Patent Publication Number: US-2019200431-A1

Title: Drive device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. national stage of PCT/JP2017/023560 filed Jun. 27, 2017, which claims priority of Japanese Patent Application No. JP 2016-132600 filed Jul. 4, 2016. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a drive device. 
     BACKGROUND 
     JP 2003-338396A discloses a drive device that drives an incandescent light bulb. In this drive device, one end of the switch is connected to a power supply source, and the incandescent light bulb is disposed in a current path of current flowing from the other end of the switch. The incandescent light bulb is driven by switching the switch on and off in an alternating manner. The drive device according to JP 2003-338396A detects the value of voltage output by the power supply source. 
     A duty, which is the ratio of a target power value to the value of power consumed in the case where the value of the voltage output by the power supply source is a detection value and the switch is on, is calculated. The duty for switching the switch on and off is changed to the calculated duty. The average value of the power consumed by the incandescent light bulb therefore stabilizes at the target power value, regardless of the value of the voltage output by the power supply source. This prevents the incandescent light bulb from flickering. 
     Light-emitting diodes are becoming widespread as light-emitting units installed in vehicles. The intensity of the light emitted by a light-emitting diode fluctuates depending on the average value of current supplied to the light-emitting diode. When the average value of the current supplied to a light-emitting diode is stable, the intensity of the light emitted by the light-emitting diode stabilizes as well. The average value of the current is a value averaged over a finite constant period. 
     If a light-emitting unit is a light-emitting diode, there is a problem in that the light-emitting diode becomes more likely to flicker when the duty for switching a switch on and off is changed in order to stabilize the average value of the power consumed by the light-emitting diode. 
     Accordingly, an object of the present disclosure is to provide a drive device in which a light-emitting diode is not likely to flicker. 
     Advantageous Effects of Disclosure 
     According to the present disclosure, a light-emitting diode is not likely to flicker. 
     SUMMARY 
     A drive device according to one aspect of the present disclosure includes a drive unit, the drive unit switching a switch, one end of which is connected to one end of a battery, on and off in an alternating manner to drive a light-emitting diode disposed in a current path of current flowing from another end of the switch, and the drive device further including: a voltage detection unit that detects a battery voltage value at the one end of the battery; a calculation unit that calculates a duty on the basis of a detection value detected by the voltage detection unit; and a changing unit that changes a duty of the switching of the switch on and off to the duty calculated by the calculation unit, wherein when the battery voltage value is a detection value detected by the voltage detection unit, the duty calculated by the calculation unit is a ratio of a target current value to the value of current flowing in the current path when the switch is on. 
     Note that the present disclosure can be realized not only as a drive device including such characteristic processing units, but also as a drive method that takes the characteristic processes as steps, a computer program that causes a computer to execute those steps, and so on. Additionally, the present disclosure can be realized as a semiconductor integrated circuit that implements some or all of the drive device, as a drive system that includes the drive device, and so on. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating the primary configuration of a power source system according to a first embodiment. 
         FIG. 2  is a flowchart illustrating a drive start process sequence. 
         FIG. 3  is a flowchart illustrating a duty change process sequence. 
         FIG. 4  is a flowchart illustrating a drive stop process sequence. 
         FIG. 5  is a graph illustrating an example of transitions in the value of voltage applied to a light emission circuit. 
         FIG. 6  is a graph illustrating an example of transitions in the value of voltage applied to a light emission circuit in a second embodiment. 
         FIG. 7  is a block diagram illustrating the primary configuration of a power source system according to a third embodiment. 
         FIG. 8  is a graph illustrating a relationship between duty and a detection value. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     First, embodiments of the present disclosure will be described as examples. The embodiments described hereinafter may be at least partially combined as desired. 
     A drive device according to one aspect of the present disclosure includes a drive unit, the drive unit switching a switch, one end of which is connected to one end of a battery, on and off in an alternating manner to drive a light-emitting diode disposed in a current path of current flowing from another end of the switch, and the drive device further including: a voltage detection unit that detects a battery voltage value at the one end of the battery; a calculation unit that calculates a duty on the basis of a detection value detected by the voltage detection unit; and a changing unit that changes a duty of the switching of the switch on and off to the duty calculated by the calculation unit, wherein when the battery voltage value is a detection value detected by the voltage detection unit, the duty calculated by the calculation unit is a ratio of a target current value to the value of current flowing in the current path when the switch is on. 
     According to this aspect, the battery voltage value at one end of the battery is detected, a duty is calculated on the basis of the detected detection value, and the duty for switching the switch on and off is changed to the calculated duty. Here, the calculated duty is, when the battery voltage value is the detected detection value, the ratio of the target current value to the value of current flowing in the current path when the switch is on. Accordingly, the average value of the current flowing in the light-emitting diode stabilizes at the target current value regardless of the battery voltage value, and the light-emitting diode is less likely to flicker. 
     In a drive device according to an aspect of the present disclosure, assuming that the battery voltage value is a prescribed voltage value, the target current value is a value of current flowing in the current path when the switch is on; and the prescribed voltage value is less than or equal to a lower limit value of a fluctuation range of the battery voltage value. 
     According to this aspect, the prescribed voltage value is less than or equal to the lower limit value of the fluctuation range of the battery voltage value, and thus the average value of the current flowing in the current path can be adjusted to the target current value by changing the duty for switching the switch on and off. 
     A drive device according to an aspect of the present disclosure further includes a diode disposed in a second current path in which current flows from the other end of the switch, wherein the drive unit also drives a second light-emitting diode disposed in the second current path by carrying out the switching in an alternating manner; and a width of a voltage drop arising in one or more diodes disposed in the current path when current flows in the current path substantially matches a width of a voltage drop arising in a plurality of diodes disposed in the second current path when current flows in the second current path. 
     According to this aspect, the light-emitting diode disposed in the current path, as well as the second light-emitting diode disposed in the second current path, are driven by switching the switch on and off in an alternating manner. The width of the voltage drop arising in one or more diodes disposed in the current path when current flows in the current path substantially matches the width of the voltage drop arising in a plurality of diodes disposed in the second current path when current flows in the second current path. Accordingly, the average value of the current flowing in the second light-emitting diode disposed in the second current path also stabilizes regardless of the battery voltage value. 
     A drive device according to an aspect of the present disclosure further includes: a second diode disposed in a third current path in which current flows from the other end of the switch, wherein the drive unit also drives an incandescent light bulb disposed in the third current path by carrying out the switching in an alternating manner; and the second diode is used to stabilize a value of power consumed by the incandescent light bulb. 
     According to this aspect, both the light-emitting diode disposed in the current path and the incandescent light bulb disposed in the third current path are driven by switching the switch on and off in an alternating manner. The duty for switching the switch on and off is the above-described ratio, but disposing the second diode in the third current path makes it possible to realize a configuration in which the value of power consumed by the incandescent light bulb stabilizes at the target power value. In this case, the intensity of the light emitted by the incandescent light bulb stabilizes, and the incandescent light bulb is less likely to flicker. 
     DETAILS OF EMBODIMENT OF PRESENT DISCLOSURE 
     A specific example of the drive device according to embodiments of the present disclosure will be described hereinafter with reference to the drawings. Note that the present disclosure is not intended to be limited to these examples, and is defined instead by the scope of the appended claims. All changes that fall within the same essential spirit as the scope of the claims are intended to be included therein as well. 
     First Embodiment 
       FIG. 1  is a block diagram illustrating the primary configuration of a power source system  1  according to a first embodiment. The power source system  1  is favorably installed in a vehicle, and includes a drive device  10 , a light emission circuit  11 , a battery  12 , and a starter  13 . 
     The drive device  10  is connected separately to one end of the light emission circuit  11  and a positive terminal of the battery  12 . One end of the starter  13  is also connected to the positive terminal of the battery  12 . Another end of the light emission circuit  11 , a negative terminal of the battery  12 , and another end of the starter  13  are grounded. 
     The light emission circuit  11  includes a diode D 1 , N (where N is a natural number) light-emitting diodes L 1 , L 1 , . . . , L 1 , and a resistor R 1 . These are connected in series within the light emission circuit  11 . The diode D 1  and the N light-emitting diodes L 1 , L 1 , . . . , L 1  have the same forward directions. In the diode D 1  and the N light-emitting diodes L 1 , L 1 , . . . , L 1 , the anode is connected to the drive device  10  side, and the cathode is connected to the grounded side. 
     It should be noted that in the light emission circuit  11 , the order in which the diode D 1 , the N light-emitting diodes L 1 , L 1 , . . . , L 1 , and the resistor R 1  are connected from the drive device  10  side is not limited to the order illustrated in  FIG. 1 . It is sufficient for the diode D 1 , the N light-emitting diodes L 1 , L 1 , . . . , L 1 , and the resistor R 1  to be connected in series in the light emission circuit  11 . 
