Patent Publication Number: US-2010127676-A1

Title: Power source apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to a power source apparatus, such as an AC adaptor, to perform power output through a cable. 
     BACKGROUND ART 
     There is a power source apparatus, such as an AC adaptor, to generate a predetermined DC power from an AC power source to supply the generated DC power to an external apparatus through a cable. A general AC adaptor is configured so as to detect the output voltage and output current thereof on the side of the adaptor main body to perform the output control thereof. 
     DISCLOSURE OF THE INVENTION 
     Problem to be solved by the Invention 
     However, in a power source apparatus to supply DC power to an external apparatus through a cable, the supplied voltage thereof somewhat falls owing to the wiring resistance of the cable and the contact resistance of a connector. Moreover, the fall of the output voltage disperses according to the magnitude of the output current and the connected state of the connector and is not constant. 
     Accordingly, it is general for a conventional system to set the output voltage of the AC adaptor to be a little higher than a predetermined voltage so that an apparatus to receive power supply may drop the output voltage to a necessary voltage with a regulator circuit to use the dropped voltage on the apparatus side if the apparatus needs an accurate power source voltage. 
     Moreover, in recent years, it is considerable that the accurate control of the supply voltage from the AC adaptor becomes more difficult owing to the recent requirements to the AC adaptor for a portable telephone such as thinning the power supply line thereof to increase the flexibility of the cable thereof and miniaturizing the connector thereof. 
     The present invention is directed to provide a power source apparatus capable of supplying accurate electric power even if the resistance of its power supply line and the contact resistance of its connector exist. 
     According to a first aspect of the present invention, there is provided a power source apparatus ( 10 :  FIG. 1 ) including a connector for power output, the apparatus performing the power output through a cable connected to the connector, the apparatus including: a power source circuit ( 11 ) capable of changing an output thereof; a control circuit ( 12 ) to perform output control of the power source circuit; and a first detection circuit ( 14 ) including an input terminal for detection connected to wiring on a side of a tip of the cable to perform detection pertaining to an output quantity (for example, a voltage detection or a current detection) of electric power, wherein the output control is performed based on a detection signal of the first detection circuit, the detection signal fed back to the control circuit. 
     Preferably, the first detection circuit is disposed on the side of the tip of the cable or in the connector. 
     According to such an aspect, the detection for the output control is performed on the tip side of the cable. Consequently, even if the wiring resistance of the cable is large, the accurate output control excluding the influences of the wiring resistance can be performed. The wiring resistance is also added to the detection line of the first detection circuit and the output line of a detection signal, but the currents flowing through the detection line and the output line of the detection signal can be miniaturized to a neglectable degree in comparison with the current flowing through the wiring for power output. Consequently, no errors are caused in the output control owing to the wiring resistance. Thus, accurate power supply can be performed to an external apparatus to be connected, and it becomes possible to use the output of the power source apparatus directly as a power source voltage without the necessity of providing a regulator circuit or the like into the external apparatus. 
     According to a second aspect of the present invention, there is provided a power source apparatus ( 10 A: FIG.  3 ) including a connector for power output, the apparatus performing the power output through a cable connected to the connector, the apparatus comprising: a power source circuit ( 11 ) capable of changing an output thereof; a control circuit ( 12 ) to perform output control of the power source circuit; a first detection circuit ( 14   a ) to perform detection pertaining to an output quantity (for example, a voltage detection or a current detection) of electric power to feed back a detection signal to the control circuit; and a control connection terminal (T 2 ) provided in the connector, the control connection terminal connected to an input terminal for detection of the first detection circuit, wherein a voltage at a predetermined node of an external apparatus connected to the power source apparatus through the control connection terminal is input into the first detection circuit, and the output control of the control circuit is performed based on the detection signal of the first detection circuit. Here, preferably, the first detection circuit is disposed on a side of a tip of the cable. 
     According to such an aspect, since the detection point of the output quantity of the power source apparatus can be set at the node of a power source line or the like on the side of the external apparatus, the accurate output control of the power source apparatus excluding the influences of the wiring resistance of the cable and the contact resistance of the connector can be performed. 
     Preferably, the power source apparatus further includes: a switch circuit (SW 1 ) to switch a connection of the input terminal for detection of the first detection circuit ( 14   a :  FIG. 5 ) between the control connection terminal (T 2 ) and wiring (N 1 ) for power output; and a connection detection circuit ( 18 ) to detect a connection of the connector to the external apparatus, wherein the input terminal for detection is switched to a side of the control connection terminal by the switch circuit when the connection of the external apparatus is detected, and the input terminal for detection is switched to the wiring for power output by the switch circuit when the connection of the external apparatus is not detected. 
     By such a configuration, the abnormal rising and instability of the output of the power source apparatus owing to the inexistence of any detection signals at the time of no connection of any external apparatus can be prevented. 
