Abstract:
A flash circuit for use in a camera, the flash charging circuit comprising a flash light discharge circuit having a light emitting element electrically connected to a flash capacitor; a flash triggering circuit connected to the flash light discharge circuit with the flash triggering circuit having a trigger signal generating circuit generating a signal that enables energy from the energy storage device to be converted into light by the light emitting element; a voltage conversion circuit connected between a battery and the flash capacitor to convert energy from a source voltage into a higher voltage to charge the flash capacitor; a timer circuit to cause the voltage conversion circuit to operate when the voltage at a timing capacitor is within a range of voltages, said timer circuit having a time constant circuit connected to the timing capacitor to discharge energy stored in the timing capacitor at a predetermined rate; a reset circuit that resets the timing capacitor voltage to a voltage that is within the range.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a photographic flash circuit, and, more particularly to a flash circuit having a voltage boosting circuit for charging a photoflash capacitor. 
     BACKGROUND OF THE INVENTION 
     It is well known to use electronic flash circuits to provide artificial illumination of a scene to improve the appearance of a photographic image. Because cameras are typically portable many electronic flash circuits draw energy from portable batteries such as chemical batteries. Charging circuits are used to convert battery voltage into a higher voltage that can charge a flash capacitor so that it stores sufficient energy to cause a flash tube to discharge enough light to illuminate the scene. Chemical batteries provide a fixed amount of power to such flash circuits and, therefore, needless operation of the flash charging circuit is to be avoided to prevent premature exhaustion of the chemical batteries during a photography session. 
     U.S. Pat No. 4,522,479 entitled “Flash Apparatus with Power Supply Control Device” filed in the name of Yamada et al. on Dec. 24, 1983 discloses a power supply control device for use in a flash apparatus. The disclosed power supply control device automatically cuts off power to a flash apparatus by turning off a power switch at a predetermined time after the power switch is turned “on”. This prevents the waste of electrical energy that can arise from, for example, the careless failure to turn the power switch off. The power supply control device is provided with a timer circuit that sets the predetermined period of time and is arbitrarily resettable with a manually operable power switch. The circuit of the &#39;479 patent incorporates a number of expensive electrical components, including integrated circuits such as operational amplifiers and a one shot multivibrator. This makes such a circuit expensive. This circuit may be useful for flash circuits of the type that are incorporated in expensive products such as single lens reflex cameras and separable flash units of the type that are typically used with SLR type cameras. What is needed is a less expensive circuit for use in lower cost cameras and one-time use cameras. 
     SUMMARY OF THE INVENTION 
     A flash circuit for use in a camera, the flash charging circuit comprising a flash light discharge circuit having a light emitting element electrically connected to a flash capacitor; a flash triggering circuit connected to the flash light discharge circuit with the flash triggering circuit having a trigger signal generating circuit generating a signal that enables energy from the energy storage device to be converted into light by the light emitting element; a voltage conversion circuit connected between a battery and the flash capacitor to convert energy from a source voltage into a higher voltage to charge the flash capacitor; a timer circuit to cause the voltage conversion circuit to operate when the voltage at a timing capacitor is within a range of voltages, said timer circuit incorporating said timing capacitor in a time constant circuit that discharges energy stored in the timing capacitor at a predetermined rate; a reset circuit that resets the timing capacitor voltage to a voltage that is within the range; wherein the range of voltages is higher than the battery voltage and wherein the reset circuit first applies battery voltage to the timing capacitor and then applies voltage generated by the voltage conversion circuit to charge the timing capacitor to a voltage that is higher than the battery voltage. 