     In the power source system  1 , current flows from the positive terminal of the battery  12  and through the drive device  10  and the light emission circuit  11  in that order. When current flows in the light emission circuit  11 , the N light-emitting diodes L 1 , L 1 , . . . , L 1  in the light emission circuit  11  emit light. The light emitted by the N light-emitting diodes L 1 , L 1 , . . . , L 1  gains intensity as the average value of the current flowing in the light emission circuit  11  increases. Here, the average value of the current is a value averaged over a finite constant period, e.g., over one cycle of a switch signal, which will be described later. 
     The battery  12  supplies power not only to the light emission circuit  11 , but also to the starter  13 . The starter  13  is a motor for starting an engine (not shown). When the battery  12  supplies power, current flows in the battery  12  through an internal resistor (not shown), and the voltage drops in the internal resistor. The width of the voltage drop arising in the internal resistor is greater the greater the value of the current flowing through the internal resistor is. The battery  12  outputs voltage through the internal resistor. The value of the voltage output by the battery  12 , i.e., the voltage value at the positive terminal of the battery  12 , differs depending on whether or not the starter  13  is operating. When the starter  13  is operating, a high current flows through the internal resistor of the battery  12 , and thus the value of the voltage output from the battery  12  is low. When the starter  13  is stopped, a low current flows through the internal resistor of the battery  12 , and thus the value of the voltage output from the battery  12  is high. 
     When the battery  12  supplies power to the light emission circuit  11 , the width of the voltage drop arising in the internal resistor of the battery  12  is sufficiently low. As such, the value of the voltage output by the battery  12  experiences almost no fluctuation depending on whether or not the battery  12  is supplying power to the light emission circuit  11 . 
     The value of the voltage output by the battery  12  fluctuates each time the starter  13  operates and each time the starter  13  stops operating. In the following, an upper limit value of a fluctuation range pertaining to the value of the voltage output by the battery  12  will be represented by Vt, and a lower limit value of the fluctuation range will be represented by Vb (&lt;Vt). When the starter  13  is operating, the value of the voltage output by the battery  12  is the lower limit value Vb, whereas when the starter  13  is stopped, the value of the voltage output by the battery  12  substantially matches the upper limit value Vt. 
     Drive signals that instruct driving of the N light-emitting diodes L 1 , L 1 , . . . , L 1 , and stop signals that instruct the driving of the N light-emitting diodes L 1 , L 1 , . . . , L 1  to be stopped, are input to the drive device  10 . 
     When a drive signal is input, the drive device  10  intermittently connects the positive terminal of the battery  12  with the one end of the light emission circuit  11 . As a result, current is supplied from the battery  12  to the light emission circuit  11 , and the N light-emitting diodes L 1 , L 1 , . . . , L 1  emit light. 
     When a stop signal is input, the drive device  10  cuts the connection between the battery  12  and the light emission circuit  11 . As a result, the supply of current from the battery  12  to the light emission circuit  11  is stopped, and the N light-emitting diodes L 1 , L 1 , . . . , L 1  stop emitting light. 
     The drive device  10  includes a switch  20 , a drive circuit  21 , a voltage detection unit  22 , and a microcomputer  23 . One end of the switch  20  and the voltage detection unit  22  are connected to the positive terminal of the battery  12 . The other end of the switch  20  is connected to one end of the light emission circuit  11 . Both the drive circuit  21  and the voltage detection unit  22  are also connected to the microcomputer  23 . The switch  20  is a field effect transistor (FET), a bipolar transistor, a relay contact point, or the like. 
     When the switch  20  is on, current flows from the positive terminal of the battery  12 , to the switch  20 , and then to the light emission circuit  11 , in that order. The light emission circuit  11  is disposed in a current path of current flowing from the other end of the switch  20 . When the switch  20  is off, no current flows from the positive terminal of the battery  12  to the light emission circuit  11 . 
     The switch signal is input to the drive circuit  21  from the microcomputer  23 . The switch signal includes a high-level voltage and a low-level voltage. 
     If, while the switch signal is being input from the microcomputer  23  to the drive circuit  21 , the voltage of the switch signal switches from the low-level voltage to the high-level voltage, the drive circuit  21  switches the switch  20  from off to on. If the voltage of the switch signal switches from the high-level voltage to the low-level voltage in the same situation, the drive circuit  21  switches the switch  20  from on to off. As such, the switch  20  is on while the switch signal is at the high-level voltage, and the switch  20  is off while the switch signal is at the low-level voltage. The drive circuit  21  keeps the switch  20  off while no switch signal is being input to the drive circuit  21  from the microcomputer  23 . 
     The voltage detection unit  22  detects the value of the voltage output by the battery  12 , and outputs, to the microcomputer  23 , analog detection value information expressing a detection value Vs that has been detected. 
     The microcomputer  23  starts outputting the switch signal to the drive circuit  21  upon a drive signal being input to an input unit  34 , and stops outputting the switch signal upon a stop signal being input to the input unit  34 . The microcomputer  23  adjusts the duty of the switch signal output to the drive circuit  21  on the basis of the detection value Vs, which expresses the detection value information input from the voltage detection unit  22 . 
     In the switch signal, the switch from the low-level voltage to the high-level voltage or the switch from the high-level voltage to the low-level voltage occurs cyclically. The period of the high-level voltage in a single cycle of the switch signal is referred to as a “high-level period”, and the duty is the ratio of the high-level period to a single cycle of the switch signal. The duty is expressed as a percentage (%). The duty is greater than or equal to 0% and less than or equal to 100%. A duty of 0% indicates that the switch signal is at the low-level voltage for the entire cycle, whereas a duty of 100% indicates that the switch signal is at the high-level voltage for the entire cycle. 
     The microcomputer  23  includes a control unit  30 , a storage unit  31 , an A(Analog)/D(Digital) conversion unit  32 , input units  33  and  34 , and an output unit  35 . The control unit  30 , the storage unit  31 , the A/D conversion unit  32 , the input unit  34 , and the output unit  35  are connected to a bus  36 . In addition to the bus  36 , the A/D conversion unit  32  is connected to the input unit  33 , and the input unit  33  in turn is connected to the voltage detection unit  22 . In addition to the bus  36 , the output unit  35  is connected to the drive circuit  21 . 
     The analog detection value information from the voltage detection unit  22  is input to the input unit  33 . When the analog detection value information has been input from the voltage detection unit  22 , the input unit  33  outputs the input analog detection value information to the A/D conversion unit  32 . The A/D conversion unit  32  converts the analog detection value information input from the input unit  33  into digital detection value information. The digital detection value information resulting from the conversion by the A/D conversion unit  32  is obtained by the control unit  30 . The detection value Vs expressed by the detection value information obtained from the A/D conversion unit  32  by the control unit  30  matches or substantially matches the value of the voltage output by the battery  12  at the point in time when the detection value information is obtained. 
     The drive signal and the stop signal are input to the input unit  34 . When the drive signal or the stop signal is input to the input unit  34 , the input unit  34  notifies the control unit  30  to that effect. 
     The output unit  35  outputs the switch signal to the drive circuit  21 , changes the duty of the switch signal, and stops the output of the switch signal in response to instructions from the control unit  30 . 
     The storage unit  31  is non-volatile memory. A control program P 1  is stored in the storage unit  31 . 
     The control unit  30  includes a central processing unit (CPU) (not illustrated). By executing the control program P 1  stored in the storage unit  31 , the CPU of the control unit  30  executes a drive start process, a duty change process, and a drive stop process. The drive start process is a process of starting the driving of the N light-emitting diodes L 1 , L 1 , . . . , L 1 . The duty change process is a process of changing the duty of the switch signal output from the output unit  35  to the drive circuit  21 . The drive stop process is a process of stopping the driving of the N light-emitting diodes L 1 , L 1 , . . . , L 1 . The control program P 1  is a computer program for causing the CPU of the control unit  30  to execute the drive start process, the duty change process, and the drive stop process. 
     Note that the control program P 1  may be stored in a storage medium E 1  so as to be readable by a computer. In this case, the control program P 1  read out from the storage medium E 1  by a readout device (not shown) is stored in the storage unit  31 . The storage medium E 1  is an optical disk, a flexible disk, a magnetic disk, a magneto-optical disk, semiconductor memory, or the like. The optical disk is a CD (Compact Disc)-ROM (Read Only Memory), DVD (Digital Versatile Disc)-ROM, a BD (Blu-ray (registered trademark) Disc), or the like. The magnetic disk is a hard disk, for example. Additionally, the control program P 1  may be downloaded from an external device (not shown) connected to a communication network (not shown), and the downloaded control program P 1  may be stored in the storage unit  31 . 
     Flag values are also stored in the storage unit  31 . A flag has a value of either 0 or 1. A flag having a value of 0 indicates that the driving of the N light-emitting diodes L 1 , L 1 , . . . , L 1  is stopped. A flag having a value of 1 indicates that the N light-emitting diodes L 1 , L 1 , . . . , L 1  are being driven. The values of the flags are set by the control unit  30 . 
       FIG. 2  is a flowchart illustrating the drive start process sequence. The control unit  30  executes the drive start process when a drive signal is input to the input unit  34 . In the drive start process, first, the control unit  30  obtains the digital detection value information from the A/D conversion unit  32  (step S 1 ), and calculates a current duty (step S 2 ). Like the duty of the switch signal, the current duty is expressed as a percentage (%). 
     The following Expression 1 is stored in the storage unit  31 . 
         Ti= 100·( Vc−Vd 1− Vf 1)/( Vs−Vd 1− Vf 1)  (1) Vc=Vb
 