     Preferably, the power source apparatus further includes a second detection circuit ( 20 :  FIG. 7 ,  FIG. 8 ) to perform detection pertaining to the output quantity of the electric power on a side of a main body of the power source apparatus in relation to the cable, wherein the control circuit ( 12 ) performs the output control by using the detection signal of the first detection circuit ( 14   a ) with priority to a detection signal of the second detection circuit when the detection signal of the first detection circuit exists. 
     By such a configuration, the output control based on the detection of the predetermined node in the external apparatus is performed when the external apparatus is connected to the power source apparatus, and the output control based on an internal detection signal is performed when no external output apparatus is connected to the power source apparatus. Consequently, the abnormal rising and instability of the output of the power source apparatus owing to no existence of detection signals can be prevented. 
     Preferably, the power source apparatus further includes a switch circuit ( 21 :  FIG. 7 ) to transmit the detection signal of the first detection circuit to the control circuit when the detection signal of the first detection circuit exists, and to transmit the detection signal of the second detection circuit to the control circuit when the detection signal of the first detection circuit does not exists. 
     Alternatively, the first detection circuit ( 14   a :  FIG. 8 ) and the second detection circuit ( 20 ) are configured to displace the detection signals from a reference value when detection values pertaining to the output quantity of the electric power exceed respective set values (V 1 , V 2 ) of the first and second detection circuits, and the set values of the first and second detection circuits are set to satisfy the following relation: (the set value of the first detection circuit)&lt;(the set value of the second detection circuit). 
     By such a configuration, the control circuit can use the detection signal of the first detection circuit with priority to the detection signal of the second detection circuit to perform the output control of the power source apparatus. 
     Preferably, the power source apparatus further includes: a third detection circuit ( 14   b :  FIG. 10 ) having an input terminal for detection, the input terminal connected to the control connection terminal; a switch circuit ( 24 ) to switch a connection of the control circuit between outputs of the first and third detection circuits selectively to transmit a selected output to the control circuit; a second detection circuit ( 20 ) to perform detection pertaining to the output quantity of the electric power to output a detection signal to the control circuit; a stop circuit ( 26 ) capable of stopping/continuing output of the second detection circuit; and a control section to detect existence of the output of the switch circuit to perform operation control of the stop circuit and the switch circuit, wherein the first to third detection circuits are configured to displace their output values from a reference value when detection values pertaining to the output quantity of the power exceed respective set values of the first to third detection circuits, and the set values are set to satisfy the following relation: (the set value ( 71 ) of the first detection circuit)&gt;(the set value (V 2 ) of the second detection circuit)&gt;(the set value (V 3 ) of the third detection circuit). 
     Preferably, the control section stops the stop circuit ( 26 ) to continue the output of the second detection circuit ( 20 ), and switches the switch circuit ( 24 ) to a side of the third detection circuit ( 14   b ) when the output of the switch circuit ( 24 ) does not exist, and the control section makes the stop circuit ( 26 ) operate to stop the output of the second detection circuit ( 20 ), and switches the switch circuit ( 24 ) to a side of the first detection circuit ( 14   a ) when the output of the switch circuit ( 24 ) exists. 
     According to such an aspect, when no external apparatus are connected to the power source apparatus, the output control of the power source apparatus based on the detection signal of the second detection circuit is performed, and the output voltage of the power source apparatus can be arbitrarily set to, for example, a lower output voltage and an output voltage by which the standby power of the power source apparatus becomes the minimum. Moreover, when an external apparatus is connected to the power source apparatus, the output control of the power source apparatus based on the detection signal of the second detection circuit is switched to the output control of the power source apparatus based on the detection signal of the first detection circuit by the operation of the third detection circuit, and consequently necessary power output can be performed. 
     Preferably, the power source apparatus further includes a detection circuit ( 15 :  FIG. 4 ) for protection to perform detection pertaining to the output quantity of the electric power on the side of the main body in relation to the cable, wherein the control circuit stops or lowers the output from the power source circuit when the detection circuit for protection detects the output quantity equal to or more than a predetermined output quantity. 
     By adding such a configuration, it can be performed to protect the power source apparatus lest its output should exceed rated power. 
     Incidentally, although the marks indicating the correspondence relations with embodiments have been shown in parentheses in the description in this clause, the present invention does not restricted to those indicated components of the embodiments. 
     According to the present invention, the following advantages can be obtained. That is, a power source apparatus can supply accurate electric power to a connected external apparatus even if a relatively large wiring resistance exists in a cable for outputting the electric power and even if a connection resistance exists in a connector of the power source apparatus. Moreover, as the result, even if the external apparatus requires an accurate power source voltage or an accurate power source current, the power source apparatus can enable the external apparatus to use the supplied electric power from the power source apparatus directly without any necessity of being provided with a regulator circuit for adjusting the power source voltage. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  This is a block diagram showing the configuration of an AC adaptor according to a first embodiment of the power source apparatus of the present invention. 
