Abstract:
A mobile apparatus is provided with a first charging circuit, and a charging device is provided with a second charging circuit. The charging device charges a secondary buttery of the mobile apparatus through three pairs of terminals, that is, the first input/output terminals of DC voltage, second input/output terminals of a charging voltage, and third input/output terminals connected to ground. With this arrangement, an effective charging operation is executed with minimum loss. Since the secondary buttery is charged only when the mobile apparatus is installed on the charging device and a power switch of the mobile apparatus is turned off, data stored in the mobile apparatus can be protected against destruction and the secondary buttery can be charged in safety.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a charging system for charging a secondary buttery provided in a mobile apparatus.  
       BACKGROUND OF THE INVENTION  
       [0002]     Conventionally, a charging system is used, in which a secondary buttery of a hand-held apparatus is charged by a cradle (charging device) when the hand-held apparatus is installed onto the cradle. As such charging system, there has been proposed two types of configurations. According to one configuration, a charge control circuit is provided in the cradle, and according to another, the charge control circuit is provided in the hand-held apparatus.  
       SUMMARY OF THE INVENTION  
       [0003]     According to one aspect of the invention there is provided a charging system which comprises a first charging circuit which is provided in a mobile apparatus having a secondary buttery and a power-source circuit, and a second charging circuit which is provided in a charging device for supplying power to the first charging circuit, wherein the second charging circuit provided in the charging device comprises a first output terminal for outputting a DC voltage, a second output terminal for outputting a charging voltage for the secondary buttery of the mobile apparatus, a third terminal connected to ground; and a charge-control unit for causing the second terminal to output DC voltage as a charging voltage, and wherein the first charging circuit provided in the mobile apparatus comprises: a first input terminal for receiving the DC voltage when connected with the first output terminal of the second charging circuit, a second input terminal for receiving the charging voltage when connected with the second output terminal of the second charging circuit, a third input terminal to be connected to ground when connected with the third output terminal of the second charging circuit, a switching control unit for switching a power supply source to the power-source circuit of the mobile apparatus from the secondary buttery to the first input terminal to apply the charging voltage received by the second input terminal to the secondary buttery, when the mobile apparatus is installed on the charging device, a detecting unit for detecting whether or not the mobile apparatus has been installed on the charging device, a power-source control unit for generating and applying a power-source control signal to the power-source circuit of the mobile apparatus based on the resultant of detection by the detecting unit, a charge-control signal generating unit for generating a charge-control signal based on the resultant detection by the detecting unit and the power-source control signal generated by the power-source control unit, and a conduction switching unit for making conduction between the second input terminal and the secondary buttery of the mobile apparatus based on the charge-control signal generated by the charge-control signal generating unit. Further, according to another aspect of the invention, there is provided a charging system which comprises a first charging circuit which is provided in a mobile apparatus having a secondary buttery and a power-source circuit, and a second charging circuit which is provided in a charging device for supplying power to the first charging circuit, wherein the second charging circuit provided in the charging device comprises a first output terminal for outputting a DC voltage; a second output terminal for outputting a charging voltage for the secondary buttery of the mobile apparatus, a third terminal connected to ground, and a charge-control unit for causing the second terminal to output DC voltage as a charging voltage, and wherein the first charging circuit provided in the mobile apparatus comprises a first input terminal for receiving the DC voltage when connected with the first output terminal of the second charging circuit, a second input terminal for receiving the charging voltage when connected with the second output terminal of the second charging circuit, a third input terminal to be connected to ground when connected with the third output terminal of the second charging circuit, a switching control unit for switching a power supply source to the power-source circuit of the mobile apparatus from the secondary buttery to the first input terminal to apply the charging voltage received by the second input terminal to the secondary buttery, when the mobile apparatus is installed on the charging device, a detecting unit for detecting whether or not the mobile apparatus has been installed on the charging device, a power-source control unit for generating and applying a power-source control signal to the power-source circuit of the mobile apparatus based on the resultant of detection by the detecting unit, and for generating a charge-control signal based on on/off state of the power of the mobile apparatus, and a conduction switching unit for making conduction between the second input terminal and the secondary buttery of the mobile apparatus based on the charge-control signal generated by the charge-control signal generating unit. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]      FIG. 1  is a view illustrating a structure of a charging system  1000  for a camera (camera charging system) according to a first embodiment of the present invention.  
         [0005]      FIG. 2  is a block diagram showing a configuration of a digital camera  110 .  
         [0006]      FIG. 3  is a view showing a circuit configuration of a charging circuit  20  in a charging device.  
         [0007]      FIG. 4  is a view showing a circuit configuration of a charging circuit  10  of an apparatus.  
         [0008]      FIG. 5  is a view showing a circuit configuration of a charging circuit  30  of an apparatus.  
         [0009]      FIG. 6  is a view showing a circuit configuration of a charging circuit  40  of an apparatus.  
         [0010]      FIG. 7  is a view showing a circuit configuration of a charging circuit  50  of an apparatus.  
         [0011]      FIG. 8  is a view showing a circuit configuration of a charging circuit  60  of an apparatus.  
         [0012]      FIG. 9  is a view showing a circuit configuration of a charging circuit  70  of an apparatus. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]     Now, preferred embodiments of a charging system for a camera according to the present invention will be described in detail with reference to the accompanying drawings.  
