Electronic flash device for photographic camera

Details Electronic flash device for photographic camera Electronic flash device for photographic camera

2000-03-23

2001-11-13

Perkey, W. B. (Department: 2851)

Photography

With object illumination for exposure

Having flash built into camera

C396S203000, C396S206000, C396S267000

Reexamination Certificate

active

06317566

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electronic flash device for a photographic camera, and, more particularly, to an electronic flash device with law specific power consumption for a camera.
2. Description Related to the Prior Art
Cameras and lens-fitted film units have built-in electronic flashes for convenience of taking pictures indoors or under law ambient subject brightness. Such an electronic flash device has a need of charging a main capacitor up to a specified charged level of voltage. When the main capacitor is completely charged up, a neon lamp connected to both terminals of the main capacitor is energized or turned on to emit light for providing an indication that the electronic flash device is ready to flash. An electronic flash device equipped with a light emitting diode (which is hereinafter referred to as LED) as used for an indicator instead of the neon lamp has been proposed in, for example, Japanese Unexamined Patent Publication No. 8-115796 filed by the same applicant of this application and placed on the market, the LED needs a build-up voltage of 1.8 or higher to turn on and emit light. However, electromotive force of a battery that is usually used in cameras and lens-fitted film units is about 1.5 volts which is too low to energize directly the LED. The electronic flash device disclosed in the above mentioned publication energizes the LED with a voltage that is provided by a blocking oscillator that is constituted by an oscillating transistor and an oscillating transformer and well known in various form to those in the art. Reference is made to
FIG. 6
for the purpose of providing a brief background that will enhance an understanding of the operation of a circuit of the electronic flash device disclosed in the above-mentioned publication.
Referring to
FIG. 6
, the electronic flash device includes a blocking oscillator that is comprised of an oscillating transistor
60
and an oscillating transformer
61
. The oscillating transistor
60
repeatedly increases/decreases a primary current I
1
which flows through a primary winding
61
a
of the oscillating transformer
61
so as to generate an electromotive force and a counter electromotive force across secondary and third windings
61
b
and
61
c,
respectively. When an electromotive force builds up, a main capacitor
63
is charged with a secondary current I
2
which flows through a rectifier diode
62
from the secondary winding
61
b.
While a charging switch
64
remains turned on or closed, a battery
66
can start to supply current I
0
through a resistor
65
and the third winding
61
c
of the oscillating transformer
61
to a base of the transistor
60
, as a result of which the transistor
60
is turned conductive to admit the primary current I
1
to flow therthrough. This causes the secondary and third windings
61
b
and
61
c
to produce the secondary and third currents
12
and
13
, respectively. These currents I
2
and I
3
are added to the current I
0
supplied originally from the battery
66
with the result of increasing the base current of the oscillating transistor
60
, which leads to a further increase in the primary current I
1
, so that the base current reaches a peak current instantaneously due to a further increase in the secondary current I
2
. On the other hand, when the primary current I
1
reaches a peak level and then stops increasing, each winding,
61
a,
61
b
and
61
c
generates a counter electromotive force which is opposite in direction to the electromotive force. The counter electromotive force across the secondary and third windings
61
b
and
61
c
cause a reduction in the base current of the oscillating transistor
60
, which results in a reduction in the primary current I
1
correspondingly. In consequence, there occurs a further increase in the counter electromotive force, which leads an instantaneous reduction in the base current to a bottom level. As a result, when the counter electromotive force disappears, the oscillating transistor
60
is brought into conductive, so as to repeat the same operation.
As described above, the LED
67
for providing an indication of completion of charging a main capacitor is connected to both ends of the tertiary winding
61
c
which gives ON/OFF oscillation to the transistor
60
by amplifying the amplitude of a base current of the transistor
60
with a current which is generated as a current
13
when electromotive force is generated across the tertiary winding
61
c
or as a current (−I
3
) opposite in direction to the current I
3
when counter electromotive force is generated across the tertiary winding
61
c.
In order to energize LED
67
to emit light when the main capacitor
63
attains a specified charged voltage, the utilization is made of a potential present at one of the opposite ends of the tertiary winding
61
c
that changes in proportional to a charged voltage of the main capacitor
63
.
In the case where, although it has no concern in installation of a light emitting diode, the tertiary winding is used to control, increase or reduce, the base current of the oscillating transistor by connecting the tertiary winding to the light emitting diode, the current I
0
supplied from the battery is not supplied as a base current to the base transistor and is, however, cancelled out by the current (−I
3
) when counter electromotive force is generated across the tertiary winding. That is to say, the battery wastes power by letting the current I
0
to flow. Besides the current I
0
is rather large as the resister used in the circuit through which the current I
0
flows has a relatively low resistance such as 200 ohms. Accordingly, the electronic flash device described above unnecessarily consumes electric power and causes the battery to waste easily its power. In the case where a current is directly supplied as a base current to the oscillating transistor from the battery by way of a resister having resistance of about 200 ohms in place of amplifying the amplitude of a base current of the oscillating transistor through the tertiary winding, the oscillating transistor remains turned ON and does not implement oscillation. Otherwise, in the case where a current is supplied as a base current to the oscillation transistor from the battery through a resister having a high resistance of, for example, 1 K ohms, while the oscillating transistor implements oscillation, it is turned nonconductive with counter electromotive force generated across the secondary winding. In consequence, when the counter electromotive force across the secondary winding becomes weak due to a rise in the charged voltage of the main capacitor to a somewhat high level, the oscillating transistor remains conductive due to a reduction in amplitude of the base current, as a result of which charging stops before the main capacitor attains a specified charged voltage and the oscillating transistor is continuously supplied with a current from the battery.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an electronic flash device with a ready-to-flash indicator by a light emitting diode which is less expensive than a neon lamp.
Another object of the present invention is to provide an electronic flash device which provides a reduction in consumption of a battery.
The above objects of the present invention are achieved by an electronic flash device comprising an oscillating transformer which has a primary winding, a secondary winding and a tertiary winding connected to one another in inductive coupling and is operative to increase or decrease a primary current across the primary winding so as to generate induction currents across the secondary winding and the tertiary winding and thereby to charge the main capacitor of the electronic flash device with the secondary current across the secondary winding, an oscillating transistor operative to amplify the primary current in accordance with a base current supplied thereto, a controlling transistor operative to control the base c

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