         FIG. 2  This is a graph showing an output characteristic of the detection circuit shown in  FIG. 1 . 
         FIG. 3  This is a block diagram showing the configuration of an AC adaptor  10 A of a second embodiment of the present invention. 
         FIG. 4  This is a block diagram showing the configuration of an AC adaptor  10 B of a third embodiment. 
         FIG. 5  This is a block diagram showing the configuration of an AC adaptor  10 C of a fourth embodiment. 
         FIG. 6  This is a circuit diagram showing the concrete circuit examples of the connection detection circuit and the switch circuit shown in  FIG. 5 . 
         FIG. 7  This is a block diagram showing the configuration of an AC adaptor  10 D of a fifth embodiment. 
         FIG. 8  This is a block diagram showing a modification of the AC adaptor  10 D of the fifth embodiment. 
         FIG. 9  This shows a graph of output characteristics of the first detection circuit  14   a  and the second detection circuit  20  shown in  FIG. 20 . 
         FIG. 10  This is a block diagram showing the configuration of an AC adaptor  10 E of a sixth embodiment. 
         FIG. 11  This is a block diagram showing the configuration of the AC adaptor  10 F of a seventh embodiment. 
     
    
    
     
         
         
           
               10 ,  10 A- 10 F AC adaptor 
               11  SW power source circuit 
               12  control circuit 
               13  detection receiving circuit 
               14 ,  14   a  detection circuit 
             h 1  power supplying line 
             h 2  ground line 
             h 3  detection signal line 
             R 1 -R 3  wiring resistances 
             T 0 , T 1  power source output terminal 
             T 2  control connection terminal 
               15  detection circuit for protection 
               16  stop control circuit 
             SW 1  switch circuit 
               18  connection detection circuit 
               20  second detection circuit 
               21  switch circuit 
               21   a  adding circuit 
               24  switch circuit 
               26  stop circuit 
           
         
       
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     In the following, the preferred embodiments of the present invention will be described with reference to the attached drawings. 
     First Embodiment 
       FIG. 1  is a block diagram showing the configuration of an AC adaptor according to a first embodiment of the power source apparatus of the present invention. 
     The AC adaptor  10  of the present embodiment is a power source apparatus connected to a piece of set equipment, such as a portable telephone, through a connector to supply electric power to the set equipment through a cable. 
     The AC adaptor  10  is equipped with a SW power source circuit  11 , a control circuit  12 , a detection circuit  14 , and a detection receiving circuit  13 . The SW power source circuit  11  receives the input of AC power to perform current output controlled by a switching operation of a transistor. The control circuit  12  changes the switching frequency of the SW power source circuit  11  and changes the on-period of the switching element of the SW power source circuit  11  to perform the output control of the SW power source circuit  11 . The detection circuit  14  detects the output voltage and the like of the AC adaptor  10  for the output control thereof to feed back the detected signal. The detection receiving circuit  13  is, for example, a reception buffer to receive the detection signal from the detection circuit  14  to output the received detection signal to the control circuit  12 . 
     The AC adaptor  10  is composed of an adaptor main body section mounted with the SW power source circuit  11 , a cable for supplying electric power, which cable extends from the adaptor main body section, and a connector provided at the tip of the cable. A power supplying line h 1 , a ground line h 2 , and a control signal line h 3  are wired in the cable, and wiring resistances R 1 -R 3  are added to the lines h 1 -h 3 , respectively. Moreover, power source output terminals T 0  and T 1  are provided to the connector. 
     The detection circuit  14  is disposed on the tip side of the cable for supplying electric power (or in the connector) and is configured to perform the detection of the output voltage of the AC adaptor  10  at a node N 1  near to the output terminal T 1  of the power supplying line h 1  as a detection point. 
       FIG. 2  shows an output characteristic graph of the detection circuit. 
     The detection circuit  14  divides the voltage at an input terminal for detection with, for example, dividing resistors and compares the divided voltage with a reference voltage. Then the detection circuit  14  amplifies the voltage difference between the divided voltage and the reference voltage with an error amplifier to output the amplified voltage difference. Then, as shown in  FIG. 2 , the detection circuit  14  has an output characteristic as follows: the detection output thereof is set to a reference value (for example, of a voltage value of zero) when the divided voltage is lower than the reference voltage; the detection output is raised when the divided voltage becomes in the neighborhood of the reference voltage; and the detection output is heightened according to the voltage difference between the divided voltage and the reference voltage when the divided voltage exceeds the reference voltage. The set voltage Vs at which the detection output has risen to reach a constant value can be arbitrarily selected by selecting the division ratio of the dividing resistors. 