         [0014]      FIGS. 1 through 3  are views showing the charging system for a camera (camera charging system) used commonly in the embodiments of the invention.  
         [0015]      FIG. 1  is a view showing a structure of a camera charging system  1000  according to the embodiment of the present invention.  FIG. 2  is a block diagram showing a configuration of a digital camera  110 .  FIG. 3  is a view showing a charging circuit  20  in a charging device.  
         [0016]     Each embodiment of the present invention comprises a modified circuit of the charging circuit of an apparatus (shown in  FIG. 2 ) in the camera charging system.  
       First Embodiment  
       [0017]     The first embodiment of the present invention will be described with reference to  FIGS. 1 through 4 .  FIGS. 1 through 3  are views showing the configuration of the camera charging system.  FIG. 4  is a view showing a circuit configuration of the charging circuit  10  of an apparatus.  
         [0018]     As shown in  FIG. 1 , the camera charging system  1000  according to the embodiment of the present invention, which serves as a charging system, comprises the digital camera  110  as a mobile apparatus and a charging device (cradle)  200  for charging the secondary buttery of the digital camera  110 .  
         [0019]     In the present embodiment of the invention, the digital (still) camera  110  is described as the mobile device. But the mobile device is not limited to the digital camera  100 , but any device which uses at least a secondary battery and can be charged by a charging device may be used as the mobile device in the embodiments. A digital movie camera, PDA (Personal Digital Assistant), a cellular phone, PHS (Personal Handy-phone System), HT (Handy Terminal), a portable audio/video player/recorder (MD, cassette, CD, DVD (Digital Versatile Disk), a large capacity recording medium using a blue laser, a semiconductor memory), a portable game machine, a portable HDD (hard disk drive), a car navigation apparatus and the like are used as the mobile devices in the present embodiment. This is the same for other embodiments.  
         [0020]     As shown in  FIG. 2 , the digital camera  110  comprises an image pick-up section  1  including CCD (Charge Coupled Device), a display section  2  such as LCD (Liquid Crystal Display) and an organic EL (Electro Luminescent) display, for displaying various screens, an operation section  3  which is operated by a user, and a control section  4  such as a main micro computer for controlling the sections in the camera  110 . Further, digital camera  110  has a power source circuit  101  for supplying power to the image pick-up section, display section, operation section and controlling section, and the charging circuit  10  of an apparatus, serving as the first charging circuit.  
         [0021]     As shown in  FIG. 4 , the charging circuit  10  of an apparatus has three terminals P 1 , P 2 , and P 3  serving as the first, second and third input terminal, and a switching control section including resistors  11  and  12 , a diode  13 , and FET (Field Effect Transistor)  14 , and a secondary battery  15 .  
         [0022]     A series connection of the resistors  11  and  12  is connected between the terminal P 1  and ground (GND). It is assumed that the resistors  11  and  12  have resistances of R 1  and R 2 , respectively and that it is true that the resistance R 2  is sufficiently higher than the resistance R 1 . The diode  13  is connected between the terminal P 1  and the power source circuit  101  in the forward direction from the terminal P 1  to the power source circuit  101 . The secondary battery  15  is connected between the terminal P 2  and GND. It is assumed that a voltage drop across the diode  13  is Vf 1 .  
         [0023]     FET  14  is of a P-channel type. FET  14  has a gate electrode connected to a connection point of the resistors  11  and  12 , a drain electrode connected to the terminal P 2 , and a source electrode connected to the power source circuit  101 . The FET  14  includes a parasitic diode  141  connected in the forward direction from the terminal P 2  to the power source circuit  101 . It is assumed that a voltage drop across the parasitic diode  141  is Vf 2 . The terminal P 3  is connected to GND.  
         [0024]     The charging device  200  is provided with a charging circuit  20  in a charging device, which serves as the second charging circuit. As shown in  FIG. 3 , the charging circuit  20  in a charging device comprises the first, second and third output terminals Q 1 , Q 2  and Q 3 , which are connected to the terminals P 1 , P 2 , and P 3 , respectively, a charge control section including a charge control IC  21 , diode  22 , transistor  23  and resistor  24 , and a thermistor  25  serving as a temperature detecting section, and LEDs  26 ,  27  (Light Emitting Diode) serving as a notifying section. There is also provided a DC power source (for example, combination of the commercial AC power source and AD converter for converting a power supply from the AC power source, and AC adopter).  
         [0025]     The diode  22  is provided between DC power source and the terminal Q 1  in the forward direction from DC power source to the terminal Q 1 . The transistor  23  has the emitter connected to the DC power source, the collector connected to the terminal Q 2  through the resistor  24 , and the base connected to the charge control IC  21 . The both ends of the resistor  24  are connected to the charge control IC  21 .  
         [0026]     The thermistor  25  detects a temperature of the charging device  200  and the digital camera  110 . The terminal Q 3  is connected to GND. LEDs  26 ,  27  are connected in parallel between the charge control IC  21  and GND. The charge control IC  21  reads a current value flowing through the resistor  24  and receives a temperature signal for the thermistor  25 , and on-off controls a base voltage of the transistor  23  in accordance with the voltage drop across the resistor  24  and the temperature signal to adjust a current (charging current) supplied from DC power source to the terminal Q 2 . The charge control IC  21  can control the LEDs  26  and  27  separately to turn on the same.  
