Abstract:
An electronic flash unit has alternative (dual) capacitor switching to reduce flash ready time for flash firing.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to the field of photography, and in particular to electronic flash units for use in cameras. More specifically, the invention relates to an electronic flash unit with alternative (dual) capacitor switching to reduce flash ready time. 
     BACKGROUND OF THE INVENTION 
     Electronic flash, as used in photography, is produced by an instantaneous electric discharge between two (anode and cathode) electrodes in a gas-filled glass tube. In practice, the electrical energy for the discharge is stored in a capacitor. The main components of an electronic flash unit are therefore the power supply, the capacitor, the triggering circuit, and the flash tube itself usually with a reflector. 
     At present, electronic flash units are built into a broad spectrum of cameras, from disposable one-time-use cameras to sophisticated single-lens reflex cameras. Typically, they use a high-frequency oscillator to convert the dc voltage from a battery to an ac voltage that&#39;s stepped up in a transformer. This high voltage is rectified back to dc and stored in a flash (main) capacitor connected to a flash tube filled with xenon and small amounts of other gases. A monitoring circuit ignites a flash ready light when the stored voltage in the flash capacitor approaches a peak capacity. The stored voltage cannot discharge through the flash tube until the xenon becomes ionized. This happens when the camera&#39;s shutter-flash synchronization switch closes, dumping the charge from a small trigger capacitor into an ignition coil connected to the flash tube, which produces a brief burst of voltage applied to the glass wall of the flash tube via a third (triggering) electrode. The xenon is then ionized to provide a conductive path for the stored voltage from the flash capacitor. 
     Since the electronic flash unit usually has only one flash capacitor, flash ready time is limited by the time it takes to recharge the flash capacitor to its storage capacity following each flash illumination. Prior art U.S. Pat. No. 5,640,624 issued Jun. 17, 1997 discloses a high-speed flash charging system that accelerates flash charging time. The flash charging system includes an energy charging section with three capacitors and a trigger section with two capacitors for energizing the flash tube. The three capacitors are successively discharged as needed to charge the two capacitors. 
     SUMMARY OF THE INVENTION 
     An electronic flash unit comprising: 
     a flash tube; 
     first and second energy-storing flash capacitors, each one having an identical storage capacity which is sufficient to energize the flash tube for the same amount of flash illumination when either one of the first and second capacitors discharges its stored energy through the flash tube; and 
     a controller alternatively connecting the first and second capacitors to the flash tube to permit each one of the first and second capacitors to individually discharge its stored energy through the flash tube. 
     Preferably, a flash charging circuit charges each one of the first and second capacitors to their identical storage capacity, and the controller connects the flash charging circuit to the first capacitor when the first capacitor has discharged its stored energy through the flash tube and connects the flash charging circuit to the second capacitor when the second capacitor has discharged its stored energy through the flash tube. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded rear perspective view of a one-time-use camera with a built-in electronic flash unit according to a preferred embodiment of the invention; 
     FIG. 2 is a partially-exploded, partially-assembled front perspective view of the camera shown in FIG. 1; 
     FIG. 3 is a schematic diagram of the electronic flash unit; and 
     FIGS. 4 and 5 are a flow chart depicting operation of the electronic flash unit. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is disclosed as being embodied preferably in a one-time-use camera with a built-in electronic flash unit. Because the features of a one-time-use camera with a built-in electronic flash unit are generally known, the description which follows is directed in particular only to those elements forming part of or cooperating directly with the disclosed embodiment. It is to be understood, however, that other elements may take various forms known to a person of ordinary skill in the art. 
     Referring now to the drawings, FIGS. 1 and 2 show one-time-use camera  10  including a plastic opaque main body part  12 , a plastic opaque front cover part  14 , and a plastic opaque rear cover part  16 . The front cover part  14  and the rear cover part  16  house the main body part  12  between them and are connected releasably to one another and to the main body part  12  via known hook-in-hole connections. 
     As is typical, the main body part  12  has a rearwardly open cartridge receiving chamber  18  for a conventional film cartridge  20  and a rearwardly open film supply chamber  22 . See FIGS. 1 and 2. During manufacture, a filmstrip  24  is prewound from the film cartridge  20  into an unexposed film roll  26  which is placed in the film supply chamber  22 . A rearwardly open backframe opening  28  is located between the cartridge receiving chamber  18  and the film supply chamber  22  for exposing successive frames of the filmstrip  24  when ambient light is received through a front taking lens  30  on the main body part  12 . 