     The control circuit  12  increases the output of the SW power source circuit  11  when a detection signal fed back is the reference value. On the other hand, when the detection signal fed back becomes larger than a predetermined voltage, the control circuit  12  lessens the switching frequency of the SW power source circuit  11  or shortens the on-period of the switching element thereof by the amount of the excess of the detection signal over the predetermined voltage, and the control circuit  12  thereby reduces the output of the SW power source circuit  11 . The control circuit  12  performs the output control of the AC adaptor  10  so as to keep the voltage at the detection point of the detection circuit  14  to the set voltage Vs. 
     According to the AC adaptor  10  having the configuration mentioned above, since the detection point of the detection circuit  14  is set on the tip side of the cable, even it the wiring resistances R 1  and R 2  of the power supplying line h 1  and the ground line h 2 , respectively, become comparatively larger, accurate power output without the influences of the increase of the wiring resistances R 1  and R 2  can be performed. Although the control signal line h 3  includes the wiring resistance R 3  similarly, the current flowing through the detection circuit  14  can be set to be a very small value in comparison with the current of the power supplying line h 1 , and consequently the influences of the wiring resistance R 3  can be reduced to a negligible level. 
     Consequently, accurate power supply to a connected external apparatus can be performed, and even if the external apparatus requires an accurate power source voltage or an accurate power source current, it is possible to enable the external apparatus to use the supplied electric power from the AC adaptor  10  directly. Thus an advantage of no necessity of providing any regulator circuit to the apparatus to which the electric power is supplied can be obtained. 
     Incidentally, although the detection circuit  14  is provided on the tip side of the cable, a similar advantage can be obtained by the configuration in which the detection circuit  14  is provided on the adaptor main body side and the input terminal for detection of the detection circuit  14  is connected to the node N 1  on the side of the output terminal T 1  of the power source supplying line h 1  though the signal control line h 3 . 
     Second Embodiment 
       FIG. 3  is a block diagram showing the configuration of an AC adaptor  10 A of a second embodiment of the present invention. 
     The AC adaptor  10 A of the second embodiment adds a control connection terminal T 2  to the connector to be connected to an external apparatus, and the detection input terminal for detection of a detection circuit  14   a  is connected to the control connection terminal T 2  to perform the output control of the SW power source circuit  11  based on the detection of an arbitrary node in the external apparatus. 
     The detection circuit  14   a  keeps its detection output to a reference value (for example, a voltage value of zero) when the voltage at the input terminal for detection is lower than a predetermined reference voltage (for example 1V); raises the detection output from the reference value when the voltage at the input terminal for detection becomes in the neighborhood of the reference voltage; and heightens the detection output by the amount of excess of the detection output over the reference voltage when the voltage at the input terminal exceeds the reference voltage. 
     Moreover, the control circuit  12  receives the detection output to perform the control of increasing and decreasing the output quantity of the SW power source circuit  11  similarly to the first embodiment. 
     In such a configuration, for example, if a predetermined set voltage (for example, 6V) is necessary at an arbitrary node of the power source line of an external apparatus, then the external apparatus is configured in advance so as to be provided therein with division resistors to divide the voltage at the node to drop the voltage to the reference voltage (1 V), and is further configured so that the division point of the division resistors may be connected to the control connection terminal T 2 . Thereby, the AC adaptor  10 A can perform necessary output control to enable to supply the stable and accurate set voltage (6 V) to the node of the external apparatus. 
     Moreover, if the external apparatus needs a predetermined set current at a certain node on the power source line, then the external apparatus is configured in advance so as to be provided therein with a resistor to convert the current flowing the node into a voltage, a circuit to amplify the voltage on both the ends of the resistor up to the reference voltage (1 V), and the like, and further is configured so that the output terminal of the circuit may be connected to the control connection terminal T 2 . Thereby, the AC adaptor  10 A performs needed output control to enable to supply the stable and accurate set current to the node of the external apparatus. 
     As described above, according to the AC adaptor of the present embodiment, since it is possible to set the detection point of a voltage or current by the detection circuit  14   a  at an arbitrary node in the external apparatus of a connection destination, an accurate voltage or current can be supplied to the external apparatus with the influences of the wiring resistances R 1 -R 3  of the cable and the contact resistances of the connection terminals T 0  and T 2  excluded, even if these resistances exist. 
     Incidentally, also in the present embodiment, the detection circuit  14   a  may not be provided on the tip of the cable, but may be configured to be provided on the adaptor main body side and to extend the signal line of the control connection terminal T 2  to connect the signal line to the input terminal for detection of the detection circuit  14   a.    
     Third Embodiment 
       FIG. 4  is a block diagram showing the configuration of an AC adaptor  10 B of a third embodiment. 
     The AC adaptor  10 B of the third embodiment is provided with a detection circuit  15  for protection and a stop control circuit  16  for protection in order not to exceed a rated output in addition to the configuration of the second embodiment. 
     The detection circuit  15  for protection is disposed on the side of the adaptor main body, and is configured so as to raise a detection output when the output voltage or output current of the AC adaptor  103  exceeds the rated voltage or rated current of the AC adaptor  103 , respectively. 