         [0027]     Now, a charging operation by the camera charging system  1000  will be described. The charging operation performed while the digital camera  110  is installed in the charging device  200  (first charging operation to be performed while camera is installed on charging device) and the charging operation performed while the digital camera  110  is uninstalled from the charging device  200  (first charging operation to be performed while camera is uninstalled from charging device) will be described separately.  
         [0028]     First Charging Operation to be performed While Camera is Installed on Charging Device:  
         [0029]     When the digital camera  110  is installed on the charging device  200 , the terminals P 1 , P 2  and P 3  in  FIG. 4  are connected with the terminals Q 1 , Q 2  and Q 3  in  FIG. 3 , respectively, and then a DC voltage is applied to the terminal P 1  through the diode  22  and the terminal Q 1 .  
         [0030]     A voltage equivalent to DC voltage reduced by the voltage drop Vf 1  across the diode  13  is applied to the source electrode of FET  14 . Since it is true that the resistance R 1  is sufficiently lower than the resistance R 2 , the source voltage becomes lower than the gate voltage, bringing FET  14  to an off state. Since FET  14  is made off, the terminal P 2  is connected to the secondary battery  15 .  
         [0031]     At this time, since DC voltage is applied to the power source circuit  101  through the diode  13  in the charging circuit  10  of an apparatus, the power source is continuously supplied to the digital camera  110  without power interruption, even though the digital camera is installed on the charging device  200  while the digital camera is in operation. Further, the digital camera can be made on or off, while the digital camera  110  is installed on the charging device  200 .  
         [0032]     The terminal P 2  is connected to the positive terminal of the secondary buttery  15  in the digital camera  110 , and the charge control IC  21  makes the transistor  23  on, and current of charging (charging current) is monitored by measuring the voltage drop across the resistor  24 , and is supplied to the secondary buttery  15  through the terminal Q 2 . At this time, the charge control IC  21  turns on LED  26 , informing the user that the charging operation of the secondary butter  15  of the digital camera  11  has started. The charge control IC  21  can change the base voltage of FET  14  anytime.  
         [0033]     When the charging current under monitor decreases to a value not more than a preset value, the charge control IC  21  determines that the secondary buttery  15  has been charged to the full, and turns on LED  27 , informing that the secondary buttery  15  has been charged to the full. When the thermistor  25  detects an abnormal temperature, the charge control IC  21  makes the transistor  23  off, ceasing the charging operation.  
         [0034]     First Charging Operation to be Performed While Camera is Uninstalled from Charging Device:  
         [0035]     Operation to be performed while the digital camera  110  is uninstalled from the charging device  200  will be described. When the digital camera  110  is uninstalled from the charging device  200 , a voltage equivalent to a supply voltage of the secondary buttery  15  reduced by a voltage drop Vf 2  across the parasitic diode  141  is applied to the power source circuit  101 . Since the terminal P 1  is open at this time, and the gate electrode of FET  14  is grounded via the resistor  12  ( FIG. 4 ), FET  14  goes on. When FET  14  is turned on, the power source voltage of the secondary battery  15  of the digital camera  110  is supplied to the power source circuit  101 .  
         [0036]     In the present embodiment, an arrangement using the thermistor  25  has been described, but the temperature detecting unit is not limited to the thermometer and another temperature detecting unit such as a platinum thermometer (platinum temperature resistor) may be used in place of the thermistor  25 . Further, two LEDs  26 ,  27  are used in the embodiment to inform the charging state, but an audio outputting unit may be used to inform the charging state. Single or not less than three units of informing unit for informing the charging state may be used. These are the same for other embodiments to be described hereafter.  
       Second Embodiment  
       [0037]     A second embodiment of the invention will be described with reference to  FIG. 5 .  FIG. 5  is a view illustrating a circuit configuration of a charging circuit  30  of an apparatus.  
         [0038]     A camera charging system according to the present second embodiment of the invention comprises a digital camera  120  and the same charging device as the charging device  200  of the first embodiment. The digital camera  120  (not shown) has the substantially same configuration as the digital camera  110  in the first embodiment, and comprises an image pick-up unit, display unit, operation unit, control unit and the same power source circuit as the power source circuit  101  of the first embodiment. The digital camera  120  further comprises the charging circuit  30  of an apparatus shown in  FIG. 5 , in place of the charging circuit  10  of an apparatus (shown in  FIG. 4 ) in the first embodiment.  
         [0039]     As shown in  FIG. 5 , the charging circuit  30  of an apparatus comprises a detecting unit including a digital transistor  31 , resistor  32  and a micro-computer power source regulator  33 , and a power source control unit including a subsidiary micro-computer  34  in addition to the elements of the charging circuit  10  of an apparatus.  
         [0040]     The base electrode of the digital transistor  31  is connected through a resister  311  to the connecting point between the resistor  11  and the gate electrode of FET  14 . The emitter electrode of the digital transistor  31  is connected to GND and also to the base electrode through a resistor  312 . The collector electrode of the digital transistor  31  is connected to the subsidiary micro-computer  34  and to the micro-computer power source regulator  33  through a resistor  32 . It is assumed that the collector of the digital transistor is supplied with a signal CHGINB, which is previously pulled up to a high side by the subsidiary micro-computer  34 .  