     A flash charging circuit for use in a camera, the flash charging circuit comprising a flash light discharge circuit comprising a light emitting element electrically connected to a flash capacitor; a flash triggering circuit connected to the flash light discharge circuit with the flash triggering circuit having a trigger signal generating circuit generating a signal that enables energy from the flash capacitor to be converted into light by the light emitting element; 
     a voltage conversion circuit connected between a battery and the flash capacitor to convert energy from a battery voltage into a higher voltage to charge the flash capacitor; a timer circuit to cause the voltage conversion circuit to operate when the voltage at a timing capacitor is within a range of voltages, said timer circuit incorporating the timing capacitor in a time constant circuit that discharges energy stored in the timing capacitor at a predetermined rate; and a reset circuit having a thyristor connected to the timing capacitor, a gate of said thyristor being triggered upon operation of a shutter of the camera, with the thyristor being connected to the battery and conducting energy from the battery to the timing capacitor when the thyristor is triggered to charge the timing capacitor to a voltage no greater than the battery voltage but within the range of voltages; wherein said voltage conversion circuit further supplies voltage pulses to the thyristor to charge the timing capacitor to a voltage higher than the battery voltage when the voltage conversion circuit is operated and wherein the thyristor turns off when the voltage at the timing capacitor approaches the voltage of said pulses. 
     In yet another aspect of the invention, what is provided is a photographic flash circuit The flash circuit has a light emitting element connected to a flash capacitor, a flash triggering circuit which causes the light emitting element to convert energy from the flash capacitor into light and a voltage conversion circuit for converting a low battery voltage into a higher voltage to charge said flash capacitor, with the voltage conversion circuit having an oscillation transistor and at least one other transistor in an oscillation current path, said oscillation transistor oscillating during voltage conversion. A diode is connected to more than one transistor to suppress any voltage spikes at both transistors that appear during oscillation at the transistors to which the diode is connected. 
     In still another aspect of the invention a photographic flash charging circuit is provided. The photographic flash circuit comprises a light emitting element connected to a flash capacitor, a flash triggering circuit which causes the light emitting element to convert energy from the energy storage capacitor into light, and a timer control circuit adapted to cause the voltage conversion circuit to operate for a timing period and then automatically shut off, with the timer control being reset to the beginning of the timing period by actuation of the flash triggering circuit, with the timer control circuit having a timing period determined as a function of the voltage to which a timing capacitor is charged when the timer control circuit is reset. Test points are provided across the timing capacitor so that a testing circuit can determine conditions at the timing capacitor during the testing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows one embodiment of a flash circuit in accordance with the present invention. 
     FIG. 2 shows another embodiment of a flash circuit in accordance with of the present invention. 
     FIG. 3 shows still another embodiment of a flash circuit in accordance with present invention. 
     FIG. 4 shows still another embodiment of a flash circuit in accordance with present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows one embodiment of a flash circuit  10  in accordance with present invention. Flash circuit  10  is low-cost and therefore is particularly useful in a low-cost one time use camera. Flash circuit  10  comprises a voltage boosting circuit  12 , a timer circuit  14 , a reset circuit  16 , a flash discharge circuit  18 , and flash triggering circuit  22 . 
     Voltage boosting circuit  12  comprises an oscillation step-up transformer  24  having a core  25 , a primary winding  26  a secondary winding  28 , and a feedback winding  30 . Voltage boosting circuit  12  also has an oscillation transistor  32 , a control switch transistor  34 , a feedback current limiting resistor  38 , a high-voltage rectifier diode  40 , a resistor  41  and a light emitting diode  42 . A battery  45  is also provided, and in the embodiment shown in FIG. 1, battery  45  comprises a single 1.5 volt battery. Battery  45  can take any variety of forms, for example, batteries of different size and/or voltage rating can be used as well as combinations of more than one battery. 
     Timer circuit  14  comprises a timing capacitor  44 , time constant resistors  46 ,  48 , and transistor  50 . 
     Reset circuit  16  comprises latch transistors  54  and  56 , connected to latch each other on, momentary contact switch  58 , holding capacitor  60  and series resistor  62  and flash sync detecting diode  64 . In the embodiment shown in FIG. 1 reset circuit  16  also comprises resistors  80 ,  90 ,  92  and  98 . 
     Discharge circuit  18  includes a flash capacitor  20  electrically connected to a flash tube  66 . Flash tube  66  conducts electrical energy provided by flash capacitor  20  when a preferred potential exists at an electrode  68  on flash tube  66 . Electrical energy is stored in flash capacitor  20  by voltage boosting circuit  12  in a manner that will be described in greater detail below. 