     The stop control circuit  16  is configured to receive the output of the detection circuit  15 , and to output a signal for stopping output to the control circuit  12  when the received output exceeds a predetermined threshold value. Moreover, the stop control circuit  16  is configured to continue to output the signal for stopping output until the input of AC electric power is broken to reset the circuit when the stop control circuit  16  has once output, the signal for stopping output. 
     When the control circuit  12  receives the input of the signal for stopping output from the stop control circuit.  16 , the control circuit  12  controls the SW power source circuit  11  so as to stop the power output thereof or so as to output a lower voltage regardless of the magnitude of the detection signal. 
     By such a configuration, it is possible to protect the external apparatus and the internal circuits of the AC adaptor  10 B by stopping the output thereof or by lowering the output voltage thereof within a range of not exceeding the rated output even if the output voltage or output current thereof abnormally rises owing to some defects. 
     Fourth Embodiment 
       FIG. 5  is a block diagram showing the configuration of an AC adaptor  10 C of a fourth embodiment. 
     The AC adaptor  10 C of the fourth embodiment is equipped with a switch circuit S 1  to switch the connection of the input terminal for detection of the detection circuit  14   a  between the control connection terminal T 2  and the node N 1  of the power source supplying line h 1 , and a connection detection circuit  18  to detect whether the connector is connected to an external circuit or not in addition to the configuration of the second embodiment, and the AC adaptor  10 C is configured to switch the switch circuit SW 1  based on a detection result of the connection detection circuit  18 . 
     The connection detection circuit  18  can be configured so as to detect the existence of the connection of the connector circuit to the external circuit by detecting, for example, whether a voltage is applied to the control connection terminal T 2  or not. Then, the connection detection circuit  18  is configured to switch the connection of the switch circuit SW 1  to the side of the control connection terminal T 2  when the connection exists, and to switch the connection of the switch circuit SW 1  to the side of the node N 1  when there is no connection. 
       FIG. 6  shows the concrete circuit examples of the connection detection circuit  18  and the switch circuit SW 1 . 
     Various systems, such as a contact point system switch and a semiconductor switch, can be applied to the switch circuit SW 1 . If semiconductor switch is used, for example, bipolar transistors Q 1  and Q 2 , as shown in  FIG. 6 , can be applied as the switch circuit SW 1 . That is, the emitter terminal and collector terminal of the transistor Q 1  are connected to the node N 1  and the input terminal for detection of the detection circuit  14   a , respectively, and consequently the turning on and off of the transistor Q 1  are enabled by the base voltage. Moreover, the emitter terminal and collector terminal of the transistor Q 2  are connected to the control connection terminal T 2  and the input terminal for detection of the detection circuit  14   a , respectively, and consequently the turning on and off of the transistor Q 2  are enabled by the base voltage. 
     Moreover, the connection detection circuit  18  can be configured with a bipolar transistor Q 3  connected between the base of the transistor Q 2  and the ground. The base of the transistor Q 3  is connected to the control connection terminal T 2 . 
     By such a configuration, when a voltage is applied to the control connection terminal T 2 , the transistor Q 3  is tuned on to turn on the transistor Q 2  in the switch circuit SW 1 . Thereby, the input terminal for detection of the detection circuit  14   a  can be switched to the side of the control connection terminal T 2 . Moreover, when no voltages are applied to the control connection terminal T 2 , the transistor Q 3  is turned off to turn on the transistor Q 1  in the switch circuit SW 1 . Thereby, the input terminal for detection of the detection circuit  14   a  can be switched to the side of the node N 1 . 
     According to the AC adaptor  10 C configured as described above, when an external apparatus is connected thereto, the detection circuit  14   a  performs the detection of a predetermined node in the external apparatus through the control connection terminal T 2 , and the output control of the AC adaptor  10 C can be performed based on the detection output of the detection circuit  14   a . On the other hand, when the connection of the external apparatus is taken off, the control terminal of the detection circuit  14   a  is connected to the node N 1  of the power source supplying line h 1 . Consequently, the disadvantages such as the abnormal rising of the output voltage of the AC adaptor  10 C and the instability of output voltage can be avoided. 
     Incidentally, the concrete configurations of the switch circuit SW 1  and the connection, detection circuit  18  are not restricted to those shown in the circuit diagram of  FIG. 6 , but any circuit configuration may be applicable as long as the circuit configuration can realize the operations described above. 
     Fifth Embodiment 
       FIG. 7  is a block diagram showing the configuration of an AC adaptor  10 D of a fifth embodiment. 
     The AC adaptor  10 D of the fifth embodiment is further configured so as to transmit a detection signal from a second detection circuit  20  to the control circuit  12  to stabilize the power output of the AC adaptor  10 D when the output of the first detection circuit  14   a  does not exist by the taking-off of the connection with an external apparatus, in addition to the configuration of the second embodiment. 