     Expression 1 is an expression for calculating a current duty Ti. The “·” represents multiplication. Vc is a prescribed voltage value. The prescribed voltage value Vc is set to the lower limit value Vb of the fluctuation range of the value of the voltage output by the battery  12 . As described earlier, the detection value Vs is a detection value expressed by the detection value information obtained from the A/D conversion unit  32  by the control unit  30 . Additionally, as illustrated in  FIG. 1 , a voltage value Vd 1  represents the width of a voltage drop arising in the diode D 1  when current flows in the current path in which the light emission circuit  11  is disposed. A voltage value Vf 1  represents the width of a voltage drop arising in the N light-emitting diodes L 1 , L 1 , . . . , L 1  when current flows in the current path. The lower limit value Vb is a constant value. 
     Expression 1 can be rewritten as the following Expression 2 by dividing both the numerator and denominator in the right side of Expression 1 by a resistance value r 1  of the resistor R 1 . 
         Ti= 100·(( Vc−Vd 1− Vf 1)/ r 1)/(( Vs−Vd 1− Vf 1)/ r 1)  (2)
 
     Assuming that the value of the voltage output by the battery  12  is the prescribed voltage value Vc (=Vb), (Vc−Vd 1 −Vf 1 )/r 1  represents, when the switch  20  is on, a current value Ic 1  flowing in the current path in which the light emission circuit  11  is disposed. 
     Furthermore, assuming that the value of the voltage output by the battery  12  is the detection value Vs expressed by the detection value information obtained from the A/D conversion unit  32  by the control unit  30 , (Vs−Vd 1 −Vf 1 )/r 1  represents, when the switch  20  is on, a current value Is 1  flowing in the current path in which the light emission circuit  11  is disposed. 
     The current duty Ti is the ratio of the current value Ic 1  to the current value Is 1 . The current value Ic 1  corresponds to a target current value. 
     In step S 2 , the control unit  30  calculates the current duty Ti by substituting, into Expression 1, the detection value Vs expressed by the detection value information obtained in step S 1 . 
     After executing step S 2 , the control unit  30  makes an instruction to the output unit  35  to start the output of the switch signal (step S 3 ). Here, the duty of the switch signal output by the output unit  35  is set to the current duty calculated in step S 2 . As described above, the drive circuit  21  switches the switch  20  on when the voltage of the switch signal switches from the low-level voltage to the high-level voltage, and switches the switch  20  off when the voltage of the switch signal switches from the high-level voltage to the low-level voltage. The drive circuit  21  repeatedly switches the switch  20  on and off in an alternating manner in accordance with the voltage of the switch signal. As a result, current is supplied to the N light-emitting diodes L 1 , L 1 , . . . , L 1  of the light emission circuit  11 , and the N light-emitting diodes L 1 , L 1 , . . . , L 1  emit light. 
     As described above, the N light-emitting diodes L 1 , L 1 , . . . , L 1  are driven by the drive circuit  21  repeatedly switching the switch  20  on and off in an alternating manner. The drive circuit  21  therefore functions as a drive unit. The duty of the switch signal corresponds to the duty at which the switch  20  is turned on and off. 
     The average value of the current flowing in the current path in which the light emission circuit  11  is disposed is a value obtained by dividing the product of the current value Is 1  and the current duty Ti calculated in step S 2  by 100, i.e., the current value Ic 1 . The intensity of the light emitted by the light-emitting diodes L 1  is an intensity corresponding to the current value Ic 1 . 
     After executing step S 3 , the control unit  30  sets the value of the flag to 1 (step S 4 ), and ends the drive start process. 
       FIG. 3  is a flowchart illustrating the duty change process sequence. The control unit  30  executes the duty change process periodically. The control unit  30  first determines whether or not the value of the flag is 1 (step S 11 ). As described earlier, a flag having a value of 1 indicates that the N light-emitting diodes L 1 , L 1 , . . . , L 1  are being driven, whereas a flag having a value of 0 indicates that the driving of the N light-emitting diodes L 1 , L 1 , . . . , L 1  is stopped. When the value of the flag is 1, the output unit  35  outputs the switch signal to the drive circuit  21 . 
     If it is determined that the value of the flag is not 1, i.e., the value of the flag is 0 (S 11 : NO), the control unit  30  ends the duty change process. 
     If it is determined that the value of the flag is 1 (S 11 : YES), the control unit  30  obtains the detection value information from the A/D conversion unit  32  (step S 12 ), and calculates the current duty by substituting the detection value Vs expressed by the obtained detection value information into Expression 1 (step S 13 ). Then, the control unit  30  changes the duty of the switch signal output by the output unit  35  to the current duty calculated in step S 13  (step S 14 ), and ends the duty change process. The control unit  30  therefore functions as a calculation unit and a changing unit. 
     As described earlier, the control unit  30  executes the duty change process periodically. Therefore, the duty of the switch signal is changed so that each time the value of the voltage output by the battery  12  fluctuates, the average value of the current flowing in the current path in which the light emission circuit  11  is disposed takes on the current value Ic 1 . As a result, the average value of the current flowing in the N light-emitting diodes L 1 , L 1 , . . . , L 1  stabilizes at the current value Ic 1  regardless of the value of the voltage output by the battery  12 . The intensity of the light emitted by the N light-emitting diodes L 1 , L 1 , . . . , L 1  therefore stabilizes, and the N light-emitting diodes L 1 , L 1 , . . . , L 1  are less likely to flicker. 
     A configuration in which a DC-DC converter is connected between the positive terminal of the battery  12  and the end of the switch  20  on the battery  12  side is conceivable as a configuration that stabilizes the average value of the current flowing in the N light-emitting diodes L 1 , L 1 , . . . , L 1  at the current value Ic 1  regardless of the value of the voltage output by the battery  12 . With this configuration, the DC-DC converter appropriately adjusts a step-up width or a step-down width to transform the voltage output by the battery  12  to a constant voltage, and outputs the transformed voltage toward the switch  20 . Compared to this configuration, the drive device  10  does not require a DC-DC converter. The drive device  10  is therefore compact, and can be manufactured at low cost. 
       FIG. 4  is a flowchart illustrating the drive stop process sequence. The control unit  30  executes the drive stop process when the stop signal is input to the input unit  34 . The control unit  30  first instructs the output unit  35  to stop the output of the switch signal (step S 21 ). As described earlier, the drive circuit  21  keeps the switch  20  off while the output unit  35  is stopping the output of the switch signal. After executing step S 21 , the control unit  30  sets the value of the flag to 0 (step S 22 ), and ends the drive stop process. 
       FIG. 5  is a graph illustrating an example of transitions in the value of the voltage applied to the light emission circuit  11 . The vertical axis represents the voltage value, and the horizontal axis represents time.  FIG. 5  illustrates transitions in the voltage value when the output unit  35  outputs the switch signal and the N light-emitting diodes L 1 , L 1 , . . . , L 1  are being driven. 