     Flash triggering circuit  22  comprises flash trigger transformer  70 , a trigger capacitor  72 , trigger capacitor charging resistor  74 , and a flash sync switch  76 . Flash triggering circuit  22  provides the preferred potential at electrode  68  in response to closure of flash sync switch  76  so that closure of flash sync switch  76  causes a flash of light to be discharged from flash tube  66  when flash capacitor  20  is appropriately charged. 
     The following sections will now describe the operation of the embodiment of FIG.  1 . In the embodiment shown in FIG. 1, a charging cycle begins when the momentary contact switch  58  is closed. The closure of switch  58  creates a current that passes through a current limiting resistor  80  to create a voltage that forward biases latch transistor  56 . This initiates a timing period during which timing capacitor  44  is charged. Latch transistor  54  and latch transistor  56  are connected to one another, collector to base, such that turning “on” one transistor of the pair will turn “on” the other and latch transistors  54  and  56  will keep each other forward biased as long as current is supplied to the emitter of latch transistor  54 . This behavior is similar to a thyristor. In an alternative embodiment, a thyristor can be used in place of latch transistors  54  and  56 . 
     Two sequential events take place after momentary contact switch  58  is momentarily closed. The first is that latch transistor  54  and latch transistor  56  turn “on”. This allows timing capacitor  44  to charge to battery voltage minus the voltage drop of latch transistors  54  and  56 . The second event begins when timing capacitor  44  is charged sufficiently to forward bias transistor  50 , as control switch transistor  34  and oscillation transistor  32  turn “on”, causing current to flow from battery  45  through primary winding  26  of step up transformer  24 . Under this “second event” condition, an oscillation cycle will begin. At this point, all transistors  32 ,  34 ,  50 ,  54 , and  56  are turned “on.” 
     The following is a description of one oscillation cycle. Each oscillation cycle begins with current flowing through oscillation transistor  32  and primary winding  26  of step up transformer  24 . The amount of current flowing in this way increases at a rate determined by the inductance of primary winding  26  and causes a corresponding increase in the magnetic flux in a core  25  of step up transformer  24 . A corresponding current is induced in secondary winding  28  and feedback winding  30 . The secondary voltage is stepped up to a high voltage and rectified by diode  40  to charge flash capacitor  20 . When core  25  is saturated, current drops in the secondary winding  28  and feedback winding  30 . A relatively low voltage positive flyback pulse is generated at the collector of oscillation transistor  32  as the flux in step-up transformer  26  collapses. This is the end of one oscillation cycle. 
     Negative voltage pulses are simultaneously generated at the emitter of control switch transistor  34  and at the base of oscillation transistor  32 . A clamp diode  84  clamps these negative pulses, protecting both oscillation transistor  32  and control switch transistor  34  from damage due to excessive reverse bias. The use of a single diode, clamp diode  84 , to protect both transistors is one factor that enables flash circuit  10  to provide protection for oscillation transistor  32  and control switch transistor  34  at a low cost. 
     The oscillator feedback used to drive the base of oscillation transistor  32  has two components. The first component is from transformer feedback winding  30 . This signal is current limited by current limiting resistor  38  and its magnitude is relatively constant for the duration of the time required to charge flash capacitor  20 . The second component of the oscillation feedback is the flash capacitor charging current from the negative terminal of flash capacitor  20 . This current is high when charging begins on a discharged flash capacitor  20 . This current then decreases exponentially as flash capacitor  20  charges. These two currents are added together at the emitter of control switch transistor  34 . 
     By itself, first component of the oscillator feedback will sustain oscillations at a minimum battery current, will keep flash capacitor  20  at fill charge and will illuminate light emitting diode (LED)  42 . However, both the first and second components are required to charge flash capacitor  20  to a requisite voltage to enable desirable flash discharge. This method of driving the base of oscillation transistor  32  has the advantage of using less energy from battery  45  after the flash capacitor  20  is charged, because oscillation is sustained only to maintain the flash capacitor  20  at flash ready voltage and to illuminate the LED  42 . 