         [0041]     The micro-computer power source regulator  33  serves to adjust the power source voltage to be supplied to the subsidiary micro-computer  34 . The micro-computer power source regulator  33  is connected to a connecting point between the source electrode of FET  14  and the diode  13  to be supplied with the power source voltage, and adjusts the supplied power source voltage to apply the adjusted voltage to the subsidiary micro-computer  34 .  
         [0042]     The subsidiary micro-computer  34  is grounded and outputs a power-source control signal (signal PWCTL) to the power source circuit  101 . The power-source control signal (signal PWCTL) is used in the power source circuit  101  to control the power source.  
         [0043]     The charging operation in the camera charging system of the present embodiment will be described similarly to in the first embodiment, that is, the charging operation performed while the digital camera  120  is installed in the charging device  200  (second charging operation to be performed while camera is installed on charging device) and the charging operation performed while the digital camera  120  is uninstalled from the charging device  200  (second charging operation to be performed while camera is uninstalled from charging device) will be described separately.  
         [0044]     Second Charging Operation to be Performed While Camera is Installed on Charging Device:  
         [0045]     When the digital camera  120  is installed on the charging device  200 , FET  14  in the charging circuit  30  of an apparatus ( FIG. 5 ) is turned off and a voltage equivalent to a DC voltage at the terminal P 1  reduced by a voltage drop Vf 1  across the diode  13  is applied to the power source circuit  101  in the same way as in the first charging operation performed while installed on charging device. Even though the digital camera  120  in operation is installed on the charging device  200 , the voltage equivalent to a DC voltage at the terminal P 1  reduced by a voltage drop Vf 1  is continuously supplied to the power source circuit  101  without power interruption.  
         [0046]     Simultaneously, since DC voltage at the terminal P 1  is applied to the base electrode of the digital transistor  31  through the resistors  11  and  311 , the digital transistor  31  goes on and the collector voltage (signal, CHGINB) to the digital transistor  31  is changed for a high level to a low level. The signal, CHGINB serves as a detection signal of detecting connection between the digital camera  120  and the charging device  200 .  
         [0047]     Receiving DC voltage supplied from the terminal P 1  through the micro-computer power source regulator  33 , the subsidiary micro-computer  34  works at all times. The subsidiary micro-computer  34  monitors the signal CHGINB and makes a power-source control signal PWCTL off when the digital camera is connected to the charging device  200 . When the power-source control signal PWCTL is made off, the power source of the digital camera  120  is turned off, whereby the display unit, image pick-up unit and control unit connected to the power source circuit  101  are made off.  
         [0048]     The terminal P 2  connected to the positive terminal of the secondary buttery  15  and a charging operation of the secondary buttery  15  starts as in the same manner as in the first charging operation performed while installed in the charging device. During the charging operation, the subsidiary micro-computer  34  monitors the signal CHGINB at all times even though the power source of the digital camera  120  is turned off. Therefore, even when the power switch of the digital camera is turned on while the digital camera  120  is charged by the charging device  200 , the digital camera  120  is not made active. The above mentioned arrangement allows a charging current to be supplied to the secondary charging buttery  15  in a normal state.  
         [0049]     In the same manner as in the first charging operation while installed on the device, the secondary buttery  15  is charged from the terminal P 2 , while the charging control IC  21  monitors the full charge of the secondary buttery  15  and an abnormal temperature detected by the thermister  25 .  
         [0050]     Second Charging Operation to be Performed While Camera is Uninstalled from Charging Device:  
         [0051]     Charging operation to be performed while the digital camera  120  is uninstalled from the charging device  200  will be described. When the digital camera  120  is uninstalled from the charging device  200 , a voltage equivalent to the supply voltage of the secondary buttery  15  reduced by a voltage drop Vf 2  across the parasitic diode  141  is supplied to the power source circuit  101 , and FET  14  is turned on.  
         [0052]     Simultaneously, the base electrode of the digital transistor  31  is connected to ground, turning the digital transistor  31  off. When the digital transistor  31  is turned off, the signal CHGINB changes from a low level to a high level, whereby the subsidiary micro-computer  34  determines that the digital camera  120  is uninstalled from the charging device  200 .  
         [0053]     A power source voltage of the secondary buttery  15  of the digital camera  120  is applied to the power source circuit  101  in the same manner as in the first charging operation performed while uninstalled from the charging device. The digital camera  120  returns to its normal state, and is normally activated by turning on the power source by a user.  
         [0054]     In the second embodiment, when the signal PWCTL is made off, the control unit, image pick-up unit, display unit connected to the power source circuit  101  are also turned off. The arrangement is not limited to the mentioned above, but may be modified such that, for example, when the signal PWCTL is made off, a part of the display unit and operation unit is kept in operation. This will be also applied to a fifth and seventh embodiment.  
         [0055]     In the present second embodiment of the invention, an arrangement may be changed such that the subsidiary micro-computer  34  is connected to the power switch of the operation unit, and the signal PWCTL is made on or off in response to on/off operation of the power switch. This arrangement may be applied to the fifth and seventh embodiment.  