     A known optical viewfinder  32  has a pair of front and rear viewfinder lenses  34  and  36  for viewing a subject to be photographed, and supports a shutter release button  38  that is manually depressed to initiate momentary shutter-opening. 
     A film winding thumbwheel  40 , rotatably supported on the main body part  12 , above the cartridge receiving chamber  18 , radially protrudes partially from an elongate narrow opening  42  in the rear cover part  16  and has a depending coaxial stem, not shown, in coaxial engagement with an exposed top end  44  of a film take-up spool inside the film cartridge  20 . The film winding thumbwheel  40  has a continuous alternating series of peripheral-edge symmetrical teeth  46  that readily permit the thumbwheel to be manually grasped or fingered to incrementally rotate the thumbwheel in a film winding direction, i.e. counterclockwise in FIGS. 1 and 2, to similarly rotate the film take-up spool inside the film cartridge  20 . This is done in order to wind an exposed frame of the filmstrip  24  from the backframe opening  28  into the film cartridge  20  after each film exposure, and to move a fresh frame of the filmstrip from the unexposed film roll  26  to the backframe opening. Each time the filmstrip  24  is advanced slightly greater than a frame width, A metering lever or other device, not shown engages one of the teeth  46  to prevent rotation of the thumbwheel  40 . 
     When the filmstrip  24  is wound completely into the film cartridge  20 , a bottom cover-door  48  for the cartridge receiving chamber  18  is disengaged from the main body part  12  to permit the film cartridge to be removed from the cartridge receiving chamber. 
     As shown collectively in FIGS. 1-3, an electronic flash unit  50  on the main body part  12  has a flash circuit board  52  on which is included the following components. 
     (a) A conventional flash charging circuit  54  including a battery  56 , a high-frequency oscillator  58 , a transformer  60  and a rectifier  62 . The high-frequency oscillator  58  converts the dc voltage from the battery  56  to an ac voltage that&#39;s stepped up in the transformer  60 . The rectifier  62  then rectifies the high voltage back to dc. 
     (b) A conventional glass-walled flash tube  64  filled with xenon and small amounts of the gases. 
     (c) First and second conventional energy-storing flash (main) capacitors  66  and  68 , each one having an identical storage capacity which is sufficient to energize the flash tube  64  for the same amount of flash illumination when either one of the first and second capacitors discharges its stored energy through the flash tube. 
     (d) A known-type charge switching circuit  70  having mutually exclusive first and second states to alternatively connect each one of the first and second flash capacitors  66  and  68  to the flash charging circuit  54 , for the flash charging circuit to charge whichever one of the flash capacitors that is connected to the flash charging circuit to its storage capacity. Only one of the first and second flash capacitors  66  and  68  can be connected to the flash charging circuit  54  at a time. The charge switching circuit  70 , when not being used, is biased to a neutral (or third) state intermediate the first and second states in which neither one of the first and second flash capacitors  66  and  68  is connected to the flash charging circuit  54 . 
     (e) Respective known-type charge/discharge monitors  72  and  74  that determine the individual charge-state, i.e. charged or discharged, of the first and second flash capacitors  66  and  68 . 
     (f) A known-type discharge switching circuit  76 , similar to the charge switching circuit  70 , has mutually exclusive first and second states to alternatively connect each one of the first and second flash capacitors  66  and  68  to the flash tube  64 , for whichever one of the flash capacitors that is connected to the flash tube to discharge its stored energy through the flash tube. Only one of the first and second flash capacitors  66  and  68  can be connected to the flash tube  64  at a time. The discharge switching circuit  76 , when not being used, is biased to a neutral (third) state intermediate the first and second states in which neither one of the first and second flash capacitors  66  and  68  is connected to the flash tube  64 . 
     (g) A conventional flash ready light  78 . 
     (h) A known-type biased-open shutter-flash synchronization switch  80 . 
     (i) A known-type biased-open flash charge switch  82  which must be closed to initiate flash charging when the camera  10  has not been used for awhile, i.e. the charge switching and discharge switching circuits  70  and  76  are in their respective neutral states. 
     (j) A known-type controller, such as a microprocessor,  84  for controlling operation of the electronic flash unit  50 . 
     Operation 
     The method of operating the electronic flash unit  50  is shown in FIGS. 4 and 5. 
     Beginning in FIG. 4, when a flash charge button  86 , which is an integral button portion of the front cover part  14 , is manually depressed, it closes the flash charge switch  82 . 
     The controller  84 , sensing closure of the flash charge switch  82 , orders the charge switching circuit  70  to change from its neutral state to its first state in order to connect the first flash capacitor  66  to the flash charging circuit  54 . 
