. When 
the shutter operation changes from the opening operation to a closing 
operation, an exposure control circuit 26 in the form of a CMOS type 
integrated circuit produces an actuating signal which is applied through 
the connection terminals c' and c and a resistor 28 to a gate electrode of 
a thyristor 27. Thyristor 27 is thus turned on and the charge on the 
trigger capacitor 23 is given to the primary winding of trigger 
transformer 25 in the form of an impulse, inducing a high voltage of 
damped oscillation By this induced voltage, the flash discharge tube 13 is 
excited. The charge stored on the capacitor 8 is then discharged through 
the tube 13. Thus, the required amount of flash light for the proper 
exposure value is emitted. 
Assume that after the storage capacitor 8 has been charged to a voltage 
necessary to maintain the amount of flash light emitted from the flash 
discharge tube 13 at a proper level, without making a flash exposure, the 
main switch 9 is manually opened so that pole 9s contacts contact 9b. 
Operation of the booster circuit I stops. At the same time, the series 
circuit of the neon lamp 14 and resistor 15 is short-circuited at the ends 
thereof through the contacts 9b and 9c of the main switch 9. Because the 
anode of the neon lamp 14 is grounded, the neon lamp 14 is turned off, and 
the trigger circuit is rendered inoperative. Assume again that soon after 
that, that is, before the charge on the storage capacitor 8 all 
disappears, The electronic flash device is detached from the camera. The 
auxiliary switch is opened, but the charge on the storage capacitor 8 
continues discharging through the diode 16 connected in parallel with the 
auxiliary switch 10. Therefore, the potential at the anode of the neon 
lamp 14 is maintained at a higher level than that at the circuit earth 
terminal, this potential is only equal to the forward potential drop 
(about 0.6 volts) of the diode 16. Therefore, the neon lamp 14 is no 
longer lit. 
It will be understood from the foregoing that when the main switch 9 is 
switched from its ON to its OFF state, or when chattering takes place at 
the transit of the main switch 9 to the ON state, as has been described 
above, a surge voltage is produced between the line A and the line B. 
However since, in the embodiment of the invention illustrated in FIG. 3, 
all the signal terminals a', b' and c' are connected to the line B either 
directly or through a circuit element such as the resistor 28 and 
transistor 20, the surge voltage between the lines A and B is never 
applied across the camera. Even when the exposure control circuit 26 in 
the camera is constructed of CMOS type integrated circuit units, there is 
no possibility of causing any withstand voltage damage to them. 
Another advantage is that after the main switch 9 has been turned off, 
there is no need for the photographer to wait for the charge on storage 
capacitor 8 to fully discharge before the auxiliary switch 10 can be 
turned off without causing the neon lamp 14 to be lit. Therefore, after 
the electronic flash device is ready for the next flash exposure, deciding 
not to proceed with the device, the photographer may remove the device 
from the camera as soon as the main switch is turned off. Even in this 
case, no spurious indication that the electronic flash device is operative 
again is presented by the neon lamp, as the neon lamp remains off. This is 
important when the electronic flash device is of the type described. If 
the neon lamp were lit again at the time of removal of the device from the 
camera, the photographer would think something was wrong because even 
though the main switch had been turned off before the device was detached 
from the camera, the electronic flash device would appear as if it were 
starting to operate again. 
In FIG. 3, the diode 16 corresponds to the one-way semiconductor element of 
the invention, and the neon lamp 14 corresponds to the indicator lamp for 
detecting when the storage capacitor is fully charged. 
Also, according to the invention, the series-connected circuit of main 
switch 9 and auxiliary switch 10 is not included in the primary loop of 
the booster circuit I, which primary loop contains the primary winding P 
of oscillation transformer 5, but is included in the secondary loop of 
relatively smaller current, thereby giving an additional advantage that 
the influence in increase of the resistance at the interface between the 
switch contacts 9s and 9a and between the contacts 10s and 10a on the 
performance of the DC-DC converts can be lessened. 
As has been described in greater detail above, according to the present 
invention, the auxiliary switch is connected in series to the main switch 
at such a position that it is farther from the negative terminal of the 
battery than the main switch, and the one of the contacts of the auxiliary 
switch which is connected to a signal terminal is connected to the 
negative pole of a storage capacitor, whereby the circuit connecting the 
signal terminal to another signal terminal is not cut off when either the 
main switch or the auxiliary switch is turned off. With the electronic 
flash device left attached to the camera, therefore, the surge voltage 
produced by changing the main switch from the ON to the OFF state does not 
cause withstand voltage damages of the electronic circuit in the camera as 
it is applied backward to the camera. Further, the signal terminal 
connected to one of the contacts of the auxiliary switch may be used as 
the common circuit earth of the other signal terminals, thereby reducing 
the necessary number of parts. This is advantageous as to compactness and 
lowering production cost. 
Further in the aforesaid arrangement of the constituent parts, when a 
one-way semiconductor element is connected in parallel to the auxiliary 
switch and oriented so that its forward direction coincides with the 
direction in which the charging completion indicator lamp is 
short-circuited, the indicator lamp is maintained short-circuited by the 
one-way semiconductor element against the turning off of the auxiliary 
switch. Therefore, the opening of the main switch followed immediately by 
detaching the electronic flash device from the camera while the storage 
capacitor is fully charged allows the indicator lamp to remain off, 
thereby preventing the photographer from thinking the electronic flash 
device had started operating again.