     As illustrated in  FIG. 5 , the value of the voltage output by the battery  12  is the upper limit value Vt while the starter  13  is stopped. The duty of the switch signal is less than 100% while the starter  13  is stopped, and the drive circuit  21  repeatedly switches the switch  20  on and off in an alternating manner. The upper limit value Vt of the value of the voltage output by the battery  12  is applied to the light emission circuit  11  when the switch  20  is on, whereas the value of the voltage applied to the light emission circuit  11  is 0 V when the switch  20  is off. The average value of the voltage applied to the light emission circuit  11  is the prescribed voltage value Vc (=Vb), and the average value of the current flowing in the current path in which the light emission circuit  11  is disposed is the current value Ic 1 . 
     When the value of the voltage output by the battery  12  becomes the lower limit value Vb due to the starter  13  operating, the duty of the switch signal becomes 100%, and the switch  20  is kept on. The average value of the voltage applied to the light emission circuit  11  is the prescribed voltage value Vc (=Vb), and the average value of the current flowing in the current path in which the light emission circuit  11  is disposed is the current value Ic 1 . 
     As described above, the average value of the current flowing in the N light-emitting diodes L 1 , L 1 , . . . , L 1  stabilizes at the current value Ic 1  regardless of the value of the voltage output by the battery  12 . 
     According to the drive device  10 , the prescribed voltage value Vc is the lower limit value Vb of the fluctuation range of the value of the voltage output by the battery  12 . Therefore, the average value of the current flowing in the current path in which the light emission circuit  11  is disposed can be adjusted to the current value Ic 1  by changing the duty of the switch signal. 
     Second Embodiment 
     In the first embodiment, the prescribed voltage value Vc is set to the lower limit value Vb of the fluctuation range of the value of the voltage output by the battery  12 . However, the prescribed voltage value Vc is not limited to the lower limit value Vb, and may be less than or equal to the lower limit value Vb. 
     Hereinafter, points of the second embodiment that are different from the first embodiment will be described. Configurations aside from those described hereinafter are the same as in the first embodiment. As such, constituent elements that are the same as in the first embodiment will be given the same reference signs as in the first embodiment, and descriptions thereof will be omitted. 
       FIG. 6  is a graph illustrating an example of transitions in the value of the voltage applied to the light emission circuit  11  according to the second embodiment. The vertical axis represents the voltage value, and the horizontal axis represents time. Like  FIG. 5 ,  FIG. 6  illustrates transitions in the voltage value when the output unit  35  outputs the switch signal and the N light-emitting diodes L 1 , L 1 , . . . , L 1  are being driven. 
     The second embodiment differs from the first embodiment in that the prescribed voltage value Vc is a voltage value that is less than the lower limit value Vb of the fluctuation range of the value of the voltage output by the battery  12 . 
     In the second embodiment, the duty of the switch signal is less than 100% both while the starter  13  is stopped and while the starter  13  is operating, and the drive circuit  21  repeatedly switches the switch  20  on and off in an alternating manner. The duty of the switch signal is changed so that the average value of the voltage applied to the light emission circuit  11  becomes the prescribed voltage value Vc. 
     The lower limit value Vb of the fluctuation range of the value of the voltage output by the battery  12  is lower than the value of the voltage output by the battery  12  while the starter  13  is stopped, i.e., is lower than the upper limit value Vt of the fluctuation range of the value of the voltage output by the battery  12 . Accordingly, the duty of the switch signal being output by the output unit  35  while the starter  13  is running, i.e., the percentage of one switch signal cycle occupied by a period in which the switch  20  is on, is higher than the duty of the switch signal being output by the output unit  35  while the starter  13  is stopped. 
     The drive device  10  according to the second embodiment configured as described above provides the same effects as those of the drive device  10  according to the first embodiment. Thus, according to the drive device  10  of the second embodiment as well, the average value of the voltage applied to the light emission circuit  11  is adjusted to the prescribed voltage value Vc, and the average value of the current flowing in the current path in which the light emission circuit  11  is disposed is adjusted to the current value Ic 1 , by changing the duty of the switch signal. 
     Third Embodiment 
       FIG. 7  is a block diagram illustrating the primary configuration of the power source system  1  according to a third embodiment. 
     Hereinafter, points of the third embodiment that are different from the first embodiment will be described. Configurations aside from those described hereinafter are the same as in the first embodiment. As such, constituent elements that are the same as in the first embodiment will be given the same reference signs as in the first embodiment, and descriptions thereof will be omitted. 
     The power source system  1  according to the third embodiment can also be favorably installed in a vehicle. The power source system  1  according to the third embodiment includes a light emission circuit  40  and an incandescent light bulb  41  in addition to the constituent elements of the power source system  1  according to the first embodiment. One end each of the light emission circuit  40  and the incandescent light bulb  41  is connected to the drive device  10 . The other ends of the light emission circuit  40  and the incandescent light bulb  41  are grounded. 
     The light emission circuit  40  includes a diode D 2 , M (where M is a natural number) light-emitting diodes L 2 , L 2 , . . . , L 2 , and a resistor R 2 . These are connected in series within the light emission circuit  40 . The diode D 2  and the M light-emitting diodes L 2 , L 2 , . . . , L 2  have the same forward directions. In the diode D 2  and the M light-emitting diodes L 2 , L 2 , . . . , L 2 , the anode is connected to the drive device  10  side, and the cathode is connected to the grounded side. 
     It should be noted that in the light emission circuit  40 , the order in which the diode D 2 , the M light-emitting diodes L 2 , L 2 , . . . , L 2 , and the resistor R 2  are connected from the drive device  10  side is not limited to the order illustrated in  FIG. 7 . It is sufficient for the diode D 2 , the M light-emitting diodes L 2 , L 2 , . . . , L 2 , and the resistor R 2  to be connected in series in the light emission circuit  40 . 
     In the power source system  1  according to the third embodiment, current flows from the positive terminal of the battery  12 , through the drive device  10 , and to each of the light emission circuits  11  and  40  and the incandescent light bulb  41 . 
     When current flows in the light emission circuit  40 , the M light-emitting diodes L 2 , L 2 , . . . , L 2  in the light emission circuit  40  emit light. The light emitted by the M light-emitting diodes L 2 , L 2 , . . . , L 2  gains intensity as the average value of the current flowing in the light emission circuit  40  increases. Here, the average value of the current is a value averaged over a finite constant period, e.g., over one cycle of the switch signal. 
     The incandescent light bulb  41  emits light when current flows in the incandescent light bulb  41 . The light emitted by the incandescent light bulb  41  gains intensity as the average value of the power consumed by the incandescent light bulb  41  increases. Here, the average value of the power is a value averaged over a finite constant period, e.g., over one cycle of the switch signal. 