       Third Embodiment  
       [0056]     A third embodiment of the present invention will be described with reference to  FIG. 6 .  FIG. 6  is a view illustrating a circuit configuration of a charging circuit  40  of an apparatus. A camera charging system according to the present third embodiment of the invention comprises a digital camera  130  and the same charging device as the charging device  200  of the first embodiment. The digital camera  130  (not shown) has the substantially same configuration as the digital camera  110  of the first embodiment, and comprises an image pick-up unit, display unit, operation unit, control unit and the same power source circuit as the power source circuit  101  of the first embodiment. The digital camera  130  further comprises the charging circuit  40  of an apparatus shown in  FIG. 6 , in place of the charging circuit  10  of an apparatus (shown in  FIG. 4 ) in the first embodiment. Further, the digital camera  130  comprises the charging circuit  40  of an apparatus shown in  FIG. 6  in place of the charging circuit  10  (shown in Fig. of an apparatus in the first embodiment.  
         [0057]     Since the terminals P 1 , P 2  and P 3  are exposed on an external surface of the digital camera  130 , there is a fear that they can be shunted due to some causes. For this reason, the charging circuit  40  of an apparatus has a function of preventing a short circuit between the terminals P 1  and P 2  or a short circuit in the secondary buttery  15 . The charging circuit  40  of an apparatus further comprises a diode  41  serving as a rectifier. The diode  41  is connected between the terminal P 2  and the secondary battery  15  in a forward direction from the terminal P 2  to the secondary battery  15 .  
         [0058]     Similarly to in the first embodiment, a third charging operation performed in the charging system camera while the digital camera is installed on the charging device and a third charging operation while the digital camera is uninstalled from the charging device will be described separately.  
         [0059]     Third Charging Operation to be Performed While the Camera is Installed on Charging Device:  
         [0060]     When the digital camera  130  is installed on the charging device  200 , FET  14  is turned off in the charging circuit  40  of an apparatus, and the power source circuit  101  is supplied with a voltage equivalent to DC voltage at the terminal P 1  reduced by a voltage drop Vf 1  across the diode  13  in the same way as in the first charging operation performed while the camera is installed on the charging device. In this case, even if the digital camera  130  in operation is installed on the charging device  200 , a voltage equivalent to DC voltage reduced by the voltage drop Vf 1  is applied to the power source circuit  101  without power interruption.  
         [0061]     The terminal P 2  is connected to the positive terminal of the secondary buttery  15 , and charging operation starts, charging the secondary buttery  15  from the terminal P 2  through the diode  41  in the same manner as in the first charging operation performed while the camera is installed on the charging device. Further, in the same manner as in the first charging operation performed while installed on the device, the secondary buttery  15  is charged from the terminal P 2 , while the charging control IC  21  monitors the full charge of the secondary buttery  15  and an abnormal temperature detected by the thermister  25 .  
         [0062]     Third Charging Operation to be Performed While Camera is Uninstalled from the Charging Device:  
         [0063]     Charging operation to be performed while the digital camera  130  is uninstalled from the charging device  200  will be described. When the digital camera  130  is removed from the charging device  200 , a voltage equivalent to the supply voltage of the secondary buttery  15  reduced by a voltage drop Vf 2  across the parasitic diode  141  is applied to the power source circuit  101 , and FET  14  goes on in the same way as in the first charging operation performed while the camera is installed on the charging device.  
         [0064]     A voltage of the secondary buttery  15  in the digital camera  130  is supplied to the power source circuit  101  in the same manner as in the first charging operation performed while the camera is installed on the charging device. The diode  41  prevents a voltage of the secondary buttery  15  from appearing at the terminal P 2 .  
       Fourth Embodiment  
       [0065]     A fourth embodiment of the present invention will be described with reference to  FIG. 6 .  FIG. 6  is a view showing a circuit configuration of a charging circuit  50  of an apparatus.  
         [0066]     A camera charging system according to the present fourth embodiment of the invention comprises a digital camera  140  and the same charging device as the charging device  200  of the first embodiment. The digital camera  140  (not shown) has the substantially same configuration as the digital camera  110  of the first embodiment, and comprises an image pick-up unit, display unit, operation unit, control unit and the same power source circuit as the power source circuit  101  of the first embodiment. The digital camera  140  further comprises the charging circuit  50  of an apparatus shown in  FIG. 5 , in place of the charging circuit  10  of an apparatus (shown in  FIG. 4 ) in the first embodiment.  
         [0067]     In the charging circuit  40  according to the third embodiment of the invention, there is a fear that a voltage at the terminal P 2  becomes a little low to charge the secondary buttery  15  due to the voltage drop Vf 3  across the diode  41 . The charging circuit  50  of an apparatus has a function of preventing the charging voltage at the terminal P 2  from going low. The charging circuit  50  of an apparatus comprises a switcher or FET  51 , digital transistor  52  and resistor  53  in addition to the elements of the charging circuit  10 .  
         [0068]     FET  51  is a P-channel type transistor. The gate electrode of the FET  51  is connected to the positive terminal of the secondary buttery  15 , and the drain electrode is connected to the terminal P 2  and the source electrode to the positive terminal of the secondary buttery  15 . FET  15  includes a parasitic diode  511  connected in a forward direction from the terminal P 2  to the positive terminal of the secondary buttery  15 .  