     Accordingly, the AC adaptor  10 D is equipped with a the second detection circuit  20 , which is provided in the adaptor main body to detect the output voltage thereof to output a second detection signal S 2 , and a switch circuit  21  to switch the output signal thereof between a detection signal S 1  of the first detection circuit  14   a  and a detection signal S 2  of the second detection circuit  20  to output the switched detection signal to the control circuit  12 . 
     The switch circuit  21  receives the two detection signals S 1  and S 2  to output either of them to the control circuit  12 , and is configured so as to operate to output the detection signal S 1  preferentially when the input of the detection signal S 1  from the first detection circuit  14   a  exists. 
     According to such a configuration, when an external apparatus is connected to the AC adaptor  10 D, the output control of the AC adaptor  10 D is performed based on the detection signal S 1  of the detection circuit  14   a  to enable the accurate power supply. On the other hand, when the connection of the external apparatus is taken off, the detection signal S 2  of the second detection circuit  20  is transmitted to the control circuit  12 , and consequently the disadvantages of the abnormal rising and instability of the output of the SW power source circuit  11  owing to the inexistence of any detection, signals can be avoided. 
       FIG. 8  shows a modification of the AC adaptor  10 D of the fifth embodiment. 
     Several patterns of the switch circuits can be applied to the configuration of the switch circuit  21  of the fifth embodiment. For example, one of the patterns of the switch circuits is a circuit of detecting the existence of the first detection signal S 1  to switch the connection of the signal lines based on the detection result, and another of the pattern of the switch circuits is a configuration as an addition circuit  21   a  shown in  FIG. 8 . Next, the case of the configuration as the addition circuit  21   a  will be described. 
     The addition circuit  21   a  is a circuit to add the voltage values of two analog signals to output the added voltage value. To put it concretely, the addition circuit  21   a  may be a circuit using an operational amplifier to add the voltages, or may be a circuit including only resistors to add the voltages without using any operational amplifiers because the addition values do not need to be pretty accurate. 
       FIG. 9  shows a graph of output characteristics of the first detection circuit  14   a  and the second detection circuit  20 . 
     Moreover, if the addition circuit  21   a  is used, it is required to set the output characteristics of the first and second detection circuits  14   a  and  20  to predetermined characteristics. That is, the characteristics are set as follows. First, as shown in  FIG. 9 , the output characteristics of the first and second detection circuits  14   a  and  20  set their outputs to a reference value (for example, a voltage value of zero) in the range of the detection voltages being less than the set voltages V 1  and V 2 , respectively. The output characteristics raise the output values when the detection voltages approach the set values, and raise the output values by the excesses of the detection voltages over the set voltages V 1  and V 2  when the detection voltages exceed the set voltages V 1  and V 2 , respectively. Furthermore, the set voltage V 2  of the second detection circuit  20  is previously set to be a value larger than the set voltage V 1  of the first detection circuit  14   a . That is, the set voltages are set to “V 2 &gt;V 1 +ΔV” (where ΔV indicates a voltage drop of a wiring resistance, a connector contact resistance, and the circuits between them). 
     By setting the set voltages V 1  and V 2  as above, when the output voltage detection voltage) of the AC adaptor  10 D is becoming larger, the detection signal S 1  of the first detection circuit  14   a  rises first to be output to the control circuit  12  through the addition circuit  21   a . Then, the control to suppress the output voltage is performed based on the detection signal S 1 , and the output voltage is stabilized in the neighborhood of the set voltage V 1  of the first detection circuit  14   a.    
     At this time, since the output voltage of the AC adaptor  10 D is kept at a voltage lower than the set voltage V 2  of the second detection circuit  20 , the detection signal S 2  of the second detection circuit  20  becomes almost zero, and the output of the addition circuit  21   a  is substantially the same value of the output value of the first detection circuit  14   a . Consequently, the addition circuit can be regarded as the circuit to output the detection signal of the first detection circuit  14   a  preferentially. 
     On the other hand, if the external apparatus is taken off, then the output of the first detection circuit  14   a  becomes nonexistent, and consequently the output voltage of the AC adaptor  10 D rises and also the detection signal S 2  of the second detection circuit  20  rises. Then, the detection signal S 2  is output to the control circuit  12  through the addition circuit  21   a , and the output control based on the detection signal S 2  is performed. Then, the output voltage of the AC adaptor  10 D is stabilized in the neighborhood of the set voltage V 2 . 
     As described above, the AC adaptor  10 D can be configured as follows. That is, when the detection signal S 1  of the first detection circuit  14   a  exists, the output control based on the detection signal S 1  is preferentially performed. When the detection signal S 1  of the first detection circuit  14   a  does not exist, the output control based on the detection signal S 2  of the second detection circuit  20  is performed. Then, by such output control, the advantage capable of avoiding the disadvantage of the abnormal rising and instability of the output of the SW power source circuit  11  owing to the inexistence of any detection signals even when the connection of the external apparatus is taken off. 