     Like the first embodiment, in the third embodiment, the battery  12  supplies power to the starter  13  as well, and thus the value of the voltage output by the battery  12  fluctuates. When the battery  12  supplies power to the light emission circuits  11  and  40  and the incandescent light bulb  41 , the width of the voltage drop arising in the internal resistor of the battery  12  is sufficiently low. As such, the value of the voltage output by the battery  12  experiences almost no fluctuation depending on whether or not the battery  12  is supplying power to the light emission circuits  11  and  40  and the incandescent light bulb  41 . 
     As in the first embodiment, when the starter  13  is operating, the value of the voltage output by the battery  12  is the lower limit value Vb, whereas when the starter  13  is stopped, the value of the voltage output by the battery  12  substantially matches the upper limit value Vt. 
     In the third embodiment too, the drive signal and the stop signal are input to the drive device  10 . In the third embodiment, the drive signal instructs the driving of the N light-emitting diodes L 1 , L 1 , . . . , L 1 , the M light-emitting diodes L 2 , L 2 , . . . , L 2 , and the incandescent light bulb  41 . The stop signal instructs the driving of the N light-emitting diodes L 1 , L 1 , . . . , L 1 , the M light-emitting diodes L 2 , L 2 , . . . , L 2 , and the incandescent light bulb  41  to stop. 
     When a drive signal is input, the drive device  10  intermittently connects the positive terminal of the battery  12  with the one end of each of the light emission circuits  11  and  40  and the incandescent light bulb  41 . As a result, current is supplied from the battery  12  to the light emission circuits  11  and  40  and the incandescent light bulb  41 , and the N light-emitting diodes L 1 , L 1 , . . . , L 1 , the M light-emitting diodes L 2 , L 2 , . . . , L 2 , and the incandescent light bulb  41  emit light. 
     When the stop signal is input, the drive device  10  stops the supply of current from the battery  12  to the light emission circuits  11  and  40  and the incandescent light bulb  41 . As a result, the N light-emitting diodes L 1 , L 1 , . . . , L 1 , the M light-emitting diodes L 2 , L 2 , . . . , L 2 , and the incandescent light bulb  41  stop emitting light. 
     The drive device  10  according to the third embodiment includes diode circuits  50  and  51  in addition to the constituent elements of the drive device  10  according to the first embodiment. One end each of the diode circuits  50  and  51  is connected to the light emission circuit  11 -side end of the switch  20 . The other end of the diode circuit  50  is connected to one end of the light emission circuit  40 . The other end of the diode circuit  51  is connected to one end of the incandescent light bulb  41 . 
     The diode circuit  50  includes J (where J is a natural number) internal diodes A 2 , A 2 , . . . , A 2  provided within the drive device  10 . These are connected in series within the diode circuit  50 . The J internal diodes A 2 , A 2 , . . . , A 2  all have the same forward direction. In the J internal diodes A 2 , A 2 , . . . , A 2 , the anode is connected to the switch  20  side, and the cathode is connected to the light emission circuit  40  side 
     Likewise, the diode circuit  51  includes K (where K is a natural number) internal diodes A 3 , A 3 , . . . , A 3  provided within the drive device  10 . These are connected in series within the diode circuit  51 . The K internal diodes A 3 , A 3 , . . . , A 3  all have the same forward direction. In the K internal diodes A 3 , A 3 , . . . , A 3 , the anode is connected to the switch  20  side, and the cathode is connected to the incandescent light bulb  41  side. 
     As in the first embodiment, by executing the control program P 1  stored in the storage unit  31 , the CPU of the control unit  30  in the microcomputer  23  executes a drive start process, a duty change process, and a drive stop process. The control unit  30  executes the drive start process when a drive signal is input to the input unit  34 , and executes the drive stop process when a stop signal is input to the input unit  34 . The control unit  30  executes the duty change process periodically. 
     In the third embodiment, the drive start process is a process of starting the driving of the N light-emitting diodes L 1 , L 1 , . . . , L 1 , the M light-emitting diodes L 2 , L 2 , . . . , L 2 , and the incandescent light bulb  41 . The drive stop process is a process of stopping the driving of the N light-emitting diodes L 1 , L 1 , . . . , L 1 , the M light-emitting diodes L 2 , L 2 , . . . , L 2 , and the incandescent light bulb  41 . 
     The details of the drive start process, the duty change process, and the drive stop process in the third embodiment are the same as in the first embodiment. 
     In the third embodiment, a flag having a value of 0 indicates that the driving of the N light-emitting diodes L 1 , L 1 , . . . , L 1 , the M light-emitting diodes L 2 , L 2 , . . . , L 2 , and the incandescent light bulb  41  is stopped. A flag having a value of 1 indicates that the N light-emitting diodes L 1 , L 1 , . . . , L 1 , the M light-emitting diodes L 2 , L 2 , . . . , L 2 , and the incandescent light bulb  41  are being driven. 
     When the drive start process has been started by the control unit  30 , the switch signal is output from the output unit  35  to the drive circuit  21 , and the drive circuit  21  repeatedly switches the switch  20  on and off in an alternating manner in accordance with the voltage of the switch signal output from the output unit  35 . As a result, current flows from the light emission circuit  11 -side end of the switch  20  through the light emission circuit  11 , current flows from the light emission circuit  11 -side end of the switch  20  through the diode circuit  50  and the light emission circuit  40 , and current flows from the light emission circuit  11 -side end of the switch  20  through the diode circuit  51  and the incandescent light bulb  41 . 
     As described above, according to the power source system  1  of the third embodiment, three current paths are provided in which current flows from the light emission circuit  11 -side end of the switch  20 . The light emission circuit  11  is disposed in the first current path. The diode circuit  50  and the light emission circuit  40  are disposed in the second current path. The diode circuit  51  and the incandescent light bulb  41  are disposed in the third current path. 
     The current path in which the diode circuit  50  and the light emission circuit  40  are disposed corresponds to a second current path. The current path in which the diode circuit  51  and the incandescent light bulb  41  are disposed corresponds to a third current path. Each of the K internal diodes A 3 , A 3 , . . . , A 3  functions as a second diode. 
     When current flows in the current path in which the light emission circuit  11  is disposed, the N light-emitting diodes L 1 , L 1 , . . . , L 1  emit light. When current flows in the current path in which the light emission circuit  40  is disposed, the M light-emitting diodes L 2 , L 2 , . . . , L 2  emit light. When current flows in the current path in which the incandescent light bulb  41  is disposed, the incandescent light bulb  41  emits light. 
     The N light-emitting diodes L 1 , L 1 , . . . , L 1 , the M light-emitting diodes L 2 , L 2 , . . . , L 2 , and the incandescent light bulb  41  are driven by the drive circuit  21  repeatedly switching the switch  20  on and off in an alternating manner. While the drive circuit  21  keeps the switch  20  off, no current is supplied to the N light-emitting diodes L 1 , L 1 , . . . , L 1 , the M light-emitting diodes L 2 , L 2 , . . . , L 2 , and the incandescent light bulb  41 , and the driving thereof is stopped. The light-emitting diodes L 2  function as second light-emitting diodes. 
     As described in the first embodiment, in the drive start process and the duty change process, the duty of the switch signal is set and changed using Expression 1. Accordingly, the duty of the switch signal is adjusted so that the value of the current flowing in the current path in which the light emission circuit  11  is disposed is the prescribed current value Ic 1 . 
     According to the power source system  1  in the third embodiment, the following Expression 3 holds true. 
         Va 2+ Vd 2+ Vf 2= Vd 1+ Vf 1  (3)
 