         [0069]     The digital transistor  52  has the base electrode connected to the terminal P 1  through a resistor  521  and the collector electrode connected to the gate electrode of FET  51 . The emitter electrode of the digital transistor  52  is connected to grounded and also to the base electrode through a resistor  522 .  
         [0070]     Similarly to in the first embodiment, a fourth charging operation performed in the camera charging system  1000  while the digital camera is installed on the charging device and a fourth charging operation performed while the digital camera is uninstalled from the charging device will be described separately.  
         [0071]     Fourth Charging Operation to be Performed While the Camera is Installed on the Charging Device:  
         [0072]     When the digital camera  140  is installed on the charging device  200 , FET  14  is turned off in the charging circuit  50  of an apparatus, and the power source circuit  101  is supplied with a voltage equivalent to DC voltage at the terminal P 1  reduced by a voltage drop Vf 1  across the diode  13  in the same way as in the first charging operation performed while the camera is installed on the charging device.  
         [0073]     When DC voltage at the terminal P 1  is applied to the base electrode of the digital transistor  52  through the resistor  521 , the digital transistor  52  goes on. When the digital transistor  52  is turned on, the gate voltage of FET  51  is grounded and goes low, turning FET  51  on. At this time, even though the digital camera  140  in operation is installed on the charging device  200 , a voltage equivalent to DC voltage reduced by the voltage drop Vf 1  is applied to the power source circuit  101  without power interruption in the same manner as in the first charging operation performed while the camera is installed one the charging device.  
         [0074]     Since the terminal P 2  is connected to the positive terminal of the secondary buttery  15  and FET  51  is turned on, charging operation starts, charging the secondary buttery  15  from the terminal P 2  through FET  51 , in the same manner as in the first charging operation performed while the camera is installed on the charging device. The secondary buttery  15  is charged from the terminal P 2  while the charge control IC  21  monitors the full charge of the secondary buttery  15  and an abnormal temperature detected by the thermister  25 , as in the same manner as in the first charging operation performed while the camera is installed on the charging device.  
         [0075]     Fourth Charging Operation to be Performed While the Camera is Uninstalled from the Charging Device:  
         [0076]     Operation to be performed while the digital camera  140  is uninstalled from the charging device  200  will be described. When the digital camera  140  is removed from the charging device  200 , a voltage equivalent to the supply voltage of the secondary buttery  15  reduced by the voltage drop Vf 2  across the parasitic diode  141  is applied to the power source circuit  101 , in the same manner as in the first charging operation performed while the camera is uninstalled from the charging device, and FET  14  goes on.  
         [0077]     Since the terminal P 1  is in an open state and no DC voltage is applied to the base electrode of the digital transistor  52 , the digital transistor  52  remains off. Further, since the gate and source voltage are kept equivalent, FET  51  remains off. As a result, the power source voltage is applied to the power source circuit  101  from the secondary battery  15  of the digital camera  140  in the same manner as in the first charging operation performed while the camera is uninstalled from the charging device. But since FET  51  remains off, the buttery voltage of the secondary butter  15  does not appear at the terminal P 2 .  
       Fifth Embodiment  
       [0078]     A fifth embodiment of the present invention will be described with reference to  FIG. 8 .  FIG. 8  is a view showing a circuit configuration of a charging circuit  60  of an apparatus.  
         [0079]     A camera charging system according to the present fifth embodiment of the invention comprises a digital camera  150  and the same charging device as the charging device  200  of the first embodiment. The digital camera  150  (not shown) has the substantially same configuration as the digital camera  110  of the first embodiment, and comprises an image pick-up unit, display unit, operation unit, control unit and the power source circuit  101  of the first embodiment. The digital camera  150  further comprises the charging circuit  30  of an apparatus shown in  FIG. 5 , in place of the charging circuit  10  of an apparatus (shown in  FIG. 4 ) in the first embodiment. Further, the digital camera  150  comprises the charging circuit  60  of an apparatus shown in  FIG. 8  in place of the charging circuit  10  of an apparatus (shown in  FIG. 4 ) in the first embodiment. As shown in  FIG. 8 , the charging circuit  60  of an apparatus comprises a digital transistor  31 , a resistor  32 , the micro-computer power source regulator  33 , the subsidiary micro-computer  34 , FET  51 , a digital transistor  52 , NOR circuit  61  serving as a charge-control signal generator, and a resistor  61  in addition to the elements of the charging circuit  10  of an apparatus.  
         [0080]     The transistor  31 , resistor  32 , the micro-computer power source regulator  33 , and the subsidiary micro-computer  34  are connected in the same way as in the charging circuit  30  of an apparatus of the second embodiment. The subsidiary micro-computer  34  has an output terminal (output terminal of a power-sour control signal PWCTL) which is connected to ground through the resistor  62 , and further to the power source circuit  101  and one terminal of NOR circuit  61 . The collector electrode (output terminal of a signal CHGINB) of the digital transistor  31  is connected to the other terminal of NOR circuit  61 .  