     When the charge/discharge monitor  72  informs the controller  84  that the first flash capacitor  66  is fully charged, the controller orders the charge switching circuit  70  to change from its first state to its second state in order to disconnect the first flash capacitor  66  from the flash charging circuit  54  and alternatively connect the second flash capacitor  68  to the flash charging circuit, and orders the discharge switching circuit  76  to change from its neutral state to its first state in order to connect the first flash capacitor to the flash tube  64 . 
     When the charge/discharge monitor  80  informs the controller  84  that the second flash capacitor  68  is fully charged, the controller  84  turns the flash ready light  78  on. 
     The camera  10  is now ready for flash use. 
     When the shutter release button  38  is manually depressed to initiate momentary shutter-opening, the shutter-flash synchronization switch  80  is briefly closed. The controller  84 , sensing closure of the shutter-flash synchronization switch  80 , orders a conventional trigger circuit (not shown) including a trigger capacitor and an ignition coil to produce a brief burst of voltage applied to the glass wall of the flash tube  64 . The xenon in the flash tube  64  is then ionized, which provides a conductive path for the stored voltage in the first flash capacitor  66 . 
     When the charge/discharge monitor  72  informs the controller  84  that the first flash capacitor  66  has discharged its stored voltage through the flash tube  64 , the controller turns the flash ready light  78  off, orders the charge switching circuit  70  to change from its second state to its first state in order to disconnect the second flash capacitor  68  from the flash charging circuit  54  and alternatively connect the first flash capacitor  66  to the flash charging circuit, and orders the discharge switching circuit  76  to change from its first state to its second state in order to disconnect the first flash capacitor from the flash tube  64  and alternatively connect the second flash capacitor to the flash tube. 
     Then, the controller  84  turns the flash ready light  78  on. 
     If the shutter release button  38  is again depressed to initiate momentary shutter-opening, the shutter-flash synchronization switch  80  is briefly closed. The controller  84 , sensing closure of the shutter-flash synchronization switch  80 , orders the trigger circuit to produce a brief burst of voltage applied to the glass wall of the flash tube  64 . The xenon in the flash tube  64  is then ionized, which provides a conductive path for the stored voltage in the second flash capacitor  68 . 
     When the charge/discharge monitor  74  informs the controller  84  that the second flash capacitor  68  has discharged its stored voltage through the flash tube  64 , the controller turns the flash ready light  78  off. 
     When the charge/discharge monitor  72  informs the controller  84  that the first flash capacitor  66  is fully charged, the controller orders the charge switching circuit  70  to change from its first state to its second state in order to disconnect the first flash capacitor from the flash charging circuit  54  and alternatively connect the second flash capacitor  68  to the flash charging circuit, and orders the discharge switching circuit  76  to change from its second state to its first state in order to change from its second state to its first state in order to disconnect the second flash capacitor from the flash tube  64  and alternatively connect the first flash capacitor to the flash tube. 
     Then, the controller  84  turns the flash ready light  78  on. See ( 2 ) in FIG.  4 . 
     This cycle from ( 2 ) in FIG. 4 to ( 2 ) in FIG. 5 can then be repeated. 
     The invention has been described with reference to a preferred embodiment. However, it will be appreciated that variations and modifications can be effected by a person of ordinary skill in the art without departing from the scope of the invention. For example instead of alternatively switching between the two flash capacitors  66  and  68 , the electronic flash unit  50  could be expanded to alternatively switch between three or more flash reflectors. 
     PARTS LIST 
       10 . camera 
       12 . main body part 
       14 . front cover part 
       16 . rear cover part 
       18 . cartridge receiving chamber 
       20 . film cartridge 
       22 . film supply chamber 
       24 . filmstrip 
       26 . unexposed film roll 
       28 . backframe opening 
       30 . front taking lens 
       32 . optical viewfinder 
       34 . front viewfinder lens 
       36 . rear viewfinder lens 
       38 . shutter release button 
       40 . film winding thumbwheel 
       42 . narrow opening 
       44 . top spool end 
       46 . teeth 
       48 . bottom cover-door 
       50 . electronic flash unit 
       52 . flash circuit board 
       54 . flash charging circuit 
       56 . battery 
       58 . oscillator 
       60 . transformer 
       62 . rectifier 
       64 . flash tube 
       66 . first flash capacitor 
       68 . second flash capacitor 
       70 . charge switching circuit 
       72 . charge/discharge monitor 
       74 . charge/discharge monitor 
       76 . discharge switching circuit 
       78 . flash ready light 
       80 . shutter-flash synchronization switch 
       82 . flash charge switch 
       84 . controller 
       86 . flash charge button