     Here, a voltage value Va 2  represents the width of a voltage drop arising in the J internal diodes A 2 , A 2 , . . . , A 2  when current flows in the diode circuit  50 . A voltage value Vd 2  represents the width of a voltage drop arising the diode D 2  when current flows in the light emission circuit  40 . A voltage value Vf 2  represents the width of a voltage drop arising in the M light-emitting diodes L 2 , L 2 , . . . , L 2  when current flows in the light emission circuit  40 . 
     As described in the first embodiment, the voltage value Vd 1  represents the width of a voltage drop arising the diode D 1  when current flows in the light emission circuit  11 . A voltage value Vf 1  represents the width of a voltage drop arising in the N light-emitting diodes L 1 , L 1 , . . . , L 1  when current flows in the light emission circuit  11 . 
     Accordingly, the left side of Expression 3 represents the width of a voltage drop arising in the J internal diodes A 2 , A 2 , . . . , A 2 , the diode D 2 , and the M light-emitting diodes L 2 , L 2 , . . . , L 2  when current flows in the current path in which the diode circuit  50  and the light emission circuit  40  are disposed. The right side of Expression 3 represents the width of a voltage drop arising in the diode D 1  and the N light-emitting diodes L 1 , L 1 , . . . , L 1  when current flows in the current path in which the light emission circuit  11  is disposed. 
     Note that even if Expression 3 does not strictly hold true, it is sufficient for the sum of the voltage values Va 2 , Vd 2 , and Vf 2  to substantially match the sum of the voltage values Vd 1  and Vf 1  within a range in which Expression 3 can be viewed as holding true. When a difference (an absolute value) between the sum of the voltage values Va 2 , Vd 2 , and Vf 2  and the sum of the voltage values Vd 1  and Vf 1  is, for example, less than or equal to 0.2 V Expression 3 is considered to hold true, that is, the sum of the voltage values Va 2 , Vd 2 , and Vf 2  is considered to substantially match the sum of the voltage values Vd 1  and Vf 1 . 
     Using Expression 3, Expression 1 can be rewritten as the following Expression 4. 
         Ti= 100·( Vc−Va 2− Vd 2− Vf 2)/( Vs−Va 2− Vd 2− Vf 2)  (4)
 