     Sixth Embodiment 
       FIG. 10  is a block diagram showing the configuration of an AC adaptor  10 E of a sixth embodiment. 
     The AC adaptor  10 E of the sixth embodiment is configured so as to supply a high voltage, such as 30 V, when an external apparatus is connected, and so as to set its output voltage to a low voltage, such as 10 V, when the connection of the external apparatus is taken off. 
     The aforesaid configuration of  FIG. 8  can stabilize the output voltage of the AC adaptor  10 D by performing the output control thereof based on the detection signal S 2  of the second detection circuit  20  when the external apparatus is taken off, and, on the other hand, the configuration is obliged to set the set voltage V 2  of the second detection circuit  20  to be larger than the set voltage V 1  of the first detection circuit  14   a  and consequently the output voltage when the external apparatus is taken off becomes higher. Consequently an AC adaptor for outputting a high voltage has a problem in which the waiting voltage of the AC adaptor when an external apparatus is taken off becomes very high. 
     The AC adaptor  10 E of the sixth embodiment is configured so that the waiting voltage thereof when the external apparatus connected to the AC adaptor  10 E is taken off therefrom can be set to a lower voltage even if the AC adaptor  10 E is the one for outputting a high voltage. 
     In order to add the aforesaid function, the AC adaptor  10 E of the present embodiment is equipped with an auxiliary detection circuit  14   b  having an input terminal for detection connected to the control connection terminal T 2 , a switch circuit  24  to selectively output either of the output of the first detection circuit  14   a  and the output of the auxiliary detection circuit  14   b , a time constant circuit to delay the switching timing, and a stop circuit  26  to stop the operation of the second detection circuit  20  in addition to the first detection circuit  14   a  having the input terminal for detection connected to the control connection terminal T 2  and the second detection circuit  20  to perform the detection of the output voltage of the SW power source circuit  11  in the adaptor main body. 
     The set voltage of each of the first detection circuit  14   a , the second detection circuit  20 , and the auxiliary detection circuit  14   b  is set, as described with regard to the fifth embodiment. Then, the AC adaptor  10 E has the output characteristic as follows. That is, when the detection voltage of each of the detection circuits  14   a ,  20  and  14   b  is smaller than the corresponding set voltage, the detected output thereof is set to a reference value (for example, a voltage value of zero); when the detection voltage becomes in the neighborhood of the set voltage, the AC adaptor  10 E raises its output; and when the detection voltage exceeds the set voltage, the AC adaptor  10 E increases its output according to the excess of the detection voltage over the set voltage. 
     The set voltage V 1  of the first detection circuit  14   a  is set to, for example, 30 V, which is necessary for the apparatus of an output destination, and the set voltage V 2  of the second detection circuit  20  is set to 10 V, which is appropriate as a waiting voltage. Moreover, the set voltage V 3  of the auxiliary detection circuit  14   b  is set to, for example, 8 V, which is smaller than the set voltage V 2 . 
     Next, the operation of the AC adaptor  10 E of the configuration described above will be described. 
     First, in the state in which no external apparatus are connected to the AC adaptor  10 E, there are no detection outputs of the first and auxiliary detection circuits  14   a  and  14   b , and the detection output of the second detection circuit  20  is output to the control circuit  12  and the output control based on the detection output is performed. Consequently, the output voltage of the AC adaptor  10 E is controlled to the set voltage V 2  (=10V) of the second detection circuit  20 . 
     When an external apparatus is connected to the AC adaptor  10 E, an output voltage of 10 V is applied to the input terminal for detection of the auxiliary detection circuit  14   b , and consequently the detection output of the auxiliary detection circuit  14   b  rises. Thus the detection signal is transmitted to the control circuit  12  through the switch circuit  24  and the detection receiving circuit  13 . Then, the output control is performed based on the detection signal, and the output voltage of the AC adaptor  10 E is dropped to the set voltage V 3  (=8 V) of the auxiliary detection circuit  14   b.    
     Moreover, when a detection signal of a predetermined voltage value or more is output from the auxiliary detection circuit  14   b , the switch circuit  24  makes the time constant circuit  25  operate, and the detection receiving circuit  13  makes the stop circuit  26  operate to stop the operation of the second detection circuit  20 . The detection receiving circuit  13  makes the stop circuit  26  operate to stop the operation of the second detection circuit  20  during a period of the continuation of the input of the detection signal of a certain value or more. 
     Next, a short delay time has elapsed from the output of the detection signal of the auxiliary detection circuit  14   b , a single indicating the elapse of the delay time is output from the time constant circuit  25 , and the switch circuit  24  switches the connection thereof from the output of the detection signal of the auxiliary detection circuit  14   b  to the output of the detection signal of the first detection circuit  14   a . Consequently, the detection signal of the first detection circuit  14   a  is output to the control circuit  12  in the state in which the second detection circuit  20  is in its stop state. Then, the output voltage rises up to the set voltage V 1  (=30 V) of the first detection circuit  14   a.    