     The reverse voltage amplitude of the oscillation pulses on feedback winding  30  are proportional to the voltage on flash capacitor  20 . The number of turns in feedback winding  30  is chosen so that the reverse oscillation pulse voltage will begin to illuminate LED  42  when flash capacitor  20  is charged to flash ready voltage. This flash ready voltage can be, for example, about 300 volts. Thus, LED  42  indicates to the photographer when flash circuit  10  is ready to take a flash picture. 
     During the first event (previously described), the emitter of latch transistor  54  and the collector of oscillation transistor  32  are at a voltage established by the voltage at battery  45 . The second event is defined when a series of oscillations begins at oscillation transistor  32 . Transistor  32  begins to oscillate when the magnetic field in core  25  of step up transformer  24  saturates and collapses inducing a periodic pulse signal known as a flyback signal in secondary winding  28  and feedback winding  30 . The flyback signal is present during half of the oscillation cycle and takes the form of pulses at the collector of oscillation transistor  32  and at the emitter of latch transistor  54 . During the other half of the oscillation cycle the collector of oscillation transistor  32  and the emitter of latch transistor  54  are at the saturation voltage of oscillation transistor  32 . The saturation voltage is substantially less than the base-emitter voltage drop of transistor  32 . 
     Latch transistors  54  and  56  are forward biased by the flyback pulses and transfer energy from the flyback pulses to charge timing capacitor  44 . Transistors  54  and  56  turn off in between successive flyback pulses because the voltage at the emitter of transistor  54  (the saturation voltage of transistor  32 ) is less than the voltage on timing capacitor  44 . 
     Timing capacitor  44  is eventually charged to the voltage of the flyback pulses minus the combined voltage drops of latch transistors  54  and  56 . At this point the current through transistors  54  and  56  approaches zero and the transistors turn off, leaving timing capacitor  44  charged. 
     The time duration that latch transistors  54  and  56  conduct flyback pulses is a function of flyback pulse amplitude, frequency and harmonics which may be present on the flyback pulses. To reduce the influence of these factors, holding capacitor  60  and resistor  62  can be connected across the base-emitter of transistor  54 . Holding capacitor  60  maintains transistors  54  and  56  “on” in-between flyback pulses. 
     As the collector current approaches zero, the current from holding capacitor  60  to the base of transistor  54  also drops, this current being supplied from the collector of transistor  56 . When capacitor  60  is discharged, latch transistors  54  and  56  turn off, leaving timing capacitor  44  charged. Resistor  62  controls the discharge rate of holding capacitor  60 . This establishes the time that transistors  54  and  56  are held “on”. This improves the repeatability of the charging time of timing capacitor  44 , which is otherwise dependent on flyback pulse amplitude (which is a function of battery energy level), frequency and harmonics present on the flyback pulses. 
     Resistors  90  and  92  insure that transistors  54  and  56  will stay off in the absence of forward bias. Resistor  100  insures that control switch transistor  34  will stay off in the absence of forward bias. Resistor  102  limits the current to the base of control switch transistor  34 . Resistor  48  provides a slow discharge path for timing capacitor  44  and resistor  46  provides a trickle bias for timing transistor  50 , which stays “on” until the charge on timing capacitor  44  falls below the forward base to emitter bias of timing transistor  50 . When this occurs, transistor  50  turns “off”, which in turn turns “off” control switch transistor  34  and oscillation transistor  32 . This ends the timing period. As transistors  54  and  56  have previously turned off, all of transistors  32 ,  34 ,  50 ,  54 , and  56  are “off”. 