     Furthermore, Expression 4 can be rewritten as the following Expression 5 by dividing both the numerator and denominator in the right side of Expression 4 by a resistance value r 2  of the resistor R 2 . 
         Ti= 100·(( Vc−Va 2 −Vd 2 −Vf 2)/ r 2)/(( Vs−Va 2 −Vd 2 −Vf 2)/ r 2)  (5)
 
     Assuming that the value of the voltage output by the battery  12  is the prescribed voltage value Vc (=Vb), (Vc−Va 2 −Vd 2 −Vf 2 )/r 2  is, when the switch  20  is on, a current value Ic 2  flowing in the current path in which the diode circuit  50  and the light emission circuit  40  are disposed. 
     Furthermore, assuming that the value of the voltage output by the battery  12  is the detection value Vs expressed by the detection value information obtained from the A/D conversion unit  32  by the control unit  30 , (Vs−Va 2 −Vd 2 −Vf 2 )/r 2  is, when the switch  20  is on, a current value Is 2  flowing in the current path in which the diode circuit  50  and the light emission circuit  40  are disposed. 
     Accordingly, the current duty Ti is the ratio of the current value Ic 1  to the current value Is 1 , and the ratio of the current value Ic 2  to the current value Is 2 . The value of the current flowing in the light emission circuit  11  is adjusted to the prescribed current value Ic 2  by the control unit  30  executing the drive start process and the duty change process. 
     As described thus far, according to the third embodiment, the diode circuit  50 , i.e., the J internal diodes A 2 , A 2 , . . . , A 2 , are provided. Accordingly, the width of the voltage drop arising all the diodes disposed in the current path of the current flowing from the light emission circuit  11 -side end of the switch  20  to the light emission circuit  40  is adjusted to match or substantially match the width of the voltage drop arising in all the diodes disposed in the current path of the current flowing from the light emission circuit  11 -side end of the switch  20  to the light emission circuit  11 . This makes it possible to supply current to both the light emission circuits  11  and  40  in a stable manner. Additionally, the current value Ic 2  can be set to a current value different from the current value Ic 1  by adjusted in the resistance value r 2  of the resistor R 2 . 
     According to the power source system  1  in the third embodiment, when the M light-emitting diodes L 2 , L 2 , . . . , L 2  are being driven, the average value of the current flowing in the light emission circuit  40  stabilizes at the current value Ic 2  regardless of the value of the voltage output by the battery  12 . The intensity of the light emitted by the M light-emitting diodes L 2 , L 2 , . . . , L 2  therefore stabilizes, and the M light-emitting diodes L 2 , L 2 , . . . , L 2  are less likely to flicker. 
     The intensity of the light emitted by the incandescent light bulb  41  stabilizes when the average value of the power consumed by the incandescent light bulb  41  stabilizes. Accordingly, the intensity of the light emitted by the incandescent light bulb  41  stabilizes when the duty of the switch signal is adjusted to a power duty Tp calculated through the following Expression 6. Like the duty of the switch signal and the current duty, the power duty Tp is expressed as a percentage (%). 
         Tp= 100·(( Vc−Va 3) 2 /( Vs−Va 3) 2 )  (6)
 
     Here, a voltage value Va 3  is the width of a voltage drop arising in the K internal diodes A 3 , A 3 , . . . , A 3  when current flows in the incandescent light bulb  41 . 
     Expression 6 can be rewritten as the following Expression 7 by dividing both the numerator and denominator in the right side of Expression 6 by a resistance value r 3  of the incandescent light bulb  41 . 
         Tp= 100·(( Vc−Va 3) 2   /r 3)/(( Vs−Va 3) 2   /r 3)  (7)
 
