Flash unit

Details Flash unit Flash unit

2000-02-25

2002-02-05

Perkey, W. B. (Department: 2851)

Photography

With object illumination for exposure

Having flash built into camera

C396S203000, C396S267000

Reexamination Certificate

active

06345156

ABSTRACT:

FIELD OF THE ART
The present invention relates to a flash device, and more particularly to a flash device which is suitable for being incorporated in a lens-fitted photo film unit that contains a roll of photo filmstrip and is provided with a simple photographic mechanism including a taking lens.
BACKGROUND ARTS
When the subject brightness is so low that a proper exposure would not be provided without any artificial illumination, a flash device is often used to project light toward the subject synchronously with the shutter release. Since compact cameras and lens-fitted photo film units have an inexpensive lens system of a relative large f-number, most of them are provided with a built-in flash device. To make a flash photography, it is necessary to charge the main capacitor up to the set voltage prior to the shutter release. The conventional flash devices start charging in response to an actuation of a flash charge switch.
A flash circuit has recently been known, for example from JPA 7-122389, wherein once a flash charge switch is turned on the main capacitor continues to be charged even after the flash charge switch is turned off. The flash circuit stops charging while the main capacitor is at the set charge voltage. Hereinafter, this type of flash device will be referred to as an autostop flash circuit.
An example of autostop flash circuit is shown in
FIG. 24
, which has fundamentally the same configuration as that disclosed in JPA 7-122389, except some minor differences. In the flash circuit of
FIG. 24
, when a flash charge switch
200
is turned on, an oscillation transistor
201
is activated and starts oscillating due to positive feedback of an oscillation transformer
202
. The oscillation causes an increase in a primary current that flows through a primary coil
202
a
, i.e. a collector current that flows to the collector of the oscillation transistor
200
. As a result, an electromotive force induces a current through a secondary coil
202
b
, and the current charges a main capacitor
204
through a rectifying diode
203
.
Since the collector current flows through the oscillation transistor
201
, a latching transistor
205
is turned on. Thereafter when the increment of the primary current goes down, a back electromotive force is generated in the secondary coil
202
b
, and the current fed back from the oscillation transformer
202
to the oscillation transistor
201
, i.e. base current of the oscillation transistor
201
, begins to decrease. However, a voltage from a battery
206
is applied to the base of the oscillation transistor
201
through the latching transistor
205
as being in the ON state, the oscillation transistor
201
is not completely turned off. Therefore, the primary current starts flowing again, thereby the oscillation transistor
201
continues to oscillate and charge the main capacitor
204
.
A Zener diode
207
with a Zener voltage of 300V is provided for starting conducting a Zener current when the main capacitor
204
is charged up to a set voltage of 300V. Because a base current is applied to a base of a stopping transistor
208
due to the Zener current, the stopping transistor
208
is turned on. When the stopping transistor
208
is turned on, the emitter and the base of the oscillation transistor
201
are connected to each other, so that the oscillation transistor
201
is completely turned off, and thus the latching transistor
205
is turned off. In this way, the oscillation stops to stop charging the main capacitor
204
when the main capacitor
204
is charged up to the set voltage.
In the above autostop flash circuit, since the charge voltage of the main capacitor is applied to the Zener diode to conduct the Zener current for activating the stopping transistor when the charge voltage reaches the set value, the Zener diode must have a high Zener voltage, e.g. 300V, in correspondence with the set charge voltage. As the Zener diode with high Zener voltage is expensive, it raises the cost of the flash circuit. In addition, the conventional autostop flash circuit requires a lot of space for mounting various elements as above which are necessary for the automatic continuation and stopping of charging.
Meanwhile, a lens-fitted photo film unit having a flash device incorporated therein is widely known. Since the flash device for the lens-fitted photo film unit is required to be inexpensive and compact, the flash switch is constituted of a metal blade and contact chips formed on a flash circuit board such that the metal blade is brought into contact with the contact chips through a manual operation member, to close the flash circuit and thus cause the flash circuit to start charging. Conventionally, the manual operation member is a push button or a sliding button. Some of the conventional manual operation members are provided with a fastening mechanism by which the metal blade is kept in contact with the contact chips once the operation member is operated for a moment. Others are designed to bring the metal blade into contact with the contact chips only while the photographer operates the manual operation member.
The flash device of the lens-fitted photo film unit has been provided with a light emission element for indicating completion of charging the flash device. In the conventional lens-fitted photo film unit, the light emission element is placed behind an indication window formed through a rear wall of the lens-fitted photo film unit, or is placed in connection to a light guide that conducts light from the light emission element to a view field of a viewfinder.
Where the charge switch operation device is provided with the fastening device, the flash device keeps charging so long as the operation device is in the ON position. If the operation device is left in the charging position after the photography is terminated, the battery is wasted. As a result, the battery runs down and it becomes impossible to use the flash device before all of the available exposures are carried out. Such trouble will be prevented if only the photographer checks the position of the operation member or the light from the light emission element at the conclusion of photography. However, according to the configurations of the conventional lens-fitted photo film units, it is not easy to know the switching condition of the operation member at a glance, or the charge condition indicating light is not visible from the outside of the lens-fitted photo film unit. Therefore, the photographer can fail to reset the charge switch to the OFF position.
In view of the foregoing, a prime object of the present invention is to provide an autostop flash device which cuts the cost and space without lowering reliability and stability.
A further object of the present invention is to provide a flash device which is effective to prevent the photographer from forgetting to turn off the flash charge switch, and is also suitable especially for use in a lens-fitted photo film unit.
DISCLOSURE OF THE INVENTION
In a flash device comprising an oscillation circuit that starts oscillating when a flash charge switch is turned on, wherein the oscillation circuit is comprised of an oscillation transformer having a primary coil connected to a power source and a secondary coil connected to a main capacitor, and the secondary coil is inductively coupled to the primary coil such that a high voltage current is induced in the secondary coil while the oscillation circuit oscillates, and that the main capacitor is charged with the high voltage current up to a set charge voltage,
the present invention is comprised of a tap point located at an intermediate position of the secondary coil, the tap point having a potential that changes proportionally to the charge voltage across the main capacitor; a Zener diode connected to the tap point, to conduct a Zener current when the potential at the tap point reaches a value that corresponds to the set charge voltage of the main capacitor; and a stopping transistor activated by the Zener current to stop the oscillation circuit from oscillating and thus stop cha

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