     Moreover, when the external apparatus is taken off from the AC adaptor  10 E in the state of the output of the voltage of 30 V, the output of the first and auxiliary detection circuits  14   a  and  14   b  become nonexistent, and the stop control signal output from the detection receiving circuit  13  to the stop circuit  26  is negated. Consequently, the second detection circuit  20  operates to drop the output voltage of the AC adaptor  10 E to the set voltage V (=10 V) of the second detection circuit  20 . 
     As described above, according to the AC adaptor  10 E of the present embodiment, the following advantages can be obtained. That is, even if no external apparatus are connected to the AC adaptor  10 E, a detection signal is output to the control circuit  12  to enable the control circuit  12  to stabilize the output control thereof. Furthermore, the output voltage at the time when no external apparatus are connected to the AC adaptor  10 E can be set to a low voltage. 
     Incidentally, although the AC adaptor  10 E has been described as the one for a high voltage output in the above description, the value of the output voltage of the AC adaptor  10 E is not especially restricted to the one mentioned above, and also the set voltages V 1 , V 2 , and V 3  of the detection circuits  14   a ,  20 ,  14   b , respectively, are not restricted to the concrete values mentioned above. For example, the value of the output voltage of the AC adaptor  10 E when no external apparatus are connected thereto can be set to the one which makes the waiting power of the AC adaptor  10 E to be the lowest. 
     Seventh Embodiment 
       FIG. 11  is a block diagram showing the configuration of the AC adaptor  10 F of a seventh embodiment. 
     The AC adaptor  10 F of the seventh embodiment is an example of using the first detection circuit  14  provided in the tip of the cable in the configuration of the first embodiment as a detection circuit for protection (referred to as a second protection detection circuit  14 ). 
     The AC adaptor  10 F of the present embodiment is equipped with a voltage detection circuit  28  for output control and a first protection detection circuit  29  to perform the detection of the output voltage and output current of the AC adaptor  10 F lest the output voltage and the output current should exceed the maximum rated voltage and the maximum rated current, respectively, on the side of the adaptor main body in addition to the configuration of the first embodiment. 
     Moreover, the AC adaptor  10 F is equipped with a switch circuit  30  to switch the detection signal between those of the second protection detection circuit  14  at the tip of the cable and the voltage detection circuit  28  to output the switched detection signal, and a synthesis circuit  31  to perform the synthesis of the detection signals from the switch circuit  30  and the first protection detection circuit  14 . The AC adaptor  10 F is configured to transmit the output of the synthesis circuit  31  to the control circuit  12  to perform the output control of the AC adaptor  10 F. 
     The voltage detection circuit  28 , the first protection detection circuit  29 , and the second protection detection circuit  14  are provided with the set voltages V 1 , V 2 , and V 3 , respectively, and severally have an output characteristic, as shown in  FIG. 2 , in which, when each of the detection signals of the detection circuits  28 ,  29 , and  14  becomes the neighborhood of each of the set voltages V 1 , V 2 , and V 3  or more, the detection signal is raised. 
     Each of the set voltages V 1 , V 2 , and V 3  are set as follows: the set voltage V 1  is set to the output voltage at the normal time; the set voltage V 2  is set to a value of the maximum rated value satisfying the relation of “V 2 &gt;V 1 ”; and the set voltage V 3  is a value for abnormality protection satisfying the relation of “V 3 &gt;V 2 .” 
     Moreover, each of the switch circuit  30  and the synthesis circuit  31  has a circuit configuration of analogously adding each detection signal together. 
     According to the AC adaptor  10 F configured as above, the output control thereof is performed based on the detection output of the voltage detection circuit  28  at the normal time, and the voltage output at the normal time is performed. But if the power source output terminals T 0  and T 1  are short-circuited together, or if the voltage detection circuit  28  gets out of order, the detection output of the first protection detection circuit  29  is transmitted to the control circuit  12 , and the control of not outputting the output of the AC adaptor  10 F exceeding the maximum rated value thereof is performed. 
     Furthermore, if the first protection detection circuit  29  goes wrong, the second protection detection circuit  14  operates, and the output control of the AC adaptor  10 F is performed lest the output voltage thereof should exceed the voltage V 3  for protection. Thus double protection can be added lest any excessive voltage should be output to an external apparatus. 
     Although the best modes of the present invention have been described above, the present invention is not restricted to the first to seventh embodiments. For example, although the AC adaptors receiving AC power to output DC power have been described as the power source apparatus in the embodiments described above, the configuration of the present invention is not restricted to such ones. Moreover, the detailed configurations and operation systems shown in the embodiments can be suitably changed without departing from the spirit and scope of the invention. 
     The present invention can be applied to a power source apparatus to perform power output through a cable, such as an AC adaptor.