         [0081]     FET  51 , the digital transistor  52 , and resistor  53  are connected in the same manner as in the charging circuit  50  of an apparatus in the fourth embodiment. But the base electrode of the digital transistor  52  is connected to an output terminal of NOR circuit  61  through a resistor  521 . A signal CHGCTL is output from the NOR circuit  61 .  
         [0082]     Similarly to in the first embodiment, a fifth charging operation performed in the camera charging system while the digital camera is installed on the charging device and a fifth charging operation performed while the digital camera is uninstalled from the charging device.  
         [0083]     Fifth Charging Operation Performed While Camera is Installed on Charging Device:  
         [0084]     When the digital camera  150  is installed on the charging device  200 , FET  14  is turned off in the charging circuit  60  of an apparatus, and the power source circuit  101  is supplied with a voltage equivalent to DC voltage at the terminal P 1  reduced by a voltage drop Vf 1  across the diode  13  in the same way as in the first charging operation performed while the camera is installed on the charging device.  
         [0085]     Simultaneously, the digital transistor  31  is turned on and the collector voltage (signal CHGINB) of the transistor  31  goes to a low level form a high level. Mnitoring signal CHGINB, the subsidiary micro-computer  34  outputs a signal PWCTL of a low level in response to installation of the camera on the charging device  200 .  
         [0086]     Receiving the signal CHGINB of a low level and signal PWCTL of a low level at its input terminals, NOR circuit  61  outputs a signal CHGCTL of a high level, which is input to the base electrode of the digital transistor  52 . Then, the transistor  2  is turned on, bringing FET  51  on in the same manner as in the fourth operation performed while the camera is installed on the charging device. At this time, even if the digital camera  150  in operation is installed on the charging device  200 , a voltage equivalent to DC voltage reduced by the voltage drop Vf 1  is applied to the power source circuit  101  without power interruption in the same manner as in the first operation performed while the camera is installed on the charging device. The terminal P 2  is connected to the positive terminal of the secondary buttery  15 , and since the FET  51  is turned on, charging operation starts for charging the secondary buttery  15  from the terminal P 2  through FET  51 , in the same manner as in the first charging operation performed while the camera is installed on the charging device. Further, in the same manner as in the first charging operation performed while installed on the device, the secondary buttery  15  is charged from the terminal P 2 , while the charging control IC  21  monitors the full charge of the secondary buttery  15  and an abnormal temperature detected by the thermister  25 .  
         [0087]     NOR circuit  61  outputs a signal CHGCTL of a high level to carge the secondary buttery  15 , only when the signals CHGINB and PWCTL are low, that is, when the digital camera  150  is installed on the charging device  200  and when the power of the digital camera is turned off.  
         [0088]     Fifth Charging Operation to be Performed While Camera is Uninstalled from Charging Device:  
         [0089]     Operation to be performed while the digital camera  150  is uninstalled from the charging device  200  will be described. When the digital camera  150  is removed from the charging device  200 , a voltage equivalent to the supply voltage of the secondary buttery  15  reduced by the voltage drop Vf 2  across the parasitic diode  141  is applied to the power source circuit  101 , and FET  14  goes on, in the same manner as in the first charging operation performed while the camera is uninstalled from the charging device.  
         [0090]     Simultaneously, the digital transistor  31  is turned off, and the signal CHGINB changes from a low level to a high level, whereby the subsidiary micro-computer  34  determines that the digital camera  150  has been uninstalled from the charging device  200  in the same manner as in the operation performed while the camera is uninstalled from the charging device. Then, the output signal CHGCTL of NOR circuit  61  goes low, bringing the base voltage of the digital transistor  52  to a low level, turning the transistor  52  off, and also turning FET  51  off, in the same manner as in the fourth operation performed while the camera is uninstalled from the charging device.  
         [0091]     As a result, the power source voltage is applied to the power source circuit  101  from the secondary battery  15  of the digital camera  150  in the same manner as in the first charging operation performed while the camera is uninstalled from the charging device. But since FET  51  remains off, the buttery voltage of the secondary butter  15  does not appear at the terminal P 2 .  
       Sixth Embodiment  
       [0092]     A sixth embodiment of the present invention will be described with reference to  FIG. 9 .  FIG. 9  is a view showing a circuit configuration of a charging circuit  70  of an apparatus.  
         [0093]     A camera charging system according to the present sixth embodiment of the invention comprises a digital camera  160  and the same charging device as the charging device  200  of the first embodiment. The digital camera  160  (not shown) has the substantially same configuration as the digital camera  110  of the first embodiment, and comprises an image pick-up unit, display unit, operation unit, control unit and the same power source as the power source circuit  101  of the first embodiment. The digital camera  160  further comprises the charging circuit  70  of an apparatus shown in  FIG. 9 , in place of the charging circuit  10  of an apparatus (shown in  FIG. 4 ) in the first embodiment.  
         [0094]     As shown in  FIG. 9 , the charging circuit  70  of an apparatus comprises a digital transistor  31 , a resistor  32 , the micro-computer power source regulator  33 , the subsidiary micro-computer  34 A, FET  51 , a digital transistor  52 , and resistors  53  and  62  in addition to the elements of the charging circuit  10  of an apparatus.  