     Typically batteries of the type used as battery  45  in a flash circuit  10  are less efficient at low temperature. Therefore the amount of time that is required to charge flash capacitor  20  will be longer at low temperature. It may therefore be desirable to provide features in flash circuit  10  that automatically provide a longer timing period when flash circuit  10  is operated in low temperature environments. Accordingly, in one embodiment of the present invention, resistor  48  is a temperature dependent resistor, such as a thermistor or other temperature dependent variable resistor. This embodiment of resistor  48  is selected so that the resistance provided by resistor  48  increases as ambient temperature decreases. This, in turn provides a longer timing period when this embodiment of flash charging circuit  10  is operated at a low temperature and a shorter timing period at normal temperatures. 
     Switch  76  comprises shutter sync contacts  94  and  96 , which close when a shutter (not shown) in a camera (not shown) that uses flash circuit  10  opens to expose a film (not shown) in the camera. When this occurs, trigger capacitor  72  is charged to the flash capacitor voltage through resistor  74 . When sync switch  76  closes, trigger capacitor  72  discharges into the primary winding of trigger transformer  70 , generating a very high voltage pulse at a secondary winding of trigger transformer  70 , which is applied to contact  68  and which provides sufficient potential to initiate conduction in flash tube  66 . Flash capacitor  20  then discharges through flash tube  66 , which emits light. The timing period comprises both the charging and discharging of timing capacitor  44 . 
     The timing period is automatically restarted when sync switch  76  is closed. Diode  64  is normally reverse biased by the high voltage charge on capacitor  20 . The minimum voltage on flash capacitor  20  will be the voltage at battery  45  minus the forward voltage drop of rectifier diode  40 . This minimum voltage is sufficient to reverse bias flash sync detecting diode  64 . 
     When sync switch  26  closes, flash sync detecting diode  64  is forward biased and current flows through a current limiting resistor  98  to turn “on” latch transistor  54 . This begins the automatic timing period reset after flash by initiating the first sequential event described above. However, it will be appreciated that the closure time of sync switch  76  is about 0.1 millisecond, which is much shorter than a closure time that can be provided by a person pressing momentary switch  58  to initiate charging. The closure time of sync switch  76  is also shorter than the time required for flyback pulses to appear at the emitter of latch transistor  54  and is thus shorter than the time required to complete the first sequential event. This problem is solved in reset circuit  16  by transistors  54  and  56 , which are kept “on” by the bias from holding capacitor  60  which is charged during the conduction of flash sync detecting diode  64 , thus insuring completion of the first sequential event. 
     FIG. 2 shows another embodiment of flash circuit  10  of FIG.  1 . In this embodiment flash circuit  10  has a filtering capacitor  110  connected across the base-emitter of latch transistor  54  and a filtering capacitor  112  across the base-emitter of latch transistor  56 . Small amplitude impulse noise or static electricity can trigger these transistors “on” and initiate an undesired timing period. Filtering capacitors  110  and  112  reduce the sensitivity of flash circuit  10  to impulse noise without substantially increasing the cost of this embodiment of flash circuit  10 . 
     FIG. 3 shows another embodiment of a flash circuit  10 . In the embodiment of FIG. 3 a charge limiting circuit  114  is added to the embodiment shown in FIG. 1 to further improve the battery efficiency of flash circuit  10 . As described above, in flash circuit  10  the oscillator feedback signal to the base of oscillation transistor  32  has two components. The first component is from transformer feedback winding  30 . The second component is the flash capacitor charging current from the negative terminal of flash capacitor  20 . These two currents are added together through the emitter of control switch transistor  34 . The first component alone will sustain oscillations after flash capacitor  20  is charged but both current components are necessary to drive oscillation transistor  34  sufficiently to charge flash capacitor  20 . 
     In the embodiment of FIG. 3, additional battery energy savings are obtained by electronically disconnecting the flash capacitor  20  from charging circuit  12  after flash capacitor  20  reaches a voltage determined by a zener diode  120  which can establish a voltage limit of, for example, 320 volts. This is done by disconnecting the second current component from the emitter of oscillation transistor  32  and allowing the first current component to sustain oscillations, at minimum battery current, for the duration of the timing period. This prevents the charger from using battery energy to charge flash capacitor  20  to a voltage higher than necessary. Even though flash capacitor  20  charging is terminated, ready light LED  42  is illuminated for the duration of the timing period. 