     Assuming that the value of the voltage output by the battery  12  is the prescribed voltage value Vc (=Vb), (Vc−Va 3 ) 2 /r 3  represents a power value Pc consumed by the incandescent light bulb  41  when the switch  20  is on. 
     When the value of the voltage output by the battery  12  is the detection value Vs expressed by the detection value information obtained from the A/D conversion unit  32  by the control unit  30 , (Vs−Va 3 ) 2 /r 3  represents a power value Ps consumed by the incandescent light bulb  41  when the switch  20  is on. As described earlier, the detection value Vs is a value within the fluctuation range of the value of the voltage output by the battery  12 . 
     The power duty Tp is calculated by dividing the power value Pc by the power value Ps. 
     If the duty of the switch signal is adjusted to the power duty Tp calculated using Expression 6 in the drive start process and the duty change process, the average value of the power consumed by the incandescent light bulb  41  is a value obtained by dividing the product of the power value Ps and the power duty Tp by 100, i.e., the power value Pc. 
     Accordingly, when the duty of the switch signal is adjusted to the power duty Tp calculated in Expression 6 in the drive start process and the duty change process, the average value of the power consumed by the incandescent light bulb  41  stabilizes at the power value Pc regardless of the value of the voltage output by the battery  12 . When the average value of the power consumed by the incandescent light bulb  41  is stable, the intensity of the light emitted by the incandescent light bulb  41  stabilizes, and the incandescent light bulb  41  is less likely to flicker. 
       FIG. 8  is a graph illustrating a relationship between the power duty Tp and the detection value Vs. When the detection value Vs is the prescribed voltage value Vc (=Vb), the power duty Tp expressed by Expression 7 is 100%. The power duty Tp drops as the detection value Vs rises from the prescribed voltage value Vc (=Vb). The graph of the power duty Tp in which the horizontal axis represents the detection value Vs is a graph that protrudes downward. 
     As described earlier, a voltage value Va 3  expresses a width of a voltage drop arising in the K internal diodes A 3 , A 3 , . . . , A 3  when current flows in the current path in which the incandescent light bulb  41  is disposed. Assuming a constant detection value Vs in the fluctuation range of the value of the voltage output by the battery  12 , the power duty Tp rises as the voltage value Va 3  drops, and the power duty Tp drops as the voltage value Va 3  rises. 
     The graph of the current duty Ti in which the horizontal axis represents the detection value Vs is, like the graph of the power duty Tp, a graph that protrudes downward. In the third embodiment, the graph of the current duty Ti matches or substantially matches the graph of the power duty Tp when the detection value Vs is a value within the fluctuation range of the value of the voltage output by the battery  12 . At any detection value Vs within the fluctuation range, when a difference (an absolute value) between the power duty Tp and the current duty Ti is, for example, less than or equal to 2%, the graph of the current duty Ti can be considered to substantially match the graph of the power duty Tp. 
     Like the first embodiment, the duty of the switch signal is adjusted to the current duty calculated by the control unit  30  in the third embodiment as well. However, by providing the diode circuit  51 , i.e., the K internal diodes A 3 , A 3 , . . . , A 3 , the graph of the power duty Tp can be caused to match or substantially match the graph of the current duty Ti, which makes it possible to realize a configuration in which the value of the power consumed by the incandescent light bulb  41  stabilizes at the power value Pc. According to the third embodiment, the value of the power consumed by the incandescent light bulb  41  is stable, and thus the intensity of the light emitted by the incandescent light bulb  41  stabilizes and the incandescent light bulb  41  is less likely to flicker. 
     As described above, the K internal diodes A 3 , A 3 , . . . , A 3  are used to stabilize the value of the power consumed by the incandescent light bulb  41 . 
     With the drive device  10  according to the third embodiment, the duty of the switch signal is adjusted in the same manner as in the first embodiment, and thus the drive device  10  according to the third embodiment achieves the same effects as those achieved by the drive device  10  according to the first embodiment. 
     In the third embodiment, the prescribed voltage value Vc is not limited to the lower limit value Vb of the fluctuation range of the value of the voltage output by the battery  12 , and may be any value less than or equal to lower limit value Vb. Accordingly, the prescribed voltage value Vc may be a voltage value less than the lower limit value Vb, as in the second embodiment. Even in this case, the drive device  10  achieves the same effects as those described above. Additionally, the light emission circuit  40  need not include the diode D 2 , and may be constituted by the M light-emitting diodes L 2 , L 2 , . . . , L 2  and the resistor R 2 , for example. In this case, the voltage value Vd 2  is treated as 0 V. 
     In the first to third embodiments, the load that causes the value of the voltage output by the battery  12  to fluctuate is not limited to the starter  13 , and may be any load to which a comparatively large amount of current is supplied. Furthermore, the number of loads to which power is directly supplied from the battery  12  is not limited to one, and may be two or more. In this case, the value of the voltage output by the battery  12  is the lower limit value Vb when all of the loads to which power is directly supplied from the battery  12  are operating, and is the upper limit value Vt when all of the loads to which power is directly supplied from the battery  12  are stopped. 
     Furthermore, the light emission circuit  11  need not include the diode D 1 , and may be constituted by the N light-emitting diodes L 1 , L 1 , . . . , L 1  and the resistor R 1 , for example. In this case, the voltage value Vd 1  is treated as 0V. 
     The first to third embodiments disclosed here are intended to be in all ways exemplary and in no ways limiting. The scope of the present disclosure is defined not by the foregoing descriptions but by the scope of the claims, and is intended to include all changes equivalent in meaning to and falling within the scope of the claims. 
     FIG.  1 ,  7   
     
         
           10  DRIVE DEVICE 
           23  MICROCOMPUTER 
           30  CONTROL UNIT 
           31  STORAGE UNIT 
         P 1  CONTROL PROGRAM 
           32  A/D CONVERSION UNIT 
           33  INPUT UNIT 
           35  OUTPUT UNIT 
           34  INPUT UNIT 
            SWITCH SIGNAL 
           22  VOLTAGE DETECTION UNIT 
           21  DRIVE CIRCUIT 
           13  STARTER 
            DRIVE SIGNAL 
            STOP SIGNAL 
       
    
     FIG.  2   
     
         
            DRIVE START PROCESS 
            START 
         S 1  OBTAIN DETECTION VALUE INFORMATION 
         S 2  CALCULATE CURRENT DUTY 
         S 3  START OUTPUTTING SWITCH SIGNAL 
         S 4  SET FLAG VALUE TO  1   
            END 
       
    
     FIG.  3   
     
         
            DUTY CHANGE PROCESS 
            START 
         S 11  IS FLAG VALUE  1 ? 
         S 12  OBTAIN DETECTION VALUE INFORMATION 
         S 13  CALCULATE CURRENT DUTY 
         S 14  CHANGE DUTY OF SWITCH SIGNAL 
            END 
       
    
     FIG.  4   
     
         
            DRIVE END PROCESS 
            START 
         S 21  STOP OUTPUT OF SWITCH SIGNAL 
         S 22  SET FLAG VALUE TO  0   
            END 
       
    
     FIG.  5 ,  6   
     
         
            VOLTAGE VALUE 
            STARTER OPERATES 
            STARTER STOPS OPERATING 
            CYCLE 
            TIME 
       
    
     FIG.  8   
     
         
            POWER DUTY 
            LOWER 
            HIGHER 
            FLUCTUATION RANGE 
            DETECTION VALUE