         [0095]     The digital transistor  31 , resistor  32 , micro-computer power source regulator  33 , and subsidiary micro-computer  34 A are connected in the same way as the digital transistor  31 , resistor  32 , micro-computer power source regulator  33 , and subsidiary micro-computer  34  included in the charging circuit  30  of an apparatus of the second embodiment. The FET  51 , digital transistor  52 , and resistors  53  are connected in the same manner as those in the charging circuit  50  in an apparatus of the fourth embodiment. The output terminal (power source control signal PWCTL) of the subsidiary micro-computer  34 A is connected to ground through the resistor  62  and also to the power source circuit  101 . The subsidiary micro-computer  34 A outputs a voltage signal CHGCTL to the base electrode of the digital transistor  52  through a resistor  521  for controlling timing of charging of the secondary buttery  15 . The subsidiary micro-computer  34 A is also connected to the power switch of the operation unit.  
         [0096]     Sixth operation performed while the camera is installed on the charging device in the present camera charging system, and sixth operation performed while the camera is uninstalled from the charging device will be described separately in the same manner as in the first embodiment.  
         [0097]     Sixth Charging Operation to be Performed While Camera is Installed on Charging Device:  
         [0098]     When the digital camera  160  is installed on the charging device  200 , FET  14  is turned off in the charging circuit  70  of an apparatus, and the power source circuit  101  is supplied with a voltage equivalent to DC voltage at the terminal P 1  reduced by a voltage drop Vf 1  across the diode  13  in the same way as in the first charging operation performed while the camera is installed on the charging device.  
         [0099]     Simultaneously, the digital transistor  31  is turned on and the collector voltage (signal CHGINB) of the transistor  31  goes to a low level form a high level. The subsidiary micro-computer  34 A monitors the signal CHGINB. When it is determined that the signal CHGINB is low and the power source of the digital camera  160  is on, bringing the camera  160  in operation, the subsidiary micro-computer  34 A outputs the signal PWCTL of a low level, whereby the digital transistor  52  is not turned on, and also FET  51  is not turned on. As a result, the secondary buttery  15  is not charged.  
         [0100]     When the signal CHGINB is low and the power is turned off, bringing the digital camera  160  out of operation, the subsidiary micro-computer  34 A outputs the power-source control signal PWCTL of a low level, and thereafter outputs the signal CHGCTL of a high level. When the signal CHGCTL of a high level is input to the base electrode of the digital transistor  52 , the digital transistor  52  is turned on, making FET  51  on in the same manner as in the fourth charging operation while the camera is installed on the charging device. At this time, even if the digital camera  160  in operation is installed on the charging device  200 , a voltage equivalent to DC voltage reduced by voltage drop Vf 1  is applied to the power source circuit  101  without power interruption in the same manner as in the first charging operation performed while the camera is installed on the charging device.  
         [0101]     Since the terminal P 2  is connected to the positive terminal of the secondary buttery  15  and FET  51  is turned on, the charging operation starts to charge the secondary buttery  15  from the terminal P 2  through FET  51 , in the same manner as in the first charging operation performed while the camera is installed on the charging device. The secondary buttery  15  is charged from the terminal P 2  while the charge control IC  21  monitors the full charge of the secondary buttery  15  and an abnormal temperature detected by the thermister  25 , as in the same manner as in the first charging operation performed while the camera is installed on the charging device.  
         [0102]     When the user turns on the power switch of the digital camera  160  while the camera is installed on the charging device  200 , the subsidiary micro-computer  34 A detects that the power switch of the digital camera  160  has been turned on, and makes the signal CHGCTL low, ceasing the charging operation of the secondary buttery  15 . Further, the subsidiary micro-computer  34 A makes the signal PWCTL high, allowing the user to turn on the digital camera  160 .  
         [0103]     Sixth Charging Operation to be Performed While the Camera is Uninstalled from the Charging Device:  
         [0104]     Operation to be performed while the digital camera  160  is uninstalled from the charging device  200  will be described. When the digital camera  160  is removed from the charging device  200 , a voltage equivalent to the supply voltage of the secondary buttery  15  reduced by the voltage drop Vf 2  across the parasitic diode  141  is applied to the power source circuit  101 , in the same manner as in the first charging operation performed while the camera is uninstalled from the charging device, and FET  14  goes on.  
         [0105]     Simultaneously, the base electrode of the digital transistor  31  is connected to ground, and the digital transistor  31  is turned off. When the digital transistor  31  is turned off, the signal CHGINB changes from a low level to a high level, whereby the subsidiary micro-computer  34 A monitors or judges that the digital camera  160  has been uninstalled from the charging device  200 . Then, the subsidiary micro-computer  34 A makes the base voltage of the digital transistor  52  low, turning the same transistor  52  off, whereby FET  51  is turned off in the same manner as in the fourth operation performed while the camera is uninstalled from the charging device.  
         [0106]     As a result, the power source voltage is applied to the power source circuit  101  from the secondary battery  15  of the digital camera  160  in the same manner as in the first charging operation performed while the camera is uninstalled from the charging device. But since FET  51  remains off, the buttery voltage of the secondary butter  15  does not appear at the terminal P 2 . Note that the detailed configuration and operation of the elements included in the camera charging systems of the above mentioned embodiments may be changed within the spirit and scope of the present invention.