     While flash capacitor  20  is charging, transistor  122  is biased “on” by resistor  124  and transistor  126  is not conducting. Transistor  122  thus conducts the charging current from a negative terminal of flash capacitor  20  to the emitter of control switch transistor  34 , providing normal flash capacitor charging. Zener diode  120  conducts at flash ready voltage and forward biases transistor  126 , which is normally biased “off” by resistor  128 . Resistor  130  limits the current to the base of transistor  126 . When transistor  126  turns “on” it biases transistor  122  “off”, which effectively disconnects the negative terminal of flash capacitor  20  from the emitter of control switch transistor  34 . The oscillation of oscillation transistor  32  is sustained only by current from feedback winding  30  and ready light LED  42  is illuminated. The battery current is minimum because flash capacitor  20  is not being charged. 
     As the flash capacitor discharges, zener diode  120  will cease conduction and transistors  122  and  124  will reconnect flash capacitor  20 , which will recharge to the zener voltage. This cycle will repeat until the timing period ends, maintaining the flash capacitor at the zener voltage while conserving battery energy. Therefore the embodiment of FIG. 3 always charges flash capacitor  20  to the same voltage, a voltage established by zener diode  120 , with either a strong or weak battery. 
     FIG. 4 shows still another alternative embodiment of the circuit of FIG.  1 . In this embodiment holding capacitor  60  and series resistor  62  are omitted. In this configuration transistors  54  and  56  operate to recharge the timing capacitor  44  and thus reset the timing period. Transistors  54  and  56  both turn “on” for each flyback pulse and turn “off” when each flyback pulse terminates. Latch transistors  54  and  56  cease turning “on” when timing capacitor  44  charges to the amplitude of the flyback pulses because of the diminished voltage drop across latch transistors  54  and  56 . This ends the reset period for timing capacitor  44  and timing capacitor  44  will begin discharging. 
     As is also shown in FIG. 4, optional test points  132  and  134  are provided across timing capacitor  44  so that a testing circuit from a testing circuit (not shown) can determine conditions at the timing capacitor. Depending on the testing regimen applied, the testing circuit (not shown) can determine the condition at the timing capacitor by measuring the voltage on timing capacitor  44 , also the testing circuit can modify the state of timing capacitor  44  by changing the time constant associated with timing capacitor  44  or by shorting the timing capacitor  44  to prematurely terminate the timing period. Also the testing circuit (not shown) can force timing capacitor  44  to a predetermined voltage to quickly move the timing period to a certain state. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     Parts List 
       10  flash circuit 
       12  voltage boosting circuit 
       14  timer circuit 
       16  reset circuit 
       18  flash discharge circuit 
       20  flash capacitor 
       22  flash triggering circuit 
       24  step up transformer 
       25  core of step up transformer 
       26  primary winding 
       28  secondary winding 
       30  feedback winding 
       32  oscillation transistor 
       34  control switch transistor 
       38  limiting resistor 
       40  voltage rectifier diode 
       41  resistor 
       42  light emitting diode 
       44  timing capacitor 
       46  current limiting resistor 
       45  battery 
       48  time constant resistor 
       50  timing transistor 
       54  latch transistor 
       56  latch transistor 
       58  momentary contact switch 
       60  holding capacitor 
       62  series resistor 
       64  flash sync detecting diode 
       66  flash tube 
       70  flash trigger transformer 
       72  trigger capacitor 
       74  trigger capacitor charging resistor 
       76  flash sync switch 
       80  current limiting resistor 
       84  clamp diode 
       90  resistor 
       92  resistor 
       94  flash sync switch contact 
       96  flash sync switch contact 
       98  current limiting resistor 
       100  resistor 
       102  resistor 
       110  filtering capacitor 
       112  filtering capacitor 
       114  charge limiting circuit 
       120  zener diode 
       122  transistor 
       124  resistor 
       126  transistor 
       128  resistor 
       130  current limiting resistor 
       132  test point 
       134  test point